Those resistors are shown as 1 ohm on the schematic.  If so, you have idle currents of around 88mA per output device.  That seems kind of hot, 10 to 20mA would be a better target.  Adjust bias to get around .01 to .02V across each of those resistors.
Then again check for DC at the output.  If it stays around the 10mV you have now, that is good.

.01v dc on the output

r737 - .0873v
r726 - .0885v
r727 - .0889v
r738 - .0882v

With no load connected, is there any DC at the output?  What is the DC voltage drop across each of R737, R738 ?

Sorry, didn't see the now obvious "click or drag files here" button.

Here's an updated schematic and voltages. I just copied the vbe multiplier from another site, probably not wise to copy paste parts of a bias circuit that I don't fully understand, but seemed like a good idea at the time. The bias trim pot seems to keep the ratio about the same, so I can make either side match fairly close to the schematic voltages, just not at the same time. Both pairs of output transistors measure the same, so I don't think it's just a mismatch.  I have already tried a few different transistor substitutions that didn't seem to change much. 

How much each output device conducts is related to the bias circuit.
As you have made modifications, the stock schematic does not show the whole picture.
Post a schematic that includes all your circuit changes.

In the 90s, I went into an electronics store in Hollywood, and saw some amplifier boards hang packaged for sale that looked suspiciously like the circuit in my own Pignose. Same 4 transistor transformer coupled module, looked like it would be a drip-in fit. I believe it was Calrad branded packaging and sold as a 3 or 5 Watt amplifier module for less than $10 as I recall. Interestingly, there was also a larger 10 or 12 Watt version sold right next to it for a few bucks more that looked more or less like the exact same circuit, just scaled up. 

I agree, as an amplifier, the circuit was pretty antiquated well before the 90s when I saw it, and I've no idea why one would want such a thing. Except maybe to drop in replace an identical failed module.

Hello, first post here, apologies if I am missing something or in the wrong spot.

I am working on a k200b poweramp that had a blown output section. Most of the transistors were bad, replaced q1-4 q703-9 and the majority of associated parts. The diode cr702 was blown. I didn't have a suitable chassis mount diode so I replaced the bias string with a transistor vbe multiplier, which seems to be working fine.  The amp now works and passes audio, but I can't get the output transistor voltages on each side to match.  When the collectors of q3 and q4 is -.1v the emitter of q1 and q2 is .125v.  All of the resistors are 5% or better, 1% where indicated on the schematic.

I'm not too experienced with ss power amps, so please let me know if i'm screwing up the nomenclature or leaving out obvious info.  Any help is greatly appreciated. Thanks!
schematic

The KiCad schematic was supposedly traced out by someone from their amp, I would trust the other 2 more as well.
If you cant see the schematics in full scale then right click on the small picture and open in a new tab or window and it should come out larger.

IF it is the real early version such as used by Ike and Tina Turner, not the later Pantera era, back in the day "everybody copied the Twin Reverb idea" so Guitar and Bass preamps were the same (clean, no distortion), only the Guitar one added Reverb and Tremolo.

So if you find the old Guitar one, it will help you.

I have them somewhere, in some old hard disk, haven´t seen them for many years.

Quote from: franvelasco on April 10, 2025, 03:25:05 AMI have a Pathfinder 10 Bass and I'm thinking on getting the guitar one. I suspect the PB10 is the same circuit without the distortion channel, as it sounds great with a telecaster, but I don't really have a way to know because the schematic is impossible to find.

From what I've seen in the modded guitar version, removing C12 gives more clarity and the Bright switch in the bass version does something quite similar.

Anyone has a clue on what's going on? Maybe there's a way to make the LEDs work to add an overdrive channel?

Thanks!

Looking at schematics kindly posted by Tassieviking:

1) the black on white ones look reasonable, "should" work, but I can not trust the coloured Eagle/Kicad type one.

In any case, you can use the Guitar one with Bass (and a suitable speaker) by just not pushingb the Distortion button, now for the Bass one it is not practical, just use any distortion pedal up front.