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  1. Replies
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    Some if I missed it, but have you eliminated a...

    Some if I missed it, but have you eliminated a bad tube/s, eg with poor heater-cathode insulation, playing some part in this?
  2. It was a long time ago, sorry, canít recall the...

    It was a long time ago, sorry, canít recall the detail, but it got worse and provoked me to track down the culprit. I was surprised it was a tube fault.
    It may have been one of GTs ĎMullardí...
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    Just to note that increasing the g2 voltage will...

    Just to note that increasing the g2 voltage will increase plate current under all conditions, idle all the way up to fully overdriven. So more current drawn from the PT and through the OT.
    Of...
  4. I seem to remember having a 12AX7 that made that...

    I seem to remember having a 12AX7 that made that quiet motorboat noise, try subbing out each in turn with a good one.
  5. Yes, it does sound excellent, well done...

    Yes, it does sound excellent, well done :thumbsup:
    I'd be interested to see as much info as you can get around to measuring on the power supply / power amp, eg OT ratios, PT HT Vac and Iac at open...
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    Yes, a bad mismatch could result in the hot tube...

    Yes, a bad mismatch could result in the hot tube over dissipating, which increases grid current, which increases plate current and so on.
    With shared cathode bias (which Merlin advises can add even...
  7. Assuming itís an oscillation issue (scoped it?),...

    Assuming itís an oscillation issue (scoped it?), Iím not keen with the way the regular Marshall layout brings the OT primary wires through the chassis into such close proximity with the tone...
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    Yes, but 330k is a far higher value than...

    Yes, but 330k is a far higher value than necessary.
  9. Replies
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    Try this thread...

    Try this thread https://ampgarage.com/forum/viewtopic.php?f=6&t=20341

    There's something on robrob's site too, his wiring diagrams are usually nice.
  10. Replies
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    Check that thereís no Vdc on the power tube...

    Check that thereís no Vdc on the power tube control grids (terminal 5).
    Grid stoppers there would be good practice. Probably omitted from the original design to reduce cost.
  11. Replies
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    550

    Thereís no time like now / itís never too late,...

    Thereís no time like now / itís never too late, to remedy that.
    My view is that for jobs like this they are as essential as a functioning multimeter.
  12. Bear in mind that the HT current draw when...

    Bear in mind that the HT current draw when overdriven may be significantly higher than at the onset of sine wave clipping.

    Hereís a nice tube type info database, donít even need to hit Ďreturní...
  13. Yes, my understanding is that it's good practice...

    Yes, my understanding is that it's good practice to twist wiring carrying a balanced signal, it's something I've always done, even if it's only a turn or 2 along a short run. Obviously the longer the...
  14. Cheaply made amps tend to have OTs that are...

    Cheaply made amps tend to have OTs that are barely adequate for the application, and may get pushed close to saturation when overdriven. Increasing the cathode bypass cap value (along with the...
  15. Replies
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    Your drawing isn't consistent with the use of...

    Your drawing isn't consistent with the use of dots to indicate circuit connections, so it's unclear whether crossing lines are connected or not.
  16. Replies
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    Silicon back up diodes in series with the plates...

    Silicon back up diodes in series with the plates of a tube rectifier are fine, no switching needed, the silicon diodes automatically 'take over' if the tube has any kind of short type of fault. They...
  17. Replies
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    Leech PA120 Service

    A friend wants to put his old 70s Leech PA120 4 channel PA amp made in the late 70s back into service.
    He tried and popped its 5A mains fuse, then tried a 2A fast blow in it, which popped straight...
  18. Regarding the 'cut' control, power amp negative...

    Regarding the 'cut' control, power amp negative feedback will tend to diminish its effect, and it may work at odds to the existing presence control.

    '9) Pentode / Triode Half Power Switch - short...
  19. Thread: Fender 68 prri

    by pdf64
    Replies
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    Yes, the resistance of the volt meter will form a...

    Yes, the resistance of the volt meter will form a potential divider with the 1M grid leak resistor and draw current through it, pulling the grid voltage down whilst itís being measure thus. Evidence...
  20. Replies
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    Just to note that the Fender spec is 470 ohm 1...

    Just to note that the Fender spec is 470 ohm 1 watt. I've never known resistors of this spec to fail in this application other than as collateral damage from a tube short. My perception is that their...
  21. Replies
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    Note that your HT voltage at full load will...

    Note that your HT voltage at full load will increase with a real GZ34, and with that, the amp's power output increases, significantly so when overdriven.
    I found that out the hard way with a Fender...
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    As you've got a 1k shared resistor feeding the 2...

    As you've got a 1k shared resistor feeding the 2 individual screen grid resistors, it's fine for them to be 470.
    470 ohm may be too low if there wasn't that shared 1k resistor feeding them.
    With...
  23. Replies
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    Ok, good, I noticed it was missing from one of...

    Ok, good, I noticed it was missing from one of layouts you posted, so assumed that was how your amp was.
    One day you may feel our frustration at the absence of an accurate schematic!
    Just a...
  24. Replies
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    Yes, Iíve read other accounts of very high value...

    Yes, Iíve read other accounts of very high value shared resistors being used, eg 8k2; they must have crippled the ampís power output!
    I wonder if they had trainees assembling them and poor QC, hence...
  25. Replies
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    Yes, the standard 1k shared feeding 470 ohm...

    Yes, the standard 1k shared feeding 470 ohm individual. I tried the 1k shared feeding 100 ohm individual, and couldnít see/hear/measure any benefit.
  26. Replies
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    Donít go on my say so please, try it with and...

    Donít go on my say so please, try it with and without, to find out what works best for you.
    That cap makes the bright channel brighter, but at the expense of making the normal channel muffled.
    I...
  27. Replies
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    Yes, plus I forgot to mention that I changed the...

    Yes, plus I forgot to mention that I changed the coupling caps between the LTP and power tube grids to 47nF (were 0.1uF).
    Also, move the NFB wire from the 8 to the 16 ohm terminal of the impedance...
  28. Replies
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    Is the amp fitted with a grounded power cord,...

    Is the amp fitted with a grounded power cord, that has its earth wire properly bonded to the amp chassis?
    The board ecaps are all original; maybe they all tested good when serviced but I'm rather...
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