Ampeg SVT II owners, help troubleshoot me plz!

[Phil_Collins]

Well... bad news. My first practice since acquiring the amp, and it takes a colossal dump in the middle of the first song. The volume just drops to near nothing, and if you turn the volume knob back a forth a bit, it would kick back on for short bursts of time. Is this possibly a bad preamp tube? The power tubes appeared to be glowing normally, and showed no sign of trouble. It didn't feel like it was emanating too much heat, the back cage was warm to the touch and the fan ran,silently at that. Anybody have this problem before?

I only played it for a few minutes in the store... and at practice I let it warm up for at least a solid 1/2 hour before I played it. I don't think I had the volume higher then 3 or 4, so I don't think I was pushing the amp all that hard.

I'll probably end out taking it back where I got it from and see about them fixin' it, unless someone here already knows what to do.

 

[rodl2005]

This is a SVT2PRO is it??? Not a old SVT2? In either case..... ???????? Sorry. Just wanted to clear up-pro or ___ !

 

[i_got_a_mohawk]

Im pretty sure he has an SVT-II.

Well, the half hour is excessive for warm up, but thats not going to harm the amp.

Just because the tubes are glowing normally, all that means is the filaments are working right.

However, because it seems to work again if you move the volume back and forth, is it maybe a dodgy volume pot?

Possibly a loose connection near the pot (slight movement of the pot fixing it briefly).

The joint between the preamp and power amp seems to be notorious in these amps, so what to try, is running a cable from the "preamp out" to the "power amp in". See if that works.

Just some ideas to work with.
I dare say it could be a preamp tube, if you have a spare 12AX7 and 12AU7 you could swap them in and out of the sockets one at a time (making sure to fully switch the amp off between each swap), and see if its a preamp tube.

 

[squish]

i've had a bad preamp tube, but it was obvious (cracked in pieces). i've had other stuff like solder joints on the ckt board and input jacks on the back go bad too. even though these jacks weren't being used, they are still in the signal path and made the sound cut out intermittently. there are quite a few points of potential failure on this amp. seek a professional.

 

[Phil_Collins]

Im pretty sure he has an SVT-II.

Well, the half hour is excessive for warm up, but thats not going to harm the amp.

Just because the tubes are glowing normally, all that means is the filaments are working right.

However, because it seems to work again if you move the volume back and forth, is it maybe a dodgy volume pot?

Possibly a loose connection near the pot (slight movement of the pot fixing it briefly).

The joint between the preamp and power amp seems to be notorious in these amps, so what to try, is running a cable from the "preamp out" to the "power amp in". See if that works.

Just some ideas to work with.
I dare say it could be a preamp tube, if you have a spare 12AX7 and 12AU7 you could swap them in and out of the sockets one at a time (making sure to fully switch the amp off between each swap), and see if its a preamp tube.

It's not a wiggling of the pot, as much as it is turning it all the way up, then down, then back up, and most of the time, it'll pop back on, 30sec later, it's near dead again.

I'll just take it back and let them figure it out.

 

[Tim1]

I had an identical problem with my SVT2 nonpro. What was worse was that it was so intermittent that whenever I took it to a tech it was fine. Eventually it became a permanent problem and turned out to be a problem with one of those white push-in switches. Sold the amp shortly after for other reasons (fell in love with a 400+). Long and the short - it could be any number of things, probably a tube or a dry joint, but as long as you can demonstrate the problem a good techie should be able to fix it.

 

[Phil_Collins]

well, I went back to the store I got it at, and their tech will be in tomorrow. I guess I just have to sit tight and wait. I've got so many gigs in March that I need this head. It's like viagra or cialis for my bass compared to the Fender pa100 I have to use now. It's so frustrating. I just hope it's back before SXSW, that's all I gotta say.

 

[lofreq64]

I had the same symptoms with mine when I first got it. After a few attempts by different techs to figure it out, it turned out the problem was an intermittent connection between the preamp and the power amp. Good luck- once the problem is sorted out you will love this amp.