DC Filaments
If you run grounded cathode, passive grid with tetrodes, I don't see a
problem with a cheap switcher for the fils - provided it goes into
constant current, not foldback, as Rich says. It needs shielding,
although I picked up a 6v 20A switcher for $20 at a flea market which is
fine - running it on the desk in front of the rx and I can't hear it -
more than can be said for the PSU for the Apple laptop! I guess a time
delay is needed to get over the longer warm up time. But I heard a story
once (which I can't trace) that running high power fils on DC can lead
to some strange corrosion effects on the filament internally - sounds
unlikely to me - anyone any info?
Bias adjustment
I'm still having trouble with Rich's idea on biasing. What's wrong with
setting the bias and screen for the correct levels and NOT driving into
grid current? (By suitable adjustment of the exciter) If you can't wind
back the exciter output, then granted you have a problem, but don't most
exciters have a power control function? If the thing is so badly
designed that it then then gives power spikes, I guess there's a
problem, but I don't like Rich's approach.
Parasitics.
I'm getting confused. Is Rich saying that if you use a conventional coil
with a shunt resistor, RF currents can heat the resistor so it goes high
in value and then the parasitic can start, with whatever results are
seen, while a nichrome wire parasitic suppressor won't do this?
The worst tubes I've ever found for parasitics were 572B's and worse, a
Mullard /Phillips tube called a QV08-100. That had effectively 3
cathode/grid assemblies from 807's paralleled up, each cathode/grid
assembly laid horizontal, and then stacked vertically, and a big two
piece carbon anode. They were swine to cure!
73
Peter G3RZP
--
FAQ on WWW: http://www.contesting.com/ampfaq.html
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
Search: http://www.contesting.com/km9p/search.htm
|