...I'm just quoting a 60 year old schematic which says + or - 20%, not + or - 10%.
I acknowledge that, however a literal reading of that ubiquitous schematic note doesn't make sense to me. Seriously, let's apply some common sense / critical thinking here; how can it be acceptable for the key power supply voltages of a correctly functioning amp, supplied with its nominal mains voltage, to vary +/-20% from the levels intended by the designer?
Valves don't acquire a whole different set of ratings by virtue of being plugged into an amp that has such a schematic note.
My interpretation of that note is that it applies to downstream voltages, ie that are significantly affected by component tolerance and the particular characteristics of the valve fitted. Thereby avoiding 1000 tech support cases per day from folks querying why eg the preamp anode voltage in their amp measures 190V when the schematic has 170V noted.
With regard to this thread, the key point here is that amp will be getting the mains voltage it was intended for when its heater voltages are close to nominal.
If the heater voltage is off by 5% or more, then my view is that to accommodate the normal expected fluctuations in mains voltage, serious consideration should be given to taking measures to get them closer, eg bucking transformer.
If the heater voltage is off by 10% or more, then urgent remedial action should be taken, and serious consideration given to taking the amp out of service, especially for long periods of high power usage, until that's resolved.
I just bought a completely unmolested '63 6G9B Tremolux.
...I measured 450Vdc @ 43.6mA / 450 @ 46,1mA on the power tubes....
The 6G9B schematic notes 365VDC at the HT and 6L6 anodes, so 450V is clearly a lot higher than that (23%
).
Being a 1963, it may have been fitted with a PT that had a revised spec compared to that of the schematic (drawn March 1962).
Your PT is stamped both 125P6A (6G9A type) and 68409 (6G9B type), may both be the same spec, as both HTs are noted as 365V.
But the AA763 (drawn Sept 1963) had a HT of 415V, 6L6 anode of 410V.
So even a later spec PT may not explain it.
Really, I think that the best thing would be to check your 6.3V and 5V heater voltages, as supplying the amp with a much higher voltage than it was intended for may cause significant stress to the PT, valves and OT.
Bearing in mind that even without any overvoltage, your 50Hz mains will be causing the PT to operate closer to the saturation point of its magnetic circuit. A USA domestic PT would probably have been designed for 60Hz operation; for the same degree of resilience / efficiency, 50Hz operation really requires the mag circuit to be uprated (though 60Hz transformer generally work fine at 50Hz, provided there are no other significant stress factors).
https://el34world.com/charts/Schematics/files/Fender/Fender_tremolux_6g9b_schem.pdfhttps://el34world.com/charts/Schematics/files/Fender/Fender_tremolux_aa763_schem.pdf
Topic: Step down transformer question (Read 3991 times)