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  1. Replies
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    Well, the mod suggested by Chuck results in 10k...

    Well, the mod suggested by Chuck results in 10k grid stoppers - and the original circuit effectively has 220k grid stoppers.
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    I would use naphta (lighter fluid) or white...

    I would use naphta (lighter fluid) or white spirit, but be aware of high combustibility.
  3. I was involved in safety approval of power...

    I was involved in safety approval of power supplies and am not in favor of modifying a power supply that doesn't conform to safety standards.



    What do you mean with "magnetized"?
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    There seems to be a misconception. While a grid...

    There seems to be a misconception. While a grid (series) stopper resistor (1k to 10k is sufficient) can avoid oscillation, a large grid (shunt) resistor does not.
  5. Sorry, doesn't open here.

    Sorry, doesn't open here.
  6. From the paper: "Sealing out corrosive ...

    From the paper:

    "Sealing out corrosive gases can help to protect metal surfaces against atmosphericcorrosion. Corrosion inhibitors/lubricants applied at any sliding contact ...
  7. In fact almost all plated (nickel, chrome, gold)...

    In fact almost all plated (nickel, chrome, gold) covers I tested got slightly attracted to a neo magnet, but typically not enough to lift the cover. Most unplated covers showed essentially no...
  8. So the culprit should be the macbook's power...

    So the culprit should be the macbook's power supply.




    From what you describe above this looks like a potential safety issue.
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    Thanks. There seems to be sufficient distance...

    Thanks.
    There seems to be sufficient distance between the PT and the tank. I would use shielded wiring inside the chassis connecting to the reverb jack.
  10. That seems to confirm that the problem is...

    That seems to confirm that the problem is restricted to the V2/V1 section. As pulling V1 makes it worse, it seems to be somehow involved as well. What happens if you ground the V1 grid?

    Did you...
  11. Where does the "grounding of V2A" happen? Can't...

    Where does the "grounding of V2A" happen? Can't see it from the preamp schematic. The grids of V2A and V1A seem to be connected?

    Does the oscillation at the plate of V2B stop when you pull any of...
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    But that's typical for the non-inverting triode...

    But that's typical for the non-inverting triode in a LTPI. However the tube has 2 inputs, cathode (non-inverting) and grid (inverting).
  13. Lanolin is wool wax and I don't think you want it...

    Lanolin is wool wax and I don't think you want it on high current contacts.
  14. Looking at the picture, I don't think there is a...

    Looking at the picture, I don't think there is a problem with the switch at all. The contact area looks shiny and like others said, all it might need is a little Kontakt 60 (followed by Kontakt 61)...
  15. Means that it probably contains silicone grease,...

    Means that it probably contains silicone grease, which doesn't easily wash off with soap.

    Often Vaseline (petroleum jelly) is used on contacts. But there are more specialized products sold e.g. by...
  16. There is certainly something like contact grease....

    There is certainly something like contact grease. E.g.: https://www.amazon.com/LubriMatic-11755-Dielectric-Electrical-Contact/dp/B001446LP4

    The grease found inside pots is typically for...
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    Yes, it's definitely a differential amplifier...

    Yes, it's definitely a differential amplifier (LTPI). Note coupled cathodes.
    With a scope you should be able to see equal but out-of phase signals at both plates - even if you disconnect the NFB.
  18. Volentieri :) Typical eddy current effects are...

    Volentieri :)

    Typical eddy current effects are increased losses and reduction of inductance. Both effects increase with frequency.

    My notes state that a Seth Lover cover increased the PU's...
  19. Chrome plated Gibson T-Top covers had no copper...

    Chrome plated Gibson T-Top covers had no copper layer and measure exactly like original PAF covers. Seymour Duncan Seth Lover Covers (nickel plated) have an intermediate copper layer and show...
  20. So the -6dB from disconnecting the cathode bypass...

    So the -6dB from disconnecting the cathode bypass cap (as suggested by Dave H) is not sufficient?
  21. ...and never completely tighten the 2 screws that...

    ...and never completely tighten the 2 screws that secure the pouch.
  22. Preamp oscillation in the audio range typically...

    Preamp oscillation in the audio range typically involves 2 inverting gain stages. So I suspect some unwanted coupling between V2A and V2B. This could be due to a bad C19 or a bad ground connection.
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    Some good info contained here:...

    Some good info contained here: http://www.valvewizard.co.uk/bias.html

    Might get a little tricky to get -50V from standard circuit values and lowish 400V HT. But with some resistor value...
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    Yes, considerably. My x1.4 formula only applies...

    Yes, considerably. My x1.4 formula only applies with smoothing cap and without load. Without smoothing cap many meters won't give a meaningful DC reading as the voltage is varying heavily.



    I...
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    Now this seems very low. Is this with a reservoir...

    Now this seems very low. Is this with a reservoir ecap connected??

    I don't think it is possible to produce a negative voltage from a HT tap when using a bridge rectifier, so your proposal won't...
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    One thing is for sure: An output/speaker fuse...

    One thing is for sure: An output/speaker fuse would be a very bad idea when there is an output (auto-)transformer.
  27. Magnet type and DCR have little effect PU filter...

    Magnet type and DCR have little effect PU filter response. The dominating parameter is inductance (+cable capacitance). Actually DCR is rather useless - once you know inductance.
    Just listen to the...
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    Good point!

    Good point!
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