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  1. Replies
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    Power tube grid drive/biasing is not clear. I...

    Power tube grid drive/biasing is not clear. I don't see coupling caps.
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    The settings of tone and volume pots change the...

    The settings of tone and volume pots change the grid circuit impedance.
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    This at least shows that there is an influence of...

    This at least shows that there is an influence of grid circuit impedance. Next I would connect a variable resistor of 500k or 1M directly between the 12AU7 grid and ground (short leads!) and...
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    That sounds like a promising idea. But first I...

    That sounds like a promising idea. But first I would try to verify that lowering grid impedance actually cures the issue (see my proposals above).
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    Apparently only if its input is terminated by a...

    Apparently only if its input is terminated by a low enough impedance (signal generator).
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    This seems to confirm my theory above. Now you...

    This seems to confirm my theory above.
    Now you may try to lower grid impedance by wiring a suitable resistor (or even a small capacitor of say 10pF) directly between 12AU7 grid and ground.

    Or...
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    The 12AU7 is part of the global NFB loop and thus...

    The 12AU7 is part of the global NFB loop and thus part of the power amp. Your are using heavy NFB and I think the power amp is close to the edge of instability.
    It is possible that the stability...
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    The left tube in this circuit is an ordinary...

    The left tube in this circuit is an ordinary cathode follower DC coupled to a common grid stage. The cathode follower has low output impedance of maybe 1k. As the cathode output is loaded by the...
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    As an LC ripple filter is more effective than an...

    As an LC ripple filter is more effective than an RC filter, a choke makes sense where high ripple rejection and low voltage drop are required. (Doubling inductance has the same effect as doubling...
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    I gave this uncommon circuit some more thought...

    I gave this uncommon circuit some more thought and now think that connecting the grid to the top of the tail resistor as shown actually increases cathode input impedance by what I would call...
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    Still it must be possible to get more than 70...

    Still it must be possible to get more than 70 clean "RMS" watts (more exactly: average or mean power) from 4 6L6s with the measured voltages.

    What are screen voltages at full power?

    Did you try...
  12. Has anybody had success with remagnetizing...

    Has anybody had success with remagnetizing ceramic magnets using neos?

    Ceramic magnet data specifies a charging field strength of 10,000G for saturation (4 to 5 times higher than for most...
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    If it's a comco amp, it could be acoustic...

    If it's a comco amp, it could be acoustic feedback between speaker and reverb springs. Do you also see oscillation with a dummy load instead of speaker?
  14. Higher input impedance increases sensitivity to...

    Higher input impedance increases sensitivity to electric interference and sometimes (depending on amp layout) necessitates shielding of the input wiring. So you need to decide if the tone improvement...
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    Please post schematic and a list of main...

    Please post schematic and a list of main circuit/construction changes between different versions.

    Do you have a scope?
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    The original datasheets are still available....

    The original datasheets are still available. Tubes carrying the same designation have to be compatible. A new 6L6-GC that doesn't meet the original spec. simply is no 6L6-GC.

    What I meant is, look...
  17. Not topic-related, but the first thing I would...

    Not topic-related, but the first thing I would change in this amp is raising its input impedance to at least 500k. A 47k/100k input kills PU resonance and acts as if the guitar's tone control was...
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    I would think so, as the internal cathode...

    I would think so, as the internal cathode impedance acts between cathode and signal ground, not between cathode and grid (grid current must be zero).

    Also your circuit will have very low gain, as...
  19. Sure, values are close enough.

    Sure, values are close enough.
  20. The contacts of a switching jack tend to...

    The contacts of a switching jack tend to oxidize/corrode especially when not regularly used.
  21. Does that mean that volume and amp work normal...

    Does that mean that volume and amp work normal after warm-up?

    If you turn guitar volume way down and turn amp volume fully up, is there still any change with warm-up?

    Anyway I would check the...
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    Bootstrapping to increase the input impedance of...

    Bootstrapping to increase the input impedance of a circuit works by reducing the signal current through the (external) input resistor. This is accomplished by connecting the far end of the resistor...
  23. With such limited bandwidth also the scope will...

    With such limited bandwidth also the scope will produce some frequency dependent phase shift between its channels.
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    Oh, I thought you mainly meant the large upright...

    Oh, I thought you mainly meant the large upright chassis mounting cans.
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    As the F&T website...

    As the F&T website (https://www.ftcap.de/en/products/electrolytic-capacitors/axial-lead/) doesn't show the tube amp versions, I assume that they are custom order types for big amp OEMs and amp parts...
  26. There is no standardized or established method to...

    There is no standardized or established method to measure PU sensitivity/output. So measurements are not comparable.
    Especially measurements using a multimeter are not reliable for a number of...
  27. If C13/16 are wired according to the schematic,...

    If C13/16 are wired according to the schematic, DC collector voltages must not change when connecting the diodes. Even shorting the diodes should not change DC voltages. Please check wiring.

    I...
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    Or it is actually interference from an external...

    Or it is actually interference from an external source like a cell phone, a switched mode power supply or a dimmer. Make sure that the metal sheet is securely grounded. Amp chassis should always be...
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