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Old September 12th, 2004, 07:08 PM   #1 (permalink)
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Troubleshooting My Princeton Reverb

68 PR: when I turn it on it makes some crackling and popping sounds until it warms up and then it's fine. When this is happening, I've observed that the right-hand 6V6 (when looking from the rear) has a red-hot glow on one side of the tube. By the time the noise stops (about 60 sec.) the glow is gone and everything seems OK. I've swapped out power tubes and still have the same problem, so I assume it's not a tube thing. Anyone have any suggestions as to what to do next?

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Old September 12th, 2004, 09:51 PM   #2 (permalink)
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Sounds like you may have a bias supply problem. Has the amp ever had a cap job?
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Old September 12th, 2004, 11:12 PM   #3 (permalink)
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Cap Job--Yes.

About 4 years ago I did all the caps and the amp's had really low hours since then. I used good quality parts--Spragues etc. Could it be one of the bias caps has already gone?
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Old September 13th, 2004, 02:20 AM   #4 (permalink)
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Possibly.
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Old September 13th, 2004, 06:14 AM   #5 (permalink)
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Mine dit the same thing, it is a 1980 PR with the push pull boost. I thought it was time to recap and ordered new caps. In the meantime I changed the speaker... and the crackling was gone. It's doing fine now for about a year and I'm keeping the caps in the drawer.
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Old September 13th, 2004, 10:58 AM   #6 (permalink)
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It's probably not the bias cap, as that would affect both tubes equally. It is more likely a bad solder joint or bad 220K resistor where the bias voltage is divided between both 6V6's. Once it heats up, the bad open joint probably closes again and all is well until it cools off.
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Old September 13th, 2004, 12:18 PM   #7 (permalink)
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My '67 SFPR does that a bit too. It's pretty minor, so I've always assumed that it's just the tubes warming up since it doesn't have a stand-by switch. It was just gone over with a full cap job, so it should be in tip-top operating condition otherwise. I don't worry about it.
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Old September 13th, 2004, 01:39 PM   #8 (permalink)
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Quote:
Originally Posted by Jay N.
My '67 SFPR does that a bit too. It's pretty minor, so I've always assumed that it's just the tubes warming up since it doesn't have a stand-by switch. It was just gone over with a full cap job, so it should be in tip-top operating condition otherwise. I don't worry about it.
Slight noises, OK, but I certainly would worry any time an output tube glows red. It's a danger sign for sure. I do think he should investigate it and not ignore the problem.
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Old September 13th, 2004, 01:51 PM   #9 (permalink)
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Thanks for the info...

I'm leaning toward the bad solder joint theory because I previously discovered one in that same area of the amp--the PI section.
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Old September 13th, 2004, 03:31 PM   #10 (permalink)
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In a PR, the bias supply is "borrowed" from half of the high voltage winding, unlike it's bigger brothers that have actual tranny taps. So that's why there is that large resistor in series with the bias rectifier diode. Now mind you, that large resistor, in conjunction with the bias filter cap(s), has a certain RC time constant. That's why you should NOT use a huge 100uf filter cap on the bias supply of a PR, it takes too long for the bias filter to charge, and the tubes are running too hot during that time. On a PR's bias supply, don't go over 33 uf on the filter or the RC time constant is too long.
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Old September 13th, 2004, 04:08 PM   #11 (permalink)
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On a PR's bias supply, don't go over 33 uf on the filter or the RC time constant is too long.
That's good info, thanks. I had a 50 uF put in there. I'll have to talk to my amp tech about that.
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Old September 13th, 2004, 04:09 PM   #12 (permalink)
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Quote:
Originally Posted by fullerplast
[Slight noises, OK, but I certainly would worry any time an output tube glows red. It's a danger sign for sure. I do think he should investigate it and not ignore the problem.
Good point. My tubes don't glow when they "gurgle".
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