Hi ye all,
I feel you are worrying too much about Q and the like. Make it simple,
guys! Start from the basics:
You know the operating frequency, and the total tuning capacitance
value. Take the maximum value, 127pF in this case (200 in series with
347). Calculate the capacitive reactance from this: Xr = 1/(2*PI*f*C).
Now take the maximum RF RMS voltage at the tube plate. It will be quite
close to the DC plate voltage, so for simplicity you might even cheat
and use the DC voltage. Now apply Ohm's Law, and you have the current
through the tuning caps. Compare this current to the maximum current
rated by the manufacturer of the caps (I hope you can find this value!),
not forgetting that you have two in parallel, and you are done.
If the results look very bad on the higher bands, it's because we are
assuming maximum capacitance. Try doing the math again with the actual
tuning capacitance used on each band.
For sure, I would add a healthy safety factor, but that's me. Many
commercial manufacturers don't do it, as they have to watch the cost.
Now the practice: On 18MHz, 127pF has 70 ohms. With 3000V rms of RF,
that would give 42A. Poof...! But if the amp tunes to 18MHz with just
20pF total tuning capacitance, you get less than 7A, which seems fine.
Since the amp anyway won't be able to run with a high tuning capacitance
setting on this band, it's probably OK.
And assuming it needs the full 127pF at 1.6MHz, the reactance there
would be 783 ohms, which gives you a current of 3.8A. Surely that's
fine. But clearly this amp has a low loaded Q on the low bands, unless
additional capacitance is switched in!
So, it seems you amp is designed with a sensible safety factor, but not
going overboard.
The change of coil will not significantly change the current through the
tuning caps, as long as you implemented it with the same inductances as
the original one. If its unloaded Q is higher, as it should, the added
power output will come from reduced dissipation in the coil, and the
LOADING capacitor will be exposed to a slightly higher voltage and might
need significantly different values than before.
Cheers,
Manfred.
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http://ludens.cl
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