WEM Westminster Squealing

[rf7]

I've been resurrecting this amp, and I'm close, but the volume pot causes an oscillation at one point only, almost halfway up. It's fine above and below that. Sometimes it doesn't squeal, but just hums excessively there. The pot measures ok there with a meter.

Any ideas?

WEM Westminster Mk 9 Schematic

[Gingertube]

Can be several things. Squeal is sign of positive feedback causing oscillation, The excessive hum is probably oscillation too but the oscillation is ultrasonic (causes excessive power supply currents).

1st - layout, make sure the wire from the volume pot wiper, via the 0.01uF and 27K go no where near the pin 9s of the ECL82 (6BM8).

2nd - Old bypass caps - have you replaced all the 50uF power supply bypass caps ( 3 off). Should also replace the 50uF cathode bypass cap on the ECL82 cathodes. If caps are old then they have high internal impedance and you can get positive feedback from the power supply rails.

Also check the 4K7 + 2200pF connection across the ECL82 pentode anodes. Make sure the resistor is not open circuit and the cap is not "broken".

If the above gives you no joy then the oscillation maybe caused by parasitics and you may need to add some grid stop resistors. Particularly on the ECL82 triodes (from your fault description).
1) the ECL82 triode grids, lift the wires from pins 1 of the 2 ECL82, solder a 4K7 resistor to each pin 1 then re-attach the wires to the other end of those resistors - do one at a time so you don't get confused.

2) The 12AX7 should also have gridstop resistors. Lift pin 2 and pin 7, solder 10K resistors to these pins then re attach the wires to the other end of the resistors.

3) Ditto for the ECL82 Pentodes - lift wires from pins 3 of each and add 2K2 resistors as above.

For guidance - EVERY tube should have a grid stop resistor, regardless of teh fact that many of these ols amps never had them fitted. The minimum value of grid stop resistor should be 8/gm of that tube
For 12AX7 triodes gm = 1.6 mA/V, so min resistor is 8/.0016 = 5K (use 10K)
For ECL82 triodes gm = 2.5 mA/V, so min resistopr is 8/.0025 = 3K2 (use 4K7)
For ECL82 pentodes gm=6.4 mA/V, so min resistor is 8/.0064 = 1K25 (use 2K2) CAUTION: I know that rule is good for triodes, am assuming it is also good for pentodes.

That should keep you busy.
Cheers,
Ian

[rf7]

Thanks Ian. I already re-capped the amp. I'll check out the other stuff.

[rf7]

I actually got the issue to go away. The pots were sounding crackly even though they had already been cleaned, and cleaning them again made the squeal completely disappear. The amp stayed quiet for days after that.

[Jazz P Bass]

Wow.

That's one for the books.