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  1. Pleased there was no lasting visual damage - just...

    Pleased there was no lasting visual damage - just goes to show the variety of hazards on the bench.

    Even the Williamson amp showed about 45 deg phase shift at circa 40kHz, which keeps increasing...
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    Although that article focuses on how to gauge a...

    Although that article focuses on how to gauge a fuse for the secondary side, I added some guidance towards the end on AC side fusing and how to estimate how much the inrush may get to, and how to...
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    Some additional details on fuses, and a process...

    Some additional details on fuses, and a process to select them using PSUD2, is provided in link - it is a bit more technical, but may assist....
  4. If you have a larger transformer with 9VAC...

    If you have a larger transformer with 9VAC secondary, which you want to supply the step-up Tx as well as be rectified for DC heater use, then you could estimate (as a minimum) the VA requirement of...
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    Roly was very supportive to all who met him, and...

    Roly was very supportive to all who met him, and an inspirational figure - whether it was recycling some old PC power supply, or just going that extra mile for someone.

    We had some memorable...
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    I've been using this technique:...

    I've been using this technique:
    https://www.dalmura.com.au/static/Choke%20measurement.pdf

    The jig part values can be changed to vary the size of the AC applied signal if needed, although I mainly...
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    Yeh they are quite plentiful here in Oz and have...

    Yeh they are quite plentiful here in Oz and have quite a nice high self-resonance around 10kHz, so act like an inductor a bit better than commonly used chokes.

    A slightly better fit for valve...
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    A serendipitous option is a not too old flouro...

    A serendipitous option is a not too old flouro choke for 240VAC operation and common 20W tube size - it measures about 1.6-2H and is fine for up to 300mADC (5W dissipation).
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    Sunn started in 1965 and used the Dynaco Mk3 amp...

    Sunn started in 1965 and used the Dynaco Mk3 amp block and PAM-1 for their 200s - supposedly for its relatively better bass performance (being a hi-fi amp), but also possibly it was an available kit...
  10. Quite often with vintage amps, the power...

    Quite often with vintage amps, the power transformer HT secondary CT is taken directly to chassis, rather than more appropriately taking the CT directly to the first filter cap negative terminal...
  11. The silicon diode, added in series with the valve...

    The silicon diode, added in series with the valve diode anode, is a rectifier diode that could be safely used in place of the valve diode - it is typically a 1kV 1A rated diode that is used in amps...
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    Did you re-measure the heater voltage again to...

    Did you re-measure the heater voltage again to confirm if it was indeed 2.95Vac on the EL84 - that is still a possible problem. You can also measure the DC heater voltage going to the preamp tubes...
  13. I prefer to see Cgd as a characteristic value...

    I prefer to see Cgd as a characteristic value that depends on the voltage across the capacitor. If you want Vds to transition between two states (on and off), then you have to appreciate the energy...
  14. Cgs is fairly constant with voltage change. Cgd...

    Cgs is fairly constant with voltage change. Cgd is non-linear but very significant when it comes to smps switching, as drain voltage is rapidly changing.

    Yes, you need a driver that can push/pull...
  15. There is some detailed discussion on MOV...

    There is some detailed discussion on MOV protection for output transformers in the link below - perhaps a bit technical, but hopefully provides some more insight for you:
    ...
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    Yes, as you consider deep diving in to...

    Yes, as you consider deep diving in to measurements, crest factor will be your next hurdle to jump.

    Some vintage transformer manufacturers had secondary ratings based on capacitor filtered or...
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    Yeh I'd imagine a type test may somehow include...

    Yeh I'd imagine a type test may somehow include embedded sensors, or a confident margin is used based on previous type tests of similar parts.

    With respect to 'full spec load' testing, you still...
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    The manufacturer also has the choice of wire...

    The manufacturer also has the choice of wire enamel thickness to use - typically identified by winding wire being grade 1 or 2. Thicker enamel coating means less turns per layer, so possibly a...
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    That may not be the case. It was not uncommon...

    That may not be the case. It was not uncommon for a manufacturer to get a batch of higher powered OT's and use them for lower powered models as well. The cost of a mixed batch of OT's was more than...
  20. Were you able to check how close your speaker...

    Were you able to check how close your speaker line is to 0Vdc - that will be a simple indicator that your output stage is biasing up ok.

    Was a signal on your scope looking fairly clean for...
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    So if the waveshape at 40Hz is not to your...

    So if the waveshape at 40Hz is not to your liking, and if it was being degraded at high signal level due to the drop in forward gain (and hence feedback level), you could measure that if you had a...
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    I get an invalid attachment message.

    I get an invalid attachment message.
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    What is your circuit schematic for the scope...

    What is your circuit schematic for the scope waveforms?
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    You have feedback around the output stage, within...

    You have feedback around the output stage, within which is a CR filter which drops the forward gain at a nominal 16Hz corner (0.1uF, 100k + a titch more from bias supply). I'd be expecting that...
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    You identify a minor but noticeable change in...

    You identify a minor but noticeable change in bias voltage. Perhaps that totally comes from a transformer winding level/waveform change, rather than anything related to EL34 grid conduction. You...
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    Can you show the power supply cap values, and...

    Can you show the power supply cap values, and also the bias circuit details, just for completeness?
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    "much less than 1W". The reference states "in...

    "much less than 1W". The reference states "in the order of tenths of a watt ... it will be neglected". So that heat flow path through the socket contributes to perhaps about 1% of heat flow from...
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    No probs. The 'maximum allowable voltage'...

    No probs.

    The 'maximum allowable voltage' ratings in a datasheet relate to an application's typical maximum operating level. For example, if the MOV was used across an AC mains supply then it...
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