"John L Stewart" wrote in message
...
Steve Bench published an excellant cure for the AC DHT filament hum
problem more than 10 years ago. See it at this link-
http://diyaudioprojects.com/mirror/m...ch/humbal.html
Have to say I'm not in favor of running the filaments on DC. Just
another complication to get in the way of reliability. Altho in some
instances we gotta do to.
Cheers, Joh
Though steve's method of hum compensation by injecting rectified (double
mains frequency) signal works, he is not (completely) right about the
mechanism of the hum second harmonic generation.
He attributes the 120Hz hum to the filament temperature modulation. Though
it might contribute in smaller tubes, the main cause is different -- it is
transconductance nonlinearity, the law of 3/2.
If at the pek of the AC filament voltage, say one end of the perfectly AC
balanced filament goes down by -3V and the second end goes up by +3V, the
emission current from the first end increases by a LARGER AMOUNT than
emission decreases on the second edge -- it is due to the nonlinear
current-vs-grid voltage characteristic (law of 3/2). So at the peaks of the
filament supply total emission is always higher. Thus we have a frequency
doubling effect.
This effect will be most pronounced if the tube is driven to cut-off.
According to Steve, at cut-off point there would be no plate current,
regardless of temperature modulation.
In reality, if one end of the filament goes negative, this end starts
conducting (emitting), and the other end is still not conducting (deeper
into cutoff). When the filament polarity reverses, the ends swap roles. So
at zero instantaneous filament voltage the trube is not conducting, but at
either peak one or the other end of the filament causes the plate current to
flow. In a way it is a frequency doubler using "anode-bend" effect.
To check which effect (temperature modulation or "anode-bend rectification")
contributes most, one needs to check the phase of the 2F hum. If it is in
phase with the mains -- it is anode bend effect, if it is delayed, it is
temperature modulation. Temperature is 90 degrees lagging behind because of
the thermal mass (inertia) of the filament.
Regards,
Alex