Issue with my 70s Ampeg SVT at my gig

D Bopp · #1 01-23-2011, 01:50 AM

Hi. Tonight at my gig, my mid 70s SVT head started squealing. It has been perfect for the past few months. But tonight, after about an hour of playing, it would drop in volume a little bit, and start making this horrible high-pitch squeal. I had to pull the cord out of the input to stop it.
I played the head the rest of the evening, with it sporadically making the noise. I found that when I played harder, it would happen more often.
Do you think this is an issue with the power tubes, preamp tubes, or something else?
Thanks in advance

bigbottomend · #2 01-23-2011, 04:02 AM

Microphonic tube maybe ?

MAMMOTHvolume · #3 01-23-2011, 04:03 AM

Quote:

Originally Posted by bigbottomend

Microphonic tube maybe ?

+1

lem8r · #4 01-23-2011, 06:47 AM

Positive feedback loop makes such sounds, so I vote for microphonic tube too

dincz · #5 01-23-2011, 07:02 AM

Quote:

Originally Posted by bigbottomend

Microphonic tube maybe ?

Or maybe not if it stopped when you pulled out the input cord.

kosmicwizard · #6 01-23-2011, 07:03 AM

Pre is my guess. Best of luck with finding and fixing your problem, man. Cheers.

D Bopp · #7 01-23-2011, 08:36 AM

Thanks. I hope its just a preamp tube.

anderbass · #8 01-23-2011, 09:50 AM

Did you try the other input channel?

rickdog · #9 01-23-2011, 10:58 AM

Got any spare tubes? Try swapping out tubes. If you don't have any spares, try exchanging tubes of like types. See if this affects the problem.

Another possible cause is bad power supply filter capacitors. This is plausible given the age of the amp, unless of course you've had it recapped recently. In that case, it's probably tech time, unless you have the skills (starting with not electrocuting yourself) and equipment.

D Bopp · #10 01-23-2011, 03:52 PM

I don't know when it has last been serviced. It is in very good condition for its age. I guess I'll be taking her down to Andrews Amp lab here in Atlanta soon.
I did have an issue at this same club a few months ago with my SVT-VR where the voltage was too and the head didn't kick off of standby. I did the mod for it and it was fine after that. But I wonder if that may have something to do with it as well?

JimmyM · #11 01-23-2011, 04:57 PM

i doubt it. i think a bad tube or cap are more plausible.

weekend warrior · #12 01-23-2011, 05:19 PM

Sounds like a preamp tube to me.When was the last time you had it tubed D?

nysbob · #13 01-23-2011, 08:29 PM

I've had similar things happen with the rocker switches...have you tried exercising all of them? Sometimes they get stuck between postions and ugly things can happen.

vincenzomauer88 · #14 01-23-2011, 08:35 PM

Try tubes, it could be a tube or a socket. Try taking a pencil or something similar and tapping on the tubes for microphonics. I have a SVT VR and had the same issue. Also could be a power supply too.

Ahh, tube amps, why do you have to sound so damn good?!!!

My solid state needs no maintenance, no work, so reliable.

johnk_10 · #15 01-23-2011, 08:39 PM

my first thought is that it sounds like a preamp tube problem. i've heard output tubes make some strange noises too, but the preamp is where i'd look first.

D Bopp · #16 01-23-2011, 08:48 PM

Quote:

Originally Posted by johnk_10

my first thought is that it sounds like a preamp tube problem. i've heard output tubes make some strange noises too, but the preamp is where i'd look first.

Thanks guys. And Johnk_10 I'm glad you replied.
So if I tap on the tubes with a pencil, I may hear some microphonic noises through the speakers with the standby off?

johnk_10 · #17 01-23-2011, 09:17 PM

you have to tap on them with the amp on.

D Bopp · #18 01-23-2011, 09:56 PM

Quote:

Originally Posted by johnk_10

you have to tap on them with the amp on.

Yes, that's what I meant. Thanks. I'll try this

D Bopp · #19 01-27-2011, 04:45 PM

Ok, I just powered up the head. Let it sit on standby for about 10 minutes and then flipped it off standby and played it for a minute. I didn't hear any squealing. This may be because it's not hot enough.
However, I did take the advice of tapping on the tubes with a pencil.
With the amp on and the volume on my bass turned up, I tapped the preamp tubes lightly. The very front tube kind of made a crackle sound. The same kind of sound as a dirty pot on your bass.
The preamp tubes on top behind the rocker switches and knobs all had a microphonic sound. I could hear my pencil tapping through the speakers. They all sounded the same.
So does this mean that I should replace all the preamp tubes? If so, do I need to replace the 6550s as well?

weekend warrior · #20 01-27-2011, 04:56 PM

Yes!They shouldnt make any noise when you tap on them.They need to be replaced.The pre amp tubes will last for years with out needing replaced.so, If you have not changed the power section(6550's)in the last couple of years,they will definently need to be replaced also.You need to have a good amp tech work on it.all the tube sockets should be cleaned as well.Its gonna cost you but when its done you'll be glad you spent the coin.