Trimmer pot source?

RobWilJr · 29th May 2009, 08:03 AM

My tube amp has a bad trim pot I believe. It's a 33K ohm, single turn with 3 leads. It's attached to a 22K ohm resistor. Do I necessarily have to use the same value? What about watt rating? I have no idea what the rating for this one is. Where do you get these?
See attached pic (disregard arrow, fried resistor has been replaced)

RobWilJr · 30th May 2009, 06:58 PM

OK, I found a source for the trim pots with wire leads. I can't find them in 33K though. Would a 25K or 50K work? This is a KT88 push/pull amp with 2 KT88's on each side, 50wpc, screen grid voltage is 454V, and I need to set the bias voltage to ~54V. Also, how important is the watt rating? Would 1/2 or 3/4W be sufficient?

sorenj07 · 30th May 2009, 08:34 PM

Schematic would be nice

RobWilJr · 30th May 2009, 08:44 PM

I don't have a schematic.

sorenj07 · 30th May 2009, 08:59 PM

Well can you trace the bias circuit out? At the outset I wouldn't think that it would be a problem, but I can't blindly advise you to plunk in a different pot with a clear conscience..

theoldtrout · 30th May 2009, 09:14 PM

Looking at your photograph: the right hand valve base has a very cooked looking resistor connected to it. You may have other problems with this amp.

multisync · 30th May 2009, 09:53 PM

If you have a working channel measure the voltage across the working trimpot and you can calculate the require dissipation. 50k would be a better choice esp if the adjustment puts you between the 25k and 33k value. You could put a 100k resistor across the 50k trim pot to give you appx 33k . (100k x 50k)/(100k + 50k) = 33.33k From the picture it looks like it's less than a 1 watt pot. But hey what do I know?

RobWilJr · 31st May 2009, 12:13 AM

Thanks guys. Yes, the resistor has been replaced and hopefully the only remaining issue is the trim pot, which evidently took a hit also. Exactly what I was looking for multisync

Edit: Actually, I think the faulty trim pot caused an over current condition to the tube, which went out and took the resistor with it.

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30th May 2009, 06:58 PM	#2
RobWilJr diyAudio Member Join Date: May 2009	OK, I found a source for the trim pots with wire leads. I can't find them in 33K though. Would a 25K or 50K work? This is a KT88 push/pull amp with 2 KT88's on each side, 50wpc, screen grid voltage is 454V, and I need to set the bias voltage to ~54V. Also, how important is the watt rating? Would 1/2 or 3/4W be sufficient?

30th May 2009, 08:34 PM	#3
sorenj07 diyAudio Member Join Date: May 2006	Schematic would be nice

30th May 2009, 08:44 PM	#4
RobWilJr diyAudio Member Join Date: May 2009	I don't have a schematic.

30th May 2009, 08:59 PM	#5
sorenj07 diyAudio Member Join Date: May 2006	Well can you trace the bias circuit out? At the outset I wouldn't think that it would be a problem, but I can't blindly advise you to plunk in a different pot with a clear conscience..

30th May 2009, 09:14 PM	#6
theoldtrout diyAudio Member Join Date: Aug 2008	Looking at your photograph: the right hand valve base has a very cooked looking resistor connected to it. You may have other problems with this amp.

30th May 2009, 09:53 PM	#7
multisync diyAudio Member Join Date: Jan 2008 Location: Carp	If you have a working channel measure the voltage across the working trimpot and you can calculate the require dissipation. 50k would be a better choice esp if the adjustment puts you between the 25k and 33k value. You could put a 100k resistor across the 50k trim pot to give you appx 33k . (100k x 50k)/(100k + 50k) = 33.33k From the picture it looks like it's less than a 1 watt pot. But hey what do I know?

31st May 2009, 12:13 AM	#8
RobWilJr diyAudio Member Join Date: May 2009	Thanks guys. Yes, the resistor has been replaced and hopefully the only remaining issue is the trim pot, which evidently took a hit also. Exactly what I was looking for multisync Edit: Actually, I think the faulty trim pot caused an over current condition to the tube, which went out and took the resistor with it.