Drop-in One Tube Reverb

Discussion in 'Shock Brother's DIY Amps' started by JuneauMike, Jun 20, 2019.

  1. JuneauMike

    JuneauMike Friend of Leo's

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    So this is Tubenit's one-tube reverb circuit as a drop-in for an amp. I tried to make the board as small as I could. Could y'all take a look and give some constructive criticism. This is really for conversational purposes.

    1tube reverb layout.png
    This is the circuit below. I went ahead and made space for the plate voltage resistor that may or may not be necessary. I also used Hoffman's templates for the caps and resistors so they ought to be to scale. The tank and the transformer are just standard Fender values and since I don't know what amp this would go to I don't know what plate voltages to use, and so that's going to affect everything else.

    reverb_tweaks.gif

    This is the 12DW7 tube, which is a cool little dual triode tube that has the gain factor of a 12ax7 on Number 1 triode and the current handling capacity of a 12au7 on the Number 2 triode. So the 12au7 would feed the driver stage and the 12ax7 would feed the recovery stage.

    download.png
     
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  2. anglinnr

    anglinnr TDPRI Member

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    how about as a stand alone unit?
     
  3. JuneauMike

    JuneauMike Friend of Leo's

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    From what I've read about this circuit a standalone unit would be dissapointing. If you are going to go to the trouble of building a reverb unit you should go ahead with a Revibe or something with more bells and whistles.
     
  4. tubeswell

    tubeswell Friend of Leo's

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    That ought to work methinks

    FWIW I reverse-engineered this schematic from a boutique brand amp I had in the other day. It uses one-tube reverb - very lush reverb for 1 12aX7
     

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  5. JuneauMike

    JuneauMike Friend of Leo's

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    Lazy J, by chance?

    Scratch that, its a Morgan. Do you know what kind of reverb driver they used?
     
    Last edited: Jun 21, 2019
  6. Paul-T

    Paul-T Tele-Meister

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    this is very interesting, thanks for this. I thought Tubenit's circuit was for a 12AX7? Did you change components to reflect that?
     
  7. Bitsleftover

    Bitsleftover Tele-Meister

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    I’ve retro fitted the Tubenit 12ax7 one tube reverb in 2 amps now. Both sound great. The first one I made a separate turret board for. The second I mounted all the components either directly on the tube socket or the pot body. That made it a very compact, all be it a little fiddly to install.
    I had to increase the value of the pot though to get more reverb from it. 1 Meg wasn’t exactly dripping in verb even on 10. (Or 12)


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  8. sds1

    sds1 Tele-Afflicted

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    Yeah exactly, my suggestion is go with a 2M pot for max verb.

    Also the dwell on a trimmer pot would be nice to have.
     
  9. JuneauMike

    JuneauMike Friend of Leo's

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    No, there's no real way to do that without knowing your supply voltages, but I'd bet this wouldn't be far off. Generally you're looking for strong current supply somewhere under 10mA on the driver side (the 12au7 triode) and strong gain on the recovery side. Once you've got your B+'s you can get a better idea of what your cathode needs.
     
  10. JuneauMike

    JuneauMike Friend of Leo's

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    Why didn't you just adjust the mixing resistor?
     
  11. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Did anyone ever resolve the issue that was raised (can't remember their name to give them credit) in the previous adding reverb thread about using the second grid of the LTP to insert reverb but then raising that grid far above AC ground by way of the reverb effect level pot and associated resistors? Does it affect dry signal PI function in a way that can be heard or seen on an oscilloscope?
     
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  12. Bitsleftover

    Bitsleftover Tele-Meister

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    I tried that. It didn’t have as dramatic a increase in effect level as I expected until I got to a really high value. (1meg iirc) At that point the feel of the amp had changed dramatically. It sort of sucked the life out of it!
    I went back to either 100k or 180k mixing resistor and up to a 3 meg pot and that gave me the best of both worlds. More reverb than I’ll ever use and minimal effect on the response of my amp.
    On my next build I only had a 2 meg pot laying around. It still gives more than enough reverb for me. The knob possibly has a more useable sweep than the 3 meg, which i tend to find all the useable levels are between 0-3 on the scale. Both are a massive improvement over a 1 meg pot though.
    A trim pot as a dwell control would be nice though and I will definitely include one on my next build. The tails are a little long for me on my medium pan. But that’s really just a personal taste issue.


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  13. JuneauMike

    JuneauMike Friend of Leo's

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    Do you mean this one? If so, I think maybe no.
     
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  14. JuneauMike

    JuneauMike Friend of Leo's

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    Well as I understand it, that mixing resistor and the pot make up a voltage divider that balances the mix of wet/dry. There's also a bright cap there that sounds like you could have used to bring some of the life back to the tone. I don't totally understand reverb circuits but I'm not clear how a larger pot would have helped. Maybe it added the brightness that you didn't have in the absence of a bright cap on the mixing resistor. I guess to my simple understanding, a 1M pot goes from 0-10, a 3M pot doesn't go from 0-30.

    Someone way smarter than I can weigh in there. It seems like there is a cheaper, more efficient fix there.
     
    Last edited: Jun 21, 2019
  15. JuneauMike

    JuneauMike Friend of Leo's

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    1tube-reverb-layout-dwell.jpg

    Something like this. I think it still needs a resistor to ground off the trim pot, but I don't know what size.
     
  16. sds1

    sds1 Tele-Afflicted

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    I'm using a 1M trimmer straight to ground. A resistor between the trimmer and ground would establish a minimum amount of drive into that grid; I'm not sure this has much value in the context of a set-and-forget internal trimmer.

    The schematics on Hoffman's site all use between like 100k-200k for the mixing resistor by the way. It looks like you carried over the classic Fender values. Was this intentional?
     
  17. JuneauMike

    JuneauMike Friend of Leo's

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    Yeah, mine was Fender values because that's what I know. I think Hoffman is more of a Vox/Marshall guy. I don't think the mixing resistor is a random thing, and I've got a lot to learn about how each arrived at their values.

    But as I think about it, you would want a minimum value of drive in that grid, wouldn't you? It excites the spring reverb and you want to ensure that at "0" the reverb is still sending a signal through the circuit rather than everything to ground. I guess I'm thinking about bias wiggle tremelo circuits where you need a certain level of, in that case voltage, just to be able to fire the circuit. Or am I looking at it incorrectly.
     
    Last edited: Jun 21, 2019
  18. tubelectron

    tubelectron Tele-Holic

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    Another one-tube circuit, derived from the Fender circuit. I use it in one of my amps with success...

    [​IMG]

    [​IMG]

    Note the difference in voltage between in and out circuit, so an extra gain is needed in the following stage to compensate the drop.

    -tbln
     
  19. symbiotic

    symbiotic TDPRI Member

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    The cathode resistor on V2b is 150 ohms? Am I reading that correctly?
     
  20. JuneauMike

    JuneauMike Friend of Leo's

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    Plate 1 is 318v and Plate 2 (pin 6) is 63v?! That doesn't look right. Wouldn't that imply that your current is low on the drive side and high on the recovery side? That would seem the opposite of what you would want, right?

    And do I understand correctly, you have to add another gain stage as a result (an extra triode?)? If that causes you to add another tube, or to modify your circuit to reserve another gain stage for recovery, doesn't that defeat the purpose of a one tube reverb?

    Have you tried it with another tube? Like a 12ax7? Have you looked at the specs for a 12dw7?

    Not trying to be critical, just sorta interrogating the circuit to understand it.
     
    Last edited: Jun 22, 2019
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