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Post subject: Bad power tubes?
Posted: Tue Mar 27, 2012 12:10 pm
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So i noticed recently that my amp was making a popping noise and varying volume. I took off the back cover and noticed that two of the four 6v6 power tubes would pulse when the popping noise came through the amp. One of the tubes is glowing much brighter (orangish color) on the top and bottom plates then the others. Is this a sign that i need to replace the power tubes?

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Post subject: Re: Bad power tubes?
Posted: Tue Mar 27, 2012 1:28 pm
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stratdude83 wrote:
One of the tubes is glowing much brighter (orangish color) on the top and bottom plates then the others. Is this a sign that i need to replace the power tubes?


That'd be a safe bet. I'd replace all four power tubes with a fresh, matched quartet, then set the idle bias to the correct spec.

Arjay

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Post subject: Re: Bad power tubes?
Posted: Tue Mar 27, 2012 2:54 pm
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Just outta curiosity, would those 6V6GT be reissued TungSol's?


Thanks!


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Post subject: Re: Bad power tubes?
Posted: Tue Mar 27, 2012 5:41 pm
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Retroverbial wrote:
stratdude83 wrote:
One of the tubes is glowing much brighter (orangish color) on the top and bottom plates then the others. Is this a sign that i need to replace the power tubes?


That'd be a safe bet. I'd replace all four power tubes with a fresh, matched quartet, then set the idle bias to the correct spec.

Arjay


Just bought a fresh set of JJ 6V6 matched quad set "burned in" and i should have how to bias the amp later today. I have Tung-Sol's in there now and they are good tubes but i figure i'd change things up a bit and see if i like them. We'll see if this fixes the problem or not...

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Post subject: Re: Bad power tubes?
Posted: Tue Mar 27, 2012 6:16 pm
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If it's those TungSol RI (Made in Russia) 6V6GT that are going red plate on you, I have one suggestion--- do not use them anymore. I've had 4 sets go bad on me. 2 of the sets BOTH tubes red plated and one went ballastic to the point of popping resistors and causing a bit of smoke.

I know of about half a dozen guitar amp folks with older Fender or similar amps that place a lot of voltage onto the plate (like they did in the 1950-70's). And these six had the TS either red plate or start popping resistors or both. One had the PS rail damaged. Took out a cap can, that oozed out a bit of its guts. The rep of the TS 6V6GT is kinda well-known in the hi-fi world, now too.

If you want to continue to use these tube (if this is really a TS issue with your amp). You need to drop the plate voltage down to 350VDC range. Change the grid-to-ground resistors from 220k-ohms to 100k-ohms and decrease the coupling caps from whatever your stock value is (say 0.1mfd to 0.047mfd/600VDC---may need to experiment) to compensate for time-constant change. Otherwise simple grid-toground resistor change will drasitcally effect your tone.

Shoot for idle wattage dissipation of no more than 7 watts per tube. And you might be ok.

I have not tried any TSRI 6V6GT in a good 6-8 months. So their QA and overall quality may have improved. I know they are great sounding tubes. But, I'm afraid the vintage I tried were not built to meet the demands of most combo guitars. Esp those vintage units. Dunno. Been using TAD 6V6GT and good used US made NOS 6V6GT and GTA ever since. Just a warning... Good luck with your amp and tubes.


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