Not sure how this amp is set up, but are the jacks wired or board mount?
Sometimes jacks that have insulators get replaced with non-insulated, or the isolation washers get lost.
Then you can get weird ground loops, maybe because pot bodies are also grounded.
If you can dismount the jacks but leave the pots touching the chassis it's worth a try.  Then if it helps you just need to get insulating washers for the jacks.

Thanks for the insights on the role of the preset.

I'm getting closer to source of the hum. The nice thing about this WEM amp is that the preamp just plugs into the PA/power section.

If I disconnect the preamp/controls section hum disappears and I just hear very low level noise from PA. All the electrolytics in preamp section test good for value and ESR which is remarkable after 50-odd years. When I remove the board carrying the preamp and pots from metal front panel and reconnect to PA the hum is much lower and nearly imperceptible, but if I lay the board down again on the front panel so that one or more pots metal bushes touch the panel then the hum is present. The panel isn't grounded in any way. The solder joints on back of preamp board all look excellent (I had to reflow some on PA board which was nowhere near as nicely soldered). The input sockets are all switching correctly with the hot terminals grounded with nothing plugged in (they ground to mains ground via one of the 3 plug wires connecting the preamp and power amp sections - one  ground, one signal from preamp output, one carrying 18V supply from power supply back to preamp board).

But I'll try shorting out the PA input resistor as suggested but suspect hum will stop as it stops with no signal connected from preamp. It's clearly some grounding problem, but when sorted I'll get back to lowering the dc offset as suggested above.

Quote from: Tassieviking on May 06, 2025, 11:20:12 AMI would like to add that some heat transfer paste is conductive and will cause shorts if used with heatsinks and mica washers.
The conductive heat transfer paste is often used in computers for the main processor heatsink, it makes no difference in there since there are no electrical terminals close by.

Make sure you only get non conductive heatsink paste if you get some, a small tube of really good expensive paste will last forever and ever so don't get anything cheap and nasty.

Computer builders and modders swear by "Arctic Silver" paste, claimed to contain silver dust.
I much suspect Aluminum dust instead but in any case, any metal present will be conductive.

That 100 ohm potenttiometer i9s to fine adjust output DC offsetr.

It is a primitive version of famous RCA "Brute 70" amplifier, which was a revolution when it came out in 1969-70

I built many back in the day and is the base design for most modern SS amps.

But it uses a single input transistor (later ones used a differential pair there which self balances) so 40406 needs about -0.6V on its emitter (this is shown on schematic) which is "stolen" from driver transistor 40409 emitter.

Try to reduce offset, but 200mV is good enough, not sure your hum comes from there.

It may come from poorly filtered main rails (+/-42V) or grounding issues.

What happens if you short across input 18k resistor with a bit of wire?
Does hum change in any way?

The trimmer is not for bias, it looks to me like offset or symmetry adjustment but I'm not familiar with this amp.
There is a note about reducing quiescent (idle current) by adding 560 ohm resistor across one of the bias diodes.
You can check what quiescent for the power transistors is right now by measuring voltage across the 40411's emitter resistors (shown a 0R33 and 0R25) and calculating current.
It's possible with lower idle current the trimmer will do enough to get rid of the offset.  400mV or even 200mV that you have for offset is too much.  50mV is a better target, or maybe 100mV in worst case scenario.

Excellent point sir thanks nice to hear from you.   

I know right now (I think) that all of this is gonna go together like a big 🗝� key lime pie I'm lust learning how to make the crust and missing the key ingredients 

I'm restoring a lovely WEM PA100. Just finished installing new filter caps (old ones measured fine but will future-proof it for a few more years) and the output is up to spec power but I still get a hum across the output with no signal. I measure 0.4V across the speaker when I use an 8 ohm load.

Is there any way to reduce this? The voltages on the schematic are not very helpful as they seem to show 0.2V offset 'officially', and confusingly a different voltage under load for the same point electrically, written just to the left on the power amp schematic!

There is a 100 ohm preset but I think this is for setting bias. I can turn it and the offset reduces to around 0.2V but noise level doesn't change. I can't see any preset for setting offset unless it's this one?

Would be nice to eliminate the offset/hum. Everything else in very good condition.

Thank you very much sir that was awesome 

Vbe looks "like a rock" because it is being force fed throughn am high-ish resistor (100K)

As of the wide Hfe disparity:

small signal (preamp) transistors are optimized for high gain, they don´t worry about high current because they typically will be used around 1-2 mA or so.
Low dissipation also, 300-500mW (tops).

Power transistors on the contrary are optimized for POWER     , high dissipation (70 to 120W), high current (think 10-15 Amperes), etc.

*Everything* in this World is a compromise, so the heavy price paid is abysmal Hfe.

You will find than in all power Amps, power transistors are never used by themselves, *always* driven by a medium power driver transistor, in a Darlington or Quasi Darlington configuration, so their Hfe multiply.

For example my rule of thumb for 50-100W power amps I design is a 2N3055 (Hfe 10 @ 10A) driven by a TIP 31/32 (Hfe 40 @ 1A)) for a combined Hfe of 400, go figure.

I am very conservative (which is safe), in practice combined Hfe is quite higher, but that is a guaranteed minimum value.

Later I switched to power Darlingtons, TIP142/147 (which are a sort of Industry Standard, being also used by Fender Marshall Ampeg Laney Peavey Crate, H&Kettner which should mean something.

Guaranteed Hfe @ 10A is 1000, go figure.