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So this looks fun: Matchless Spitfire

Discussion in 'Shock Brother's DIY Amps' started by JuneauMike, Jan 4, 2019.

  1. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    While I wait for the UPS driver to show, I was kicking around a future build. Something that isn't so Fendery and something that an idiot like me can figure out, toy with, or at least enjoy. I don't have a Vox amp and I love their sound, but frankly their schematics give me a headache. So I hit on this idea:

    spitfire.gif
    The reverb was stolen from Tubenit over at Hoffman's joint and he apparently stole it from someone else. What I like best about the reverb circuit is that it only uses one tube, and it's a proven circuit. In at least one iteration of his circuit he put a Dwell pot at C5, which is a possibility. I think I could lay it out in such a way that I could play with the values easily and make this into a fun little classroom for myself.

    Most of my amps have reverb, and the ones that don't get a reverb pedal or a delay pedal or some other form of syrupy goodness stacked in front of them. I just like a little reverb on everything.

    I take way more from this site than I give, so I'm offering this up as a modest contribution. FWIW. I have no ownership of any of it, it's all stolen from someone else. But if you want to build another simple amp that isn't Fender, this is as good as any other, I guess.
     
    Last edited: Jan 5, 2019
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  2. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    deleted
     
    Last edited: Jan 5, 2019
  3. Dacious

    Dacious Poster Extraordinaire

    Mar 16, 2003
    Godzone
    Hoffman has a one tube reverb add-on this might be based on. It doesn't do Fender deep reverb, but it's apparently similar to Marshall tube reverbs - subtle and shimmery.
     
  4. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    Yep. Gibson had one, Pignose had one too. Interesting stuff.
     
  5. Snfoilhat

    Snfoilhat Tele-Holic

    Age:
    37
    770
    Apr 8, 2016
    Oakland, CA
    +1 the Spitfire! For all its simplicity it has great sounds and gain to spare, so take this comment w/ that in mind: the Princeton Reverb style of dry+reverb summing like you have above acts as a voltage divider. You lose some of the gain of both wet and dry channels. In a PR, that is totally OK, because the wet has more gain than anybody ever wanted, and the dry has an extra 12AX7 gain stage that comes after the mixing resistors and before the (cathodyne) phase inverter.

    The Spitfire, in contrast, has only two gain stages, the first and the roughly 10X provided by the LTP phase inverter. This may not be the best amp to employ a Princeton Reverb style wet/dry mixing network. But the LTP is specially designed to mix two channels without those losses. You just sent the second channel (in this case the wet) into the unused grid (when not used it is grounded out by a big cap). That's how Matchless adds the reverb in the Chieftain and maybe some other amps with which I'm less familiar.

    Anyhow, if you have any doubts about the 1-tube reverb having enough wet sound, or if you want to preserve the full gain range of the original Spitfire, then you can try this alternative out.

    None of this post should be taken to imply I know better than the EL34 World folks about circuit mods. Just relating a couple ideas that have worked well for me. Whatever direction your Spitfire goes in, good luck!
     
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  6. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    Interesting. Had not thought of any of that. So you are saying something more like this:

    spitfire-reverb-mod.gif

    LTP would then be grounded at the tail of R16. The mixing (R13) and attenuation (R12) resistors would be sorta unknowns now. Or would the attenuation R go to ground? And also how would you mix the two signals now?
     
    Last edited: Jan 4, 2019
  7. FenderLover

    FenderLover Friend of Leo's

    Jun 11, 2009
    Minnesota
    Check the wiper connection on your Volume pot, and you can remove R12 and R13. Having a Matchless myself, I would recommend that circuit. They used two triodes on the reverb return, but the way you have it would be plenty for me. I seldom turn reverb up past 3-4, and less gain on the return side keeps the noise down if you don't visit Dick Dale.
     
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  8. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    Yes, of course. I just slapped the volume pot on the page, good catch. So no need for a mixing resistor in this circuit?
     
  9. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    @FenderLover, revisions made. Thank you for looking at it.

    spitfire-reverb-mod.png
     
  10. Tom Kamphuys

    Tom Kamphuys Tele-Meister

    Age:
    39
    113
    Sep 18, 2018
    The Netherlands
    The phase of the wet signal will effectively be changed by using the other side of the ltpi w.r.t. to the mixing resistor version.

    The reverb unit might make that unimportant; break the coherence. (Two lasers will show an interference pattern, two light bulbs not)

    Can anyone say some wise words about this?
     
  11. FenderLover

    FenderLover Friend of Leo's

    Jun 11, 2009
    Minnesota
    There's nothing to mix, really. The PI is effectively the mixer, having two inputs. You are now using the unused input.

    That doesn't mean you might not want to add resistance later during the tuning process. Added resistance can form a voltage divider if needed to balance the wet/dry maximum levels to your taste. On the Reverb side, there is only one triode for the recovery, so you may likely never have too much reverb, and no added resistance needed. On the dry side, better to give the PI all the juice it has, and add a grid stopper resistor to the PI later if needed. Most people using a LTPI do not use a grid stopper, though they are a good idea.
     
  12. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    Alright, thanks for the guidance. Post #9 would be the finished schematic. I'm putting this away for a while to get onto a Blues Jr rebuild (no, I don't want to turn it into a Matchless. I'm excited to try @galaxiex's very cool BJ build).

    When I order components for a project its nice to have another project in the back of my mind that I can buy a few miscellaneous parts for. That's what this project will likely be; a slow burn.
     
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  13. sds1

    sds1 Tele-Holic

    Age:
    42
    809
    May 4, 2017
    Orlando, FL, USA
    I've built that one tube reverb circuit several times now. More than enough reverb using 12ax7! Remember this if the 12at7 doesn't get you there.

    I put the dwell on a trimmer to keep the control panel simple.
     
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  14. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    Yep, 12AX7 has more gain. But I picked 12AT7 for the higher plate dissipation and larger current draw, intending to slam the reverb tank harder. But I can always drop 12AX7 in.
     
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  15. symbiotic

    symbiotic TDPRI Member

    I'm sure this has come up before in one-tube reverb discussions: 12DW7 tube for a 12AU7-spec triode to drive the reverb and a 12AX7-type triode for recovery. Anyone here actually done this?
     
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  16. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    @symbiotic, don't think I follow. A 1-tube reveb uses one triode for drive and the other triode for recovery. Are you saying the 12dw7 has 2 differently-spec'd triodes? I don't know that tube.
     
  17. symbiotic

    symbiotic TDPRI Member

    That is exactly what I am saying.
     
  18. sds1

    sds1 Tele-Holic

    Age:
    42
    809
    May 4, 2017
    Orlando, FL, USA
    Hmm so is this a tradeoff, hitting the tank harder versus amplifying the reverb signal more in the recovery stage?
     
  19. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    Yes. Not looking for overpowering reverb but I think the gain from the 12at7 will be adequate. But if not, I can swap it. And mostly I'm experimenting.

    Don't want to give the impression that I know what I'm doing here, though.

    Interesting. I was just reading about that tube. Thats a new one on me.
     
    Last edited: Jan 5, 2019
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  20. sds1

    sds1 Tele-Holic

    Age:
    42
    809
    May 4, 2017
    Orlando, FL, USA
    Me either. But for a recent build I started with a 12AT7 in the reverb spot and it wasn't enough reverb. The 12AX7 seemed to fix that. Perhaps I rushed to conclusion.

    I need to go look at that again, see what is really affected by that tube change.
     
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