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Vintage SVT ailing.

Discussion in 'Amps and Cabs [BG]' started by Wounded Paw, Jan 2, 2003.

  1. My early 70's SVT seems to be in trouble again. At practice the other day I was adjusting the mid-range and it suddenly quit on me. Not the amp just the mid-range knob. No amount of turning makes any difference in the sound and the amp seems to be giving a lot less power. It's like there is no midrange control at all and the amp is half the power it should be. It's not the first time this has happened either. When it last occurred a few years ago I had the amp repaired but the techie didn't find an exact cause of this, just various problems that needed looking at. Any ideas?
  2. ummm, I think your paw wounded the Ampeg. Sounds like it's time to send it to St. Lou for a band aid.
  3. That's an old Ampeg, so I don't think you'll gain anything by having modern Ampeg treat it. Just find a good repair place near you that understands what they'll be getting into - 600+ Volts and huge tube power!

    It's probably a bad pot or bad solder joint. That midrange pot wiper probably lost contact with the resistive material inside, and the schematic seems to say that that might kill a lot of your signal. Try switching the Mid control switch and tapping on the Mid control knob. Spraying it down with contact cleaner will probably fix it for another 10 years or so.

    Find a good tech.

  4. freebeer

    freebeer Supporting Member

    Sep 24, 2002
    Suburbs of Detroit...
    I agree with the previous thread. Find a GOOD tech and get the SVT gone through and she'll be good for many more years. I have an SVT and it had problems and it went to several different people until I finally found a guy who specialized in SVT's and since then the world has been a much better place. SVT rocks...
  5. I'm thinking it's time I learn SVT repair on my own. I can rewire my bass, build computers, wire up the whole recording studio but have always backed away from fiddling with the insides of my SVT. The problem is we have a big show coming on Saturday and I don't think I can get it in and out of the repair shop by then. Is it possible to bypass that pot altogether for now and still get the full power of the amp?
  6. Tapp


    Aug 29, 2001
    USA, Mississippi
    Paw, you can certainly learn tube amp repair yourself; I did and I'm still learning everyday! BUT, I would not learn on a vintage SVT. They're a bit more complicated than your average Fender or old Marshall circuit which would be better platforms for learning. I'm not saying it can't be done! I'm just saying please be careful inside one because an SVT will only hurt for a little while and you'll either get up or be carried off.

  7. boogiebass


    Aug 16, 2000
    Yep, good advice. Lethal voltage in that amp.
  8. Before I got the SVT I was using a V4B from about the same era but it had a number of problems. One of which was it's habit of blowing fuses at unexpected times, usually right before a set. In a bit of a rush and a little drunk I replaced a fuse without unplugging the amp, got a nice jolt up the arm and landed on my ass. Somehow we still played the set but I couldn't rilly feel my fingers til the next morning. This is definitely not typical behaviour of me but it did serve to prove a point.

    Right now I'm just wondering if I can get in and bypass that pot to have the amp in workable shape for Saturday and leave the real repair work to a pro next week sometime.
  9. I don't think you'll be able to bypass it. Dig the schematic of the preamp:


    If the wiper has lost contact, there's not a pretty way to bypass that section. You'd have to solder in (2) 22K resistors in place of the pot. Then it would be like the pot was permanently set to 12 o'clock position.

    Have you tried plugging in to the other channel to see if it sounds anemic? According to the scheme, Ch 2 should not be affected by the Mid control. If Ch 2 sounds bad too, then there are other problems. If it does not, then you know the problem is upstream from the power amp in one channel only.

    You can definitely learn tube amp repair. I love it and became a EE because of it. But I don't recommend learning on an SVT. I learned on my V4B, and that was hairy enough. If you think you're up for it, I'll happily coach you through it. First thing you're gonna need is contact cleaner to spray down the Mid pot and Mid selection switch.

  10. It seemed to work today when I briefly tried it before a band arrived for their mixing session. Channel 1 I mean and the mid control. I have some contact cleaner and I think I will see what I can do about cleaning it up tomorrow night. How do I get at the pot, from the front or the back?
    And thanx for the help.
    Wounded Paw
  11. pkr2


    Apr 28, 2000
    coastal N.C.
    There is a much easier way to clean controls than going through the back side of the pot.

    It takes a little preparation but it's definitely worth the time.

    Get a can of electronics tuner/control cleaner, a 2" to 3" X 7/16" piece of neoprene fuel line and a tube of J-B Weld or any other two part epoxy that has a thick consistency when it's mixed.

    Epoxy 1" of the little plastic tube that comes with the cleaner into the end of the neo. line. Make sure that you don't clog the little hole in the plastic tube.

    You should now have the neoprene tube reduced to the plasic tube.

    Now it's simply a matter of removing the knob, sliding the neoprene tube over the control shaft and tight against the end of the threaded bushing tha holds the pot in. The pressure from the spray can will force the fluid into the pot through the clearance between the shaft and bushing.

    I've been cleaning controls with this little gizmo for years and it works like a charm.

    IMHO WD40 should never be used to clean a control.
    The graphite material that the resistive element is made of contains a binder that any oil based cleaner MAY decompose. You may clean a hundred pots with no problem and totally and quickly ruin the 101st one.

  12. I think I'm up to the challenge. When we were getting our new studio in shape for recording we had to take every strip out of the 32 track vintage Sound Workshop console, clean all parts verrrry carefully and replace them. I've also performed a fair amount of maintenance duties on our 2" analog 24 track as well as all kinds of other repairs and maintenance in the studio. The only reason I haven't gone into my SVT before is cuz I didn't even know where to start. With your direction I think I'll be okay for something as simple as cleaning pots.
    Cheerz guyz,

    Wounded Paw
  13. pkr2


    Apr 28, 2000
    coastal N.C.

    That's a very good point, PBG.

    I first started using the idea back in the very early senenties when I was doing repairs to consumer electronics, T.V., V.C.R., stereos and that sort of thing. I haven't had any problem in several hundred uses so board contamination doesn't really seem to be a problem. There really isn't very much resistive matrix in a pot to start with and you are only flushing a minute percentage of the total amount onto the circuitry.

    In reality, I've found that cleaning is usually a temporary fix most of the time, anyway.

    At least the method I describe will get you by untill the pot can be replaced. At that time a good cleanup can be done.

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