Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: BobL on April 18, 2019, 05:06:28 pm
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I have a Blackface Princeton Reverb, and it got shorted from pin 2 of V6 to the chassis, which sparked... amp seems dead.
I thought at first it must be continuing to short from a carbon trace on the socket, so replaced both power tube sockets, tried new tubes, but no dice. Speaker hums when powered on, but that's all I get.
Getting 3.3v AC to the heaters, 450ish to pin 4. 430ish on pin 5. V8 (I've got resistors to ground to bias) on both tubes seems wacky and unstable. Voltages on the rectifier seem ok.
If I leave it on for a few w/ tubes in, I eventually get a spark and a pop in the V5 tube. If I put the tubes into a little Champ individually, they still work after all this.
What else could I have blown, and how do I figure out what was blown? I can't see any physical damage anywhere.
Thanks for any help!
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Do all the tube filaments light up?
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Do all the tube filaments light up?
Yes... though when I took the shields off of the preamp tubes to confirm them, I also found that the top had blown off of the 7025 in V1... put another tube in there and still no sound, but... that was kind of suprising.
EDIT: I actually can't say 'blown off', it may have broken off, may be unrelated... I actually really don't know.
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Getting 3.3v AC to the heaters, 450ish to pin 4. 430ish on pin 5. V8 (I've got resistors to ground to bias) on both tubes seems wacky and unstable.
Pin 5 should be about -40v. There is no V8 in a PR. Sparking 3.15V on V6-2 to ground did not blow up V1. In fact, it probably didn't hurt anything if it was just a quick spark.
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Getting 3.3v AC to the heaters, 450ish to pin 4. 430ish on pin 5. V8 (I've got resistors to ground to bias) on both tubes seems wacky and unstable.
Pin 5 should be about -40v. There is no V8 in a PR. Sparking 3.15V on V6-2 to ground did not blow up V1. In fact, it probably didn't hurt anything if it was just a quick spark.
Sorry, meant pin 8.
V5/6 - 6V6GT
Pin 2/7: 3.28AC
Pin 3: 447
Pin 4: 429
Pin 5: -394?
Pin 8 (has 1ohm, 1w to ground), I'm basically getting some variation of 0... under a volt, and not stable.
It seems that Pin 5 reading is way off.
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Pin 5 is way wrong! If it is really -394v then the bias cap is surely exploded. You need to look at the bias circuit. Only 2 resistors, a diode and an exploded cap.
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Cap in the bias circuit is 100v, 100uf, and is measuring at 94uf on my cap tester...
But in looking in that area, I think I found a bad shared ground used by the bias circuit and the power ground that got messed up when the strain relief didn't quite do its job when I was switching this amp to a new cab... Fixing that and then I'll see where I'm at.
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Cap in the bias circuit is 100v, 100uf, and is measuring at 94uf on my cap tester...
How much voltage do you measure on that cap?
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It seems that what actually blew were the bias resistors.
Fixing that ground got my pin 5 voltages right at -49v, but still no sound... had just bizarro voltages on pin 8 on each socket (14v on V6, 110v on V5!), so looking at one of those resistors, one of them looked messed up, so I replaced it, and that tube came down to 8mv... other one was still high, so replaced that one, and it came down as well. Biased back up to 22mv, and I think maybe I'm back in business.
As always, your help is greatly appreciated. You pointed me in the right direction with the bias.