Push pull switch on reverb circuit

Discussion in 'Amp Tech Center' started by isaacteller, Aug 3, 2021.

  1. isaacteller

    isaacteller TDPRI Member

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  2. corliss1

    corliss1 Poster Extraordinaire Platinum Supporter

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    With the switch not pulled out you have the standard circuit.

    With the switch "pulled" or "closed" or "engaged" or however you best think about it, the signal going into the reverb tank is split, sending part into the reverb tank, but also sending some into the next gain stage of the amp. This is how they achieved the "boost" on the later Deluxe Reverbs with that switch.

    It's not doing anything to the transformer, but it's just a good point to tap that signal.
     
  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    It doesn't boost the reverb wet signal. The dry signal before the reverb tank is at a higher voltage than elsewhere due to the reverb driver tube and reverb transformer. It uses that signal, mixing it in at V4A.
     
  4. corliss1

    corliss1 Poster Extraordinaire Platinum Supporter

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    Right, sorry if my words made it sound like it was boosting the 'verb.
     
  5. isaacteller

    isaacteller TDPRI Member

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    Thanx all for the info.
     
  6. Jowes_84

    Jowes_84 Tele-Meister

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    If I am not mistaken this is the standard boost switch they had in the later Silverface circuits - I just know the MV Pull Boost on the bigger amps and did not know about the boost on the 6v6 amps.
    Seems similar.
    Seems like most don’t like this kind of boost, including me. It is very easily converted to a Rivera-Era style boost like the Super Champ or PRII have it. Also that circuit is not overly popular, but I find it much more usable if tuned a bit.
     
  7. isaacteller

    isaacteller TDPRI Member

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    What could be done to tune it?
     
  8. Jowes_84

    Jowes_84 Tele-Meister

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    Well first of all SF amps have become very valuable and I probably wouldn’t do too much about a mint SF Deluxe Reverb except for electrolytic caps and general service.
    That said… add 10k plate resistor for reverb tube 12AT7
    Add 1nF coupling cap and connect the boost circuit to that cap between reverb Trafo primary side „minus/ground side“ anf added 10k plate R
    Change 1k resistor to 330k
    Change 220k resistor to 490k
    Sounds complicated but very easy to carry out on the board.
    Compare to Super Champ and PRII schematics
    PM me if interested in exact instructions
    I have done this to my Vibrosonic reverb and it’s a cool mod
    But again, I wouldn’t do it on a DR or any other very valuable vintage SF amp
     
  9. isaacteller

    isaacteller TDPRI Member

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    I'll try these changes and report back. Thank you.
     
  10. Greg70

    Greg70 Tele-Meister

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    Is this adaptable to a Princeton Reverb? It look like it based on the schematic.
     
  11. Jowes_84

    Jowes_84 Tele-Meister

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    Proceed with caution
    I once fell into a modding rabbit hole with my PRII and still need to undo some mods from back then
    At least keep it easily reversible if needed.
    Below a pic of a quick and dirty, hand-drawn schematic of possible modifications to the generic SF-boosts. Adaptations might be needed. And again, I would be careful with SF amps in good condition.
    If you want a high gain amp, get a high gain amp.
    I generally only work on amps that are messed up already, improving them. Mint condition I leave alone.

    image.jpg
     
  12. Jowes_84

    Jowes_84 Tele-Meister

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    Edit: double post
     
    Last edited: Aug 5, 2021
  13. isaacteller

    isaacteller TDPRI Member

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    Thank you for the drawing.
     
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