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Mod question - AA1164 to AA964

Discussion in 'Shock Brother's DIY Amps' started by ElliotKnapp, Oct 30, 2020.

  1. ElliotKnapp

    ElliotKnapp TDPRI Member

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    I recently built a AA965 Princeton non-reverb into a Vibro Champ chassis. If you haven't messed with one, there's plenty of info out there on this and other forums about how nice and clean they sound, as long as you're not expecting the exact tone of an AA1164 Princeton Reverb without a reverb tank inside. It's almost like a mini-Twin in terms of staying clean most of the way up the dial.

    I didn't realize until really getting into the build just to what degree the circuit is exactly the same as the AA1164 Princeton Reverb, but with the reverb and 3rd gain stage simply removed. So after I finished, I went back to my AA1164 Princeton Reverb, and came up with what seems to be a pretty simple but useful mod (which I haven't seen anybody mention before) that converts the circuit from AA1164 to AA964 at the flick of a switch. Basically, you put the middle a SPDT switch between the V1B plate and its plate resistor/capacitor combo. One side of the switch goes to the original connection, and the other side goes to the plate resistor/capacitor combo fed by the V3B plate . So essentially, you're bypassing the V2 12AT7 reverb tube, the V3A reverb recovery triode, and the V3B 3rd preamp gain stage (which the AA964 doesn't have). Of course, the volume is lower than an AA1164, but it's tonally a match to the AA964 I built standalone...with one slight difference.

    I was A/Bing them today, and noticed that the modded AA1164 in AA964 mode is slightly quieter than the straight-up AA964 circuit. I'm wondering if this is because mod leaves the V3 plate connected to its load resistor in parallel with the added V1B plate. So that got me wondering if I should instead use a DPDT switch and break the connection between the V3B plate and its resistor when the V1B is connected, leaving the V3B plate floating at the center switch terminal when in AA964 mode. But I don't know enough about tubes to really understand if this mod isn't potentially introducing problems for the tube and/or circuit. Is it bad for a triode to have normal grid and cathode connections but a floating plate with no connection to B+? I'm assuming it's not a problem for V2 and V3A to sit there with all connections as normal but with no chance of a signal passing through.
     
  2. elpico

    elpico Tele-Afflicted

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    Yes, it's bad for it. Same as leaving an amp on but in standby for long periods of time is bad.
     
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  3. ElliotKnapp

    ElliotKnapp TDPRI Member

    Age:
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    Of course...cathode poisoning, right? I knew I'd read that but couldn't remember the name when I was searching around.

    So, if I make sure the plate has a plate resistor and some B+ from somewhere, will it be ok? Could adding and switching to an additional resistor off one of the V1/3 power nodes that aren't in the circuit at that point save me?
     
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