Thinking of building a Harvard: using 1/2 of a 12at7 instead of 6at6?

Discussion in 'Amp Tech Center' started by randomhitz, Sep 23, 2020.

  1. randomhitz

    randomhitz Tele-Meister

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    Seems like having all the preamp sockets the same and having a wider variety of tube choices might be desirable. Any down sides?
     
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  2. Nickfl

    Nickfl Friend of Leo's

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    A 5751 would be closer to the 6at6, otherwise no downside to using a 9pin tube.
     
  3. jhundt

    jhundt Doctor of Teleocity

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    if you use a 9-pin tube, you can add vibrato and make a 5f11 Vibrolux. Or, you could use the extra tube section as a first-stage to drive the preamp a little harder.

    I have a 5f11, it sounds very nice. But I generally use a Peppermill boost pedal to add the gain before the input stage, especially at lower volume settings. The original input stage just doesn't drive the amp hard enough to satisfy many players who are more accustomed to modern amps. It is especially noticeable if you play guitars with lower-output single-coils, like I do.

    I did this on a Vox AC-15 inspired amp. It has two input jacks. The normal input jack works just like the original design. The other input jack is switched; when nothing is plugged in, the added gain-stage is grounded and out of play. But when you plug into that jack, the added stage is in play.
     
    Last edited: Sep 23, 2020
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  4. jhundt

    jhundt Doctor of Teleocity

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    I am trying to remember what happens to a pre-amp tube that is heated but has no other connections. I think - if you choose NOT to use that half - you should ground the input or something. But I can't remember right now.
     
  5. Wyatt

    Wyatt Tele-Afflicted

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    Vox, Fender and Marshall just left them unwired.

    Another option would be to wire both halves parallel.
     
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  6. schmee

    schmee Doctor of Teleocity Silver Supporter

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    I built two Harvard's and a Vibrolux using a 9 pin. No problems. I think most do that for a new build now days.
     
  7. jhundt

    jhundt Doctor of Teleocity

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    just unwired, yes, I seem to remember that, also with an old Airline. But does the heater for the unused half also remain unwired?

    You're right, Matchless (and others) always wired their tubes in parallel. I tried that wih an earlier Vox-type build, and it gave nothing useful as far as I could tell. That's the amp that I mentioned above; when I got around to rebuilding it after many years of service, I decided to turn the unused 'parallel' half into a boost-channel. To be honest, outside of testing I have never used the amp since I did that change. It sounded good and worked as planned when I tested it, though.
     
  8. Wyatt

    Wyatt Tele-Afflicted

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    On the BF/SF Bassman, the heaters on V2 are wired the identical to the others, so yes, heater current is applied to the unused triode. I'm pretty sure it's the same with Marshalls and Vox. IIRC, just heating one triode would require 12.6V.

    Parallel doesn't add any gain, but the plates splitting the pate impedance usually drives more current for what some people feel is a "fuller" sound. Mileage may vary.
     
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  9. jhundt

    jhundt Doctor of Teleocity

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    yes, I heard that before. I really didn't notice one bit of difference. But then again, my ears are not so sensitive to minor differences.
     
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