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Hi,
I have a 1978 Fender Vibro Champ; I've had it for about 30 years. I used to use it all the time but it died at one point and I put it aside (25 years ago). Recently I got into playing with tubes and pulled the thing out. After measuring all the voltages I found the power transformer was bad so I replaced that with a new Hammond 291AX (according to Hammond a better version of the 290AX). I also found a tube tester at a local antique store and bought it for testing tubes. One of the 12AX7's was bad so I replaced both of them with new tubes. I also put in a "bigger bottle" JJ 5Y3S. I had on hand an old Ruby tubes 6V6 as well as an EH 6V6, both good. After replacing the PT the amp came back to life and I've been using it as a bench amp testing some guitar effects I"m working on. After using the amp I remembered one of the reasons I stopped using it; if you turn it up past 4 or so it will "go quiet" during play. I figured just couldn't push it that hard. The other night it went quiet and smoke came out...
I opened it up and the 25uF across the bias resistor on the 6V6 had exploded and the 1K resistor at the top of the voltage divider had burned. I figured ok, screw it, I'll replace all the old caps (they may have been dry after all these years anyway). I got some new Vishay caps (replaced them all) with a higher temp rating (105 degree instead of the 85 that were in there, doesn't do anything other than maybe extend the life a bit) and replaced the 1K, 1 W with a 1K 2W. After checking everything out (all voltages correct etc.) it fired back up. As I was playing it I was watching the tubes as I turned it up. I saw that the "going quite" is actually an arc inside the 6V6. I'd say it's the tube but it doesn't matter what I put in there (the Ruby, EH or a couple of others I've got hanging around), it does this. Anyone seen this before and know what's causing it? I haven't modded the amp at all; been waiting to muck about with opening the NFB resistor etc. but not doing anything until the amp stops doing this other thing. I also recall seeing on a site (can't find it now) that the "stock" biasing may have been off from the factory and there are better values.
UPDATE EDIT: I f'd up when replacing these caps. I replaced them with the same values as shown on the board. I admit I didn't look at the voltage on the schematic and didn't measure it myself. The original cap was a 25uF, 25V. The schematic says "+21V" and the amp meaures +28V. Ok, someone please tell me who spec'd these in the first place? You NEVER rate a component like this; twice the supply voltage is minimum. So I'll be replacing this one (again) with a 50V cap... does this explain the arcing?
Thanks,
Mike T.
I have a 1978 Fender Vibro Champ; I've had it for about 30 years. I used to use it all the time but it died at one point and I put it aside (25 years ago). Recently I got into playing with tubes and pulled the thing out. After measuring all the voltages I found the power transformer was bad so I replaced that with a new Hammond 291AX (according to Hammond a better version of the 290AX). I also found a tube tester at a local antique store and bought it for testing tubes. One of the 12AX7's was bad so I replaced both of them with new tubes. I also put in a "bigger bottle" JJ 5Y3S. I had on hand an old Ruby tubes 6V6 as well as an EH 6V6, both good. After replacing the PT the amp came back to life and I've been using it as a bench amp testing some guitar effects I"m working on. After using the amp I remembered one of the reasons I stopped using it; if you turn it up past 4 or so it will "go quiet" during play. I figured just couldn't push it that hard. The other night it went quiet and smoke came out...
I opened it up and the 25uF across the bias resistor on the 6V6 had exploded and the 1K resistor at the top of the voltage divider had burned. I figured ok, screw it, I'll replace all the old caps (they may have been dry after all these years anyway). I got some new Vishay caps (replaced them all) with a higher temp rating (105 degree instead of the 85 that were in there, doesn't do anything other than maybe extend the life a bit) and replaced the 1K, 1 W with a 1K 2W. After checking everything out (all voltages correct etc.) it fired back up. As I was playing it I was watching the tubes as I turned it up. I saw that the "going quite" is actually an arc inside the 6V6. I'd say it's the tube but it doesn't matter what I put in there (the Ruby, EH or a couple of others I've got hanging around), it does this. Anyone seen this before and know what's causing it? I haven't modded the amp at all; been waiting to muck about with opening the NFB resistor etc. but not doing anything until the amp stops doing this other thing. I also recall seeing on a site (can't find it now) that the "stock" biasing may have been off from the factory and there are better values.
UPDATE EDIT: I f'd up when replacing these caps. I replaced them with the same values as shown on the board. I admit I didn't look at the voltage on the schematic and didn't measure it myself. The original cap was a 25uF, 25V. The schematic says "+21V" and the amp meaures +28V. Ok, someone please tell me who spec'd these in the first place? You NEVER rate a component like this; twice the supply voltage is minimum. So I'll be replacing this one (again) with a 50V cap... does this explain the arcing?
Thanks,
Mike T.
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