In a message dated 3/3/2005 1:33:08 PM Eastern Standard Time, Dennis12Amplify
writes:
Todd,
This brings to mind another question I have always wanted to ask, and I
believe that it is on-topic when we start talking about DC on 3-500Z tube
filaments.
If you power your filament with DC, wouldn't that mean that the filament
(cathode) had a higer DC potential on one end than the other end?
In the case of a 3-500Z, wouldn't that cause a potential difference across
the filament (cathode) of 5.0VDC?
Wouldn't that create a tapering 'hot-spot' across the filament (cathode)
during normal operation of the tube because of the difference in the bias
potential across the filament (cathode)?
Wouldn't the part of the filament that was at +5.0 VDC emit many, many, less
electrons than the part of the filament that was at ground potential?
(grounded grid)
In many 3-500Z based amplifiers, the bias system is simply a 6.0 volt zener
diode in series with the cathode to ground path, and in this case it would be
the filament to ground path. Would you 'float' the 15 ampere (per tube) DC
filament supply in that particular case?
The reason I bring all these questions up is I have seen mobile amplifiers
that DID use DC to light their tubes to eliminate the inverter and AC
transformer that would have had to be used otherwise.
Those tubes (2 or more) were series connected across the battery, so for a 3
tube amplifier I would have to believe that at least one of them was
operating at 10 to 15 volts of bias, another at 5 to 10 volts of bias, and the
last
one at 0 to 5 volts of bias.
I never got the chance to ask the owner about the longevity of his tubes or
if they ran at different shades of red during the normal operation in his
mobile.
Regards,
Dennis O.
Hi Dennis, those are all interesting questions and would need to be addressed
in a grounded-grid amplifier using DC on thoriated tungsten filaments. I
don't think the DC potential across the filaments would affect the operating
bias
as long as the DC supply is RF isolated and floating from the cathode circuit.
That would be harder to do with DC as the filament chokes would need to
withstand the DC current without saturating if you were using ferrite-core
chokes?
Iron powder chokes would probably be needed here. You would have to somehow
float the DC supply from ground to be able to apply cathode bias unless you
apply bias to the grid instead and float it below DC ground using an RF bypass
capacitor on the grid? Maybe that is how the mobile guys are doing it?Floating
an AC filament power supply is easy with chokes and transformer isolation of
the windings. It looks like a few tricks are needed to run DC on
thoriated-tungsten filament power tubes in grounded-grid service. My biggest
worry would be
shortened life of the cathode operating on DC but like Peter said switching
the DC polarity regularly should minimize that problem. 73 Todd WD4NGG
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