B+ wire/resistor layout.

Discussion in 'Shock Brother's DIY Amps' started by Mongo Park, Oct 16, 2021.

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  1. Mongo Park

    Mongo Park Tele-Afflicted

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    Maybe a dumb question but. On the Schematics a plate resistor is on the plate in the tube, hence the name. Lot of 100K's on 12ax7's. When doing this up is it possible on a 12ax7 to have a 100K resistor and 2 wires coming off the end of the resistor, one going to pin 1 and one going to pin 6. As in one 100K resistor for both plates in a 12ax7.
    This is leading me to think the same thing on the cathodes, one 1500/25uf on pin 3 and 8.
    Thanks for indulging me on this.
     
  2. Fluddman

    Fluddman Tele-Holic

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    I don't think it would work as the voltage drop across the plate resistor provides the signal for the next stage. They would need to be seperate because the voltage drop increases as the signal progresses through each preamp stage.

    I'm not sure about the cathode resistors but I suspect that may be more likely to work as its purpose is to provide a bias voltage.

    Cheers
     
  3. Mongo Park

    Mongo Park Tele-Afflicted

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    I had a hunch this would not work, thanks for explaining why.
     
  4. LightningPhil

    LightningPhil Tele-Meister

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    Doing this ties the anodes of the triodes together, which as mentioned, is usually used as their output. It can be done if you want to use both triodes in parallel to double the output current, but isn't the best option. The valves can oscillate at RF frequencies if not damped, and if an incuctive link is used between the anodes, it'll probably go a bit wild.

    So even if you do want to run a valve with both triodes in parallel, best use seperate anode resistors, then combine the outputs through resistors (can be loweish value, think a couple of k) into the load.

    There is ocasion to run triodes in parallel. If done before a tone stack, you've for twice the current to play with. There's potential for less noise by 3dB too. And if the signal is in parallel but the gain is slightly different (by using slightly different anode resistors) then the gain from mixing the 2 might be interesting.
     
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  5. NTC

    NTC Tele-Holic

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    For the cathode resistor, there are a lot of amps, certainly Fender amps, that have shared cathode resistors. For a 12AX7, the shared resistor is about 1/2 the value used for a single triode - 820 ohms instead of 1.5kohms. A 5E8A low power Twin has 4 triodes sharing a single 470 ohm resistor - and separate plate load resistors for each. Each pair is in parallel for certain input combinations.
     
  6. James Knox

    James Knox Tele-Afflicted

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    Great question and great answers! Always so much to learn - I love it.
     
  7. Mongo Park

    Mongo Park Tele-Afflicted

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    Thanks for sorting this out for me. I can use the cathode shared resistor for the next 5f2a build.
    paralleling thighs is so more th think about
     
  8. jsnwhite619

    jsnwhite619 Friend of Leo's

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    On my College Dropout with paralleled V1, I jumper across the tube twice - 3&8 tied together for the single cathode resistor, 1&6 tied together going to a single plate resistor. So many tweed amps did the same cathode trick - that's why they used 820R instead of 1500 because the parallel operation doubles to an effective 1640R cathode resistor. So, on the same idea, the 100k plate should become a 200k plate and boost the gain. I actually lowered it to a 68k on the board I think to settle things down a little.
     
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