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  1. True and motorboating typically sounds different....

    True and motorboating typically sounds different. Let's see if its still there with the GZ34. You might even use a 5U4GB for a little lower voltages if your PT supports the higher heater current.
  2. Never heard something like that with an amp....

    Never heard something like that with an amp. Sounds like a motorcycle. Is this with a GZ34? Are your filter caps rated at 500V or higher?
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    Seems you had a gassy tube that produces reverse...

    Seems you had a gassy tube that produces reverse (positive) grid current.
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    Who raised this question?. All I said is that...

    Who raised this question?.

    All I said is that it takes a DC current through the pot to make it noisy. It's a conditio sine qua non.

    Of course any current through a resistor (or contact) causes...
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    Correct me if I am wrong, but meanwhile we found...

    Correct me if I am wrong, but meanwhile we found that:

    - the power stage including OT is fine
    - speakers are fine
    - Normal channel is fine

    Does the Vibrato channel still produce ugly...
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    Why grid current? Generally a cathode resistor...

    Why grid current?

    Generally a cathode resistor counteracts thermal runaway. This is a benefit compared to fixed biasing.

    What I meant was that the hotter tube indirectly "steals" idle current...
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    I never had a shorted pot, but a short between...

    I never had a shorted pot, but a short between track and the (metal) shaft could be a safety issue.

    It suffices the reverse the order of caps and rheostats - no need for additional caps.
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    Pot noise is caused by DC current not voltage. As...

    Pot noise is caused by DC current not voltage. As there is no DC current through the pots, I see no need for changes.
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    If the increased grid voltage stays with the tube...

    If the increased grid voltage stays with the tube when you exchange power tubes, it should be the tube. If it stays with the socket, this could be the culprit.


    BTW, it just crossed my mind that...
  10. Thanks. We also abbreviate our federal states,...

    Thanks. We also abbreviate our federal states, like BY, BW, NRW, RLP etc. Of course I don't expect foreigners to know.
  11. Sorry, I goofed. Thought you were in Europe. Our...

    Sorry, I goofed. Thought you were in Europe. Our nominal voltage is 230V and import PTs or amps often are designed for 220V. So at 230V internal voltages are too high and I switch to the 240V tap if...
  12. True, but the 5F6-A has a lower value dropper...

    True, but the 5F6-A has a lower value dropper resistor (4.7k instead of 8.2k in the JTM45). Nothing wrong with 390V but on the high side for a JTM45. Does your PT have a 240V mains input tap? If...
  13. 390V PI supply voltage seems very high. What is...

    390V PI supply voltage seems very high. What is your heater voltage? (sorry if you already posted it somewhere). Some voltage overshoot before all heaters are hot is normal, but 500V is extreme. A...
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    I see. And if the speakers didn't produce ugly...

    I see. And if the speakers didn't produce ugly distortion during this test they are fine as well.
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    Why not use the dummy load? Not good for the...

    Why not use the dummy load? Not good for the speakers and your ears.

    Otherwise the signals look good. Looks like you could get around 10V clean volts at the output. 10Vrms clean sine at the output...
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    Just use the nice scope to find out. If all OT...

    Just use the nice scope to find out.

    If all OT wires are disconnected one can use the heater voltage from the same amp for testing.

    I don't really suspect the OT- based on the existing...
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    Please see my edited numbers. From the first 2...

    Please see my edited numbers. From the first 2 pictures, I calculate a power stage voltage gain of around 4.2. So 10Vrms at the output requires 2.4Vrms at the PI input.
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    The last picture with full iPod output shows...

    The last picture with full iPod output shows 0.7Vrms across the 2.8 Ohm load. This corresponds to an output power of 0.175W = 175mW.
    From your description of the problem it is unlikely that the...
  19. The CT should be yellow as it measures 175VAC to...

    The CT should be yellow as it measures 175VAC to either black or brown. Yellow is in the middle of the winding so CT .
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    I see, the relay takes it out of circuit in clean...

    I see, the relay takes it out of circuit in clean mode. Forget "common".
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    R21 would be the master pot. Whatever, check for...

    R21 would be the master pot. Whatever, check for DC across the scratchy pot(s). And yes, the usual suspects would be the plate coupling caps because the tube plates carry high DCV.
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    Just lift the coupling cap and measure grid...

    Just lift the coupling cap and measure grid voltage again.
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    Are you speaking of R7? Check for DC on the pot.

    Are you speaking of R7? Check for DC on the pot.
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    That indicates a leaky 0.01 coupling cap and...

    That indicates a leaky 0.01 coupling cap and would explain increased cathode current and voltage.
  25. Now what's the good reading? Do you need further...

    Now what's the good reading? Do you need further advice?
  26. I see, but what's your problem? I don't...

    I see, but what's your problem?



    I don't understand this. What is a bad reading?
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    We have different brands over here: 55522 ...

    We have different brands over here:

    55522

    Analog taut-band, mirror scale meters by Hartmann&Braun, DMMs by Philips PM2518X and Siemens B1023 both true RMS and 40kHz/20kHz bandwidth.
  28. In your other thread you mentioned a plate...

    In your other thread you mentioned a plate voltage of 450V. What happened?
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