Acronos P52, 40-watt Ultralinear PP tube amplifier
Reverse-engineering report based on visual inspection of photographs and cross-checking against manufacturer catalogs. Images come from the video: ACRONOS P52 Amplifier — A work of art. How it was made. and additional interior images provided by the owner of the unit (2026-10).
P52: weight ≈ 40 kg (personal comm., not verified) / 3× EPCOS B43645 610 µF / 450 V OBSERVED in silkscreen (verified on all 3 units, image 2026-10) / Hongfa HF115F relay OBSERVED (probable soft-start/inrush) / blue Bourns trimmers OBSERVED on PCB-S (function ND) / B+, bias, NFB, %UL, Zout, DF, P@1% = ND. Absolute priority: measure the P52's Zout.
GETTING STARTED GUIDE — NO PRIOR KNOWLEDGE NEEDED (Basic guide G1–G6)
GETTING STARTED GUIDE — NO PRIOR KNOWLEDGE NEEDED
If you have never opened a tube amplifier, start here. Only high-school physics (logarithms, electrical power, capacitors, impedance) is needed to understand why this report states its figures so carefully. The technical details start in Part I.
Basic guide: reading this report without being a tube expert
Central idea: no one has measured the Acronos P52, so we don't know how it performs. We do know what is visible inside and what a specific speaker in a specific room requires. With those two pieces and a bit of physics, we can reason without inventing data.
An amplifier takes the weak signal from the player (on the order of 1 V) and converts it into a signal with the voltage and current needed to drive the speaker. In this equipment, thermionic tubes do this in two blocks: a preamp, which increases the signal voltage, and a power stage, which provides the current. Everything is fed by a power supply that produces a high DC voltage, called B+.
Player → Preamp → Power stage → Output transformer → Speaker
(≈ 1 V) (voltage) (current) (adapts impedances) (4–8 Ω)
↑
Power supply (B+)In the power stage, the tubes work in pairs in push-pull: while one conducts more current, the other conducts less, and their currents combine in the output transformer. This has two physical advantages: the DC components of the two tubes cancel in the core (they do not permanently magnetize it) and many even harmonics cancel, which reduces distortion. The class of operation (A, AB, B) and the actual bias mode of the P52's 6550s are ND: they cannot be deduced from photographs.
The output transformer acts as an "adapter": a power tube needs to see a load of thousands of ohms and a speaker has only 4–8 Ω. With a turns ratio n, the impedance is multiplied by n². Illustrative example (not a P52 datum): with n = 25, an 8 Ω speaker is seen from the tubes as 25² · 8 = 5000 Ω.
In an ultralinear (UL) arrangement, the screen grids of the tubes are connected to taps on the transformer. It is a compromise: pentode mode gives more power and triode mode gives less distortion. The manufacturer declares PP UL with 6550 (CONFIRMED (manufacturer)); the %UL tap of the P52 is ND.
This report is not a measurement: it starts from photographs of the manufacturer's video, component catalogs and public documentation. That is why every claim carries an evidence label (the full legend is in the margin, and the method in §1):
| Label | What it means | Example in this report |
|---|---|---|
| OBSERVED | Directly visible in the photographs. | Silkscreen B43645-S7617-M1 on the HV capacitors. |
| PROBABLE | Reasonable physical inference, pending confirmation. | That the three capacitors are in parallel. |
| CALCULATED | Result of a formula from known data. | 35.8 W to reach 95 dB at 3 m. |
| MEASURED-EXT | Measured by an external laboratory. | Published figures for other equipment (context). |
| ND | Cannot be stated with current evidence. | P52's Zout and P@1%. |
To know whether an amplifier "has enough", you must start with the speaker and the listening position. The decibel is a logarithmic scale: +10 dB multiplies power by 10 and +3 dB multiplies it by 2 (100.3 ≈ 1.995). It is calculated as follows, with a generic reference system:
- Sensitivity. The reference speaker produces 89 dB with 1 W at 1 m. HYPOTHESIS for calculation, not measured.
- Distance. The level drops by 20·log₁₀(d) dB as you move away. At 3 m: SPL(1 W, 3 m) = 89 − 20·log₁₀(3) = 89 − 9.54 = 79.46 dB.
- Power. To reach a level L: P = 10^((L − 79.46)/10). For 95 dB it is 35.8 W with a single speaker.
- Stereo. With two speakers, each channel contributes half the acoustic power (−3 dB): 17.9 W per channel.
- Peaks. Music has peaks above its average level. A crest factor of 15 dB multiplies power by 101.5 ≈ 31.6. That is why you must size for the peak and not for the average: 80 dB average with 95 dB peaks.
Tubes need a high DC voltage (B+) obtained by rectifying the mains. In Spain the mains is 50 Hz and, with a full-wave rectifier (a KBU1010 bridge is observed in the P52), the ripple appears at 100 Hz. The filter capacitors charge on the voltage peaks and discharge between them. Since Q = C·ΔV and the time between peaks is 1/f, the voltage drop is approximately:
ΔV ≈ I / (f · C) (approximation: assumes constant current throughout the period and somewhat overestimates the actual ripple)
Example with 200 mA and a 610 µF capacitor at 100 Hz: 0.2 / (100 · 610·10⁻⁶) ≈ 3.28 V peak-to-peak ripple. Three identical capacitors in parallel would add up to 1830 µF and give ≈ 1.09 V, but that only occurs if they are in parallel, which has not been verified ND.
In addition to the capacitors, the P52 carries two toroids compatible with an inductive filter (C-L-C type). A coil opposes rapid current variations with a reactance XL = 2π·f·L. Illustrative example (not a P52 datum): a 1 H coil presents 628 Ω at 100 Hz. Without knowing L, its resistance and the load, one cannot quantify how much ripple they actually eliminate. Detail in §4.2.
A "6 Ω" speaker does not always have 6 Ω: its impedance changes with frequency. The reference speaker drops to a minimum of 4.9 Ω around 1.4 kHz. An amplifier that behaves as a voltage source delivers more current when the impedance drops: with constant voltage, the current into 4.9 Ω is 6/4.9 ≈ 1.22 times that into 6 Ω.
This is where the amplifier's output impedance (Zout) comes in, acting as a series resistance with the speaker. The damping factor is DF = Zload / Zout. Illustrative example (not a P52 datum): with Zout = 1.5 Ω, the voltage that reaches the speaker is that of the divider Vspk = V · Zload / (Zload + Zout). Over 6 Ω it loses 1.94 dB and over 4.9 Ω it loses 2.32 dB: the frequency response partly follows the speaker's impedance curve (0.38 dB difference between the two points). That is why measuring the P52's Zout is the absolute priority.
Zout threshold used by the report: for a relative variation of the response between Zmin and Zmax ≤ 1 dB, Zout ≤ 0.9 Ω. For an absolute loss over Zmin ≤ 1 dB, Zout ≤ 0.5 Ω (recommended design target).
- There are no published measurements for the P52. It has a careful-looking construction, but that does not equal performance: B+, bias, feedback, Zout and P@1% remain undemonstrated.
- The weight (≈ 40 kg) suggests larger transformers, but it is not proof of quality or power, and it is also not verified.
- Neither a photograph nor a table decides whether the amplifier "is better": that requires measurements (THD+N vs power, Zout, response with real load, noise). They are proposed in the Annex (questionnaire for JC Valvular).
Practice exercises:
- A speaker has 92 dB/W/m. What level does it produce with 1 W at 2 m?
- With the 89 dB/W/m speaker of this guide, what power is needed for 100 dB peak at 3 m with a single speaker?
- A filter has 2000 µF and the amplifier draws 300 mA. What is the ripple at 100 Hz?
See solutions
- 92 − 20·log₁₀(2) = 92 − 6.02 ≈ 86 dB.
- 10^((100 − 79.46)/10) ≈ 113 W.
- 0.3 / (100 · 0.002) = 1.5 V.
PART I — ACRONOS P52 REPORT
Map of what we know. From the general to the specific: architecture → boards → power stage → AC input → synthesis. Evidence levels are declared in §1 and applied in each section.
0. Executive summary
20-second read
What it is: ultralinear push-pull with 2× 6550 per channel, 40 W nominal. Preamp 6Ж32П (EF86) + ECC803S (dual triode); gain + phase-inversion function PROBABLE; exact phase-inverter topology ND until traced. Varnished EI transformers in a chassis with metal partition OBSERVED.
What has been identified: three EPCOS/TDK B43645 610 µF / 450 V capacitors — silkscreen B43645-S7617-M1 OBSERVED verified on all three units (image 2026-10); Hongfa HF115F power relay (probable soft-start/inrush) OBSERVED; blue Bourns trimmers on PCB-S (function ND); two toroids with function ND (three coexisting hypotheses); adjustable auxiliary rail PROBABLE with MOSFET + TL431 + Bourns; two relays with separate functions (inrush and mute/DC protection) PROBABLE; power-supply architecture with three open readings {rails with series R/L · single rail with local bypass · single rail without bypass} HYPOTHESIS.
What is NOT demonstrated: actual B+ voltage, effective bank capacitance, exact bias mode of the 6550s, global NFB, ultralinear percentage, P@1%, Zout or DF. The matching of the 6550 quartet is also unverified. The function of the blue trimmers on PCB-S remains untraced.
General description
The Acronos P52 is a 40 W nominal push-pull ultralinear tube amplifier with 6550s in the power stage. The signal PCB (PCB-S) is built on FR4 with blue Bourns 3296Y multi-turn (100 ppm/°C) as visible trimmers — their exact electrical function is ND — fourteen MKP CEC C2N 630-1000 V capacitors plus two paper-in-oil (PIO) capacitors. The power supply PCB (PCB-P) contains three EPCOS/TDK B43645 610 µF / 450 V (silkscreen verified), two toroidal elements of ND function, a Hongfa HF115F relay (probable soft-start/inrush) and independent protection and timing circuitry.
Provisional conclusion
The architecture is consistent with a tube design of greater complexity than a minimal schematic. However, none of the blocks that determine the amplifier's real behavior (B+, 6550 bias, NFB, %UL, Zout, P@1%) is demonstrated. The report stays in the field of provisional reverse engineering, not in that of verified measurements.
1. Scope, method and limitations
1.1 Source of the evidence
- Visual inspection of photographs from the manufacturer's video JC Valvular: ACRONOS P52 Amplifier — A work of art. How it was made.
- Additional interior images (2026-10) provided by the owner of the unit: they verify the silkscreen on the three EPCOS B43645 units, identify the Hongfa HF115F relay, confirm the blue Bourns trimmers on PCB-S and document the complete open-chassis layout.
- Cross-checking against manufacturer catalogs: TDK/EPCOS, Bourns, TI (TL431), JJ Electronic (tubes), Hongfa, Nichicon, Rubycon, Schaffner, Würth.
- Owner's reference system: Crossover A v2.4 MR-A-01 (6 Ω nom / Zmin 4.9 Ω / 89 dB/W/m), listening at 3 m.
1.2 Role glossary
In this report, "owner" is used in different contexts. To avoid ambiguity, the following roles are fixed:
| Role | Who they are | What they contribute |
|---|---|---|
| Reviewer | Author of the report (maty @nauscopio) | Reference system, calculations, report structure |
| Reference system owner | Owner of the Crossover A v2.4 MR-A-01 | Sensitivity, Zmin, listening conditions (89 dB/W/m, 3 m) |
| P52 unit owner | Person with physical access to an Acronos P52 unit | Declaration of the Schaffner inlet, additional interior images (2026-10) — EPCOS silkscreen, Hongfa relay, blue trimmers on PCB-S |
| Manufacturer | JC Valvular (Juan Calatayud) | Manufacturing video, 40 W commercial datasheet, design and components |
| Weight informant | Blog reader | Personal communication: P52 ≈ 40 kg, not verified |
2. Overall architecture — big picture
2.1 Block diagram with evidence levels
230 V MAINS
│
+-- [EMI cap obs.], NTC, RF-EMI filter (no L/N choke identified)
│
+-- Power transformer
│
↓
RECT KBU1010
│
↓
C HV EPCOS B43645 610/450 --- OBSERVED (silkscreen x3 verified)
│
↓
L1 toroidal --- FUNCTION ND (3 hypotheses)
│
↓
C / HV rail --- PROBABLE
+-----------------+
│ │
power stage auxiliary
6550 / OPT MOSFET + TL431 2.495 V
+ blue Bourns
REGULATING FUNCTION PROBABLE
voltage and destination ND
possible relation to 6550 bias HYPOTHESIS
[HYPOTHESIS]: three open readings of power-supply architecture
a) separate rails with series R or L (power / driver / input)
b) single rail with local bypass (capacitors in parallel, no R or L)
c) single rail without local bypass
The number of B+ wires entering PCB-S and their routing are ND.
Hongfa HF115F relay OBSERVED (probable soft-start/inrush)
+ Finder mute/DC PROB + NE555 + NE5532 (functions PROB)
Blue Bourns trimmers OBS on PCB-S, function ND
(individual 6550 bias? DC balance of the inverter? signal-stage adjustment?)
Blacks on PCB-P: possible DC threshold and RC timing (constants ND)
6550 without a clearly associated trimmer OBS on PCB-S → fixed bias
by divider, by cathode, or from auxiliary rail PROB;
method and matching of the quartet, ND
2.2 Overall topology
The Acronos P52 is a stereo ultralinear push-pull, with 2× 6550 per channel, 40 W nominal per channel. The small-signal stage relies on a 6Ж32П (EF86) as input pentode and an ECC803S (dual triode, JJ Electronic A.F. twin triode) for gain + phase inversion. The chassis houses three varnished EI transformers, with a metal partition separating the power supply from PCB-S OBSERVED.
2.3 Functional blocks
| Block | Contents | Section | Overall status |
|---|---|---|---|
| AC input and RF/EMI | Capacitors after IEC, NTC, PE, no L/N choke identified | §6 | PROBABLE general structure; class X/Y ND |
| Power supply board (PCB-P) | KBU1010, EPCOS B43645 HV, toroids, 10W10KJ, MOSFET + TL431, Hongfa HF115F relay, Songle and Finder relays, NE555/NE5532, blue and black Bourns | §4 | PROBABLE by components; exact functions ND |
| Small-signal board (PCB-S) | EF86, ECC803S, 14 MKP CEC C2N, 2 PIO, blue Bourns 3296Y, blue precision resistors, signal diodes, 2 small electrolytics | §3 | PROBABLE layout; exact function of trimmers and topology ND |
| Power stage | 4× 6550, EI OPT, bias | §5 | Bias mode, NFB, %UL ND |
2.4 What determines real behavior and why it is ND
- Actual B+ voltage and its behavior under load.
- Bias mode of the 6550s (and therefore the class of operation).
- Global feedback (NFB): presence and depth.
- Ultralinear percentage (%UL) of the OPT taps.
- P@1% and THD+N vs power curve.
- Zout and damping factor (DF).
- Effective capacitance of the HV bank.
- Matching of the 6550 quartet.
- Power-supply architecture: number and separation of rails (§4.6).
- Exact function of the blue trimmers on PCB-S: number, destination (6550 bias vs signal stage) and adjustment method.
3. Small-signal board (PCB-S)
3.1 Observed components
JC Valvular FR4 PCB with four noval holes for the EF86 and the two ECC803S triodes. Fourteen MKP CEC C2N (630-1000 V) plus two paper-in-oil (PIO) capacitors. Green MOX resistors 10K28 2 W and white 1 W. Blue Bourns 3296Y trimmers (100 ppm/°C, 25-turn) observed — their exact number and electrical function are ND. No black Bourns on PCB-S.
In the images provided (2026-10) the following are also observed: blue precision resistors (MOX or metal film), signal diodes, and two small electrolytic capacitors near the noval sockets. The function of these small electrolytics is ND; the most probable reading is cathode bypass (local decoupling of the cathode resistor), not rail decoupling. They are treated as neutral evidence regarding the rail-separation hypothesis (§4.6), not as evidence in favor.
The density of components and the symmetrical layout of the two channels are OBSERVED and consistent with a careful build; the absence of a trimmer clearly associated with each 6550 in the available images leaves the bias question open (§3.2).
3.2 Probable topology
The ECC803S is a dual triode (JJ Electronic A.F. twin triode): two independent triode systems in a single noval envelope. With one ECC803S per channel and one EF86 per channel, the classic combinations are:
- EF86 (input) → ECC803S as LTP with both triodes. This is the classic combination in this type of amplifier: the EF86 provides voltage gain and the ECC803S differential pair performs the phase inversion with high common-mode gain. MAIN HYPOTHESIS, not observed in the photographs.
- EF86 (input) → one triode of the ECC803S as gain stage → the other as cathodyne or driver. ALTERNATIVE HYPOTHESIS.
- EF86 (input) → one triode of the ECC803S as gain stage → the other as an incomplete second LTP (with asymmetric load). Unusual, but not impossible. MINOR HYPOTHESIS.
Probable reading (common to all three hypotheses):
RCA → EF86 (pentode, Ra 100 kΩ)
→ C → ECC803S (dual triode)
[hypothesis 1: LTP with both triodes]
[hypothesis 2: gain + cathodyne/driver]
[hypothesis 3: gain + asymmetric driver]
→ 2× C → 6550 grids
Gain budget as a discrimination method (estimate, not measurement). The manufacturer's commercial datasheet declares an input sensitivity for the P52. Although that figure carries no published condition, it allows an order of magnitude: if the input sensitivity is, for example, on the order of 1 Vrms for full power and the output stage needs on the order of 30–50 Vpeak at the 6550 grids, the total required gain is on the order of 30–50× (≈30–34 dB). That is compatible with EF86 (gain ≈100–200×) followed by an ECC803S LTP (gain ≈10–20×), with global feedback reducing the closed-loop gain. The gain budget is compatible with hypothesis 1 (EF86 → LTP), but does not prove it: per-stage gain measurements are needed. PROBABLE.
| Hypothesis | How it works | Practical implication | Evidence |
|---|---|---|---|
| Individual bias with 4 trimmers | Four blue Bourns trimmers on PCB-S individually adjust the bias of each 6550. | Maximum maintainability: allows compensating the quartet without replacement. | HYPOTHESIS — requires counting trimmers and tracing tracks. |
| Fixed bias by resistive divider | Fixed negative voltage by fixed-value resistive divider (the blue trimmers would have another function). | Requires adequate matching of the quartet when replacing tubes. | PROBABLE; mode ND. |
| Cathode bias | Power cathode resistors with decoupling. | Partial self-adjustment; verify cathode dissipation. | ND — not identified. |
| Bias from auxiliary rail | Adjustment from PCB-P (MOSFET + TL431 + blue Bourns). | Would allow adjustment without specific trimmers on PCB-S. | HYPOTHESIS — requires tracing. |
3.3 Probable function of the blue Bourns on PCB-S
The blue Bourns 3296Y (100 ppm/°C, 25-turn, 0.5 W) are the only visible trimmers on PCB-S. Their exact number has not been counted in the available photographs: this is an ND datum. Their exact electrical function is also ND; by position, they could be involved in the DC adjustment of the small-signal stage, of the phase inverter or (if there are four) in the bias of the 6550s. Counting the trimmers is one of the pending actions with the greatest discriminating power over the topology.
3.4 Cross-reference
The exact bias mode is developed in §3.2 and §5.2. The trimmers are cataloged in §4.5 and Part II §7. The power-supply architecture is developed in §4.6.
PCB-S images
4. Power supply board (PCB-P)
On PCB-P the following are visually identified: KBU1010 (10 A rectifier), input MKP capacitors, two toroids, a resistor marked 10W10KJ, three large EPCOS/TDK B43645 (silkscreen B43645-S7617-M1 verified on all three units, image 2026-10), a 100 µF / 450 V capacitor, TO-220 heatsinks, TL431, Hongfa HF115F power relay OBSERVED (probable soft-start/inrush), Songle SRA and Finder relays, NE5532 + NE555, and two blue Bourns + two black.
Additionally, in the images the following are observed: two small green resistors next to the white resistor (possible discharge pair or divider), diodes and transistors in the right area, a small transformer (possible common-mode choke or auxiliary transformer) and several auxiliary electrolytics (blue and black) scattered around, compatible with more complex energy management than a simple single rail.
Direct reading of the silkscreen in the 2026-10 image confirms the marking on all three units of the HV bank:
The TDK catalog documents the B43645 family in at least two variants:
| Family | Format | Typical ESR | Admissible ripple @ 60 °C / 105 °C | Note |
|---|---|---|---|---|
| B43645 variant A | 35 × 40 mm | 170 mΩ | 5.39 A / 2.09 A | TDK datasheet marked as preliminary data |
| B43645 variant B | 30 × 50 mm | 160 mΩ | 6.02 A / 2.33 A | Alternative geometry documented by TDK |
Four possible readings of the wiring (all HYPOTHESES except the first)
| Reading | Description | Status |
|---|---|---|
| Three in parallel | The three B43645 form a single 1,830 µF nominal bank on the main B+ rail. | PROBABLE — simplest reading, consistent with a common bank |
| Per-channel banks + shared | Two capacitors are the L and R power banks, and the third a shared signal or bias bank. | HYPOTHESIS |
| Two for power + one for signal | Two capacitors form the per-channel power bank and the third is a shared driver/input bank. | HYPOTHESIS |
| One per rail (power/driver/input) | One capacitor per supply rail. Barely plausible for driver and input: a signal rail does not need 610 µF / 450 V. The report already observes a 100 µF / 450 V capacitor that fits that function better. | HYPOTHESIS — compatible with §4.6, but does not raise the weight of the three-rail hypothesis |
Note: the numerical coincidence "three capacitors / three rails" proves nothing. The number of capacitors and the number of rails are independent data. The "one per rail" reading is documented for completeness, not as an argument in favor of rail separation.
Why a C-L-C structure would reduce ripple at 100 Hz
X_L = 2π · f · LAt f = 100 Hz, if the toroids are series inductances of a C-L-C and their reactance at 100 Hz is significant compared to the impedances of the rest of the filter, they can provide additional ripple attenuation. Without knowing L, DCR, associated capacitance and load, the actual attenuation of the P52 cannot be quantified.
Two toroidal elements are observed in the power-supply area, associated with capacitors, not at the 230 V mains input. Their physical position makes it very unlikely that they are common-mode chokes of the input EMI filter. The images do not allow their silkscreen or connection to be seen.
| Functional hypothesis | Technical basis | Status |
|---|---|---|
| Audio-frequency DC filtering (B+ rail choke) | Position on PCB-P associated with capacitors and not at the IEC input | PROBABLE by position · ND by function |
| HF/EMI or ringing suppression | Reduced physical size; damping of rectifier switching spikes | CANDIDATE HYPOTHESIS |
| Local filtering of auxiliary DC lines | Low-voltage decoupling for filaments or bias | CANDIDATE HYPOTHESIS — the least substantiated |
10W10KJ resistor
On PCB-P the following are observed: TO-220 heatsinks, TL431 (reference ~2.495 V) and two blue Bourns 100 ppm/°C. The combination MOSFET + TL431 + Bourns is compatible with an adjustable rail.
| Color / position | Temp. coefficient | Probable function | Evidence |
|---|---|---|---|
| Blue (PCB-S) | 100 ppm/°C | Small-signal stage adjustment / possible DC balance of the inverter / if there are four, individual bias of the 6550s | OBSERVED the trimmer · exact number ND · function ND |
| Blue (PCB-P) | 100 ppm/°C | Regulated rail (with MOSFET + TL431); possible relation to 6550 bias — exact function ND | PROBABLE |
| Black (PCB-P) | ND | Possible threshold/timing or other control function | HYPOTHESIS |
4.6.1 The question
The provided images show PCB-S with many small-signal components (14 MKP capacitors, 2 PIO, precision resistors, diodes, small electrolytics near the sockets, blue trimmers). They also show PCB-P with the power bank, toroids, Hongfa relay and auxiliary electrolytics. The question is whether the P52 electrically separates the supply of the 6550s, the ECC803S and the EF86, or whether they all share a single B+ rail.
In a 40 W push-pull, the four 6550s draw a pulsating current: the instantaneous demand varies with the music signal, and that variation modulates the B+ rail. If that modulation reaches the small-signal stages, it introduces intermodulation, loss of linearity and unwanted channel coupling. Rail separation is a classic technique to avoid this.
4.6.2 The three possible architectures
There are not two options (separate / not separate), but three physically distinct architectures:
(a) RAILS WITH SERIES R OR L
Main B+
│
├── B+ POWER ──────────► 6550 (high current, pulsating)
│
├── R1 + C3 ───────────► B+ DRIVER (ECC803S)
│
└── R2 + C4 ───────────► B+ INPUT (EF86)
Series resistors R1 and R2, with C3 and C4 to ground, form
low-pass filters that DO attenuate the modulation from the
power stage. fc = 1/(2π·R·C), typically a few Hz.
(b) SINGLE RAIL WITH LOCAL BYPASS
Main B+
│
└── (no series R or L) ──► all tubes
│
├── C_local next to each socket
The capacitor in parallel reduces the local impedance at
high frequency, but does NOT isolate audio-frequency
modulation: without series R or L, the pulsating current
of the power stage keeps flowing through the same rail
up to the signal stages.
(c) SINGLE RAIL WITHOUT LOCAL BYPASS
Main B+ ──► all tubes, without local capacitors
next to the sockets.
4.6.3 What is observed in the P52
| Evidence | What it suggests | Status |
|---|---|---|
| Common filter bank on PCB-P (three EPCOS B43645 with the same silkscreen) | Single main B+ rail or parallel per-channel banks | OBSERVED |
| In the available photographs no power resistors in series with B+ branches are observed on PCB-P | Probable absence of architecture (a) on PCB-P | OBSERVED (limited to available photographs) · inference PROBABLE |
| Two small electrolytic capacitors near the noval sockets of PCB-S | Most probable reading: cathode bypass (local decoupling of the cathode resistor), not rail decoupling | HYPOTHESIS — neutral evidence regarding rail separation, not in favor |
| Several auxiliary electrolytics (blue and black) scattered on PCB-P, in addition to the main HV bank | Possible more complex energy management than a single rail without bypass; compatible with (a) or (b) | HYPOTHESIS — weak evidence, not conclusive |
| In the available photographs no power resistors in series with the B+ lines are observed on PCB-S | Probable absence of architecture (a) on PCB-S | OBSERVED (limited) · inference PROBABLE |
| Cable bundle between PCB-P and PCB-S | Cannot count wires or trace route | ND |
4.6.4 The three architecture hypotheses
| Architecture | Description | Evidence in favor | Evidence against | Status |
|---|---|---|---|---|
| (a) Rails with series R or L | B+ power → 6550; B+ driver → ECC803S with RC network; B+ input → EF86 with RC network. Possible per-channel separation (L/R). | It is the classic technique in high-quality designs; the toroids could be part of the filtering; the scattered auxiliary electrolytics are compatible; the coincidence of three capacitors and three rails is suggestive. | No power resistors in series are observed in the photos; the filter bank appears common; the 3/3 coincidence proves nothing. | HYPOTHESIS — not raised by the numerical coincidence |
| (b) Single rail with local bypass | Single B+ for all tubes, with local capacitors next to the sockets (no series R or L). | Common filter bank; absence of power resistors in series in the photos; the small electrolytics fit cathode or local rail bypass. | Audio-frequency modulation is not attenuated without series R or L. | PROBABLE — most conservative reading with current evidence |
| (c) Single rail without local bypass | Single B+, no local capacitors next to the sockets. | No local capacitors of clear value are observed next to the power sockets. | Two small electrolytics are observed near the noval sockets; compatible with cathode bypass. | MINOR HYPOTHESIS |
4.6.5 B+ ripple in push-pull: cancellation in the OPT
In a push-pull stage, the B+ ripple present on the plates of the two output tubes appears in phase at the two ends of the output transformer primary. The OPT largely rejects that common-mode component: most of the ripple is not transferred to the secondary. This means that B+ ripple on the plates is not the main source of audible noise in a well-designed PP.
What is critical is the ripple that reaches the driver and phase-inversion stages: those stages work in differential mode with respect to the signal, and any rail modulation becomes a spurious signal that is amplified and reaches the speaker. That is why rail separation (architecture a) makes sense precisely in the driver and input, not in the power stage. And that is why the question in §4.6 is not cosmetic: it defines whether the driver and the phase inversion are protected from rail modulation.
4.6.6 How the question would be resolved
To close this question, more photographs are not enough: tracing and measurement are needed. The specific steps are:
- Photo of the underside of PCB-S (the solder side), where the tracks are visible and one can count how many B+ conductors enter.
- Photo of the connector or B+ entry point to PCB-S, to count wires and see whether there are power resistors in series.
- Count the Bourns trimmers on PCB-S (Annex P14): if there are four, they are the 6550 bias; if there are two, they are the signal-stage adjustment.
- Voltage measurement at the B+ entry point to PCB-S and at the plate pins of EF86 and ECC803S, at rest and with signal.
- Ripple measurement at each point with an oscilloscope (AC coupled) at rest and with signal.
4.6.7 Relation to the toroids
If the two toroids on PCB-P are DC filtering chokes in the main B+ rail, they are already contributing to reducing rail modulation. In that case, rail separation would be a complementary measure. If the toroids are HF/EMI suppressors, rail separation would have a clearer basis, because there would be no audio-frequency inductive filtering in the main rail.
5. Power stage
5.1 Tubes and dissipation
Four 6550s are observed (2 per channel), visually identified as Tung-Sol in the manufacturer's video. The plate dissipation in operation is ND: it requires measuring V_plate-cathode · I_cathode. The procedure is detailed in Part II §8.
5.2 Bias mode
Four working hypotheses now coexist since blue trimmers were observed on PCB-S (image 2026-10): individual bias with four trimmers, fixed bias by resistive divider, cathode bias, or bias from a regulated auxiliary rail (§4.4). Full development in §3.2. The exact mode remains ND until track tracing, trimmer counting (P14) or asking the manufacturer. The class of operation (A, AB, B) is also ND: it is deduced from the bias, which is not demonstrated.
5.3 Open questions
| Parameter | Status | Section where it is treated |
|---|---|---|
| Global NFB | ND | Annex P6 |
| %UL | ND | Annex P7 |
| Zout and DF | ND | Annex P2 |
| P@1% and THD+N vs power curve | ND | Annex P1 |
| 6550 quartet matching | ND | Part II §8 |
| Power-supply architecture | HYPOTHESIS (a) R/L rails · PROBABLE (b) single rail with local bypass | §4.6, Annex P10 |
| Phase-inverter topology | HYPOTHESIS (EF86 → main LTP); ND confirmation | §3.2 |
| Function of the blue trimmers on PCB-S | OBSERVED · function ND | §3.3, §4.5, Annex P14 |
5.4 Output transformers
Three varnished EI transformers are observed (power supply + two outputs), with a metal partition separating the power supply from PCB-S OBSERVED. Primary impedance, turns ratio, leakage inductance and frequency behavior are ND without measurement.
6. AC input and RF/EMI — observed status
6.1 Observed status of the input filter
In the available photographs, capacitors of approximately 0.1 µF / 1000 V are observed after the IEC connector, compatible with an EMI suppression function. The X/Y safety class is not established by the visible capacitance and rated voltage. No L/N common-mode choke is identified. The two toroids analyzed in §4.3 are not mains RF/EMI elements, given their physical position on PCB-P.
6.2 PE and chassis
The chassis appears directly bonded to PE PROBABLE.
6.3 Y capacitors
They are not conclusively identified in the photographs; their presence is PROBABLE by position, ND by tracing.
6.4 What this report does NOT propose to modify here
8. Conclusion and traceability
8.1 Acoustic model — conditional scenarios
The power needed for 95 dB depends on how the speaker's sensitivity has been defined: 1 W/1 m, 2.83 V/1 m, or a direct measurement at the listening distance.
| Reading / condition | Level at listening position | Power for 95 dB | Margin over 40 W |
|---|---|---|---|
| 89 dB @ 1 W / 1 m; listening at 3 m, free field | ≈79.5 dB | ≈35.8 W | +0.5 dB |
| 89 dB @ 2.83 V / 1 m; 5.3 Ω (1.51 W); listening at 3 m, free field | ≈79.5 dB | ≈54.1 W | −1.3 dB |
| 89 dB SPL with 1 W measured directly at 3 m | 89 dB | ≈4.0 W | +10.0 dB |
Calculation: power = 10^((95 − level per 1 W)/10), margin = 10·log10(40/power). Methodological note: this table is mono in free field.
8.2 Synthesis of the report
PCB-S shows an elaborate construction, with multiple coupling and filtering capacitors, blue Bourns trimmers observed (exact number and function ND) and blue precision resistors. In the power supply, three EPCOS/TDK B43645 whose silkscreen is B43645-S7617-M1, 450 V, 610 µF — OBSERVED verified on all three units (image 2026-10). The effective capacitance of the assembly remains ND: the wiring admits at least four readings (three in parallel, per-channel banks + shared, two power + one signal, one per rail), and only the first would give 1,830 µF.
The two toroids in the power-supply area admit three coexisting functional readings: audio-frequency DC filtering (PROBABLE by position, ND by function), HF/EMI or ringing suppression (CANDIDATE HYPOTHESIS), or local filtering of auxiliary lines (CANDIDATE HYPOTHESIS). Their real function is ND until L, DCR, I_sat are measured and the connection traced. The scattered auxiliary electrolytics on PCB-P (observed in 2026-10) suggest more complex energy management than a single rail without bypass, without closing the question.
The Hongfa HF115F relay is OBSERVED by its position, compatible with a soft-start or inrush-limiting system; its exact function remains PROBABLE until traced.
The combination MOSFET + TL431 + Bourns makes a regulated auxiliary rail plausible, but its voltage and destination remain ND; a relation to the biasing of the 6550s is not ruled out.
The power-supply architecture admits three physically distinct readings: (a) separate rails with series R or L, (b) single rail with local bypass (capacitors in parallel, no R or L), (c) single rail without local bypass. The most conservative reading with current evidence is PROBABLE single rail with local bypass. The numerical coincidence "three capacitors / three rails" does not raise the weight of hypothesis (a): the number of capacitors and the number of rails are independent data. Resolution requires tracing the underside of the PCB, counting trimmers and measuring voltage and ripple at the key points (§4.6).
The ECC803S is a dual triode. With one ECC803S per channel and one EF86 per channel, the classic combination is EF86 (input) → ECC803S as LTP with both triodes MAIN HYPOTHESIS; the gain budget is compatible with it, but does not prove it. The alternatives (gain + cathodyne, gain + asymmetric driver) remain open. The range of hypotheses for the bias has widened with the observation of the blue trimmers on PCB-S: four candidates coexist (individual bias, fixed divider, cathode or auxiliary rail). The class of operation (A, AB, B) is ND, because it is deduced from the bias. The Bourns on PCB-P have function PROBABLE by position, ND by tracing. Global feedback, ultralinear percentage, P@1%, Zout and DF also remain ND.
8.3 What cannot be concluded from this report
- That the P52 actually meets the 40 W at a given THD — THD+N vs power measurement is missing.
- That the effective filtering capacitance is 1,830 µF — it depends on the wiring, with four open readings.
- That the B+ voltage is 450 V or any other — not measured.
- That the bias mode of the 6550s is fixed by divider, by cathode, from auxiliary rail, or individual with four trimmers, nor what the class of operation is.
- That the blue trimmers on PCB-S have a bias function, DC balance of the inverter or signal-stage adjustment, nor exactly how many there are.
- That the 6550s are matched — not verified.
- That global NFB exists or its depth — not documented.
- That the ultralinear percentage is the usual 43% reference in PP UL.
- That the exact phase-inverter topology is LTP, cathodyne or another.
- That the two toroids have a determined function — three coexisting hypotheses.
- That there are separate supply rails with series R or L — it is a HYPOTHESIS, not an observed fact.
- That the blue or black Bourns have functions determined by their color or position.
- That the input capacitors are of X/Y safety class.
- That the Hongfa, Songle and Finder relays perform specific functions — plausible, not demonstrated.
- That the P52 sounds better or worse than other amplifiers — outside the scope of this report.
PART II — SPECIAL EDITION: WHICH COMPONENTS TO CHANGE OR ADD?
PART II — SPECIAL EDITION: WHICH COMPONENTS TO CHANGE OR ADD?
Phase 0 — Characterization (absolute prerequisite) → Protocol A safety (HV, differential probe, common-mode) → B power supply/start-up (B+ at rest/load, ripple, EPCOS temp, B4 ESR, B5 toroids, B6 auxiliary rail, B7 NE555 vs filaments sequence, B8 inrush, B9 Hongfa relay and auxiliary relays B9 + B2b cathode current of each 6550) → C performance (NFB, stability, THD, P@1%, Zout/DF, response, C6 noise with weighting and 50/100 Hz spectrum, C7 4/8 Ω THD+N) → D components (C2N, D2 resistors, D3 Bourns, D4 wiring, D5 EMI, D6 brand substitution) → S4.
20-second read
What it is: conditional protocol for a possible special edition of the Acronos P52. It is not a list of improvements nor a catalog of boutique components.
What it contains: for each area, first the current status of the Acronos P52 and what remains to be known; then, when specific components were proposed, they are collected as AUDIOPHILE PROPOSAL — S3. Interventions justified by safety, reliability, EMI or performance are not S3: they are classified according to their purpose.
What it is NOT: it is not recommended to replace anything in the Acronos P52 without prior measurement. The possible sonic difference of a boutique component is not a demonstrated fact.
Key phrase: the eventual special edition of the Acronos P52 is not defined by a list of boutique components; it is defined by what characterization demonstrates deserves to be changed and by what can subsequently be verified.
Safety warning before any measurement
Procedure
- Disconnect from mains and ensure it cannot be re-energized.
- Wait, without assuming discharge.
- Verify residual voltage on the bank with an instrument and probes rated for that voltage.
- If it persists, discharge with a dimensioned resistor, never with a screwdriver.
- Re-verify after a few minutes.
- Only then, with the equipment unplugged, attach the clips.
- To measure the live drop: energize, turn off and read without touching.
- Use a high-voltage differential probe and rated probes; specify differential and common-mode voltage.
S0–S4 Scale — Definition
| Label | Meaning | Purpose |
|---|---|---|
| S0 — Reference | No intervention: either because the device meets spec, or because there is not enough evidence to intervene | Valid outcome of the process |
| S1 — Reliability | Changes justified by reliability or safety | Verifiable problem |
| S2 — Performance | Changes with demonstrable electrical improvement | Measurable Δ |
| S3 — Listening | Substitutions evaluated through controlled listening | Reproducible perceptual difference |
| S4 — M&M | Characterization, matching and documentation | Traceability |
0.1 — Minimum instrumentation
- HV differential probe with maximum differential voltage, maximum common-mode voltage, category, maximum input voltage and connection conditions to the oscilloscope specified.
- Rated probes for the voltage and measurement category.
- Oscilloscope with declared bandwidth (≥20 MHz; ≥100 MHz for RF/EMI).
- Dimensioned discharge resistor.
- IR or contact thermometer.
- LCR/ESR meter with bank discharged and isolated.
1. Coupling capacitors
14 × MKP CEC C2N + 2 PIO — OBSERVED (image 2026-10). Individual function — ND.
Audiophile proposal — S3 · LISTENING HYPOTHESIS
| Reference | Type | Status |
|---|---|---|
| Mundorf MCap EVO Oil / Supreme | High-end MKP | AUDIOPHILE PROPOSAL |
| ClarityCap CSA / MR | High-end MKP | AUDIOPHILE PROPOSAL |
| Jupiter HT | Modern paper-oil | AUDIOPHILE PROPOSAL |
| V-Cap OIMP / CuTF | Paper-oil / Teflon | AUDIOPHILE PROPOSAL |
| Solen Fast Cap | Economical MKP | AUDIOPHILE PROPOSAL |
| Auricap | Economical MKP | AUDIOPHILE PROPOSAL |
2. Power supply capacitors
3 × EPCOS/TDK B43645 610 µF / 450 V — OBSERVED silkscreen B43645-S7617-M1 verified on all three units (image 2026-10). Wiring — ND (four open readings, see §4.2).
About voltage margin: this report adopts the guideline that the actual B+ does not exceed 85–90% of the 450 V rating (i.e., B+ ≤ 382–405 V). That guideline is a longevity decision of this report, not a manufacturer standard. TDK/EPCOS datasheets document service life as a function of voltage and temperature; they do not impose a 90% limit. Applying this guideline:
- If B+ ≤ 405 V: the bank operates within the adopted longevity margin. Replacement for listening or improvement: S3.
- If B+ > 405 V: the bank operates above the adopted margin. Replacement moves from S3 to S1 (reliability), without implying that the manufacturer considers the equipment out of specification.
Audiophile proposal — S3 · LISTENING HYPOTHESIS
| Reference | Status |
|---|---|
| Mundorf MLytic AG / HC | AUDIOPHILE PROPOSAL — verify 610 µF / ≥450 V / format |
| Nichicon KG / Fine Gold | DISCARD DUE TO VOLTAGE — series ≤100 V |
| Nichicon KW | DISCARD DUE TO VOLTAGE — KW: 6.3–100 V |
| Rubycon MXG / MXH | S3 — MXH not verified |
| Jensen / ASC | AUDIOPHILE PROPOSAL — verify voltage/capacitance/format |
3. Resistors
In the Acronos P52: green MOX 2 W (10K28) and white 1 W — OBSERVED. Function and actual dissipation — ND.
Audiophile proposal — S3 · LISTENING HYPOTHESIS
| Function | Reference | Status |
|---|---|---|
| Plate and signal cathode | Takman Rey, Carbon Film, Vishay Dale CMF / RN | AUDIOPHILE PROPOSAL |
| Feedback and grid stoppers | PRP, Shinkoh, Amtrans AMRG | AUDIOPHILE PROPOSAL |
| Power (bias, cathodes) | Mills, Ohmite non-inductive | AUDIOPHILE PROPOSAL |
4. Decoupling and bypass
In the Acronos P52: function and values — ND. A 100 µF / 450 V capacitor is documented OBSERVED, along with several small electrolytics near the sockets of PCB-S and several auxiliary electrolytics scattered on PCB-P (image 2026-10). The most probable reading for the small ones on PCB-S is cathode bypass (see §4.6.3).
Audiophile proposal — S3 · LISTENING HYPOTHESIS
- Add or improve local decoupling of the signal stage with low-inductance MKP or polypropylene.
- In the regulated auxiliary rail (if confirmed): improve filtering and bypass of the MOSFET + TL431.
- If measurement confirms that the architecture is single rail with local bypass (b) or without bypass (c), study series RC networks for driver and input (§4.6). Classification: S2 if a measurable Δ in ripple is demonstrated, not S3.
5. Internal wiring
In the Acronos P52: twisted looms — OBSERVED. Gauge, material, routing — ND.
Audiophile proposal — S3 · LISTENING HYPOTHESIS
- Replace signal wiring with OCC copper or silver (Neotech, DH Labs, Mundorf).
- HV and filament wiring with adequate gauge and Teflon or silicone insulation.
- Maintain twisting and routing to minimize pickup.
6. Power supply and protections — PSU V2 and DC Blocker
PSU V2 definition
PSU V2 is the proposed power-supply architecture for a possible revision of the P52, conditioned on prior characterization (Phase 0). Its topology is:
ORIGINAL EI TRANSFORMER
│
▼
SILICON RECTIFICATION (KBU1010)
│
▼
C1 MODERATE ─── first capacitor after the rectifier
│
▼
HT CHOKE ───── audio-frequency inductive filtering
│
▼
C2 MAIN ────── main capacitor bank
│
├── B+ POWER ────► 6550
│
├── R1 + C3 ─────► B+ DRIVER (ECC803S)
│
└── R2 + C4 ─────► B+ INPUT (EF86)
The four capacitors of PSU V2 are: C1 (first capacitor after the rectifier, moderate), C2 (main bank after the choke), C3 (driver rail decoupling), C4 (input rail decoupling).
HT filter choke
If measurement shows that the toroids are not audio-frequency chokes, adding a 5–10 H choke between C1 and C2 is studied. If they already are, it is reevaluated to avoid redundancy.
Common-mode choke
Only studied if a conducted EMI or background noise measurement demonstrates a problem attributable to the mains input.
X and Y capacitors
The capacitors observed after the IEC do not have an established X/Y class without marking or tracing. If Y capacitors are added, consider leakage current to ground (touch current) as a safety limit.
Relays
A Hongfa HF115F relay is observed on PCB-P (probable soft-start/inrush) and Songle SRA and Finder relays are reported. Gold-contact relays are for low-level signal and are not suitable for switching power.
Soft-start and start-up sequence
Before modifying: measure inrush current, HV bank charging time, NE555 delay vs filaments, mute relay behavior and Hongfa HF115F relay behavior. Adjust the NE555 delay to 40–60 s if the current value does not guarantee that the 6550 filaments reach their emission temperature before B+ is applied (S1 — tube safety).
10W10KJ resistor
Marking 10W10KJ, probable interpretation 10 kΩ / 10 W / ±5%. Function — ND.
DC Blocker at the mains input — S1 proposal
A DC Blocker at the IEC input is a circuit of two diodes and two electrolytic capacitors in antiparallel that blocks the DC component that may be present in the mains.
Why it is relevant in a large tube amplifier: EI transformers with iron cores are vulnerable to mains DC offset. An offset of just a few hundred mV can asymmetrically shift the core's hysteresis cycle, causing:
- Asymmetric core saturation.
- Audible mechanical hum (hum) unrelated to power-supply ripple.
- Increased quiescent current in the primary.
- Additional transformer heating.
Classification: S1 — Reliability. It is not an audiophile proposal (S3) nor a measurable performance improvement (S2). It is a reliability and transformer-longevity intervention.
Cost: two silicon rectifier diodes (for example, 1N5408) and two electrolytic capacitors of 10,000–22,000 µF / 10 V in antiparallel. Not much more.
Implementation condition: first measure the actual DC offset on the owner's mains. If the offset is negligible (<50 mV), the DC Blocker contributes nothing and should not be installed. If the offset is significant (>200 mV) or if the transformer already presents mechanical hum not attributable to power-supply ripple, installation is justified.
Bias servo — out of scope
NFB switch — discarded due to redesign
Proposals classified by purpose
| Proposal | Classification |
|---|---|
| DC Blocker at IEC input (two diodes + two electrolytics in antiparallel) | S1 — transformer reliability. Conditional on measuring mains DC offset |
| Adjust NE555 delay to 40–60 s | S1 — 6550 cathode protection |
| PSU V2: HT filter choke (5–10 H) + rail separation (R1/C3, R2/C4) if the toroids are not audio-frequency chokes and modulation in driver/input is high | S2 if a measurable Δ is demonstrated — not S3 |
| Common-mode choke at mains input (Würth WE-CMB or equivalent) | S2 if a measurable Δ is demonstrated — not S3 |
| Improve or add safety Y capacitors | S1 (safety) — not S3. Verify touch current |
| Replace relays with Panasonic, Omron or gold-contact | Low-level signal: S3 · Power/protection: S1. Gold not suitable for power |
| NFB switch (0/6/12 dB) | DISCARDED — topology redesign |
7. Trimmers (Bourns)
Blue 100 ppm/°C documented (3296). Black: temperature coefficient ND. The exact number of trimmers on PCB-S has not been counted (see Annex P14): if there are four, they are almost certainly the individual bias of the 6550s, and that would change §3.2 and §5.2; if there are two, they would be intended for the signal stage or the phase inverter.
| Color / position | Coeff. | Probable function | Evidence |
|---|---|---|---|
| Blue (PCB-S) | 100 ppm/°C | Small-signal stage adjustment / possible DC balance of the inverter / if there are four, individual bias of the 6550s | OBSERVED the trimmer · number and function ND |
| Blue (PCB-P) | 100 ppm/°C | Regulated rail; possible 6550 bias — exact function ND | PROBABLE |
| Black (PCB-P) | ND | Possible threshold/timing or other — exact function ND | HYPOTHESIS |
Audiophile proposal — S3 · LISTENING HYPOTHESIS
- Blue on PCB-S: Do not fix until their function is known. If they turn out to be individual bias, fixing them would eliminate the dispersion compensation between tubes.
- Blue PCB-P: S1 or S2 depending on function.
- Black: S1 or S2 depending on function.
8. S4 — Matched & Measured
S4 is not a component modification. It is a characterization and documentation layer that can be superimposed on any edition, including S0.
What it includes
- Selection and individual characterization of the four 6550s with homogeneous measurement methods.
- Verification of the installed quiescent current.
- Estimation of the plate dissipation margin V_plate-cathode · I_cathode.
- In UL, screen current is not measured without intervention; the total cathode current is documented.
- Comparison between push-pull branches.
- Comparison of regulated auxiliary voltages L/R.
- Documentation of the relevant electrical measurements.
What "matched" means
- Tube matching: prior selection by measured parameters.
- Installed current matching: verification of the actual current already mounted.
- Branch balance: push-pull comparison.
- Individual adjustment: only if the circuit allows it (depends on the real function of the blue trimmers on PCB-S).
ANNEX — TECHNICAL QUESTIONNAIRE FOR JUAN CALATAYUD (JC VALVULAR)
Annex — Technical questionnaire for Juan Calatayud (JC Valvular)
Purpose. These fourteen questions are formulated so that the designer of the Acronos P52 can answer with his own design and measurement data. They close practically all of the ND parameters of the report. They are not reverse-engineering questions: they are questions that a technical buyer needs to make a purchase decision and plan long-term maintenance.
Classification by type of response:
- Design decisions (no measurement): P3, P4, P5, P6, P7, P10, P11, P14.
- Bench measurements (require a production unit): P1, P2, P8, P9, P12.
- Operational and commercial data: P13.
Block 1 — Power and dynamic behavior
- Real power at low distortion.
The published 40 W nominal, at what THD percentage? And what is the maximum clean continuous power at 1% THD into 4 Ω and into 8 Ω?
- Output impedance and damping.
What is the measured Zout of the amplifier in the audible band and its damping factor referred to 4 Ω and 8 Ω? How does the frequency response vary over the real impedance curve of a speaker with a 4.9 Ω minimum?
- Output transformer taps.
Does the secondary have dedicated selectable 4 Ω and 8 Ω taps, or a single optimized output? What is the optimum load impedance for each tap?
Block 2 — Tubes, bias and small-signal architecture
- Biasing system.
How is the bias of the four 6550s adjusted? Are there user-accessible trimmers, individual or paired adjustment, or fixed bias by divider or cathode? What is the target quiescent current per tube? Are the blue Bourns trimmers observed on PCB-S associated with the 6550 bias or with the signal stage / phase inverter adjustment?
- Phase-inverter topology.
What topology does the ECC803S use for phase inversion: long-tailed pair (LTP), cathodyne/concertina, cathode-coupled pair, or another? If it is LTP, does it use both triodes of the ECC803S?
- Global feedback (NFB).
Does the P52 have global feedback? What is its depth in dB? How does it contribute to the Zout value answered in question 2?
- Ultralinear percentage (%UL).
What ultralinear tap percentage do the screen grids use in the output transformer? Is it the usual 43% reference or different?
Block 3 — Power supply, rails and safety
- Actual B+ voltage and dielectric margin.
What is the actual B+ DC voltage on the 6550 plates and what margin does it maintain with respect to the 450 V nominal of the EPCOS B43645 capacitors? Does the B+ vary between channels?
- HV capacitor bank discharge.
Does the supply have an automatic discharge circuit for the high-voltage bank when switched off? How long does the residual voltage take to drop below 50 V?
- Power-supply architecture and rail separation.
Is the supply of the 6550s, the ECC803S and the EF86 separated into independent rails with series resistors or chokes and their own RC networks, or do they share a common B+ rail with local bypass capacitors (no series R or L)? If separated, how many rails are there?
- Soft-start and start-up.
How does the circuit manage the high-voltage delay to protect cold cathodes and avoid inrush current at power-on? What function does the Hongfa HF115F relay perform and how many seconds is the system delay?
Block 4 — Compatibility, noise and operational data
- Residual noise and speaker compatibility.
What is the audible residual noise level (hum at 50/100 Hz and hiss) with the input short-circuited? What speaker sensitivity and minimum impedance do you recommend to obtain maximum dynamics without clipping?
- Operational data and support.
What is the actual net weight, idle and full-power consumption, warranty and service procedure, and is the buyer provided with the electrical schematic with test points for future maintenance?
- PCB-S trimmers.
How many Bourns trimmers are on PCB-S and what function does each have? If there are four, are they the individual bias of the four 6550s? If there are two, are they intended for the signal stage or the phase inverter? Or is the bias adjustment of the 6550s performed from PCB-P via the blue Bourns associated with the MOSFET + TL431?
The seven critical purchase points
| Point | Associated question | Why it is decisive |
|---|---|---|
| 1 | P1 — Power at 1% THD | Determines whether the 40 W are usable with clean dynamics or whether they are the clipping point |
| 2 | P2 — Zout / DF | Defines the stability of the response over the real impedance of the speaker |
| 3 | P4 — Bias adjustment | Defines maintainability and operating cost when changing the 6550 quartet |
| 4 | P10 — Power-supply architecture | Confirms whether the pulsating power-stage current modulates the input and driver stages |
| 5 | P9 — HV bank discharge | Determines physical safety when handling the interior of the chassis |
| 6 | P13 — Warranty and schematic | Ensures operational continuity and long-term repairability |
| 7 | P14 — PCB-S trimmers | Discriminates between alternative bias topologies; changes §3.2 and §5.2 |
ANNEX — UNVALIDATED EXTERNAL PROPOSALS
Annex — Unvalidated external proposals
Mundorf, ClarityCap, Jupiter, V-Cap, Solen, Auricap, MLytic, Jensen, ASC, Nichicon KW (discarded due to voltage for the HV bank), Rubycon MXG/MXH (MXH not verified), Fine Gold, etc. They are kept for traceability of the research process, but they do not constitute a recommendation by Nauscopio. Their eventual evaluation is entirely subject to the Phase 0 → A → B → C → D protocol and to a leveled, reproducible S3 test.
REFERENCES AND CREDITS
References and credits
Technical sources
- TDK/EPCOS — Aluminum electrolytic capacitors, snap-in, B43645 / B43655, 610 µF / 450 V. Product datasheets and ripple data. (accessed 2026-09)
- Bourns — 3296 series, 3/8" square trimpot, 25-turn, ±100 ppm/°C. Datasheet. (accessed 2026-09)
- Texas Instruments — TL431 adjustable precision shunt regulator. Datasheet. (accessed 2026-09)
- Hongfa — HF115F miniature high power relay. Datasheet. (accessed 2026-10)
- JJ Electronic — ECC803S, A.F. twin triode. Datasheet. μ=100, gm=1.6 mA/V, Ri=62.5 kΩ, "non microphonic". (accessed 2026-09)
- JJ Electronic — 6550 beam pentode. Datasheet. Wa=35 W, Ua=600 V, Ug2=400 V, If=1.6 A. (accessed 2026-09)
- JJ Electronic — KT88 beam pentode. Datasheet. Wa=42 W, Ua=800 V, Ug2=600 V, If=1.6 A. (accessed 2026-09)
- Mullard — EF86, Z729, CV2901, CV4085, small signal pentode. Historical documentation 1953: "a low hum, low-microphony pentode"; maximum hum 5 µV referred to control grid under specified test conditions; reinforced electrode structure. (historical document 1953)
- Schaffner — FN9244B / FN9280 series IEC inlet filters. Datasheet. (accessed 2026-09)
- Würth Elektronik — WE-CMB common mode chokes. Datasheet. (accessed 2026-09)
- Nichicon — KG / Fine Gold / KW series. Datasheets (confirmation of voltage limits). (accessed 2026-09)
- Rubycon — MXG / MXH series. Datasheets. (accessed 2026-09)
- Tube Amp Doctor (TAD) — EF86 / 6267 Premium Selected. Selection criteria: noise, microphonics, gain, balance, mechanical tests. (accessed 2026-09)
Sources on the Acronos P52
- JC Valvular — manufacturing process video: ACRONOS P52 Amplifier — A work of art. How it was made.
- Manufacturer's commercial datasheet — 40 W PP UL with 6550 (no THD condition).
- Blog reader — personal communication: approximate P52 weight ~40 kg. Not verified by direct measurement nor published.
- Interior images provided by the unit's owner (2026-10) — they verify the silkscreen B43645-S7617-M1 on all three EPCOS units, the presence of blue Bourns trimmers on PCB-S, the identification of the Hongfa HF115F relay, the arrangement of the 14 MKP CEC C2N, the green 10K28 resistor, the scattered auxiliary electrolytics and the complete open-chassis layout.
Image credits
- Acronos P52 images (p52-pcb-s-top.jpg, p52-chassis-interior.jpg, p52-tubes-front.jpg, p52-pcb-p-macro.jpg, p52-bourns-detail.jpg, p52-hero.jpg) — manufacturer's video JC Valvular, reproduced for technical analysis purposes.
- Additional images (2026-10) — provided by the unit's owner, reproduced for technical analysis purposes.
