Mystery amplifier

[PJL]

PS: Measuring resistors in circuit is always a problem as other resistors or leaking capacitors may impact the reading, changing the capacitors is a sensible decision if you want the amp to be reliable, and finally, the lamp limiter behaviour is normal as the lamp will glow brighter as the capacitors charge up.

[Diabolical Artificer]

The amp schematic is coming along - I've finished the left channel.

In that case your there, the right channel will be identical. Just make sure you get any power supply decoupling etc.

Andy.

[Kokotoni Will]

Here are the schematics. I wanted to make one for the right channel because I thought it might illuminate why the secondaries are wired differently. It didn't - not as far as I can tell, anyway.

Hi- res version:

https://harnessontheleftnape.blogspot.com/2025/11/mystery-amp-schematic.html

[trobbins]

Good stuff.

R15 and R20 are grid stoppers. R15 for V3 may have been incorrectly wired, or drawn.

R8-C6 is a step network for hf stability.

V2 is a cathode coupled long-tail pair phase inverter, with direct coupling from V1 anode.

R14-C7 is feedback to R4, with V1 cathode circuit including dc biasing circuit R5-C5. V1 operates pentode mode, so has lots of gain. Is R14 really 113 ohm?

That's likely a clone of a well known amp circuit.

[Diabolical Artificer]

Basically a Mullard 5-10 as I thought. Nicely drawn well done, but a bit confusing with all the wiring crossing, it's more usual to draw ground at the bottom, +HT at the top, transformers to the right, IP on the left, see attached. I find it helps when drawing schematics of unknown amps to have a known schematic, a best guess of what the circuit is based on as a reference, where to put V1 or Q1 or whatever.

Andy.

[PJL]

The Mullard 5-20 design looks like a closer fit.
https://www.r-type.org/articles/art-003d.htm

Values are different but that will be largely down to the choice of output valves that require lower HT voltage.

[Marconi_MPT4]

Yes a 5-10 modified to use MOV A2134 and possibly using 8k a-a transformer loading.

Just curious. are R16 and R21 grid leak resistor values really over 1Mohm? Data sheet suggests the maximum value for auto-bias is no greater than 0.27Mohm. Page 2 of data sheet.

https://frank.pocnet.net/sheets/126/a/A2134.pdf

Rich

[PJL]

I have added the resistor values in yellow based on the colour codes. Cant read a couple nor can I read the Welwyn green wirewounds.

[PJL]

I have marked up your component values and designations on a copy of the 5-20 schematic.
The circuit is the same but there are some value differences attributable to the lower HT and differences in open loop gain and frequency response.
C3 & C4 are the audio coupling capacitors and should be replaced before you power up..

[Kokotoni Will]

Thanks a lot, guys.

I've found that R15 goes via another 0.1uf capacitor (hidden under the board) to pin 1 of V2 (anode of V2A in the 5-20 schematic).

All the resistors, except for the Welwyn wire-wounds and the high stability ones, have drifted above their 5% tolerance. The worst are R14: 41k (should be 33k), and R40: 185k (should be 150k).

R16 and R21 are both over 1M, but should be 910k.

[PJL]

Originally Posted by Kokotoni Will

All the resistors, except for the Welwyn wire-wounds and the high stability ones, have drifted above their 5% tolerance. The worst are R14: 41k (should be 33k), and R40: 185k (should be 150k).

I think you maybe reading the colour codes incorrectly. Some of the resistors are high stability (HS) ones, for example R11 is brown-black-yellow-brown-pink which is 100K HS.

"R16 and R21 are both over 1M, but should be 910k."

They seem to be brown-black-green-gold which is 1M 5%, 910K would be white-brown-yellow. I have tried to correct your errors in the 5-20 schematic from your pictures but no guarantees.

A web search will bring up modern colour coding, not sure if someone can point us to a chart that covers vintage resistors?

The HS resistors are typically very reliable but the standard carbon resistors tend to change value over time. The builder has used higher tolerance resistors with most being 5% (gold) but these are probably the same construction as 10% (silver) and 20% (none) and selected after manufacture.

[Kokotoni Will]

I checked the values again yesterday using this calculator, which seems to be accurate:

https://www.calculator.net/resistor-calculator.html

R16 and R21 should be 910k - I've circled them in the attached photo.

I've read that 2w film resistors are a similar size to these moulded ones - would they make good replacements?

Regarding replacement coupling capacitors, would these be suitable:

https://cricklewoodelectronics.com/CFH100NH.html

[PJL]

OK, I have made a few corrections and added the values to a picture of the tag strip. This has made it a closer match to the 5-20 and allows you to check the values on the tag strip.

[Kokotoni Will]

Thanks for this.

You've marked R2 with but it is 80k in my schematic.

Should I replace R16 and R21 with 470k resistors?

You've marked R9 as 100k, where the 5-20 schematic calls for 270k. It is 150k in my amp (measures 164k). Should I replace this with a 100k resistor?

[PJL]

R2 80K is not an E12 standard value so it is likely you have read this incorrectly. The picture is not clear enough to read the colour bands, what are they?

R16, R21 look like yellow-purple-yellow-gold=470K 5%. Do you agree?

R9 looks like brown-black-yellow-gold=100K 5%. Possible it is brown-green-yellow-gold please confirm and I will make the changes to the drawing.

Once we have agreed the design values you can measure them and decide which need replacing. Are the values in your circuit the measured values?

[PJL]

We must also not forget to investigate the secondary tap differences and the consequences on feedback. Its possible to measure the turns ratio by applying 6.3V AC to the output transformer primary and measuring the AC output voltage but we can leave that to later.

[PJL]

Sorry, missed your post #132, that picture is far clearer! and I agree they are 910K. You only need to change them if they are out of tolerance.

[Diabolical Artificer]

The caps you linked to should be fine. Replace the coupling caps & power up (as advised by variac LL ), you'll be grand. Valves are very forgiving unless a resistors value is either short circuit or way over spec, say 30% your sound because this is a high voltage low current circuit. A transistor amp is different.

Unless it's easier to replace them whilst replacing the caps though, just depends. The more changes you make, the more chance of a screw up. Best to change a few things at a time, test, do some more work. Next write down your DC voltages after the amps warmed up - example - write down on a pad -
HT 1 2 3 4 whatever
V1 a
g
k
etc. Always try to keep good notes of your work, dated & with a description of work done, Andy.

[Kokotoni Will]

I've measured all the resistors out-of-circuit, and verified their design values. Here they are. Measured values are bracketed:

R1: 1M (1M)
R2: 68k (80k)
R3: 1K (1.21K)
R4: 100 (112)
R5: 1.8K (2K)
R6: 470k (472k)
R7: 120k (122k)
R8: 10k (12k)
R9: 150k (175k)
R10: 100k (101k)
R11: 100k (99.8k)
R12: 39k (50k)
R13: 1M (1.28M)
R14: 33K (40K)
R15: 4.7K (5.6K)
R16: 910K (1.1M)
R17: 470 (465)
R18: 10K (9.9K)
R19: 10K (9.6K)
R20: 4.7K (5.6K)
R21: 910K (1.23M)
R22: 470 (473)

I've just noticed that someone has scratched a big X into the base of the PSU 12-E1 valve, presumably because it doesn't work. Upon powering up the PSU, what would normally happen if this valve had failed?

[PJL]

Some parts are more critical than others, I have given them H=high, M-modest ratings and used the rating to decide if they can be left out of tolerance. I have been a little over cautious and marked those involved in feedback as H. The grid resistors on the output valves R16 & R21 have drifted quite badly and would recommend replacement as excessive grid current in the output valve can cause a runaway effect. It's up to you if you want to replace all the ones out of tolerance (OOT).

The 12-E1 is used as a series regulator and is critical to ensuring you get the correct HT voltage. If its low emissions then the voltage will drop under load but other fault conditions could cause more serious issues but it is not uncommon to find good valves erroneously marked as faulty. Best to wait till you get to the testing phase.