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  1. Replies
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    There is yet another OT core related effect. An...

    There is yet another OT core related effect. An OT's lower corner frequency depends on its primary inductance, which strongly varies with applied signal voltage. Inductance is low at low signal...
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    I may be wrong but I suspect that the amp is at...

    I may be wrong but I suspect that the amp is at the verge of instability and that the observed effects are caused by HF oscillation.
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    A lower value bias filter cap will accelerate the...

    A lower value bias filter cap will accelerate the buildup of bias VOLTAGE. But it might even somewhat slow down the ramp-up of tube idle CURRENT.
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    I also considered that possibilty but a reverse...

    I also considered that possibilty but a reverse wired bias cap would develop high leakage current. That would cause less negative bias voltage and high idle currents.
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    Same with other amp (I already asked)?

    Same with other amp (I already asked)?
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    Please answer my questions. And yes, new ecaps...

    Please answer my questions.

    And yes, new ecaps can have excessive leakage current at higher voltages which might not show with low voltage testing. After some operating time leakage reduces and...
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    Makes sense as pentodes are high impedance...

    Makes sense as pentodes are high impedance current sources. Nevertheless bass response will suffer and power tube distortion will increase by heavy current loading.
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    ..but don't seal it as it's a vent that provides...

    ..but don't seal it as it's a vent that provides voice coil cooling by air flow.
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    What pic?

    What pic?
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    I have much respect for your skills and artwork,...

    I have much respect for your skills and artwork, but need to mention that solid aluminum used as cover for magnetic PUs will considerably dull sound by eddy currents.
    For PU covers thin nickelsilver...
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    Look at the schematic. Ok, here is your...

    Look at the schematic.



    Ok, here is your real problem. Full idle current can only develop when supply voltages are fully up. I guess the voltages in your other amp ramp-up faster? Are standby...
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    As the hum signal harmonics are multiples of...

    As the hum signal harmonics are multiples of 50Hz, your problem doesn't seem to be related to power supply ripple.


    If your chassis is actually coated, make sure that all chassis connections make...
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    Check heater balancing resistors (47R) and...

    Check heater balancing resistors (47R) and connections. Make sure that heater voltage is symmetrical wrt ground.

    It would really be helpful to know if your hum is 50Hz OR 100Hz.
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    So please make sure that the pot cases connect to...

    So please make sure that the pot cases connect to the chassis.
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    In principle yes - if the OT is underdesigned. ...

    In principle yes - if the OT is underdesigned.

    Saturation doesn't require power tube clipping. It is caused by too high primary voltage amplitude at low frequencies (excessive voltage-time area)....
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    Yes, but the pot cases act as shields and should...

    Yes, but the pot cases act as shields and should connect to ground. Does hum increase when you touch them?

    Please post the complete schematic showing the heater supply.
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    The 100R resistor is a grid stopper and should be...

    The 100R resistor is a grid stopper and should be soldered directly to the tube socket.

    Is the inside surface of your chassis bare metal or coated?

    Do the pot cases connect to ground?
    ...
  18. Replies
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    THe question is if they (especially screen...

    THe question is if they (especially screen voltage) ramp up slowly as well.

    Also monitor how the bias voltage at point B changes in the first 30 seconds.
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    Results?

    Results?
  20. The cut-off frequency with a 1nF coupling cap and...

    The cut-off frequency with a 1nF coupling cap and 1G input resistance is 0.16Hz. Do you need lower?
    The cathode network of 3.6k and 22F means a corner frequency of 2Hz, same question.
  21. Replies
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    I recommend to increase R67 because this actually...

    I recommend to increase R67 because this actually improves bias supply filtering while lowering R58 has the opposite effect.
  22. Replies
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    492

    Both effects would reduce or even kill reverb...

    Both effects would reduce or even kill reverb drive but neither of them could noticeably change tube current as the tube acts as a constant current source with an equivalent internal plate source DCR...
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    If you want hotter bias (higher plate current)...

    If you want hotter bias (higher plate current) you need less (negative) bias volts. That's easy to achieve by adjusting resistors. Just increase R67 value.

    BTW, the voltage drop across R56/R57 is...
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    Just did a graphical construction of the...

    Just did a graphical construction of the operating point with Rk = 470R. My results perfectly match your measurements (4.7V/10mA) There is some extrapolation involved, so the error might be up to +/-...
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    As said above, primary DCR only affects the...

    As said above, primary DCR only affects the losses.

    Inductance depends on turns number squared, coil loop area and most important the core, which increases the air coil inductance by a factor of...
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    There seems to be a misconception. A DCR...

    There seems to be a misconception. A DCR difference at the plate of a few k won't change tube current. As the internal plate resistance of the tube is much higher than the plate load DCR, it's the...
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    Different R1 values, same results - not possible.

    Different R1 values, same results - not possible.
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    By a very rough estimation a 100R higher DCR per...

    By a very rough estimation a 100R higher DCR per primary half might reduce available output power by 0.5W in a 20W amp.
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