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  1. Have a read of...

    Have a read of http://www.valvewizard.co.uk/bridge.html
    The way I see it, on the negative going swing of its HT winding leg, the diode's anode will get pulled down to -Vp whilst its cathode is up at...
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    Is the standby hot switching the reservoir cap on...

    Is the standby hot switching the reservoir cap on to the rectifier?

    If you've traced the schematic, I'd be grateful for a copy (with one repair after another I can see you're a busy guy so...
  3. With HT windings much over 600Vac (unless used as...

    With HT windings much over 600Vac (unless used as series pairs), 1N4007 are underrated for use as backup diodes for the rectifier tube. It would be a dreadful shame for failure of the rectifier tube...
  4. The voltage across the cathode bias resistor will...

    The voltage across the cathode bias resistor will increase at high signal levels, perhaps double at full overdrive. But that would still be under 6 watts, so your resistor should stay well within...
  5. Yes, it's not critical, just need a reasonable...

    Yes, it's not critical, just need a reasonable degree of voltage and current 'headroom', so 1250V 1.5A should be fine.

    With a directly heated cathode like a 5U4, the cathodes and heater are all...
  6. The schematic is correct but if they are called...

    The schematic is correct but if they are called into use by tube failure, 1kV diodes are underrated for HT windings much over 600V.
    Need to factor in highest mains voltage (eg x 1.08) and unloaded...
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    Try JPF Amps https://jpfamps.com/ at Regent...

    Try JPF Amps https://jpfamps.com/ at Regent Sounds in Denmark St. They’re members here, probably been too busy to visit for a while.
    Fixed fee of £22 plus parts for pedals, though not sure how...
  8. As the MV is being used set high, thereby making...

    As the MV is being used set high, thereby making power tube gk diodes (rather than the 3rd gain stage) the most significant clipping point, it may be feasible to move the ‘warmth’ circuit back so...
  9. I wonder if different eras of Fenders had a...

    I wonder if different eras of Fenders had a different rake angle on their control panel? As some seem to have more room for such pots etc than others.
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    Given the direction this thread has taken, to...

    Given the direction this thread has taken, to avoid a whole lot of possibly incorrect assumptions being made about the amp in question, a full power amp schematic is essential :confused:
  11. If his settings never change, maybe just hardwire...

    If his settings never change, maybe just hardwire it to the pulled ‘boost’ mode?
    Or if that’s not acceptable, switch the boost mode with a rotary switch, eg in place of the trem speed control...
  12. I’ve used one from Weber previously...

    I’ve used one from Weber previously https://www.tedweber.com/wp105a-ppsw
    Had to file the switch body a bit to get it to fit behind the rake of the control panel, and the construction isn’t as strong...
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    Have you missed decoupling off the 'other' grid?

    Have you missed decoupling off the 'other' grid?
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    I dislike how the chassis is being used a key 0V...

    I dislike how the chassis is being used a key 0V conductor, bringing numerous opportunities for compromised, high resistance 0V to chassis connections in critical circuits. Maybe one of those is bad...
  15. I'm not convinced that the issue really is...

    I'm not convinced that the issue really is parasitic oscillation.
    I think that either may help, but, with the settings used, as the reverb drive level is almost certainly way too high (and...
  16. And also the sig gen settings, and the circuit...

    And also the sig gen settings, and the circuit node that the scope probe was connected to :)
    Looks like you were feeding it a squarewave. or otherwise overdriving V3?

    Plus, is the schematic in...
  17. Thread: Glass capacitor

    by pdf64
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    969

    Me too - LCR Components in Wales UK are a small...

    Me too - LCR Components in Wales UK are a small UK manufacturer, I've been buying their 630V low pF polystyrene caps for decades, excellent spec, they're distributed by Farnell so I guess that Newark...
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    I think that the Wa,g2 limiting value is intended...

    I think that the Wa,g2 limiting value is intended for UL applications; for this Hiwatt it's the Wa 'plate only' limit of 25W that's relevant.
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    When reading Aiken, it can be helpful to bear in...

    When reading Aiken, it can be helpful to bear in mind that he's a Marshall guy, and when he talks about power tubes / amps, he's usually coming at it from an EL34 or 84 perspective.

    Something to...
  20. The 2 resistors form a potential divider, in the...

    The 2 resistors form a potential divider, in the same way that a regular volume control is a variable potential divider https://en.wikipedia.org/wiki/Voltage_divider.
    I wouldn't remove the resistors...
  21. 'Fraid not, there's negligible signal attenuation...

    'Fraid not, there's negligible signal attenuation across R117, so reducing its value will not result in any perceivable increase in sensitivity.
    However, there's some heavy attenuation at the power...
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    If using a 100:1 scope probe, it's possible to...

    If using a 100:1 scope probe, it's possible to see the massive kV spikes on power tube plates of an amp overdriven into an inductive load; my concern is arcing across the switch body between the...
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    At idle maybe; it may cause problems if the...

    At idle maybe; it may cause problems if the switch was flipped whilst under load.

    My concern is that at high signal levels there are kV between the power tube plates of inductively loaded amps;...
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    Toggle switches aren't a good idea; too easy for...

    Toggle switches aren't a good idea; too easy for it to get knocked inadvertently, with potentially disastrous consequences; unless it's mounted internally, which would defeat the purpose.
    Schurter...
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    That seems normal; there's no reason to suspect...

    That seems normal; there's no reason to suspect the OT is faulty at this stage.

    Consider that if both plates are at 490Vdc, the difference in winding resistances is having a negligible effect, at...
  26. The parameters of ecaps that fail with age / use...

    The parameters of ecaps that fail with age / use are ESR and leakage resistance. Unless you have checked those your testing was insufficient.
  27. How exactly did you check the caps? eg ESR...

    How exactly did you check the caps? eg ESR assessment.
    Schematic file or link?
    If no schematic available then best to raise another topic in that regard in the ‘schematic request’ sub forum.
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    Copper’s resistance temperature coefficient...

    Copper’s resistance temperature coefficient looks to be 0.393% / degree C, so an increase of nearly 64C would raise resistance 25%.
    Not sure but it seems feasible that even old school paper and...
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