Bob Bruhns wrote:
Do whatever works for you.
You may actually get better low-end response with a
smaller cap, because the coupling capacitor and
modulation inductor make a high-pass L-C network that
can have a peak before rolloff, depending on impedance
and part values. This peak can supplement a sagging
low end, although rolloff below the peak will be
faster. Hopefully it will be low enough in frequency
that it does not matter.
A peaky high-pass filter will certainly mess up phase
response, which would affect low-level negative peak
clipping and such. And a high-pass filter will also
leave the modulator unloaded at the bottom end. This
loading issue should not matter much if the modulator
tubes are triodes. If your modulator tubes are
tetrodes or pentodes, you could apply some negative
feedback from the plates or the transformer output, to
lower the effective plate resistance and control the
unloaded output.
Mostly, it's the sound that matters. See what works
best and sounds best. If it doesn't screw things up,
the right capacitor value could add a few dB of
response a little bit above low-end rolloff, and that
could work perfectly well for your voice.
More capacitance will extend the low end response, the
low-end phase response will be better. The slight
low-end peak will be lower in frequency, and it may be
gone altogether. Less capacitance will make low-end
response worse, and the peak would rise in frequency
too. If you want furnace rumble, you might find a
combination of capacitor and inductor values that
produces a resonant peak for you down around 20 or 30
Hz.
Well... only because I'm using what I have available I wonder how the LC
ratio changes, if I'm using multiple inductors in series? Of course,
I'd prefer one 50HY choke at 500mA, but I don't have one of those.
Instead, I've got (3) [EMAIL PROTECTED], and a [EMAIL PROTECTED] all strung in
series. Mutual Inductance calculations? Two of the chokes are potted
and the other two are open-frame. Unfortunatly, the two open framed
chokes are -not- identical.
The total DC resistance is ~300ohms for all chokes in series, and a
total inductance of 46Hy.
The calculations are for 1500VDC @ 300mA in the final.
Final impedeance is therefore 5000ohms. The general rule-of-thumb is
8Hy per 1000ohms Z which is 40Hy. I'm covered there.
Coupling capacitance? This is where I'm stumped, because of the
multiple inductors in series.
---
73 = Best Regards,
-Geoff/W5OMR
