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SVT-CL issue

deichman

deichman

Jun 1, 2003
832
89
64
Kansas
www.endofstory.com
#1
My SVT head power tubes (all 6) are getting very hot, smells like burnt wiring and all 4 bias lights are on, both red and green. After 20-30 minutes this happens. Finally starts squealing quite loudly. Shut it down, took it to amp tech and of course he couldn't get it to replicate. Rehearsal tonight, same issue. Help!! Many thanks in advance.
 
#2
deichman said:
My SVT head power tubes (all 6) are getting very hot, smells like burnt wiring and all 4 bias lights are on, both red and green. After 20-30 minutes this happens. Finally starts squealing quite loudly. Shut it down, took it to amp tech and of course he couldn't get it to replicate. Rehearsal tonight, same issue. Help!! Many thanks in advance.
Click to expand...
Bump. Just a guess here but maybe try new preamp tubes. Surprised no one else chimed in yet.
 
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#5
Given that all six tubes or glowing about four times as bright as another head I have that I use, I just assumed it would be something other than the tubes. Would be quite unusual to me that all six would go bad at exactly the same time, but perhaps this is common in these.
 
#7
deichman said:
Given that all six tubes or glowing about four times as bright as another head I have that I use, I just assumed it would be something other than the tubes. Would be quite unusual to me that all six would go bad at exactly the same time, but perhaps this is common in these.
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Glow tells you nothing, and you still have 5 other tubes you have to worry about.
 
#8
I have no problem replacing all six tubes. I just don't want to replace all six tubes only to find out that's not the problem and there's something making these things burn so hot that they start to smell and cause noise and potentialy other damage to the amp. I'll get it back to the tech here in town. He's actually quite good he simply couldn't replicate the problem. I figured there would be somebody here on TB that has seen these exact symptoms before given there so many SVT's out there. Thanks for chiming in.
 
#13
deichman said:
When he had the head he tested every tube. None of them were bad according to his test equipment. They weren't optimum but then again they are nearly 10 years old. But none tested bad.
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Tubes can test good and still be bad. For example, a microphonic tube will often test good. Not trying to question the dude, but it happens.

You could call or email Ampeg and see what they think.
 
#14
Sounds like a tube or bias issue. Maybe a combination of both. Did you bias the tubes when you installed them? Either way, I would test each tube and replace them as a set if necessary.
 
#15
deichman said:
When he had the head he tested every tube. None of them were bad according to his test equipment. They weren't optimum but then again they are nearly 10 years old. But none tested bad.
Click to expand...

If your output tubes are almost 10 years old, you have gotten your money's worth from them. Tube "testers" 99.8% only test at reduced plate and screen voltage and results in circuit will be much different. Continuing to use old tubes you suspect, can result in damage to the amp.
 
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#17
make a cell video of it happening at your rehearsal spot and show the video to the tech just to prove that you're not nuts. post the video here too.

and it's not like you'd be throwing away your money if you bought a whole new set of tubes and that wasn't the problem, you'd still have a full backup set of tubes for when yours finally DO die.

did your tech measure the power at each critical point or did he just test the tubes and fail to replicate the problem? I'd be worried it's a failed/failing component that's making the power go wacky and if it continues it WILL blow everything up eventually.
 
#18
I'd hate to hijack the thread, but I have a similar problem. I bought a used one in January, biasing was showing red and green at any adjustment with any signal and smelled very hot, like burning. I bought six new power tubes and paid extra to have them matched and burned in (EHX). Installed them all and same thing (although it does sound better). I haven't played longer than an hour and a half, but no bad sounds or anything.

Does that mean I need to have the bias circuit board looked at?
 
#19
"Sounds like a bias circuit issue.
Output tubes could be damaged because of it,
but find out what caused it before assuming they are bad."

This
6 power tubes are not going to glow excessively red based on them all failing at once or a pre-amp tube
The problem is inside and clearly intermittent.
I like the idea of a video to show the tech
 

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