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  1. You might cover the inside of the back panel with...

    You might cover the inside of the back panel with aluminum foil to improve the shielding. Can be useful in a "noisy " environment.
  2. Looks like another "exotic" circuit :) I think...

    Looks like another "exotic" circuit :)

    I think it would work but might miss some unwanted interaction. It's probably easiest/safest to built and test or simulate it.

    Have you considered a more...
  3. It is not quite clear what you mean with noisy....

    It is not quite clear what you mean with noisy. Leaky or cracked caps can produce differend kinds of noise but they do not by themselves generate hum.

    But like any conductive surface connected to...
  4. PP capacitors have less temperature dependence of...

    PP capacitors have less temperature dependence of capacitance but often lower max. operating temperatures than polyester types.
    Electrical data of PP types is generally superior which doesn't mean...
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    Not quite. E.g. the phase shift/lag produced a...

    Not quite. E.g. the phase shift/lag produced a simple LP filter means a constant time delay for a given frequency. But signal frequency before and after the filter is the same.
    It requires a varying...
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    Lug 3 and lug 2 are not connected. There is a...

    Lug 3 and lug 2 are not connected. There is a 5.6M resistor between.
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    As already recommended, disassemble the Ultra Hi...

    As already recommended, disassemble the Ultra Hi switch and look for internal shorts between the 2 channel contact rows. If you can't repair/replace it, you can always disengage it and hardwire your...
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    Seems you wired the jack wrong. Lug #3 (hot...

    Seems you wired the jack wrong. Lug #3 (hot input) must not be connected to any other lug. I should be shorted to ground by the switch contact only. Inserting a (non-shortig) plug opens the switch...
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    A constant phase shift does not change frequency,...

    A constant phase shift does not change frequency, but varying phase does. Phase modulation means a modulation (expansion and compression) of the time scale which in turn modulates frequency. So phase...
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    Up to +/-10Hz acc. to Zollner. AC30 allows...

    Up to +/-10Hz acc. to Zollner. AC30 allows switching between pure AM (Trem) and mixed FM/AM (Vib).
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    Yes, that's the way it's supposed to be:...

    Yes, that's the way it's supposed to be: Everything grounded as long as no plug is inserted.
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    Have you tried to inquire at the BSI? Or maybe...

    Have you tried to inquire at the BSI? Or maybe this: https://webgate.ec.europa.eu/icsms/
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    I am not sure if they have to. IIRC, European...

    I am not sure if they have to. IIRC, European Legislation only requires that "The EU declaration of conformity must be made available to the surveillance authority upon request."...
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    Not generally. Some amp circuits achieve both,...

    Not generally. Some amp circuits achieve both, vibrato (pitch shift, FM) and tremolo (AM). E.g. Vibrasonic 5G13 (or Vox AC30).
  15. As pin 2 of the rectifier is B+, the 2 resistors...

    As pin 2 of the rectifier is B+, the 2 resistors constitute a typical voltage divider if screens are jumpered (looks as if they are). Screen voltage being R1/(R1+R2) times B+, with R1 to ground, R2...
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    . That makes the Ultra Hi switch suspicious....

    .

    That makes the Ultra Hi switch suspicious. The unwanted coupling between the channels I indicated in post #22 might actually be happening inside this defective switch.

    If the new input jack...
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    Yes, it's supposed to be shorted by the...

    Yes, it's supposed to be shorted by the sensitivity switch at max setting.
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    Did you check and clean the "bright" switches?...

    Did you check and clean the "bright" switches? Not sure if the bad input jack explains everything. It would require 3 fault conditions:

    1) CH1 input signal is not connecting to V1.
    2) CH1...
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    I am somewhat surprised. If someone "specializes...

    I am somewhat surprised. If someone "specializes in rewinding transformers" I'd expect him to count turns numbers of the original. And if he used the same turns numbers, voltages must be the same....
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    I was wondering what could cause such...

    I was wondering what could cause such asymmetrical channel vol interaction.

    I think it could work like this:

    Suppose something causes a short between the input of the channel 1 vol pot (where...
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    There seems to be a misunderstanding. My...

    There seems to be a misunderstanding.

    My intention was to find out what changes regarding DC voltages and amp behaviour with V1 in V2 socket and vice versa.
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    6.8K from post #11. Cathode and plate voltage are...

    6.8K from post #11. Cathode and plate voltage are consistent, so I don't suspect cathode resistance.
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    So finally we have all relevant V1/V2 voltages....

    So finally we have all relevant V1/V2 voltages. They look good except for the first triode of V1, which has 30% increased plate current. Is R2 (5.6M) ok and correctly connected?



    What does...
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    The method I am proposing requires to know the...

    The method I am proposing requires to know the orientations of the coilformers/center legs. The centerline through the coilformer defines the (main) magnetic axis. I suggest to orient the...
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    Positioning the laminations of PT and OT at right...

    Positioning the laminations of PT and OT at right angles is a good rule. But there are (at least in theory ) different ways to achieve this. The best orientatation is with laminations at right angles...
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    Feedback requires gain. The more gain the more...

    Feedback requires gain. The more gain the more likely is (unwanted) oscillation. The "2203/2204" channel can be assumed to have way more gain than the other channel, so no surprise here. Actually I...
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    Seems the "L-pad" (actually a simple power pot)...

    Seems the "L-pad" (actually a simple power pot) is not properly grounded.
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    I would start with checking all ground...

    I would start with checking all ground connections with an Ohmmeter. Should measure below 0.5 Ohm with a calibrated meter.
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