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  1. Replies
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    I think model YJC with the ground wire and...

    I think model YJC with the ground wire and (probably) voltage reduction would be the best choice. Will reduce power, though.
    Ground wire shorts the existing cathode resistor.
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    Do you have any idea which of the many Yellow...

    Do you have any idea which of the many Yellow Jackets converters this refers to? I understand that some of them reduce plate and screen voltage while others don't.
  3. Is the amp in original condition (no mods)? ...

    Is the amp in original condition (no mods)?

    What is the common plate voltage at V1/V2 pins 1? (This is the point where both channels are mixed and could possibly interact.)

    What happens/changes...
  4. Such behaviour is common to many component...

    Such behaviour is common to many component (including transformer) and contact failures and doesn't prove anything.
    I am not saying that it is likely that a transformer is defective but if so, it...
  5. Check cathode voltages (pin 3) of V1 and V2. Do...

    Check cathode voltages (pin 3) of V1 and V2. Do these change with vol settings?
  6. First thing I'd do is check if PT and OT are ok,...

    First thing I'd do is check if PT and OT are ok, as these are the most expensive/valuable parts.
    Amp could easily be converted to a Dual Showman Reverb or Bassman 135.
    ...
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    To me it is not clear up to now what kind of...

    To me it is not clear up to now what kind of "feedback" you have. Could be acoustical (meaning feedback between speaker and PU or a microphonic tube) or purely electrical. How does it manifest/sound?...
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    Yes, it's descibed here:...

    Yes, it's descibed here: https://www.yellowjacketstc.com/detailed-information
    Many different versions.
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    Good observation. I also think that the black...

    Good observation. I also think that the black lead is used to short-circuit any existing cathode resistor and use the internal resistor for biasing instead. This would allow for better center biasing.
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    Yes it will raise plate voltage. But the tube...

    Yes it will raise plate voltage. But the tube only cares about/sees plate voltage minus cathode voltage, so...
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    ??? Why not VC? I mentioned this aspect as...

    ??? Why not VC?



    I mentioned this aspect as the Yellow Jacket leaflet says "converts from class AB to class A operation", which does not hold for the VC.



    This makes the tube run very...
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    Sorry, but what is VB and what is biasing down?

    Sorry, but what is VB and what is biasing down?
  13. Replies
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    This might be a misunderstanding. My general...

    This might be a misunderstanding. My general remark above was about converting class AB designs to class A, which must decrease output power as losses significantly increase.

    But the Vibro Champ...
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    I guess it will safely work in a BF Vibro Champ...

    I guess it will safely work in a BF Vibro Champ (already operating in class A) having supply voltages like this:
    ...
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    Maybe this helps: For the tube only the...

    Maybe this helps:

    For the tube only the voltage between grid and cathode matters, consisting of DC bias and AC signal.
    The grid leak resistance always should be smaller than the internal grid...
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    Sorry, not clear what you mean (double language...

    Sorry, not clear what you mean (double language barrier). Maybe you give an example.
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    Cable thickness does not matter.

    Cable thickness does not matter.
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    OK, from tube data the gain of your 12AX7 is...

    OK, from tube data the gain of your 12AX7 is G=72. From this the input capacitance calculates as 120pF. Real value will probably be closer to 150pF, because of wiring influences.
    Worst case treble...
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    Anyway the second coupling cap and grid leak...

    Anyway the second coupling cap and grid leak resistor don't improve anything. You can't compensate a low pass filter with a high pass filter having a much lower corner frequency. A bright cap of...
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    I can't think of a reason why the frequency...

    I can't think of a reason why the frequency response should improve.
    Connecting the grid to the pot wiper eliminates the second coupling cap and actually gives slightly better bass response.
    ...
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    What benefits do expect over connecting the grid...

    What benefits do expect over connecting the grid directly to the vol pot wiper?
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    If " operational input voltage condition swing "...

    If " operational input voltage condition swing " means an AC signal, average DC cathode voltage (determing the bias point) will not change. To be precise, this only holds for linear operation....
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    A tube needs a negative bias voltage. Bias is...

    A tube needs a negative bias voltage. Bias is grid voltage minus cathode voltage. So if grid voltage is zero and cathode voltage is +1V, bias is -1V.
    A large grid leak resistor develops a negative...
  24. I just quickly checked but I think you need to...

    I just quickly checked but I think you need to load the sine output with a 15k resistor replacing the square generator input resistance to allow for ample DC current.
  25. Good idea! A cascode is another way of wiring...

    Good idea!

    A cascode is another way of wiring 2 triodes is series. Achievable gain with the 6SN7 is around 100 (a little more than with the ECC82 in Merlin's example), using a plate resistor of...
  26. The principles yes, but not the results. Clipping...

    The principles yes, but not the results. Clipping characteristics and clean headroom strongly depend on input stage gain. And the different triodes vastly differ in gain (G).

    6SN7 : single triode...
  27. In the National 1212 amp the field coil...

    In the National 1212 amp the field coil definitely serves as a PS filter choke.

    In the Supro schematic I don't see a field coil, but if there is one it is most probably also wired as filter choke.
  28. But he is using a very low 6SN7, resulting in...

    But he is using a very low 6SN7, resulting in less than 1/4 the gain of a 12AX7. Gain values with Ra = 100k and bypassed cathode resistor 6SN7: G=14, 12AX7: G=60.
    With the two 6SN7 triodes in...
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