Cathode Resistor Switch for running either 6V6 or 6L6

Discussion in 'Shock Brother's DIY Amps' started by James Knox, Oct 17, 2021.

  1. James Knox

    James Knox Tele-Afflicted

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    Swapping out a 6L6 for a 6V6 on an Octal Socket “Cathode optimized” for a 6V6 (470R or 500R) is going to get you a cold biased 6L6, right?

    So I stumbled upon this Cathode Switching idea in a build layout. I’m not really understanding the 560R and 750R Cathode resistors? Also, that the 25uf Cap is only over the 750R Cathode resistor?

    Could somebody explain this to me please?

    Here is the pic...

    AC98060F-262E-4C73-9C73-0FC20CB0FCB6.jpeg
     
  2. andrewRneumann

    andrewRneumann Tele-Afflicted

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    It appears the 560 is being switched in parallel with the 750. So effectively 320Ω for one position and 750Ω for the other. The bypass cap remains in the circuit at all times.
     
  3. James Knox

    James Knox Tele-Afflicted

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    Ok, I see that now. Still seems like weird values for either tube?
     
  4. andrewRneumann

    andrewRneumann Tele-Afflicted

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    I guess it kinda depends what the other voltages are. If they cranked up the voltage for the 6L6, then a higher value cathode resistor might be necessary for the 6V6. You got those other voltages?
     
  5. James Knox

    James Knox Tele-Afflicted

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    Looks like B+ is 385 VDC
     
  6. Telenut62

    Telenut62 Poster Extraordinaire

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    A bit high for 6V6.
     
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  7. andrewRneumann

    andrewRneumann Tele-Afflicted

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    Looks like the pic shows 367Vdc at the anode and 23Vdc at the cathode for a Vak of 344Vdc. (I guess the screen-to-cathode is sitting around 325Vdc.) Maybe they were targeting 12W for the 6V6 instead of 14W and went with the 750Ω?
     
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  8. Mongo Park

    Mongo Park Tele-Afflicted

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    Knowing that the resistors are place in parallel you should be able to pick whatever combination of values to suit your needs/situation and tweak it to your needs. Lots of on line parallel calculators.
    This could be applied to any tube you could tune your preamp tube for a middle optimal bias and have the switch take it to a hot bias or cold bias. Rob’ site has this sort of thing as well, I think he has one with three settings.
     
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  9. James Knox

    James Knox Tele-Afflicted

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    Good idea - I’ll go read some RobRob.
     
  10. radiocaster

    radiocaster Poster Extraordinaire

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    If the voltage is a little higher, it won't matter that much if the plate dissipation is not exceeded. I'd probably aim for 85%-90% at most, maybe less.

    If you really want a lower voltage you can also use a less powerful transformer and/or add a resistor or coil in the power circuit.
     
    Last edited: Oct 18, 2021
  11. Mongo Park

    Mongo Park Tele-Afflicted

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