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  1. Replies
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    That's weird indeed. I expected 310R. What do you...

    That's weird indeed. I expected 310R. What do you measure between brown and blue? Some meters have problems measuring the DCR of a large inductor.
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    Well, the measured R values look consistent. I...

    Well, the measured R values look consistent. I calculate that yout OT has 240R between brown and CT and 250R between blue and CT. The 250R is divided by the short to core into 70R + 180R. Between...
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    So we suspect a short between OT primary and core...

    So we suspect a short between OT primary and core or secondary.
    I would completely remove the OT from the amp and check with Ohmmeter.
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    That's almost impossible, especially as your amp...

    That's almost impossible, especially as your amp seems to work basically. OT DCR increases with wire temperature, though.
    Can you post DCR, idle currents, HT and plate voltage values ?

    And please...
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    Thanks, makes sense. If the primary AC flux is...

    Thanks, makes sense. If the primary AC flux is confined to the core, increasing loop area won't increase EMF. Generally very wide wire loops will produce less EMF/turn caused by the opposite phase...
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    Or you might make it an inductor load drive by...

    Or you might make it an inductor load drive by using only the existing reverb transformer's primary and adding a coupling cap.
    But that would require a high impedance reverb input transducer.
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    Yes, wrong polarity. Positive must go to the...

    Yes, wrong polarity. Positive must go to the plates. (In case of doubt just varify voltage polarity with your meter with bad cap removed before connecting a new cap.)

    Bias voltage can be adjusted...
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    No matter how Gibson called the part, if it only...

    No matter how Gibson called the part, if it only has 2 wires/lugs/terminals it is technically no transformer and can't transform anything. A transformer needs to have at least 3 terminals connected....
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    If the reverb "transformer" only has 2 lugs it is...

    If the reverb "transformer" only has 2 lugs it is no transformer but a choke/inductor. This reminds me of a reverb driver I found in a Hammond L100 organ amplifier.
    The plate resistor of the reverb...
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    Is this an online calculator? Do you have a link?...

    Is this an online calculator? Do you have a link?

    Edit: Found it, it's here: https://robrobinette.com/Tube_Bias_Calculator.htm
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    Original 60s AC30s ran the tubes at around 14W...

    Original 60s AC30s ran the tubes at around 14W plate dissipation. Also used a cathode R of 47R, but at a plate voltage of 310V (B+ = 320V).

    I would change the cathode resistor to 68R.
  12. Are you sure that the sovent of the nickel paint...

    Are you sure that the sovent of the nickel paint did not melt the wire insulation and caused a partial winding short? This would explain the dulled tone. An L(CR) meter that displays Q or D=1/Q could...
  13. Schematic? I don't think your OT is bad. But...

    Schematic?

    I don't think your OT is bad. But measure DCR on both sides of the primary wrt CT.

    Do all 4 6L6s show an idle current of 49mA?
  14. Replies
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    Because of the non-uniform spatial distribution...

    Because of the non-uniform spatial distribution of the magnetic flux, there is no simple relation between output level and coil area/turns.

    PU output is the sum of the induced signal voltages in...
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    The DCR solely depends on wire length and actual...

    The DCR solely depends on wire length and actual gauge. For a given PU assembly and wire gauge the DCR is a crude makeshift indicator of turns number and thus of inductance (frequency response) and...
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    The nice thing about the coil estimator is that...

    The nice thing about the coil estimator is that it allows to estimate the achievable turns number which is way more important than DCR.
  17. Not sure if I understand. You used conductive...

    Not sure if I understand. You used conductive paint under (metal?) covers?
  18. Replies
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    DCR is a parasitic property of conductors. All it...

    DCR is a parasitic property of conductors. All it does is produce losses and damp a PU's resonance. It completely loses its informative value if different wire gauges or different coil dimensions are...
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    While I doubt that you can exactly replicate the...

    While I doubt that you can exactly replicate the P-90 tone with alnico poles (as opposed to steel poles) I think the most important factor would be to use a similar turns number (around 10,000) to...
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    Is the footswitch cable (as well as the switch...

    Is the footswitch cable (as well as the switch itself) well shielded? Also the amp internal wiring to the footswitch is critical.
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    More turns will result in higher output and lower...

    More turns will result in higher output and lower resonant frequency. There is no relation between output and resistance.
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    As Enzo would say (and I heartily agree): Why...

    As Enzo would say (and I heartily agree): Why repair something that isn't broken? (I also own a couple of rather old Tek scopes types 547 and 453, no repairs done yet).

    Bad PS caps would make your...
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    I think Naphta (or mineral spirits) is preferable...

    I think Naphta (or mineral spirits) is preferable over any alcohol because it's water repellent, is better at dissolving greasy and waxy deposits and doesn't attack nitro lacquer. (Dan Erlewine uses...
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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...

    I would use naphta (lighter fluid) or white/mineral spirit, but be aware of high combustibility.
  26. I was involved in the design and safety...

    I was involved in the design and safety approval/certifiction of many a power supply and am not in favor of modifying a power supply that doesn't conform to safety standards.



    What do you mean...
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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.
  28. Sorry, doesn't open here.

    Sorry, doesn't open here.
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