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    Dishwashing liquid, brush and hot water? (I...

    Dishwashing liquid, brush and hot water?

    (I hope you have nickel-silver, not nickel covers as nickel would be microphonic.)
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    The lowest guitar neck resonance (typically...

    The lowest guitar neck resonance (typically around 60Hz) tends to be weaker than the higher ones at around 200Hz and 450Hz.

    Neck resonances are standing waves that use the full neck length + (part...
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    Any resonance absorbs most energy AT its resonant...

    Any resonance absorbs most energy AT its resonant frequency. A guitar (neck) has several resonant frequencies. Neck resonances can cause dead spots. Adding weight shifts resonances to lower...
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    No, the current is determined by the load. Each...

    No, the current is determined by the load. Each battery in series is forced to deliver the same current. So more batteries in series will discharge more batteries in the same time.

    Adding more...
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    See post #4. Nothing to be gained from more...

    See post #4. Nothing to be gained from more series batteries. More batteries with more diodes just wastes power and batteries.
    Personally I would just use 6 batteries, especially as some of my...
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    I was primarily referring to the "touch...

    I was primarily referring to the "touch sensitivity" of the amp when probing high voltage points. This is something I have seen with self-oscillating amps. It is likely that the circuit point being...
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    It's exactly this weird behaviour that made me...

    It's exactly this weird behaviour that made me think of oscillation from the start.
    At very high frequencies a small inductance becomes a considerable impedance.
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    No visible wave at the output doesn't mean the...

    No visible wave at the output doesn't mean the amp is not oscillating as explained above. The OT would hardly transfer something like 200kHz. So you need to look for oscillation before the OT.
    ...
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    Anyway, if a SMPS introduces noise, additional...

    Anyway, if a SMPS introduces noise, additional filtering would be required.

    OTOH, a linear regulator needs a minimum voltage drop and wastes power.

    I actually don't see the need for a...
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    ...and as the switching frequencies of lower...

    ...and as the switching frequencies of lower power SMPS are typically above 20kHz, the harmonics should be ultrasonic/inaudible.
  11. Thread: PT questions

    by Helmholtz
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    Your DCR measurements don't seem to make sense. A...

    Your DCR measurements don't seem to make sense. A heater winding with a DCR of 10R would be rather useless. Even a single 12AX7 with a heater current of 0.3A would load down the voltage by 3V. I...
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    I am no battery expert, but I don't think it is...

    I am no battery expert, but I don't think it is advisable to wire dry batteries in parallel. As the voltages between the 2 packs will hardly be completely identical, the lower voltage pack will...
  13. A cathode that is not (AC) grounded can act as a...

    A cathode that is not (AC) grounded can act as a non-inverting input or as an non-inverting output. The cathode cap provides an AC ground.
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    ...and don't forget to test C35/36 for leakage as...

    ...and don't forget to test C35/36 for leakage as suggested by Jon Snell. To test measure DCV across each R75 and R76, should read zero on both.

    What's your meter's input resistance in DCV mode?
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    Correct, but you only have -30V. I recommend to...

    Correct, but you only have -30V. I recommend to install 1R cathode resistors (or use a "bias probe") and measure idle current. You might be able to adjust AP1 for suitable bias.
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    Ok, that's not enough and explains the...

    Ok, that's not enough and explains the redplating. You might try to adjust AP1 to get more negative volts, but most probably either C47 or C46 is leaky and loading down the bias supply.
    Also check...
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    Ok now use a 10:1 probe with your scope and trace...

    Ok now use a 10:1 probe with your scope and trace back the oscillation proceeding from output to input.
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    With 1N400x diodes in power applications always...

    With 1N400x diodes in power applications always use the glass-passivated (GP) types. These have much better long time reliability/lower failure rates (FIT values) than the cheaper standard versions....
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    Does pulling the reverb driver stop it? If...

    Does pulling the reverb driver stop it?

    If yes, clean all reverb related contacts: cable jacks, plugs, jacks to chassis. Same for tank side.
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    I guess you need to email the customer service...

    I guess you need to email the customer service contact: technical.support@us.bosch.com
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    https://www.dynacord.com/?language=en

    https://www.dynacord.com/?language=en
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    Yes, Roederstein (ROE) EKU series caps are...

    Yes, Roederstein (ROE) EKU series caps are bipolar ecaps.
  23. I think it's essentially a hifi design optimized...

    I think it's essentially a hifi design optimized for lowest distortion, probably copied from some tube application handbook (also note UL operation).

    As can be seen from the uneven spacings of the...
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    Oscillation often involves more than one amp...

    Oscillation often involves more than one amp stage and I think that the reverb circuit is involved.
    So especially check the screen filter cap and its ground connection. You could temporarily wire a...
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    Seymour Duncan made these in the 80s:...

    Seymour Duncan made these in the 80s: https://reverb.com/item/11970491-seymour-duncan-the-mag-1980-s-black-pickup

    Newer version has staggered mags: https://www.seymourduncan.com/pickup/stag-mag
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    Please always post scope settings with scope...

    Please always post scope settings with scope pics.

    A 45kHz signal at the output is clear evidence of oscillation. Prime suspects are typically filter caps.
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    There is yet another OT core related effect. An...

    There is yet another OT core related effect. An OT's lower corner frequency depends on its primary inductance, which strongly varies with applied signal voltage. Inductance is low at low signal...
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    I may be wrong but I suspect that the amp is at...

    I may be wrong but I suspect that the amp is at the verge of instability and that the observed effects are caused by HF oscillation.
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