Hi,

I am trying to build a tube power oscillator running at 25MHz from a
Russian tube (GI6B), and although I have it running, I can't seem to
ramp up the power to anywhere near the level I want out of it. I am
aiming for 200 to 400 watts, but I seem to be running at just a
handful of watts. Everything I have tried that I assumed would control
this parameter seems to have failed to work...

The RF is needed not to transmit but to drive a plasma, but for now I
just want to get the tank coil running the right voltage level. But
with a B+ of about 1000V, I am only getting about 50 volts in the tank
coil.

Circuit diagram is available here http://geocities.com/peterpion/oscillator.jpg

Parameters a
Voltage = 1000V, full wave rectified and smoothed
Tank coil, 5mm copper tube, 7 turns, 5cm diameter, 7cm long, 1.3uH
Tank Cap, air dielectric, 5mm spacing, 13cm square, 31pF, 1.6mm thick
copper plates
Tank circuit is capable of taking at least 100A, very heavily soldered
Feedback coil, 5 turns 2mm copper wire, same diameter as tank coil, 5
cm length, placed on same former with gap of 3cm between tank and
feedback coil.
B+ choke 120 turns on 32mm diameter former
Bias, external 12V supply, fed via a diode to the ground side of the
grid coil, with a 33k resistor going to ground from the same point,
bypassed by a 1nF cap.

Although I have it running, both increasing or decreasing the feedback
turns reduces the tank voltage.

The tube spec is 350W anode dissipation, mu of 18-26, 2.5KV max, but
with heavy grid current requirements it seems, although I am not quite
clear on this.

http://www.nd2x.net/gi6b.html

Ive never built anything like this before, and I am lost as to where I
am going wrong. Is it that the tube has too little gain to drive
itself up to those power levels? I assumed that the low gain would
just mean that I had to couple more power back to the grid to make it
work. But if I increase the turns on the feedback winding, it actually
reduces the power - as does reducing the turns. 5 seems to be
optimum.

I have taken care to keep all leads short - very short (1 to 3 inches
mostly). Rats nest construction. Have tried several types of caps, all
apparently rated for heavy RF use. Even rolled my own caps from copper
sheet and polyethelene which work the same as the other caps.

On the scope, I can see that I never drive the grid into positive.
This seems wrong to me, but nothing I do makes it go positive. On the
plate, all I see on the scope is a tiny 50v drop (at 25MHz) when the
tube is supposedly hard on, ie the tube at the most on condition
through the cycle, still has about 950V across it. So I assume that I
am not turning the grid on enough.

I am wondering if the problem is gain, IE is there too little gain to
make it work properly? Do I need to add another tube in a darlington
type of arrangement to add enough gain to make it work better?

I want this type of free running oscillator because it should stay in
tune as I add small amounts of reactive load (the plasma during warm
up etc) - or at least thats what I believe, not being an RF guru I am
not certain of this.

If anyone can help me I would appreciate it very much. Having spent
the last 2 weeks trying, and read everything I can and searched the
web extensivley, I am at a bit of a loss as to what to do next.

I do apologise for probably stupid mistakes / questions (you cant do
that! you need a driver stage...? who knows!) but I am sure someone
must have done this before so I appeal to them to share their
experience!

Thanks, Pete