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Dual triode - can both sides be run in parallel ?

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  • HaroldBrooks
    replied
    Originally posted by Helmholtz View Post
    That would be the only way to achieve a total gain like a pentode (or more as tube gains multiply). Or use a pentode .
    I might switch the amp back to a 6J7 preamp tube (pentode). But first I have to see if that added circuit is of any use, and if it helps the amp then I will leave well enough alone.

    I'm a bit obsessive sometimes about maximizing the gain of my amps, as if I was tuning a race motor (used to do that in my youth), but I realize it can be a dead end.

    Better perhas to figure out how to modify for the best tone, and perhaps added flexibility built into the circuit with switches and nobs to control values on the fly and morph the amp as needed.

    Just some thoughts.

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  • Helmholtz
    replied
    So do you think I am better off running one triode into the other ?
    That would be the only way to achieve a total gain like a pentode (or more as tube gains multiply). Or use a pentode .

    Leave a comment:


  • HaroldBrooks
    replied
    Originally posted by mozz View Post
    6j7 is a pentode, so the circuit had to be rewired for a dual triode. Could it have been factory done?
    Perhaps... But there was a mysterious circuit addition to the amp using one of those six flat prong plugs. The circuit has an old Long Plate RCA 12AX7 and a few caps and resistors in it, and it looks as old as the amp. It may be a Tremolo, or a type of boost, or perhaps a negative feedback circuit, but those are just guesses. I will see if I can figure it out, and if I can't I will call upon you guys for help !

    The Valco factory did make a change I believe to later models of the same amp, as I saw one with a 6SQ7 and no hole in the chassis for the grid cap wire, so I believe that one to be a factory change. My "new" amp has the hole for the 6J7 grid wire, and the base of the 6SN7 is too big for the rivets of the sockets, so unless that is a newer tube, I have a feeling it was a 6J7 from the start, and then modified (quite nicely I might add) to use the two sides of the 6SN7, along with the "Mystery" add on circuit.

    My temptation is to not change it any further until I can figure out the mystery circuit, because it might end up being something cool. I have very little use for Tremolo these days, but I am fascinated by filtered Negative feedback in the form of a "Presence" or "Resonance" circuit. I also would like a tube boost built right into the amp that could be called upon as needed.

    When I get a bit better at this, I may be able to build one, or use what I have with this amp to accomplish either of those goals.

    Leave a comment:


  • HaroldBrooks
    replied
    Originally posted by Dave H View Post
    Yes, the two halves could be wired in parallel but the gain increase won't be dramatic (only a few dB)
    So do you think I am better off running one triode into the other ? I have this arrangement on another amp of mine :

    http://www.oldfrets.com/Valco/Schematics/valco_1212.pdf

    Leave a comment:


  • mozz
    replied
    6j7 is a pentode, so the circuit had to be rewired for a dual triode. Could it have been factory done?

    Leave a comment:


  • Helmholtz
    replied
    Paralleling 2 triodes does not increase the (resulting) amplification factor µ, but decreases the plate/output impedance of the circuit. This allows actual circuit gain to be closer to the theoretical µ value, using the same plate resistor as with the single tube. The latter condition may not provide good center-biasing, though.

    In any case, the upper limit for circuit gain is always the µ of the single triode.

    Parallel tubes make sense in input stages to reduce noise - and in circuits where a single tube could not deliver enough power, e.g. reverb driver, output stage.
    Last edited by Helmholtz; 11-28-2019, 01:49 PM.

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  • Dave H
    replied
    Yes, the two halves could be wired in parallel but the gain increase won't be dramatic (only a few dB)

    Leave a comment:


  • Dual triode - can both sides be run in parallel ?

    I think I've seen someone do this before, my question is, can you just parallel connect two sides of a dual triode tube, say a 6SN7 or a 12AX7 and increase gain at that point ?

    The reason I ask is I just received a pretty cool National amp in an equipment trade, and someone years ago it was fitted with a 6SN7 in place of the original 6J7 for the V1 preamp tube.

    I also believe they rewired the socket to split the two halves of the tube to operate from two separate input jacks (three inputs in total), with two separate volume controls. Wondering if I could make the both halves work together in parallel easily ? I would rather have the boost from both halves in this case, as the amplification factor on the 6SN7 is 20, and I've already jumped the channels (there are three inputs with 2 on one volume control and the other on another pot) so I like the way the more gain from the preamp sounds up front.

    Thanks for your help !

    http://www.oldfrets.com/Valco/Schematics/valco_1212.pdf
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