Well im not completley certain - heres some of my maths.

Far as I have read, for Class C operations,

Load Z should = Plate voltage/2 * Plate current

And since I am aiming for plate current of 300mA or so (for power of
300 watts at 1kV) then thats

1000/2*0.3 = about 1500 ohms (R).

Then tank Z should = load Z / Q

For a Q of 7.5, which is slightly below average I think, thats
1500/7.5 which is 200R tank Z.

Then via Xl = 2piFL, Xl is 2 * 3.14 * 25M * 1.3u which = 204R
and
via Xc = 1/2piFC, Xc is 1 / ( 2 * 3.14 * 25M * 30p) which = 212R

So the tuned circuit is going to buzz at a slightly higher F than
25MHz but only slightly, at a Z of about 208R. I think stray values
are larger than the difference here so this is not worth worrying
about.

Thats my understanding of the maths having studied it over the last
few weeks and I am pretty sure its in the right ballpark, although the
Z could be a bit high or a bit low for the tube conditions. But since
the thing buzzes at 25MHz the F is right, and if the Z is a bit off
this just means the Q is a bit off and I understand this is not a
critical parameter for power.

Also to back this up, the tank Z and tank coil and tank cap values are
about the same as you see in published designs for this tube (GI6B),
this power level, and this frequency (eg http://www.nd2x.net/yu1aw/1xgi7-6m-schem.GIF
- its a GI7B but apparently they are almost identical). Ive compared
with a few circuits for these tubes and they are all similar.

Only thing of course is that they all use grounded grid, which is not
anything like the grounded cathode I am using.

Of course if I have my tank Z wrong, this would surely cause power
problems. But as I say, I cant see anything wrong with these values,
but maybe thats my lack of knowledge...

Thanks for the help so far...

On Jul 26, 4:35 am, Doug Smith W9WI wrote:
On Wed, 25 Jul 2007 21:22:56 -0500, Bob Liesenfeld wrote:
Loading the circuit more reduces the pp voltage in the tank. And since
the tank Z is about 200R, p=e(squared)/r so p=1.6 watts in the tank
circuit.

Where are you getting the "200R" value from? (And what is 'R'?)

200R is British for 200 ohms.

(which does seem a bit low for a proper match, for 1kv plate voltage and
~300w RF out shouldn't the tank Z be closer to 3,000 ohms?)

(or is my sleep deprivation showing?)