SVT Noise

nov_1981 · Post by **nov_1981** » Sun May 06, 2007 3:53 pm

Hey everybody,
Its my first post on this forum and I just want to say thanks to everyone and what a great site this is for Ric info.

Anyway, I have an old Ampeg SVT that more and more often makes horrible cracking, thundering ear popping, noises. It doesn't seem to have a rhyme or reason, sometimes it will go an hour without making the noise, sometimes just minutes after I've turned it on. I've ruled out the possibility of a cord problem and have used different basses with always the same problem.
The only thing that works with stopping the noise after it occurs is flipping the polarity switch.

My questions are: is this a common problem with old tube amps and is it fixable.

Thanks again
Rob

charlyg · Post by **charlyg** » Sun May 06, 2007 4:05 pm

I had that before I changed output tubes, but I am fairly new to owning an SVT so wait for a real expert to come by.

admin · Post by **admin** » Sun May 06, 2007 4:14 pm

Welcome Robert. I will move this thread to the Tube Tech Forum soon.

doctorwho · Post by **doctorwho** » Sun May 06, 2007 9:13 pm

Welcome, Robert. I have a similar problem with a 1968 Fender Showman Reverb head. I have a tube tester that I will be using to test the tubes when I get a chance.

icabod · Post by **icabod** » Mon May 07, 2007 1:24 am

You may start by removing and replacing the existing tubes in they're sockets a few times, so that you re-establish a metal to metal contact with the metal on the tubes as well as in the sockets, and also clean the sockets etc. with Caig DeoxIT to remove the oxidisation that occurs....R

kenposurf · Post by **kenposurf** » Mon May 07, 2007 3:47 am

Welcome Robert...good advise here so far..first re-seat the tubes..then if needed test and replace with known good ones if any tubes that are weak..look for buldging/leaking caps...a resistor could have drifted too far off..

johnallg · Post by **johnallg** » Mon May 07, 2007 5:49 am

I'm thinking you will find a tube intermittiently on the fritz. A low level tube (preamp section) can do this also.

nov_1981 · Post by **nov_1981** » Mon May 07, 2007 9:39 am

Thanks everyone, I'm gonna take a look at the tubes.

soundmasterg · Post by **soundmasterg** » Mon May 07, 2007 9:58 am

This is often a common problem with old tube amps. I would suggest that some of the caps in the amp are bad and need to be replaced. Specifically the electrolytic caps. If they have not been replaced before, you can bet they need to be replaced now.

My SVT did this too for awhile and the problem was a bad cap in the preamp chassis, yet I could make it stop sometimes by turning the bass control way down. The amp was miswired from the factory and a jumper wire that should have been there wasn't there, so this cap had never seen any use. All the signal went through a .022uf cap instead of a .22uf, and the bass response of the amp was lousy. Once I fixed the jumper wire the bass response was better, but the noise started too. Once I replaced the bad cap, the noise was gone and the amp sounded great. Electrolytic caps need constant use to avoid drying up, and since this cap had never been used, it dried up long before the others in the amp, which are still in there btw. I plan to replace them at some point because they should be repaced, but these amps are a PITA to work on, and are dangerous too, and this is coming from someone who knows quite well how to work on these.

The electrolytic caps will dry up even under use after as little as 10 years depending on the cap and it's quality, so they should be replaced in amps that get old as something akin to doing a tune up on your car. Sometimes they get so bad, they start to leak DC and cause circuits around them to malfunction, or they can also get very bad and start to leak the paste chemicals out that are inside them. These are corrosive and toxic and can cause quite a mess inside the amp.

The problem could also be a bad tube or two, but the caps should be the first place to check. Sometimes you can tell the caps are bad because the positive side of the cap will be bubbling, but they don't always do this when they are bad. The tube sockets could be dirty and could use a cleaning, but this can be dangerous.

An Ampeg SVT is one of the most complicated musical instrument amps ever made, and is also just about the most dangerous. There are voltages in there as high as 700v, which is certainly enough to kill. I would suggest to have someone who is qualified do the work on your SVT and replace every electrolytic cap in the amp. It will sound better when done and you won't have to do the caps again for a long time. While they are at it, they can clean the sockets and check all the tubes. It isn't as simple as just replacing the caps because some of them are not made today and so a good tech will have to get creative to fit the replacement caps in there and still have it look and function correctly. Also, because of the fan, the inside of the amp is likely to be very dirty too.

nov_1981 · Post by **nov_1981** » Mon May 07, 2007 10:38 am

Greg, that's great info thanks.

Can anyone recommend a good tech in the central NJ area

soundmasterg · Post by **soundmasterg** » Mon May 07, 2007 2:35 pm

Actually, I'd suggest to take your amp to Jess Oliver, who is in NYC I believe. Jess designed the original Ampeg B15 Portaflexes, and worked at Ampeg while the SVT was being made too. He'd be the perfect person to talk to about it. Here's his email that I have for him from a year or two ago.

[email protected]

nov_1981 · Post by **nov_1981** » Tue May 08, 2007 2:37 pm

Greg, thanks again, Mr Oliver has email.

soundmasterg · Post by **soundmasterg** » Wed May 09, 2007 11:38 am

You're welcome...I hope it works out for you. The more SVT's around the merrier in my opinion! Cool amps!

charlyg · Post by **charlyg** » Wed May 09, 2007 5:01 pm

I had a strange problem Sunday. I let the batteries run out in my wireless, and had been using a cable for two weeks. I have to really adjust the SVT differently for those two scenarios, Anyways, I could not find the sweet spot with the wireless this week. I did not like my tone at all. I am thinking maybe I was overdriving the input with the wireless signal.

PS - I am not talking about the "wireless/rf" portion of the signal.

Unless one of you tells me differently, I will turn down the sensitivity/output (or whatever they happen to call it) to zero, and just turn it up enough to get output out of the amp. Some may like an excess of "front end" distortion on their notes, to me it sounds like the speaker is farting.

soundmasterg · Post by **soundmasterg** » Thu May 10, 2007 8:18 am

I've never used wireless myself so I doubt if I'd be any help at all.