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More SVT woes......

Discussion in 'Amps and Cabs [BG]' started by Tim Cole, Jan 26, 2006.

  1. Tim Cole

    Tim Cole Supporting Member

    Jun 12, 2002
    Findlay, Ohio
    One thing I noticed that didn't seem right was, a sizzling sound through the speakers when switching between standby and play. I wasn't sure if it were a problem or not....just assumed not.

    I just gave the SVT a good workout, and after about 20 minutes at gig (a LOUD gig) volume, I noticed after I'd suddenly stop playing, you'd hear that sizzling sound through the speaker for a brief moment. It was particularly worse on the lower, power draining notes versus the higher notes.

    Ignoring it and jamming on.....after a short bit, the volume would drop dramatically, and distort out into nothing but silence. A few seconds on standby then back on, and it would be fine. But only for about as long as you left it on standby before it would start to fizzle out again.

    This sounds like a problem with the caps not being able to keep up possibly? The caps are all new in it though.

  2. kringle77

    kringle77 Supporting Member

    Jul 30, 2004
    Massena NY
    I don't have a responce but I feel your pain man.
  3. THB_bassman


    Oct 11, 2005
    Tim, I'd really like to look up your whole experience thread when I have time...I recently picked up a (73) SVT, and knock on wood, so far so good.
  4. Tim Cole

    Tim Cole Supporting Member

    Jun 12, 2002
    Findlay, Ohio
    Not sure if it is possible, but I think I may have just been running it too hard to where the caps couldn't keep up?

    I had it set LOUD (giving a good workout before it goes out on a gig), with the low boost switch on as well. I realize this isn't the best "cutting" tone, but it sounds great like that in the living room.

    As said, I had the low boost on, bass set at noon, mid selector in the middle position, mid knob at about 9, treble boost off, and treble know about 9 o'clock as well. Master set at noon, which is loud enough to kill small animals. All pumped through an 810e.

    Is it possible the volatage requirements are outrunning the caps? Could the hissing sound I am hearing possibly be a leaky cap even though they are new?
  5. tombowlus

    tombowlus If it sounds good, it is good Gold Supporting Member

    Apr 3, 2003
    Fremont, Ohio
    Editor-in-Chief, Bass Gear Magazine
    I have no idea what's going on here, but I'm definitely pulling for you and I hope everything works out alright. I can't imagine driving an SVT loud enough to outrun properly spec'd caps. I mean, you would think that you would spontaneously combust from the SPL's before that would happen! :p
  6. Daywalker


    Apr 13, 2005
    These are the kinda threads that make me wanna stick with SS. I want an all tube Ampeg (CL or 2Pro) so bad, but this kinda stuff scares me to death...
  7. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    This is an older 70s, right?

    It sounds like there is a problem with the coupling between two stages. Either a funky capacitor, OR something wrong with a resistor.... Probably in a later stage if the noise isn't pretty loud.

    If a grid-to-ground resistor is bad or not connected, you can get something like that. The voltages ina tube amplifier are not regulated, and so when you play hard, they drop a bit, then recover when you stop.

    If a grid resistor is bad, that change gets coupled directly into the tube grid, and some of what you said makes sense. a crummy connection can be noisy, so..........

    Also, without the resistor, the capacitor gets a charge built up on it due to grid rectification, which messes up the tube operating point. That can make distortion that increases as you play, and ends up shutting off the stage... no sound.

    Could be something else, but it should be easy for a tech to find.... Make sure he checks it over again also, seems like that one has had a rough life.

    Bottom line is that it needs some more work. Not much, probably.....

    If its 70s vintage, it's 30-35 years old..... and probably been beat on, tossed into the truck a few hundred times, slammed around, etc, etc... not surprising it might have some problems.
  8. metron

    metron Supporting Member

    Sep 12, 2003
    Its an issue due to a 30 year old amp, not curcuit topology. You may have problems with 30 year old SS amps just the same.
  9. tadawson


    Aug 24, 2005
    Lewisville, TX
    While I agree, I think that tube gear may be more likely to have these types of problems than solid state, simply due to the greater hot/cool swings in tube gear. The thermal stresses can cause connections to fracture, and the more the temp swings, the greater the stresses . . . and yes, these type of problems can occur in SS stuff too . . . I once had to completely resolder all boards in an older solid state/CRT projection TV due to this type of failures . . . .

    In any case, I would not be afraid of either. Tube stuff is so mind numbingly easy to work on compared to SS, that any problem would likely be trivial to deal with.

    - Tim
  10. metron

    metron Supporting Member

    Sep 12, 2003
    But then thermal stress on semiconductors over time will eventually result in failure of those devices right? I was just trying to point out to Daywalker that its not necessarily a tube amp problem, more of an old amp problem. There is no reason to have concern about buying a new tube amp because of problems with an old one. Oh well... sorry for the slight sidetrack. :D
  11. tadawson


    Aug 24, 2005
    Lewisville, TX
    I was not necessarily referring to the devices themselves, but more to the interconnects - most thermal failures I have seen have been solder joints, due to the relatively soft metal of the solder . . . in most semiconductors, I think that the thermal mechanics have been considered in the wafer/package design, and I have not seen very many thermals in power transistors . . . . although I have seem some of the "all in one" modular amps, such as used in car stereos and other lower powered apps that are just plain failures waiting to happen . . . .

    - Tim