Guild Thunderbass 12DW7 question

Discussion in 'Amps and Cabs [BG]' started by PikeoftheLake, Jun 12, 2019.

  1. Hello vintage tube amp experts,

    I have a Guild Thunderbass tube amp from the 60's. It has 4 preamp tubes and 2 power tubes. When I first got it years ago, the bass channel wouldn't work. I took it into a tech and they said they had a bit of trouble with it but fixed it. Recently, when I fired it up, the Guitar channel is not working.

    Before I take it back to a tech I have a question about the preamp tubes in these. All 4 preamp tubes in the amp, at present, are 12AX7's; but some talk in online forums say that these amps had one 12DW7 tube right before the output tubes.

    Should I get a 12DW7 and try it in the last stage before the output tubes?
  2. BogeyBass

    BogeyBass Inactive

    Sep 14, 2010
    Would have to see a schematic.

    Could be lucky and just have bad tube.
    In the dead channel.

    Or any numerous number of guesses why stop working. Old sockets or connections gone bad just one of the few. Even simple as corroded input jack/ grounds.

    Swap known good tube into dead channel you already have. See what happens. Check for dirty bent contacts while your there

    12dw7 more likely be in the pre. Unless it has simple phase inverter. It's feasible if it drives output tubes. Higher current lower mu section of w7 be useful for driving output tubes. Otherwise be useful for driving tone controls
    Last edited: Jun 12, 2019
    Muckaluck likes this.
  3. beans-on-toast


    Aug 7, 2008
    If you have the 8 tube thunderbass amp (rectifier tube, two 6550 power tubes, five small signal tubes), the schematic attached should be good to use. There are two 12DW7's. If not, you have a different revision that uses 6L6GC power tubes and has a four tube preamp, one of the tubes is a 12DW7. Both schematics are attached below.

    The 12DW7 (7247) is a dual triode that is basically half 12AX7 and half 12AU7. Depending on the amps design, you might be able to substitute a 12AX7 for a 12DW7. But if the amp was designed for a 12DW7, that's what you should use. The question is, was the amp modified to use a 12AX7. You would have to look at the resistor values associated with that particular tube and compare them to the schematic to determine if the amp was modified.

    But sure, you can install a 12DW7 as the phase inverter in the revision with the four preamp tubes. But if one channel is down, I'd trace that circuit from the input. The phase inverter wouldn't be the problem.

    Attached Files:

    Last edited: Jun 12, 2019
  4. You help me see that the phase inverter wouldn't be the problem. I will try to open up the amp this weekend and see.

    Thanks so much for the help!
    beans-on-toast likes this.
  5. BassmanPaul

    BassmanPaul Inactive

    @Muckaluck - with respect neighbour, how much tube amp experience have you had? There are lethal voltages present in any tube amp even with the amp switched off. Take it back to your tech to repair the problem.

    Regards from Scarborough! :D
  6. Thanks Paul. I will definitely not tinker with the amp, just replace the 12ax7 with a 12dw7 and see what happens. I have read your warnings about the danger of working on tube amps. Would I be right in thinking you lost a friend at one point due to him working on a high voltage amplifier?
  7. BassmanPaul

    BassmanPaul Inactive

    A guitar player who thought he knew more that the designers of his amp but that was long ago. :(
  8. So sorry for the loss Paul.
  9. BassmanPaul

    BassmanPaul Inactive

    Thank you but it was so long ago.
    Muckaluck likes this.
  10. Just a note that I put a 12dw7 in the correct spot and magically the guitar channel came back!

    Love this amp by the way. Thanks everyone for the help.
    BassmanPaul, JimmyM and Wasnex like this.
  11. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Yamaha, Ampeg, Line 6, EMG
    Yes, it helps to put the 12dw7 in the right spots :D Good deal.