I would not like to build a stabilized power supply, as that would be
quite a huge machine. Anyway I shall give a look to the diagram and see
whether there would be any simple way to feed just the oscillator tube
with a separate stabilized HV.

With regard to voice peaks, using big output capacitor should help.

I am more worried about CW because, even with a big output capacitor,
voltage will tend to decrease along the duration of a dash or a dot, and
this may cause chirp.

Going back to my original question, the first test I shall do is to feed
the filaments with a transformer, while keeping the dynamotor for the HV.
I want to be sure that powering filaments on AC (instead of DC) does not
cause an unacceptable hum on the audio.

By the way, I have been looking for a spare VT-25 (10Y) tube, but I could
not find one for less than $70. Apparently that tube is one of those that
had the disgrace to fall into the esoteric hi-fi aficionados circuit, who
are prepared to pay almost any price for the promise of a "warmer" sound.

73

Tony I0JX

Oh, Wow! Been a long time ago, but more than 50 years ago, three or four of
us,
(W7KGV, W7MGG, W7LAN and I, W7LTQ) were tinkering with BC-375's,
the 28 volt version of the BC-191.
The power supply regulation with the dynamotor seemed to be almost
entirely dependant on the DC input to the dynamotor. If one had new, hunky
batteries, with very short (two feet max) very heavy (gauge # 000, maybe)
leads and
a constantly running battery charger, the CW wasn't too bad. HOWEVER,
my batteries were old and weak, dynamotor input leads were doubled up
#10 house wires, and one almost had to copy the transmitted CW with one
hand on the tuning knob. (Yeow, ditty, yeow yeow)
A long dash slowed the dynamotor to maybe half speed, and the glowing
filaments went from white hot to more like cherry red.
Later, an AC power supply was cooked up, using conventional circuitry
of the day, pair of 816 mercury vapors, "swinging" choke input to pi section
filter, maybe 4 microfarad filter condensers, a "smoothing" choke and a
50,000 ohm
bleeder resistor. The '375's filaments were lit with 10 volt AC
(transformer), and a
dropping resistor for the speech amplifier (10Y)'s filament. Seems to me,
there was
something about which side of the 10Y's filament was ground that made a
difference
in the hum level.
With that sucker, delivering a stiff 1200 or 1300 volts on the MOPA's
final VT-4C,
it's plates would turn a soft red blush on long dashes..... and would work
anyone
you could hear!
Old Chief Lynn