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Old 23rd February 2008, 05:19 PM   #1
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Default Tube glows red hot, blows fuse

Okay, forgive my total and complete ignorance when it comes to electronics.

I have a Peavey Classic 60 power amp, worked before I brought it in for service (new tubes).

Got it back from the local tech, and one of the tubes started glowing red hot to an extreme- I turned everything off and got a RMA on the matched tube set.

Put the new tube set in (Groove tubes, supposedly no bias needed for the same power rating). And immediately the same socket's tube began to glow like a red lightbulb, a strong hum came from the speaker, and then the fuse popped. All in about 5 seconds.

I can't imagine I had two bad tube sets in a row, I also read that not all the socket's contacts may not be seated properly.

I don't want to bring the unit back to the tech who serviced it, he either missed something or charged me for work he didn't do. Claimed he checked everything out before the retube and bias.

Any ideas what it could be? Thanks for your help!
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Old 23rd February 2008, 05:47 PM   #2
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Do you have a schematic for this amp?
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Old 23rd February 2008, 05:48 PM   #3
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Default Re: Tube glows red hot, blows fuse

Quote:
Originally posted by damagefactor
I don't want to bring the unit back to the tech who serviced it, he either missed something or charged me for work he didn't do. Claimed he checked everything out before the retube and bias.
Why not? He obviously did something wrong, and you are owed at least a real fix, or your money back. Let him get away with that, and he'll just do the same thing to someone else.

"Groove tubes, supposedly no bias needed for the same power rating"

I don't know about Groove Tubes, but this is nonsense. Replace the finals, and you will need to check the bias. Always. If that's their advice, then I'd shop for a new supplier.
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Old 23rd February 2008, 05:52 PM   #4
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Unfortunately, it not uncommon for this to happen ... new tubes last a short time and then go bad. The tech may have been honest when he stated that the amp worked fine.

The bad tube may have taken out a grid resistor or something else, which is now causing the new tube to burn up.

Fixing it yourself involves taking the amp apart and checking the resistors for both power tubes, as well as checking the tube socket for proper contact.

I'd recommend calling the amp tech and asking how much it would cost to bring it back in. If he's reputable, the repair should be free or the cost of parts (unless something was broken unrelated to his original repair.)

Also, specify a better quality tube, such as EH.
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Old 23rd February 2008, 05:52 PM   #5
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Default Re: Tube glows red hot, blows fuse

Quote:
Originally posted by damagefactor
(Groove tubes, supposedly no bias needed for the same power rating). Any ideas what it could be? Thanks for your help!
With one output tube burning up, you have a loss of negative control voltage on the tube's grid. This voltage is what's refered to as bias. All output tubes need it to some degree. The most likely cause is either a bad resistor that feeds the grid, or there is a very leaky or shorted coupling capacitor going to that tube.

This is not a complicated repair. I would return it to the original tech and have it made right.

Victor
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Old 23rd February 2008, 06:21 PM   #6
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Okay, I'll bring it back to the tech. Are the new tubes in the amp fried then? There was minimal time before the fuse blew.
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Old 23rd February 2008, 06:29 PM   #7
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Tubes are very forgiving and can withstand overloading somewhat. But they should be throughly tested before using. The amplifier should also be test run powered up under load for several hours by the tech in their shop before returned to you. This procedure should be standard in any competent repair facility.

Victor
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Old 23rd February 2008, 08:35 PM   #8
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Thanks, I'll inform him of my revised standards
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Old 24th February 2008, 12:36 PM   #9
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Yes he should have soak tested it for a while after service, it's standard thing really, sort of confidence check that the item repaired won't bounce back! It may be just bad luck of course that a coupling capacitor chose the wrong time to go short circuit or the transport back moved a connection and is shorting something, like the cathode bias chain directly to ground. I would imagine that the tech would be happy to have a no-cost look at it again due to the fact that you have not been able to use the amp since he last repaired it. Don't run the amp too long like this as it could lead to output transformer failure and that will cost a lot of $$$.
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Old 24th February 2008, 02:47 PM   #10
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Oh I don't plan on even plugging it in until it comes back from the tech.

I have to wait until monday to get a hold of the tech, no amp techs are on duty over the weekends.

I've had this damn amp for 5 weeks now and I've only used it for an hour before I sent it in for service.
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