Replacing the tubes on a 70s SVT

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by 6AM, Feb 4, 2006.

  1. 6AM

    6AM Supporting Member

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    I have an old 70s SVT. How hard is the amp on the non 6550 tubes in there? How often should I replace those weirdo tubes in the preamp and power section?
     
  2. Blue Strat

    Blue Strat Member

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    Are you asking if you can replace the 6550s with another type? I wouldn't try it, the plate voltage is pretty high and probably beyond what a 6L6 type can handle reliably.
     
  3. Dave C

    Dave C Gold Supporting Member

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    Many Ampegs and definitely the SVT run very high plate voltage (600+)which is hard on power tubes . Some older SVT's used 6146 power tubes if I'm not mistaken which are kind of rare these days. I'd give Mike K at KCA NOS Tubes a call if that's what you have ......he might even chime in if he sees your post. With good tubes and proper bias I'd say you should be able to get at least a couple of years out of a set of tubes depending on how hard and how often the amp gets used maybe more. With something like 14 tubes in one of those amps....I know you won't want to retube it often $$.
    Dave C
     
  4. 6AM

    6AM Supporting Member

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    :jo

    Apparently there was confusion in my question. I'm NOT talking about the power tubes. There are a bunch of 12BH7s (some in the power amp section and some in the preamp I believe) and some other small pre-amp type tube. How often do you think THOSE should be replaced?
     
  5. Blue Strat

    Blue Strat Member

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    Now your first post is totally clear:jo

    US made preamp tubes often last decades. If you've got the originals still in the amp, it would be worth having them tested but it's possible that they're still fine. I work on a lot of '60s and '70s amps with original preamp tubes that are still fine.
     
  6. VintageJon

    VintageJon Member

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    What Mike said.

    If you do have suspect 12BH7 he or I have them.

    As for the power tube "non-question" it's JJ KT-88's here as the GE 6550 is too hard to find in a matched sextet.

    Howdy Mike! See ya here in March...

    -Jon
     
  7. Geetarpicker

    Geetarpicker Member

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    I wouldn't touch any of the other tubes if it sounds good and none of them seem microphonic. Do you hear any rattles when you crank the amp? If not, I'd leave the preamp tubes alone. If the preamp circuit seems to have plenty of gain, the tone controls seem plenty responsive, and the tonal range wide from top to bottom you are probably fine to leave all the preamp tubes alone. The possible exception might be the driver tube, which can get weak and reduce the overall gain and headroom of the amp. If a tube starts to make random crackles and noise, obviously it's time to replace that specific tube. Tube quality was so good "back in the day" that many old tubes will sound better than new ones, EVEN with the decades of use on them. Power tubes are another story. They wear out much more quickly. For example I gig all the time with a '64 Fender Deluxe amp. The rectifier tube and preamp tubes are original from '64 (!), BUT the output tubes are "new old stock" RCAs.
     
  8. Danny G

    Danny G Guest

    I've had 2 microphonic preamp tubes in my late 70's SVT, so I started scouring E-Bay trying to find NOS replacements (GE's, RCA's, etc) for all of them, whether they needed replacing or not. Better to have backups ready, heh.

    I had rattling/farting, sometimes EVERY TIME the drummer stomped on his kick drum. Needless to say I couldn't use my amp for that particluar show. Haven't had any problems since the bad tubes were replaced.
     

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