SVT CL Volume Drop, all Bias lights on

mr_goodbomb · Oct 15, 2011

I dunno what is up with my SVT. I came in for practice one day, and the volume was just incredibly low. I checked the bias lights, and when I play, they all flicker, all three (or is it two? red and green?). I've had to practice with it a few times because it's all I've had, but sometimes it's a little louder, but not by much. I have to crank it pretty high to be heard at practice... which was never an issue before. I realize I'll probably have to take it to a tech, but they're so expensive around here, and far away... any ideas what the issue could be?

jmattbassplaya · Oct 15, 2011

Sounds like tubes to me. Make sure they're all snug in their sockets.

jelwood · Oct 15, 2011

jmattbassplaya said: ↑

Sounds like tubes to me. Make sure they're all snug in their sockets.
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Indeed.

B-string · Oct 15, 2011

mr_goodbomb said: ↑

I dunno what is up with my SVT. I came in for practice one day, and the volume was just incredibly low. I checked the bias lights, and when I play, they all flicker, all three (or is it two? red and green?). I've had to practice with it a few times because it's all I've had, but sometimes it's a little louder, but not by much. I have to crank it pretty high to be heard at practice... which was never an issue before. I realize I'll probably have to take it to a tech, but they're so expensive around here, and far away... any ideas what the issue could be?
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First) Download the manual, this is the information age and readily available as a download at Ampeg (sorry if it sounds grumpy but you found your way here). The bias lights are covered in the user manual and will "flicker" while playing.

Second) It could be a relatively cheap preamp tube, could be a bad output tube(s), could be an instrument cable used for speaker cable that is shorting. You could spend $400 for a tube set, with the downloaded manual get a close bias adjustment then still have to spend the money on a tech to find a cold solder joint. Which is cheaper?

Make sure you are using a Speaker cable, not an instument cable to connect your amp to your speaker cab. Try the amp with a different speaker cab, try a different instrument cable from your bass and finally does your bass have a battery that needs to be changed? These things will at least make sure you don't make a trip to an amp tech with the amp not being at fault.
)

rickdog · Oct 15, 2011

+1 on B-string's advice.

One more thing to try: connect a patch cord from the "preamp out" to the "power amp in" and see if it solves the problem. The power amp in jack has a switch in it, sometimes the contacts get corroded. Sometimes just putting a plug in the jack and pulling it out again is enough to get good contact again.

mr_goodbomb · Oct 16, 2011

B-string said: ↑

First) Download the manual, this is the information age and readily available as a download at Ampeg (sorry if it sounds grumpy but you found your way here). The bias lights are covered in the user manual and will "flicker" while playing.

Second) It could be a relatively cheap preamp tube, could be a bad output tube(s), could be an instrument cable used for speaker cable that is shorting. You could spend $400 for a tube set, with the downloaded manual get a close bias adjustment then still have to spend the money on a tech to find a cold solder joint. Which is cheaper?

Make sure you are using a Speaker cable, not an instument cable to connect your amp to your speaker cab. Try the amp with a different speaker cab, try a different instrument cable from your bass and finally does your bass have a battery that needs to be changed? These things will at least make sure you don't make a trip to an amp tech with the amp not being at fault.
)
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I appreciate that. Is there any way to test if the tubes are bad?

I have checked the cables many times just to be certain, but next time I go over, I'll bring the instrument and speaker cables from my guitar amp just to be sure.

No battery, I used Gibson Grabbers.

Funny, though, when I mentioned the flickering bias lights on another forum, they all overwhelmingly said "IT'S GOING TO EXPLODE TAKE IT TO A TECH."

4StringShooter · Oct 16, 2011

mr_goodbomb said: ↑

I dunno what is up with my SVT. I came in for practice one day, and the volume was just incredibly low. I checked the bias lights, and when I play, they all flicker, all three (or is it two? red and green?). I've had to practice with it a few times because it's all I've had, but sometimes it's a little louder, but not by much. I have to crank it pretty high to be heard at practice... which was never an issue before. I realize I'll probably have to take it to a tech, but they're so expensive around here, and far away... any ideas what the issue could be?
Click to expand...

DO have the Tech check the GROUND WIRES to the power tubes as well. Mine were ALL bad when I had it torn down, and benched. After that, with ALL new Power tubes, a biasing AND a new Pre-Amp Tube, it sounded NEW. Never been happier with it.

Oh, ...those lights on the back are ****. DO NOT trust them. Get it biased by A Tech !!!!!!!!

4StringShooter · Oct 16, 2011

B-string said: ↑

First) Download the manual, this is the information age and readily available as a download at Ampeg (sorry if it sounds grumpy but you found your way here). The bias lights are covered in the user manual and will "flicker" while playing.

Second) It could be a relatively cheap preamp tube, could be a bad output tube(s), could be an instrument cable used for speaker cable that is shorting. You could spend $400 for a tube set, with the downloaded manual get a close bias adjustment then still have to spend the money on a tech to find a cold solder joint. Which is cheaper?

Make sure you are using a Speaker cable, not an instument cable to connect your amp to your speaker cab. Try the amp with a different speaker cab, try a different instrument cable from your bass and finally does your bass have a battery that needs to be changed? These things will at least make sure you don't make a trip to an amp tech with the amp not being at fault.
)
Click to expand...

<< You could spend $400 for a tube set,>>

Just where the hell do you guys shop? A totally NEW ...FULL set of 6550s cost me 90 per PAIR....$270.00 Canadian. A new 12 AX7 was $27.00.

B-string · Oct 16, 2011

mr_goodbomb said: ↑

I appreciate that. Is there any way to test if the tubes are bad?

I have checked the cables many times just to be certain, but next time I go over, I'll bring the instrument and speaker cables from my guitar amp just to be sure.

No battery, I used Gibson Grabbers.

Funny, though, when I mentioned the flickering bias lights on another forum, they all overwhelmingly said "IT'S GOING TO EXPLODE TAKE IT TO A TECH."

The "Bias lights" can ONLY be used with NO signal. Bias IS the idle current draw through the tube. Tubes must be in a conducting state or part of the signal applied would be used to overcome the cutoff point of the tubes (non conducting, no amplification point). Since with a signal applied the tubes are no longer at idle, the bias lights will dance as the tube is in use (and no longer idle).
Tube testers help to some degree and vary in their reliability of readings. Most will not test at full voltage. In a running circuit with voltage and waveform tests is the most reliable (not a job for anyone except an experienced high voltage tube amp tech).
Hope this helps gain some understanding.
I started working in this field over 40 years ago, so this is from experience and training
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