Ampeg SVT Classic Problem
 

So I got a svt classic on craigslist yesterday. Tested it out to the max. Everything seemed fine. Got the head and a 410 ampeg he cab all for 850. I traded the cab into guitar center for 210 bucks and bought the 410hlf. Today I plugged in and the volume on the head is going up and down. Playing at a louder level after a few minutes it cuts by about 50% volume. Also, when cranked up I noticed my brand new ampeg 410hlf was distorting. Sounded like the speakers were crackling. So I hooked up a different head and cranked it up. No issues with a diff head. No crackling and the cab sounded tight. My question is...what cause the volume on the svt cl to go up amd down and if the head is in need of repair, can it cause that farty crackling noise in the cab?

Check the easy stuff first. Make sure the speaker cable and jacks on both the amp and speaker are good.

Yeah cecked all the connections. Have brand new speaker cables too. Seems to be okay. Tried other cables and had the same results too.

Could be power tube failure. What was the 4x10 that was sold with the head?

Yes preamp/driver/phase splitter or output tubes can cause this.

Ampeg 410he 8ohm cab. I traded it for the 410hlf at 4ohms because the head is rated at 4/2 ohms. The guy said he never really used them together, but at this point who knows.

This is from the early 90s. I asked if the tubes had been changed. Was told he thought they were original.

Sounds like wonky output tubes to me.

I recall seeing an SVT-CL and 410HE on the Boston craigslist for about the same price over the last few months. The ad always said it was sparingly used in a studio but I always assumed it was being sold because the owner was having trouble after running the head with an 8 ohm cab. Running a tube amp with an impedance mismatch will age the tubes at a faster rate, and running it with a higher than rated mismatch can damage the output transformer. Could be related to the problem you're having here.

Yeah. Seems like a tube issue which is what I was guessing. Thinking I might just take it in and have it looked through and fixed. Then again not sure I want to put the time into it. Might just list it for a steal and maybe somone who is a DIY'er can have at it.

If its just the power tubes its a quick and easy fix. A decent sextet of 6550s will run you around $200 and can be biased from the back of the amp. If they're original from the 90s and have seen a decent amount of use they're probably ready to be replaced after which you should have a hell of an amp.

Yeah not too bad. I just dont have the coin.

Tubes, shmubes.....

The crackling sounds as much like a crummy connection or solder joint as a bad tube. The power variation likewise, possibly more likely to be in power amp around the drive tubes ("around" them, not necessarily DUE TO the drive tubes)

Could be a jack, or something on a PWB.

Try the preamp through the other amp..... if it crackles, you know it's the pre, if not try the pre of the other amp through the power amp, and see if THAT does it.

Have you already tried patching a guitar cord between your poweramp-in & preamp-out jacks?
If this corrects the problem, you just need to clean the switching contacts in the poweramp in jack.