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  1. Replies
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    When air gets inside the tube, it cools the...

    When air gets inside the tube, it cools the filament so it no longer glows red hot. But as the filament can't reach its normal operating temperature, its resistance is lower and thus heater current...
  2. 150k resistors are safe with both 6550s and...

    150k resistors are safe with both 6550s and EL34s. 220k is too high for 6550s.
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    Without knowing the scope settings (vertical...

    Without knowing the scope settings (vertical sensitivity in V/div and time base in s/div) it is just an unidentified signal.
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    In a powered amp?? Might destroy your meter by DC...

    In a powered amp?? Might destroy your meter by DC voltage. Anyway it doesn't provide grounding. An Ohmmeter essentially injects a DC current.
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    Schematic?

    Schematic?
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    What does this mean? How do you ground with a...

    What does this mean? How do you ground with a meter?
  7. No need for a "non-inductive" resistor. The...

    No need for a "non-inductive" resistor. The inductance of any wire-wound resistor is absolutely negligible compared to the inductance of a speaker (as well as the secondary leakage inductance of the...
  8. 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...
  9. 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
  10. 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...
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    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...
  13. 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...
  15. 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.
  16. 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.
  17. 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.
  18. 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?
  19. 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.
  20. 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...
  28. 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.
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