SVT 2 Pro bias drift/clicking, static sound

[punkrockmike]

At practice tonight, i turned the amp on, left it on standby for about 10 minutes, took it off standby, checked the bias lights as i always do and everything was fine. after about an hour of playing, i heard a very faint clicking/scraping sound coming from the speakers. i checked to make sure all my cables were connected properly, then checked the bias lights and the bias control 2 lights were off. i muted the amp, unplugged the cable from the input jack, and the sound was still there. i reset the bias for bias control 2 and then finished up with practice. the sound isnt constantly there, it just randomly clicks and gives out a scratchy sound, almost like a cable being rotated in a dirty jack or something, but it even happens when the amp is muted and there is no input signal to the amp. the sound doesnt happen when the amp is in standby. when it happens, the lights on bias control 2 flicker a little, and the red light lights up when the sound happens, just like it normally does during regular playing. is this a tube problem? the bias on bias control 2 has drifted a number of times in the past few months. what could this be?

[punkrockmike]

other than this the amp is functioning just fine. everything sounds normal as far as i can tell. i have two shows coming up on the 20th and 28th and would like to try to figure out what the problem is, if there is one, well before that so i can have it up and running in time for the show.

[Sinomis666]

Take it to a tech or take a backup amp for those gigs

[JimmyM]

Sounds like you might have a tube gone ka-flooey in there.

[punkrockmike]

really? even though everything sounds fine?

[punkrockmike]

other than the faint popping/crackling that is... and the sound does not happen while im playing. i can only hear it when im not playing, or when there is no signal going into the amp.

[punkrockmike]

and also, when i turn the bias control clockwise, according to the manual, the lights go from green to red, as they should when everything is working properly

[JimmyM]

I had that happen recently in my SVT and it was a bad preamp tube. So yeah, really

[packhowitzer]

Originally Posted by punkrockmike

At practice tonight, i turned the amp on, left it on standby for about 10 minutes, took it off standby, checked the bias lights as i always do and everything was fine. after about an hour of playing, i heard a very faint clicking/scraping sound coming from the speakers. i checked to make sure all my cables were connected properly, then checked the bias lights and the bias control 2 lights were off. i muted the amp, unplugged the cable from the input jack, and the sound was still there. i reset the bias for bias control 2 and then finished up with practice. the sound isnt constantly there, it just randomly clicks and gives out a scratchy sound, almost like a cable being rotated in a dirty jack or something, but it even happens when the amp is muted and there is no input signal to the amp. the sound doesnt happen when the amp is in standby. when it happens, the lights on bias control 2 flicker a little, and the red light lights up when the sound happens, just like it normally does during regular playing. is this a tube problem? the bias on bias control 2 has drifted a number of times in the past few months. what could this be?

my svt 2pro has been doing the same thing for about a year now. I'm not gigging it at the moment, so it's not the COMPLETE end of the world. I want to take it to a tech. But it just isn't in the budget right now. So for now I just put up with it. Good to know I'm not the only one. Please keep us posted on what your issue turns out to be. good luck!

[punkrockmike]

Originally Posted by JimmyM

I had that happen recently in my SVT and it was a bad preamp tube. So yeah, really

haha alright jimmy thanks. ill throw the tubes on the tube tester tomorrow and check things out. if one of them is dead, do the preamp tubes need to be matched? or can i just take a functioning one from one of my other amps and throw it in there?

[JimmyM]

Pre tubes don't need to be biased. That's the best way to check your pre tubes...throw it in there and swap it around in the different 12ax7 slots till you find a culprit.

[Jerrold Tiers]

Flickering of the bias lights can be.......

* output tube problem

* driver tube problem

* leaky coupling capacitor

* something else

* low frequency noise from a bad cap/resistor/tube earlier in the amp

Bias drift can be any of those except the last one.

[packhowitzer]

Originally Posted by JimmyM

Pre tubes don't need to be biased. That's the best way to check your pre tubes...throw it in there and swap it around in the different 12ax7 slots till you find a culprit.

Originally Posted by Jerrold Tiers

Flickering of the bias lights can be.......

* output tube problem

* driver tube problem

* leaky coupling capacitor

* something else

* low frequency noise from a bad cap/resistor/tube earlier in the amp

Bias drift can be any of those except the last one.

Thanks for the input from two guys who know! I know with mine (which I believe is behaving the same as the OP- though perhaps for different reasons), I am certainly capable of trying Jimmy's approach to trying out different preamp tubes to hopefully find the culprit if there is a bad one.

All of Jerrold's suggestions about flickering lights and drifting biases are waaaay beyond my paygrade to diagnose and a trip to the tech is certainly in my future it looks like. Maybe when I get my tax returns....

[JimmyM]

Well it's one guy who knows and me I was just guessing. Jerrold actually worked for Ampeg as chief engineer so he's way more advanced than most of us.

[punkrockmike]

wow thanks a lot for the info guys. just dropped the amp off at the tech. should have it back by the end of next week. ill report back when i find out what the problem was. hopefully nothing too major. i talked to the tech for a bit and found out that i may have been doing more harm than good to the amp by messing with the bias controls. i've adjusted them two or three times since i had it retubed and im not sure if everything was as hot as it should have been when i did it, so i may have thrown the bias off quite a bit. he's gonna get everything back the way it should be for me. now ill have to just pretend those frickin lights arent there and stop trying to fix things that work just fine

[JimmyM]

Originally Posted by punkrockmike

wow thanks a lot for the info guys. just dropped the amp off at the tech. should have it back by the end of next week. ill report back when i find out what the problem was. hopefully nothing too major. i talked to the tech for a bit and found out that i may have been doing more harm than good to the amp by messing with the bias controls. i've adjusted them two or three times since i had it retubed and im not sure if everything was as hot as it should have been when i did it, so i may have thrown the bias off quite a bit. he's gonna get everything back the way it should be for me. now ill have to just pretend those frickin lights arent there and stop trying to fix things that work just fine

Ya, back in the day, none of our SVT's came with user bias adjustment knobs and we all did just fine. As a matter of fact, I wish they'd take all indicator lights for everything off all amps. Then it forces you to use your ears and prevents you from worrying about stuff you don't need to worry about.

[will33]

Originally Posted by JimmyM

Ya, back in the day, none of our SVT's came with user bias adjustment knobs and we all did just fine. As a matter of fact, I wish they'd take all indicator lights for everything off all amps. Then it forces you to use your ears and prevents you from worrying about stuff you don't need to worry about.

I'd agree with that up to a point, but, tube quality/consistency isn't what it was in the old days either. You can have a tech set everything up perfectly, have it checked again in a few months and things are off again.


In most cases though, yes, users aren't techs and most often do more harm than good. Maybe they should lock those controls...make you pass a test before you get the key.

[rickdog]

Originally Posted by punkrockmike

... just dropped the amp off at the tech. should have it back by the end of next week. ill report back when i find out what the problem was. hopefully nothing too major. i talked to the tech for a bit and found out that i may have been doing more harm than good to the amp by messing with the bias controls....

Good call, your tech should set it right. It will be educational for all of us to hear what the problem turned out to be.

I think Jerrold has commented on those bias lights, and suggested they're only marginally useful. Still, if you can get them both in the green then your amp is probably not going to self-destruct!

My SVT-CL had a similar problem when I got it. After it had warmed up for ten minutes or so, it would click and hum, and the bias would drift (one of the lights went red). Tapping on the output tubes with the eraser end of a pencil revealed that one was microphonic and the bias changed when I tapped. Another output tube looked pretty spent (the getter had turned almost completely grey instead of silver). A new set of Tung-Sol reissues put it right!

[punkrockmike]

pretty sure jimmy was right, as usual . its looking like a tube issue. talked to the tech today and he said a number of my power tubes were testing really weak. he hadnt had the chance to test the preamp tubes, but its looking like the power tubes were the culprit. i should have it back by tomorrow, so ill let everyone know for sure what the offical consensus is.

[punkrockmike]

aaaaand jimmy was also right about it being a preamp tube. 3 of them were bad.