I'm sure others have come across this when selecting a power transformer. For a project I'm working on, I found a power transformer with the perfect voltage and current high voltage winding and a perfect filament current rating. Problem is the filament winding does not have a center tap. I've had the same situation before and a solution I have found that cancels background filament hum is to add bypass caps from each side of the AC filament line to chassis ground. I find ceramic caps work best for bypassing and 50V multilayer ceramic caps are small enough to fit anywhere. I ground the caps to the chassis and not directly to audio ground circuits. The 120V AC power inlet third prong ground also connects to the chassis and not directly to audio grounds. Although audio grounds do connect to the chassis, all audio grounds are wired together. Copper wire is a better conductor than a chassis giving some degree of isolation between audio ground and AC ground. I have never had a hum problem (so far 😊).
Your selection of 0.47uF works for bypassing high frequencies to ground.
But 0.47uF is 5,644 Ohms of capacitive reactance at 60Hz power mains frequency.
Not a very good shunt to ground at 60, 120, and 180Hz.
At 1kHz, 0.47uF is 339 Ohms.
At 20kHz, 0.47uF is 16.9 Ohms.
The recommended two 100 Ohm resistors pseudo center tap is 100 Ohms from each lead to ground,
and covers from DC to much higher frequencies than 20kHz.
There is a very good reason that many use the dual 100 Ohm resistors to create their pseudo center tap.
In regards as to where to connect the pseudo center tap . . .
Ground Loops are Commonly Mis-understood. (both true, and the pun is intended)
But 0.47uF is 5,644 Ohms of capacitive reactance at 60Hz power mains frequency.
Not a very good shunt to ground at 60, 120, and 180Hz.
At 1kHz, 0.47uF is 339 Ohms.
At 20kHz, 0.47uF is 16.9 Ohms.
The recommended two 100 Ohm resistors pseudo center tap is 100 Ohms from each lead to ground,
and covers from DC to much higher frequencies than 20kHz.
There is a very good reason that many use the dual 100 Ohm resistors to create their pseudo center tap.
In regards as to where to connect the pseudo center tap . . .
Ground Loops are Commonly Mis-understood. (both true, and the pun is intended)
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EJ I think you are following a misguided path on how to neutralise heater voltage related hum. Perhaps you need to review how heater related hum has been managed for the last 70-80 years in millions of audio amps.
A simple 200 ohm wirewound pot, ground the wiper, and the end connections go to the filament lines.
Adjustable "hum pots" were always the way to solve issues with hum.
Adjustable "hum pots" were always the way to solve issues with hum.
The DC common-mode voltage is unpredictable. I don't know if that is a problem or a feature; it will probably automatically settle close to the cathode voltage of the valve with the worst cathode-to-heater insulation. I'm not sure what effect heater emission will have, though.
Two of the many causes of hum in an amplfier:
Power supply ground loops
Input connection and Input stage ground loop
It is not easy to make a non-negative feedback amplifier have less than 100uV of hum at the 4 or 8 Ohm output tap.
Power supply ground loops
Input connection and Input stage ground loop
It is not easy to make a non-negative feedback amplifier have less than 100uV of hum at the 4 or 8 Ohm output tap.
I probably should not have called it hum, perhaps AC buzz is more accurate. The few times I've had to use a filament winding with no center tap, caps have cleaned up the background noise.
The caps likely provide 'neutralisation' of the buzz in your amp - where the buzz arises from coupling to the input grid from each side of the heater - the coupled buzz voltage at the grid is balanced by the caps (hence it is neutralised, as a signal transfers from each side of the heater). Normally that neutralisation is done with fixed resistors, or sometimes under duress using a humdinger pot. But there are many ways for buzz/hum to couple in, so I'd suggest there is no single golden bullet for each amp.
To float the filament is waiting for strange things to happen. One or two resistors to ground ( or any elevation) is better.
I did a restoration on a Harman Kardon Citation V power amp a while back.
The filaments were tied to ground through a factory set of 33 ohm 2W resistors.
And even with Gold Lion matched outputs, and new TungSol drivers, the unit had a small barely noticable hum.
Grounding was proper, and hum lessened with the bottom cover on.
The next best thing was to replace those 33 ohms with a 200 ohm wirewound pot to "tune" the hum out.
The filaments were tied to ground through a factory set of 33 ohm 2W resistors.
And even with Gold Lion matched outputs, and new TungSol drivers, the unit had a small barely noticable hum.
Grounding was proper, and hum lessened with the bottom cover on.
The next best thing was to replace those 33 ohms with a 200 ohm wirewound pot to "tune" the hum out.
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Some Harmon Kardon and some other amplifier manufacturers use a Concertina split load phase inverter.
I had one such HK amp, and the Concertina's filament to cathode leakage introduced hum, hum harmonics, and noise
It sounded like an electric drill motor electrical noise (most likely the power mains was very dirty with noise from other appliances that were on the same power pole transformer).
Change Concertina tubes (and be sure that the Concertina's filaments are 'DC elevated').
YHMV
(Your Hum May Vary)
I had one such HK amp, and the Concertina's filament to cathode leakage introduced hum, hum harmonics, and noise
It sounded like an electric drill motor electrical noise (most likely the power mains was very dirty with noise from other appliances that were on the same power pole transformer).
Change Concertina tubes (and be sure that the Concertina's filaments are 'DC elevated').
YHMV
(Your Hum May Vary)