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  1. Replies
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    Here's a great discussion and explanation of it...

    Here's a great discussion and explanation of it https://www.guitargear.net.au/discussion/index.php?topic=30731.0


    According to the dicussion, phase shift and frequency shift are 2 sides of the...
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    271

    Yes, my previous post was in regard of both...

    Yes, my previous post was in regard of both issues raised. There’s probably some degree of interrelation between them.
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    I think it’s mainly down to the particular...

    I think it’s mainly down to the particular characteristics of the light dependant resistor (LDR) used in the opto coupler (aka trem bug). They aren’t precision devices and as they use cadmium, a now...
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    I guess a 1948 PT would be intended for 110V;...

    I guess a 1948 PT would be intended for 110V; feeding it 125V will probably result in all secondaries running too high, resulting in limiting values being exceeded. eg what are the heater voltages?...
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    149

    Is this a cathode bias amp? If so a high...

    Is this a cathode bias amp? If so a high dissipation at idle is to be expected. I’d expect the bulb to get ‘bright-ish’ but not full brightness. The latter may indicate a fault.
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    Well done, the cable look a bit more manageable...

    Well done, the cable look a bit more manageable than the 6mm dia type I envisaged.
    Be careful with its screen termination, can't quite see what you're connecting it to, but you mustn't let it ever...
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    I think that using such cable may make the job...

    I think that using such cable may make the job rather more difficult (and so likely to be unsuccessful, especially so if the operative lacks a good level of skill and experience).
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    I think it would be better not to put forward the...

    I think it would be better not to put forward the OT shunt method, as it’s needlessly hazardous. Especially so for an inexperienced operative.
    If installing 1 ohm resistor per cathode isn’t...
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    Without the diode, in the case of the fuse value...

    Without the diode, in the case of the fuse value being too high and a power tube shorting (neither of which happen often, of course), the resistor might 'fuse' and save the OT / PT etc from damage....
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    No, but I accidentally connected things like that...

    No, but I accidentally connected things like that once and the screen grid resistors (470 ohm 1 watt, 6l6GC p-p amp) began releasing the magic smoke within a few seconds of the tubes warming up and...
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    Screen grid current? With no voltage on the plate...

    Screen grid current? With no voltage on the plate it may rise to a damaging level.
  12. Well done in finding this fault :thumbsup: Not...

    Well done in finding this fault :thumbsup:
    Not sure if you've been advised of this previously, but a key part of a major service on these things is to re-establish the numerous mechanical chassis...
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    It shouldn't work like that, there is no...

    It shouldn't work like that, there is no attenuation of the #1 jack.
    Plug an open lead into it, and (measuring from the plug at the other end of the lead) there should be 1M between tip and sleeve,...
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    Yes, from your report it seems likely the...

    Yes, from your report it seems likely the connection between the amp's 0V circuit common and the sleeve of the #1 jack has failed somewhere, shouldn't be too hard to identify where that's happened...
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    Wow, that's interesting, I've not seen that...

    Wow, that's interesting, I've not seen that reported before; do you think they left the factory like that?
  16. It should be disclosed that the amp has been...

    It should be disclosed that the amp has been extensively modded ;-)
    Failing switching contacts in the fx loop return jack are a common cause of intermittent signal loss.
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    643

    I prefer a safety resistor across the secondary,...

    I prefer a safety resistor across the secondary, about 20-30 X the nominal load value.
    The low value of the feedback resistor network in most BF/SF amps might act as such a safety resistor....
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    I seem to remember that asymmetric distortion...

    I seem to remember that asymmetric distortion results in odd and even harmonics, symmetric distortion odd harmonics only.
    I think that the whole ‘even = good, odd = bad’ harmonics concept is...
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    Yes, I think a log pot would be better there.

    Yes, I think a log pot would be better there.
  20. They're just being silly ;)

    They're just being silly ;)
  21. Just noticed the above, 'coupling caps' seems...

    Just noticed the above, 'coupling caps' seems odd, was that just a terminology error, or have ecaps been used for coupling purposes?


    Confused by this, the neon couldn't flash if it's been...
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    So the shorting contact of the 1 jack is faulty....

    So the shorting contact of the 1 jack is faulty. That will result in the inputs picking up hum when nothing is plugged in, and will prevent the 6dB attenuation of the 2 input.
    So both inputs are...
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    Yes, Input 1 should be 1M input resistance, input...

    Yes, Input 1 should be 1M input resistance, input 2 134k and 6dB quieter. What input resistance do they measure? ie plug a cable in and measure the resistance between tip and screen.
  24. Yes, it’s a higher harmonics thing; the...

    Yes, it’s a higher harmonics thing; the interference from an out of balance heater circuit seems more like the ‘buzz’ from a bee, rather than the ‘hum’ of someone that knows the tune but not the...
  25. Yes, though humdingers are typically more of a...

    Yes, though humdingers are typically more of a ‘buzz balance’ than ‘hum balance’.
  26. From a contemporary schematic...

    From a contemporary schematic https://el34world.com/charts/Schematics/files/Vox/Vox_ac301978.pdf I guess that the switch is to select between 240 and 220V primary taps on the PT.
    The trimmer near...
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    I will have done, if only to check all was ok. If...

    I will have done, if only to check all was ok. If I had encountered to low gain issue you describe, I would have investigated.
    Some amps are like that, the DrZ EZG50 comes to mind as having...
  28. Maybe the board 1 ohm cathode current sensing...

    Maybe the board 1 ohm cathode current sensing resistor/s have drifted up in value a bit. Perhaps following a tube short incident.
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