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Old 29th May 2009, 09:03 AM   #1
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Default Trimmer pot source?

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)
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File Type: jpg fried resistor.jpg (95.3 KB, 219 views)
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Old 30th May 2009, 07:58 PM   #2
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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?
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Old 30th May 2009, 09:34 PM   #3
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Schematic would be nice
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Old 30th May 2009, 09:44 PM   #4
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I don't have a schematic.
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Old 30th May 2009, 09:59 PM   #5
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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..
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Old 30th May 2009, 10:14 PM   #6
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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.
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Old 30th May 2009, 10:53 PM   #7
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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?
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Old 31st May 2009, 01:13 AM   #8
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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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