Follow-Up on 20A.
I have an observation that is perplexing me. Some of my old AM Boat
Anchor gear was designed in the days when the AC mains power was lower. For
example on one of my Central Electronics 20a 's, I measured the
filament voltage (non-RMS DVM), and it came out to 6.9 vac with a AC line
voltage of 123 volts. I had to lower my ac mains with a variac to 112 vac to
get the filaments down to the nominal 6.3 volts.
This brings up a couple of issues:
1.) The filaments running ~ 10% high may have more emission, but cathode
longevity may suffer along with more likely heater to cathode shorts.
2.) The AC power transformers may not have adequate flux BH headroom, and
could start to saturate, leading to high AC input current, and hot
magnetics.
3.) The High voltage may be beyond the ratings of the components (too much
voltage) which may lead to higher heat dissipation. For example,
electrolytic capacitors may see too much voltage. This is especially true
when solid state power supplies are used with vacuum tube circuits. The
voltage soars to the peak AC value until the tube heaters warm up enough to
pull current. The higher tube element voltages may also (if unregulated)
move the DC operation points to dangerously unstable levels. For example, a
10% rise in the G2 voltage (say 400 to 440) of a Beam Power tube can
increase the DC operating point such that the plate dissipation is too high
leading to a thermal run away condition.
My first thought of attack concerning my 20A was to add resistance to the
filament wiring. This is doable so long as the ac mains voltage is
reasonably constant. The DC voltage issue can be solved in other ways, such
as regulating the G1 fixed bias, and the G2 screen voltage to my 7591 beam
power tubes. It seems that when the ac mains increases, the G1 fixed bias
goes more negative, and the screen G2 goes more positive. The sensitivity to
G2 is greater than G1, so the plate current varies quite a bit with a small
change in ac mains voltage. Just thinking about this, I could add some gain
to the G1 potential VS ac mains voltage change to cancel out the G2 effect
on plate current. A little bit of cathode bias to the 7591's might also help
with stabilizing the DC operating point as well as adding a little
degeneration (gain, and distortion reduction). This approach might be an
option to regulating both element voltages. I have plenty of headroom with
the electrolytics.
Another option would be to add a multi-tap buck/boost transformer so that I
can accommodate a variety of AC mains voltages, and still get the nominal
6.3 volts ac to the filaments. This need not be large, and could be
installed within the rig. A 12Vct 2A transformer could be used to add 6, 12,
or subtract 6, 12 volts from the AC mains input. It would require a fancy
switch (maybe 2), or network of jumpers to work however.
What I have ended up doing was to install a 5 amp variac to a 6 plug outlet
strip. I am currently running my RCA AR88 receiver, CE 20A, BC-458 VFO, and
a rack mount Kepco 200 volt power supply all off the strip. I set the variac
to 114 volts. The AC drops to 112 with my linear set to 200 watts carrier
output. Now I need to find out about my Gonset GSB-201 linear (4 X 572B),
and determine what filament voltage is on the 572B's. Hey at $50 bucks each
(today's prices) I don't want the filaments burning at 7.0 volts (6.3
nominal??). I will need a 15 amp solution to dropping the voltage to the
Gonset should the filaments be too hot.
Do any of you fine folk have this problem, and if so, how are you dealing
with it?
Jim,
WD5JKO
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