Hi Steve,
A distinction should be made between heaters & filaments, and AC & DC
output switchers.
Just about anything may be employed to run a heater. AC, DC, phase
controlled AC or a raw switcher output. Anything except DC & sine wave
AC will require a true RMS meter to check the operating voltage. From
your follow on notes, it appears that you are contemplating a tube with
a cathode.
When I built my 4CX1000A/4CX1500B amp about 20 years ago, I employed
an RO associates switching power supply with soft start plus remote
sensing at the tube socket to run the heater. This was a factory
designated 5V 20A DC supply but, both the output voltage and crowbar
adjustments had sufficient range to supply the necessary 6 volts. I
saw the switcher as an easy solution to setting and maintaining the
heater voltage, which is perhaps a bit more critical on the cathode
tubes, without having to build anything.
I was initially worried about interference from the switcher because,
though it was fully encased, the supply run in the "open" generated a
serious RF comb that extended well up into the HF region. However,
once installed in the RF deck the necessary amplifier shielding plus a
small AC line filter was sufficient to reduce those harmonics below the
ambient noise level.
Filaments are a different breed. Somewhere in the dim past, certain
large tubes were run with DC on the filaments to minimize hum. The
polarity was reversed at intervals to even out the wear.
Running a filament tube on AC will always create some hum due to
uneven emission. Terman covers the subject in some detail in all of
his radio texts from 1937 through 1955 -- look up "hum" in the index.
The short version is that the AC on the filament creates a signal at
2nd harmonic of the source. This effect is somewhat minimized due to
the interleaving of the filament structure on most tubes with less than
5KW plate dissipation. Using 60 cycle power (120Hz 2nd harmonic) it is
likely to escape unnoticed in an SSB system.
Employing an unrectified switching supply on the filament will create
some far out sidebands. I'd surmise that the amplitude would vary a
bit with the tube type. I'm going to give this try someday with a
4-400A just for curiousity. If it doesn't work out, a (silly) fix
might employ a synchronous rectifier which flips its output polarity at
a few Hz.
73 & Good afternoon,
Marv WC6W
Webpage -- http://wc6w.50webs.com/
*******************************************
-- Steve Thompson <g8gsq@eltac.co.uk> wrote:
<snip>
I'm aiming for a floating supply as the current project is a tetrode
running with the screen at ground potential. I initially thought about
a dc supply where the switching frequency can be filtered to a minimal
level, but then I thought about using the ac (more or less square wave)
instead for simplicity. Capacitive coupling from the heater to the
cathode is quite high impedance, and the cathode is bypassed to ground
so I figure that the likelyhood of heater voltage getting transferred
onto cathode-control grid bias voltage is low.
Steve
*
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