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  1. Replies
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    Valves have a limit on the max voltage between...

    Valves have a limit on the max voltage between heater and cathode. If the heater circuit is floating, no dc reference to the cathode, then we canít be sure that the voltage limit wonít be exceeded...
  2. Thread: 110v > 240v Q's

    by pdf64
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    334

    Iím a bit sceptical about about that. It would...

    Iím a bit sceptical about about that.
    It would seem risky to purposely design a 60Hz mag circuitís operation so close to saturation that 50Hz would tip it over the edge.
    Bearing in mind that...
  3. Thread: 110v > 240v Q's

    by pdf64
    Replies
    14
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    334

    I think all Fenders have had isolating power...

    I think all Fenders have had isolating power transformers.

    You may find you need to further reduce the voltage from any type of step down transformer, ie both isolating and auto, as it will likely...
  4. Thread: 110v > 240v Q's

    by pdf64
    Replies
    14
    Views
    334

    Provided that: - the 110-120V equipment has an...

    Provided that:

    - the 110-120V equipment has an isolating PT and a properly grounded chassis and power connector
    - the step down transformer maintains a proper ground connection between its input...
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    Maybe they’re dumping a lot of grid current;...

    Maybe they’re dumping a lot of grid current; what’s the Vdc across each 220k grid leak?
    Or maybe it’s oscillating; what happens to the power current if the tube in the phase splitter slot is pulled?
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    Oops, Iíll try and blame the small character size...

    Oops, Iíll try and blame the small character size on my phone, I saw the ĎHí, seemed to remember it being a Hammond PT previously, and read it as ĎHammondí
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    130C insulation?! That sounds like even Hammond...

    130C insulation?!
    That sounds like even Hammond are now using cheap wire :thumbsdown:
    See J M Fahey's run down on the topic...
  8. What exactly is meant by Ďsolder one set...

    What exactly is meant by Ďsolder one set togetherí?
  9. Replies
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    Have a read of...

    Have a read of https://www.buildyourguitar.com/resources/lemme/
    My take is that the only scenario in which a 1/4" jack cable's electrical characteristics really matter to the guitar sound is when...
  10. Replies
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    I don't think you've got that right. Have you...

    I don't think you've got that right. Have you tried monitoring the cathode or plate current of the power valves as the type 3 master is turned down? If you start out at full load, you should find...
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    Failing solder joints were a problem on the...

    Failing solder joints were a problem on the couple of these I've worked on.

    It seems rather misleading to describe this model as a reissue :mad:
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    Whatís the wall and heater Vac?

    Whatís the wall and heater Vac?
  13. Replies
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    Anything that if open circuit, would prevent...

    Anything that if open circuit, would prevent current flowing in the OT secondary circuit; eg a bad speaker, cable, connector, impedance selector.

    Another thing that could cause an amp's output to...
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    One bad power tube of a pair couldnít cause...

    One bad power tube of a pair couldnít cause complete silence. It must be something common to both; sure the speaker / OT secondary circuit is good?
  15. Perhaps a muting or protection circuit is being...

    Perhaps a muting or protection circuit is being operated / won’t disengage? It seems like you done all you can, in the absence of a schematic.
  16. Replies
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    Ah yes, I seem to remember now that it was too...

    Ah yes, I seem to remember now that it was too loud for your situation.
    By higher signal levels, I meant at the power amp, ie when it's pushing up to and past its max power output, into overdrive....
  17. The ab568 has a few changes...

    The ab568 has a few changes https://el34world.com/charts/Schematics/files/Fender/Fender_twin_reverb_ab568.pdf
    The PT looks to put out the same voltages as the ab763 so it should be simple to revert....
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    The idea is that at higher signal levels, the amp...

    The idea is that at higher signal levels, the amp will respond to, and compress, note attack. Didn't you have a 5E3? If so, a little more like that.
    As it is, the TBX's response will be as stiff and...
  19. IME, with modern tubes, the 135W models are lucky...

    IME, with modern tubes, the 135W models are lucky to put out 100W, and with the basic, original (somewhat inefficient ) speakers, they donít seem super loud.
    The hardwired bright cap on the master...
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    I think the real (but non obvious) benefit of a...

    I think the real (but non obvious) benefit of a DCCF is all about the way it shapes the waveform http://www.valvewizard.co.uk/dccf.html
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    The vintage AC15 has a C - L - C filter between...

    The vintage AC15 has a C - L - C filter between the rectifier and OT CT, whereas the TBX has a more modern arrangement where the OT CT feed is taken from direct from the reservoir cap C2. So my...
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    TBX - Stiff HT, EL84 cathodes not properly...

    TBX -
    Stiff HT, EL84 cathodes not properly bypassed https://el34world.com/charts/Schematics/files/Vox/Vox_ac1596pa.pdf
    Type 3 / crossline master vol, hodge podge preamp, neither one thing or...
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    I think the circuit is self (cathode) bias, not...

    I think the circuit is self (cathode) bias, not fixed bias.
    ie if cathode current should rise, cathode voltage will increase too, thereby reducing cathode current.
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    Most every other amp with a global nfb loop...

    Most every other amp with a global nfb loop around its power amp does that though; I guess itís a matter of magnitude, when it becomes excessive :confused:
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    How about if the awful standby isnít used? To...

    How about if the awful standby isnít used?
    To me, it looks to be doubly crap, as it may be preventing the bias supply from charging up probably and hot switching the rectifier.
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    Confused - is the amp now working right? The HT...

    Confused - is the amp now working right? The HT should have been saggy before, when it was half wave rectified, stiffer now that itís full wave.
    It would be best not to use this hot switching type...
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    Well done spotting that! I guess your amp was...

    Well done spotting that! I guess your amp was half wave rectified all these years. The HT should have been more saggy under load. The weird HT winding Vac in standby are probably due to the bias...
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    Maybe there's a problem with the pickup in the...

    Maybe there's a problem with the pickup in the buzzy guitar, such that it isn't actually bucking hum? High setting of the tone control just making that apparent.
    Check for that by swapping in a...
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