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  1. Replies
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    I can't think of a case where temporarily...

    I can't think of a case where temporarily paralleling a good capacitor with similar capacitance would hurt( one exception could be the reservoir cap after a tube rectifier where too much capacitance...
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    That in itself isn't completely consistent...

    That in itself isn't completely consistent (assuming resistor values are exact): 1.42V across 1.5k means a plate plate current of 0.95mA and a voltage drop across the plate resistor of 95V. So with a...
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    Did you measure the cathode resistors? Did you...

    Did you measure the cathode resistors? Did you try different tubes?
    Some chinese ECC83s have very high Gm, which could be a reason.
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    Some original Fender schematics show a note...

    Some original Fender schematics show a note stating voltages within +/-20%. So your cathodes voltages should roughly be within spec.
    Cathode voltage depends on tube variation (Gm), plate voltage and...
  5. Oh, this is new info. So you have separate screen...

    Oh, this is new info. So you have separate screen and HT supplies? Please try to post a drawing of both.
  6. Much less gain in triode mode. But we don't even...

    Much less gain in triode mode. But we don't even know if the amp has global NFB. Without NFB loop no chance of positive FB with OT phase reversed.
  7. Yes, but I don't think less capacitance will...

    Yes, but I don't think less capacitance will noticeably increase sag and relieve the screens. KT88s should be safe with a stiff screen supply voltage of 400V and individual screen resistors >1.5k.
    ...
  8. I think the 6A HT fuse is rated too high. A...

    I think the 6A HT fuse is rated too high. A Marshall Model 2001, 375W, 8x6550 bass amp has a 3A HT fuse. Typically the HT fuse should blow before the mains fuse. Otherwise the HT fuse doesn't provide...
  9. Yes, spice value is voltage amplitude (= peak...

    Yes, spice value is voltage amplitude (= peak value Vp) and Vp = Vpp/2.
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    Looks as if D13 is shorted (but could be other...

    Looks as if D13 is shorted (but could be other reasons as well). What is the voltage at TP42? If you lift one leg of D13 you can test it with an Ohmmeter.
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    Looks correct for the connectors up vertical...

    Looks correct for the connectors up vertical mount version. Input and output impedance around 2k.
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    D13 and D14 are 16V zeners. What are DC voltages...

    D13 and D14 are 16V zeners. What are DC voltages at TP45/TP46?
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    That's strange as auto-ranging meters typically...

    That's strange as auto-ranging meters typically display units.
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    How are you going to measure impedance? Do you...

    How are you going to measure impedance? Do you mean DCR?
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    Manufacturers often specify very low signal...

    Manufacturers often specify very low signal levels, like 4mV or 6mV input signal. Such low levels ensure that each following stage is operated within its linear range and stage gains can be exactly...
  16. For sine signals 1Vpp corresponds to 0.35Vrms and...

    For sine signals 1Vpp corresponds to 0.35Vrms and this is what an AC voltmeter will show.
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    ...and a low impedance source like a signal...

    ...and a low impedance source like a signal generator doesn't need much shielding.
  18. The 1k resistor just adds to the 470k, resulting...

    The 1k resistor just adds to the 470k, resulting in 471k +/- 10%.




    This HF filter may even have a beneficial effect, as it will attenuate residual oscillation blips. So best try both ways as...
  19. Yes, cuts treble above 16kHz.

    Yes, cuts treble above 16kHz.
  20. I strongly recommend to use shielded wires as...

    I strongly recommend to use shielded wires as they are passing the PT, which emits strong hum fields.
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    Grabbing the tube increases coupling capacitance...

    Grabbing the tube increases coupling capacitance between the plate and the PU (the human body is a conductor!) while grabbing the strings grounds your body and thus reduces coupling.
    BTW, grabbing...
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    I avoid using an electric drill on delicate...

    I avoid using an electric drill on delicate equipment. Each single slippage wears the edges of the bit and the screw head - increasing the risk for future slippage. Using a fitting manual diamond...
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    As I suspect capacitive/electric and not magnetic...

    As I suspect capacitive/electric and not magnetic coupling, any conductive material should provide adequate shielding.
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    Right, but nevertheless shields the ecaps from...

    Right, but nevertheless shields the ecaps from the tubes' heat radiation by acting as an infrared reflector.
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    I think the main source would be the power tube...

    I think the main source would be the power tube plates themselves as the rest of the circuit is typically shieded by the chassis.
    The tubes are outside the chassis and the plates have a large...
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    It doesn't even require sound or a speaker to...

    It doesn't even require sound or a speaker to excite this kind of positive feedback.
    I will be influenced by PU/guitar shielding and PU phase and of course distance between power tubes and...
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    To me this indicates that you don't have acoustic...

    To me this indicates that you don't have acoustic feedback (caused by microphonic PUs) but rather electrical feedback caused by capacitive coupling between the 6L6 plate (carrying high signal...
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    The 4µ cap in series with the tweeter constitutes...

    The 4µ cap in series with the tweeter constitutes a primitive crossover network with a crossover frequency of around 5kHz. The impedance of the cap adds to the tweeter impedance. At 1kHz the total...
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