SVT Question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by TheAmpNerd, Feb 6, 2010.

  1. TheAmpNerd

    TheAmpNerd Member

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    Hi y'all,

    Checking out the mods for the SVT some of the following questions
    came up...
    From the Tube Amp Book (4.1 ed, p. 278) and (Deluxe Revised ed, p132)
    Discuss changing the the resistor values of R6 to 220K, R7 220K,
    for use with a 12AX7 tube. However, both books show a change to
    R-25 to 4.7K resistor?

    The schematics show the R24 and R25 (4.1 ed, p. 345) and (Deluxe Revised ed, schematics, p14) to both be 47K as they connect to the p5 of the power tubes.

    Did they over look this in both pubs or do you have to compensate
    for the 12AX7 that much?
     
  2. TheAmpNerd

    TheAmpNerd Member

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    Follow up,

    I checked and another SVT that I have, has both the R24 and R25 value
    resistors as 47K. This is the later model with a stock 12AX7A in V1
    on the power tube section and V1 - V4 are 12AX7As preamp tubes.

    Go figure. The R7 value doesn't seem correct as then
    it screws up the balance. I wonder what they were thinking?
     
  3. SatelliteAmps

    SatelliteAmps Member

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    Without looking it up in the book, my guess is they overlooked some stuff. There are a number of inaccuracies about SVT mods and conversions in that book. Go by the real schematic, rather than the discussion.
     
  4. TheAmpNerd

    TheAmpNerd Member

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    Adam,

    That is kind of what I was thinking.

    I've got two here including:

    One, that has been hacked and never worked correctly since.

    Two, a driver tube went taking out an output tube and its socket,
    or it could have been a short worked it's way through the socket,
    then took out the power tube and its driver tube.

    ONE
    Is an original early 6550 amp. Changed sometime in the early 2001s and
    never completly sounded correctly since.

    [​IMG]

    TWO
    Probably the last of the original AMPEG SVTs, with different layout but
    generally following the original schematics.

    [​IMG]

    I just finished up replacing the power tube socket on V7.

    I'll post pics in a few.
     
    Last edited: Feb 8, 2010
  5. SatelliteAmps

    SatelliteAmps Member

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    We just had one that was brought in because a knob had broken off. Turned out it was one of the really early ones with the 6146's, had been converted badly by the Tube Amp Book directions (which aren't right), had some mismatched tubes, been DROPPED, then run, and had massive power overloads from the power transformer up through the power amp and into the preamp. It was a freaking nightmare. Myself and one of the guys here at my shop poured through all of those schematics for a long time making sure that everything was correct. Sounds awesome now, but man, it was a long road.
     
  6. TheAmpNerd

    TheAmpNerd Member

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    I know what you mean.

    No wonder Kevin O'Connor complains about these amps.
    They are just not obvious in their layout and have wires
    going all over the place.

    They were obviously not made with service in mind.

    So the beat goes on. :)
     
  7. donnyjaguar

    donnyjaguar Member

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    Sometimes I think these modification books keep working tech's in business.
     
  8. SatelliteAmps

    SatelliteAmps Member

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    Sometimes?
     

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