On 26 Jul, 17:57, Tim Wescott wrote:
Steve H wrote:
wrote:
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 polyethylene 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 extensively, I am at a bit of a loss as to what to do next.
I do apologize 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
With only 1KV on the anode you are unlikely to get more than 40W out.
You need to be nearer to 2.5KV with a lot of cooling to get anywhere
near 400W. Good layout is vital with this tube, they are good to 3GHz
and will take off without some sort of stopper in the anode cct.
Steve H
I was kinda wondering if maybe there wasn't some humongous GHz
oscillation that's taking up all the power, leaving none for HF. As you
point out the best way to check this would be with a spectrum analyzer,
although without it you could maybe slowly increase the plate voltage
and look for odd jumps in grid current or dives in plate current.
I looked at the data sheet and saw two grids -- is this a 'normal'
tetrode? It appeared that they were calling out negative voltages on
grid 2, which confuses me.
--
Tim Wescott
Wescott Design Serviceshttp://www.wescottdesign.com
Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details athttp://www.wescottdesign.com/actfes/actfes.html
I noticed that you do not mention any sort of shielding or shielded
room for this plasma driver. Given that situation, it would probably
not be in the best interest of this group to help you build a high
power oscillator with limited frequency stability and the possibility
of gross amounts of RF escaping into the ether to interfere with other
licensed and unlicensed users of radio spectrum, including ham radio
operators.
Having said that...there were Radio Diathermy devices made in the
1930s & 1940s era that were simply free running power oscillators,
much like what you are talking about. These old-design devices are
now illegal to operate in most countries. Similar designs are still
used for welding plastic assemblies, but these are required to operate
inside a shielded enclosure or in a totally shielded room.
To make something like this you would probably want to start with a
conventional VFO (Variable Frequency Oscillator) of the Hartley type,
then scale it up to run at the power level you want. However, in
order to not run afoul of the law, you might want to build a very
stable crystal controlled oscillator and driver section that would
then drive your power amplifier. There are specific frequencies that
are allocated for use by "incidental radiators" that are part of
industrial equipment. Check with your country's radio spectrum
regulating authority to find out what the legal operating parameters
are for your country & locality.
Arv
_._