Twin reverb reissue, with the 2k2 Rk of the reverb driver 12AT7 charred. Looks like it burned up it's cathode bypass cap also, and some other nearby components need replacing .
I'm replacing the verb xformer too, as my first guess at to the problem, if not the tube itself.
However, I'm scratching my head over why THREE preamp tubes have lost their vaccuum : V2, V3, and V5(trem) . they're not even all on the same voltage node.
What are your thoughts? I've never seen this. Amp has not had a cap job.
What happened here ?
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Smokebreak3
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What happened here ?
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martin manning
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Re: What happened here ?
Start by checking the whole power supply out?
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Stevem
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Re: What happened here ?
I would say that V2 and V5 loosing there vacuum have no relation to your problem in the reverb circuit.
If you have a veriac you can use it to prove out the the filament winding of the PT should you want to.
Remove all the tubes and on pins 2 and 7 of one of the output tubes pump in 6.3 volts
If you see 117 to 120 coming out the PTs primary then your ok.
If you have a veriac you can use it to prove out the the filament winding of the PT should you want to.
Remove all the tubes and on pins 2 and 7 of one of the output tubes pump in 6.3 volts
If you see 117 to 120 coming out the PTs primary then your ok.
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Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: What happened here ?
Watch this...seems to be a similar situation.
https://www.youtube.com/watch?v=EW7hmGPvPH4
https://www.youtube.com/watch?v=EW7hmGPvPH4
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martin manning
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Re: What happened here ?
Sure looks like it. A shorted preamp tube is pretty unusual. The burned cathode resistor may have saved the reverb trans,
Last edited by martin manning on Thu Jul 09, 2026 6:57 pm, edited 1 time in total.
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sluckey
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Re: What happened here ?
They lost their vacuum due to cracked glass resulting in rough handling. If the amp was operating when the rough handling occurred, it's posible that V3 shorted and took out that 2k2 cathode resistor. Replace with a 2k2 3W flame proof resistor mounted about 1/2" above the board.Smokebreak3 wrote: ↑Thu Jul 09, 2026 5:38 pm I'm scratching my head over why THREE preamp tubes have lost their vaccuum : V2, V3, and V5(trem) . they're not even all on the same voltage node.
This may just be a camera fish eye thing, but V1 socket shield looks bent and leaning over. Maybe due to rough handling. This could be the reason there is no tube in that socket.
...Steve
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Dr Tony Balls
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Re: What happened here ?
I once had a friend that brought me a Super Lead for repair where two of the power tubes had lost vacuum due to cracks. His amp doesnt have the back panel/screen installed and I know his band is kind of a raucous party band. I asked him, "hey what are the odds that during your last show some errant beer flew into these hot tubes and caused them to crack"? He said "yeah, the odds are pretty good...".sluckey wrote: ↑Thu Jul 09, 2026 6:56 pmThey lost their vacuum due to cracked glass resulting in rough handling.Smokebreak3 wrote: ↑Thu Jul 09, 2026 5:38 pm I'm scratching my head over why THREE preamp tubes have lost their vaccuum : V2, V3, and V5(trem) . they're not even all on the same voltage node.
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Re: What happened here ?
First thing, look at the fuse in the fuseholder. Also, did it come in with all the shields in place?
Guitar player logic: If fuse blows, put another one in. If that fuse blows, put a bigger fuse in. Repeat until no further change is noted.
I'm also going with "external factors" causing the loss of vacuum in the preamp tubes.
I have seen 12AT7 crack the base in the reverb driver slot. That's just what happens with some of the chinese and russian tubes, maybe others. The button and envelope were not joined well in the factory. I've even personally seen one newly installed, crack like that in about half an hour of test running.
Some Russian tubes are fatter around the glass and don't fit well, and the stress against the bayonet socket can crack them.
Guitar players will do weird things, like roll tubes all the time, take the back panels off, lose the shields, stack things on them in the car and break tubes.
Most of them seem to like "rolling tubes" a lot for mythology's sake. They tend to break things and wear out sockets.
I'd just go about the repair process. It's going to be expensive, with all those tubes being replaced, possibly the reverb transformer too. Ascertain whether the power transformer is still good before estimating.
Guitar player logic: If fuse blows, put another one in. If that fuse blows, put a bigger fuse in. Repeat until no further change is noted.
I'm also going with "external factors" causing the loss of vacuum in the preamp tubes.
I have seen 12AT7 crack the base in the reverb driver slot. That's just what happens with some of the chinese and russian tubes, maybe others. The button and envelope were not joined well in the factory. I've even personally seen one newly installed, crack like that in about half an hour of test running.
Some Russian tubes are fatter around the glass and don't fit well, and the stress against the bayonet socket can crack them.
Guitar players will do weird things, like roll tubes all the time, take the back panels off, lose the shields, stack things on them in the car and break tubes.
Most of them seem to like "rolling tubes" a lot for mythology's sake. They tend to break things and wear out sockets.
I'd just go about the repair process. It's going to be expensive, with all those tubes being replaced, possibly the reverb transformer too. Ascertain whether the power transformer is still good before estimating.
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Smokebreak3
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Re: What happened here ?
Turns out this used Twin was shipped from GC in May. Worked great briefly then started developing issues until it smoked. So there's the rough handling, I assume.
Once I got the board cleaned up and made sure no traces were shorted, I floated the Rk and Ck right off the socket to the 1M grid ground, and put in all new preamps and it's stable.
Luckily these ageless power tubes were (un)biased to 10mA, so he gets to keep these. As suspected, the Reverb didn't work until I swapped in a new xformer.
On that note, I seem to be replacing a lot of reverb transformers in the past year, both 60's amps and reissues. I don't know if it's coincidence or not, but I've gotten to where I stock 2 or 3
Once I got the board cleaned up and made sure no traces were shorted, I floated the Rk and Ck right off the socket to the 1M grid ground, and put in all new preamps and it's stable.
Luckily these ageless power tubes were (un)biased to 10mA, so he gets to keep these. As suspected, the Reverb didn't work until I swapped in a new xformer.
On that note, I seem to be replacing a lot of reverb transformers in the past year, both 60's amps and reissues. I don't know if it's coincidence or not, but I've gotten to where I stock 2 or 3
Re: What happened here ?
I'll say that 12AT7's are not all up to the job of running the reverb. Even a number of NOS JAN 12AT7WC/6201 will fail in that circuit. The usual hint of failure is intermittent loud crackling until the tube "rides the lightning" and goes poof. Not uncommon for it to kill the transformer in the process and break the vacuum of the tube.