Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: stratavox on June 06, 2014, 10:38:54 am
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I have an old Alamo tube amp, 5y3, 6v6, 12ax7 and a 6av6 for the trem. Sold it last weekend, and the buyer called to say if he plugs in and just strums, it's fine, no problems, but if slams a chord, or even a single note, the volume drops out completely, then returns. I have him replacing tubes this weekend, but is there anyplace else that leaps to mind as far as suspect areas? I replaced the filter caps, and all the resistors that had drifted with age. Also, no schem, in Doug's library, or anyplace else I could look...he also mentioned that manipulating the trem pots seemed to alleviate the problem somewhat...thanks.
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I would bet that if you go to Google Images you could find a schematic. There were "lots" of those made.
As an experiment, maybe try the amp with the 6AV6 pulled. Is it covered under warrantee...?
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Believe me, I LOOKED hard for the schem. Part of the problem is there's no model name, or number, anywhere on it. The tube chart was half ripped off when I got the amp, and again, it was fine for years. I've asked the buyer to swap out the 6av6, but he's out of state, so I'm trying to help by email first, and then if need be, I'll do what he wants as far as making the transaction right. It's the right thing to do...thanks for your time.
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Could be bad (high resistance) grid-leak resistor.
But he could just be playing it TOO LOUD. Beat it just to the edge of fainting, don't whip the horse past that (except for effect).
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Or it could be a dying rectifier or output tubes - esp if it 'dies' on power chords
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We'll see this week. I made arrangements to meet the buyer and get the amp back, so I can get under the hood. I'll let you know, and thanks again for the interest.
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What PRR was saying: You should know that if the amp has grid-leak bias for the input stage (a cap between the input jack and first tube grid, 3-5MΩ resistor from grid to ground, input stage cathode connected directly to ground) that big input signals will "latch up" the input and shut the tube off.
If the guy uses hot pickups (especially active pickups) or boost pedals set for very high output, gid-leak bias just doesn't work well. If this is the case, your customer probably won't change his entire setup to suit the amp, so the easiest fix is to switch to typical cathode bias for the input stage and no input coupling cap.
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And again, many thanks. I'm meeting him half- way tomorrow night, and hope to get into it Thursday...I'll post...
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Just a note- the buyer hasn't returned my calls, or messaged me- so I have no update, but when (and if) I DO get the amp back, I'll post- thanks again, all...