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)
See attached pic (disregard arrow, fried resistor has been replaced)
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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?
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..
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.
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?
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.
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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