Valve amplifier service

[Greg]

Oh dear. Sorry you’ve not yet discovered the fault. Also sorry if sharing my own experiences with you has inadvertently misdirected you.

[simon]

Not at all Greg, I appreciate the advice and the caps were the obvious place to start. I'm a little surprised that they all seem okay but it's all learning.

[Paul Barker]

Dry joint?

[Nick]

I do note that the heater supply is at 0v, while half the input valve is at 200v and the LTP is at 105v. I wonder if its heater/anode breakdown, or something simple like crud on the valve socket that is allowing heater AC to get to one or more grids. Is it 50Hz or 100Hz hum?

[simon]

It could be a dry joint I suppose. The amp's been fine as far as I know for many years, is it possible for joints to go bad like this? I'll run the iron over some appropriate joints.

[simon]

Good point on the heater potential. I'll check the manual and amp to see if there's a lift somewhere that's not shown on the schematic. I think Dave changed all the valves so the ones installed should be fine at the moment.

I need to check with the scope to see what the hum looks like, I suspect it might be 100Hz as it lowered when I switched the valves over.

Is plugging a valve in a few times good enough to clean pins and sockets?

[Ant]

Ive used a fibreglass scratch brush for the bigger sockets in the past (300b sockets, cant remember the type) and a bit of thin solid core wire just to scratch up the inside of small signal valve socket pins (b9a?)
Old 1960s sockets needed a more agressive clean but id expect the ones on that amp to be ok with a quick run through with some wire to break through any corrosion

[Nick]

I was thinking more of the state of the ceramic insulator between the pins that the pin contact themself.

[simon]

Rewetted all the solder joints around V2B (and A) where the hum disappears when I apply the probe but no change.

I can't see any heater lift - all valves are wired in series and tied to ground. But looking at the circuit and voltages again, the upper triode's cathode is at c120V so well within the 180V cathode to heater max. for an ECC83.

I broke out the scope but really don't think I should be allowed to use one .
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20230701_181842.jpg (133.5 KiB) Viewed 5678 times

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I know, it's no use nor ornament but that's the best I can get it. Frequency is notionally anywhere between 125Hz and 170Hz depending on whether it's 6ms or 8ms. But I don't really feel it's telling me much I can rely on.

I've got some contact cleaner somewhere I'll see if I can find. I'll wrestle the amp the right way up, spray the pins and insert in the socket a few times. This won't help if it's the socket insulation breaking down of course.

[Nick]

So if the hum stops when you put the probe on pin 7, I would check the signal on the two output valve grids, clearly whats going on with pin 7 is being fixed when you apply the probe.

Also, maybe worth checking the 1M resistor.

Did you try replacing C4? That is in a position where a leaky PIO would make a difference, its being driven with a 1M source. I would expect that the voltage on pin 2 and pin 7 were closer. 10v difference would indicate 10ua current, its not much, but may be enough to upset the LTP.

[Mike H]

Thanks Mike. Just to be sure I understand I think you mean remove the connection rather than bypass each with a wire?

Sorry yes I meant remove the connections, not bypass with wire!
But you're way ahead now...

[simon]

So if the hum stops when you put the probe on pin 7, I would check the signal on the two output valve grids, clearly whats going on with pin 7 is being fixed when you apply the probe.

Also, maybe worth checking the 1M resistor.

Did you try replacing C4? That is in a position where a leaky PIO would make a difference, its being driven with a 1M source. I would expect that the voltage on pin 2 and pin 7 were closer. 10v difference would indicate 10ua current, its not much, but may be enough to upset the LTP.

Yep changed all the caps, C4 is now a LCR PP. But I'll check the 1M resistor.

I'll check the output valve grids too, though whether I'll be able to pick it up on the scope is another matter. I don't recall putting the probe on the 5881 grids affecting the hum but I'll see.

[simon]

Thanks Mike. Not sure I'm ahead of anything yet

[Nick]

If it doesn't affect the hum there is more chance of seeing it. If not on the grids then try the amp output on the secondary. The shape of the hum may provide information.

[simon]

The photo of the scope was taken with the probe on the speaker terminal. I don't really think it's a fair representation of the hum I'm hearing, but I have form with a scope and hum...