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  1. Only if something shorts all metal sleeves to...

    Only if something shorts all metal sleeves to ground. If so, pin 1 should be isolated but only with 6550s!
    Just use your Ohmmeter to check. I consider it improbable, given your cathode voltage...
  2. Sorry for confusion. Please see my edited text....

    Sorry for confusion. Please see my edited text. Pin 1 and pin 8 are not internally connected
  3. Pin 1 is only connected to the metal base sleeve....

    Pin 1 is only connected to the metal base sleeve. If pin 1 and pin 8 (cathode) are connected at the socket, the metal sleeve gets connected to the cathode. So if for some reason the metal sleeve...
  4. If the mechanical adjustment doesn't suffice, I...

    If the mechanical adjustment doesn't suffice, I would check for any DC voltage across the meter terminals.
    Do we have a schematic?
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    I tried to answer this question above. As said,...

    I tried to answer this question above. As said, the necessary slanting depends on string gauge and action. Low action and thin strings don't need much slanting. Traditionally guitar setups and bridge...
  6. One resistor (base load) permanently connected to...

    One resistor (base load) permanently connected to one of the taps is sufficient. Minimum power rating depends on resistance, amp power and tap/impedance. E.g. for a 100W amp a resistor of 270 Ohm...
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    The bridge angle for best intonation depends on...

    The bridge angle for best intonation depends on string gauge and action.
    The reason for the different vibrating lengths is the following:

    Depressing a string to fret an note raises string...
  8. The resistor values for the EL34 bias circuit...

    The resistor values for the EL34 bias circuit (except the 220k grid resistors) seem to be more appropriate for your extra "cold" 6550s as well.
  9. If there's any doubt about the bias probes, I...

    If there's any doubt about the bias probes, I recommend to try alternative methods as mentioned. Always useful to have 1 Ohm cathode resistors installed.
  10. So if your bias probes and meter (battery) are...

    So if your bias probes and meter (battery) are fine, your 6550s are out of spec.
  11. Real 6550s should run VERY hot at -40V bias....

    Real 6550s should run VERY hot at -40V bias. Marshall spec was -44V with 395V plate voltage.
    What are your screen voltages (pins 4)? Should be around 390V.

    How did the EL34s measure at -38V?
  12. 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.
  13. 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...
  21. 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.
  22. 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...
  23. 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...
  24. 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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