Tube Power Oscillator not working properly
 

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

Tube Power Oscillator not working properly
 

I think the fact that you do not have a load on the output tank, is what is
limiting the power output. Power tubes require a particular load Z to
deliver 'rated' power. Right now all I see is a parallel LC tank which will
be high Z. You quote a voltage of 50V "in the tank coil". How are you
measuring this?
Can you monitor plate current? If so, that times the applied plate voltage
will give you your DC input power. Power out will always be less than
this amount.
Also, are you sure your HV supply has the current capacity for the 200-400W
you want to get out? (This may mean something like 300-600W, or more, DC
input power).
Are you taking steps to prevent radiation of the generated RF into the wider
environment? The level of power you are talking can reek havoc with other
licensed users. The FCC could pay you a visit.
Finally, BE CAREFUL! These DC voltages can kill, and you have not lived
until you have gotten an RF burn from 100W, much less 300-400W.

Bob WB0POQ
wrote in message
oups.com...
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.

Tube Power Oscillator not working properly
 

Hi Bob thanks for the attention,

Re the load, I am using a soft iron 'poker' which I bring close to /
into the tank coil to provide a load which I can easily tell when
power is delivered into. So I fire the circuit up with the scope
connected to a certain point, then increase the load by bringing the
poker closer to or into the coil. The iron looks like a very close to
resistance load, the Z varies with the proximity. So I cat test all
loads from high Z to a few ohms. If the iron gets hot, I am delivering
a decent amount of power - got some decent power with a circuit like
this running at 7 MHz with the iron getting very hot.

While doing this I monitor the voltage on the plate or tank coil with
a scope. So the 50V is the best peak to peak voltage across the tank
coil.

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.

Plate current is around 100mA but this is mostly dumped into the anode
as heat.

PSU is half amp MOT plus variac, bridge rectifier and 10uF cap. Shunts
are removed from MOT. Plenty of power available, about 1kW.

As for shielding, when running at full power the whole thing will be
screened including the load. Till then though, the tops off although I
dont think that much will radiate from the exposed leads even when the
power gets up to decent levels.

As for RF burns, yes I am rather nervous about that too... I am used
to working around up to about 20kv but this RF angle is a new thing,
so I am being cautious. Painful experience is the best way to learn
your lesson and I have had it :-(

Anyway thanks for the suggestions.



On Jul 25, 10:09 pm, "Bob Liesenfeld" wrote:
I think the fact that you do not have a load on the output tank, is what is
limiting the power output. Power tubes require a particular load Z to
deliver 'rated' power. Right now all I see is a parallel LC tank which will
be high Z. You quote a voltage of 50V "in the tank coil". How are you
measuring this?
Can you monitor plate current? If so, that times the applied plate voltage
will give you your DC input power. Power out will always be less than
this amount.
Also, are you sure your HV supply has the current capacity for the 200-400W
you want to get out? (This may mean something like 300-600W, or more, DC
input power).
Are you taking steps to prevent radiation of the generated RF into the wider
environment? The level of power you are talking can reek havoc with other
licensed users. The FCC could pay you a visit.
Finally, BE CAREFUL! These DC voltages can kill, and you have not lived
until you have gotten an RF burn from 100W, much less 300-400W.

Tube Power Oscillator not working properly
 

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'?)
Another thought is to change the L/C ratio which should give a different
loaded Q with a given poker position. This may provide more power out.

Tube Power Oscillator not working properly
 

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?)

Tube Power Oscillator not working properly
 

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

Tube Power Oscillator not working properly
 

Also... borrow a GHz Spectrum Analyser and chech that the unit is
running at the expected freq. What bandwidth is your scope ?

An anode APC is also a very good idea. A few turn of 16AWG around a 2W
4R7 maybe a good start.

Any chance of cranking Va up to 1500V just to see how much power
increases ?

Don't forget a fresh change of underwear and a fire extiguisher :)

Regards,

Mark


On Jul 26, 8:53 pm, 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 herehttp://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- Hide quoted text -

- Show quoted text -

Tube Power Oscillator not working properly
 

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?)

Tube Power Oscillator not working properly
 

Thats interesting - whats a stopper in the anode circuit? The
combination of a resistor and an inductor? And if the tube does take
off at 3GHz, will I get fried by the microwaves??!

Regarding good layout, what should I be watching out for? Is this just
a matter of short leads, or do things like the position of the tank
coil relative to the tube matter?

Re voltage, I have cranked this up to 2.2kV (its on a variac) with no
extra joy - the tube gets very hot very quick (of course) but the tank
circuit dosent get any more voltage across it.

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

Tube Power Oscillator not working properly
 

GHZ spectrum analyser would be a nice bit of kit to borrow, but I dont
know anyone with one - which is a pity. The scope is 40MHz so 25MHz is
pretty close to its max and the resolution is not the best. Waveform
is fairly symmetrical so I think the fundemental is right at 25MHz.

Ill do the APC tonight, what difference will it make?

As for 1.5kV - yup, I have had it cranked to full, 2.2kV, and no extra
power really (maybe a slight amount, but I am looking for 1kv swing
where I am only seeing 50v so its not even close). Underwear got
messy... makes the hair on the back of my neck stand up from fear to
be that close to a 10u cap charged to 2.2kv...!


Also... borrow a GHz Spectrum Analyser and chech that the unit is
running at the expected freq. What bandwidth is your scope ?

An anode APC is also a very good idea. A few turn of 16AWG around a 2W
4R7 maybe a good start.

Any chance of cranking Va up to 1500V just to see how much power
increases ?

Don't forget a fresh change of underwear and a fire extiguisher :)

Tube Power Oscillator not working properly
 

The APC is intended to be resistive (damps Q) at high freq and very
low reactance at operation freq. I should have mentioned that the 4R7
should be a carbon resistor (not wire wound).

I hope that the anode isolation inductor is mounted away, so can't
form part of the tuned circuit... What value is the inductor you are
using ?

Just for getting the anode tuning sorted out, have you considered
driving the grid with a crystal oscillator source and a suitable
buffer - this splits the problem in two. A few volts of drive should
be a good start, maybe a CMOS oscillator driving some paralleled
74AC04 buffers, that will give you 5V P-P drive into a 1:2 torridal
transformer, 10V P-P may be useful dive ?

Just out of interest, why haven't you gone for Grounded Grid
configuration ?

-Mark

On Jul 27, 1:15 am, wrote:
GHZ spectrum analyser would be a nice bit of kit to borrow, but I dont
know anyone with one - which is a pity. The scope is 40MHz so 25MHz is
pretty close to its max and the resolution is not the best. Waveform
is fairly symmetrical so I think the fundemental is right at 25MHz.

Ill do the APC tonight, what difference will it make?

As for 1.5kV - yup, I have had it cranked to full, 2.2kV, and no extra
power really (maybe a slight amount, but I am looking for 1kv swing
where I am only seeing 50v so its not even close). Underwear got
messy... makes the hair on the back of my neck stand up from fear to
be that close to a 10u cap charged to 2.2kv...!



Also... borrow a GHz Spectrum Analyser and chech that the unit is
running at the expected freq. What bandwidth is your scope ?

An anode APC is also a very good idea. A few turn of 16AWG around a 2W
4R7 maybe a good start.

Any chance of cranking Va up to 1500V just to see how much power
increases ?

Don't forget a fresh change of underwear and a fire extiguisher :)- Hide quoted text -

- Show quoted text -

Tube Power Oscillator not working properly
 

wrote in message
ps.com...
Thats interesting - whats a stopper in the anode circuit? The
combination of a resistor and an inductor? And if the tube does take
off at 3GHz, will I get fried by the microwaves??!

VHF oscillations will develop extremely high RF voltages since the
tank coil will look like a high impedance RFC and there will be
no load. Expect to see a lot of arcing.

Pete

Tube Power Oscillator not working properly
 

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 Services
http://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 at http://www.wescottdesign.com/actfes/actfes.html

Tube Power Oscillator not working properly
 

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
_._

Tube Power Oscillator not working properly
 

Anode choke is about 1.3mH, but its not mounted away, its pretty
close, about 2 inches, although its in the opposite plane to the tank
coil. Is that OK?

As for driving with a fixed generator first, it would be a good thing
to try I must admit - but its a big effort. I have no geny that will
go to 25MHz and to make one capable of providing the 20 watts or so I
would need would put me considerably back - I know shooting straight
for the goal in a case like this is harder of course, but seeing as I
have the thing running already at low power, I am hoping still...

As for the 'two grids' this seems to be bad documenting (the original
russian docs I think). There is definatly one grid - the tube has a
single grid between a pair of connections for the heater, one of which
is commoned with the cathode, and also a plate connection of course
(the heatsink). The docs showing 2 grids confused me but I assume that
its just that they couldent be bothered to redraw the picture! As I
say, theres simply not enough connections... Unless theres something I
dont know about here?

Re is it buzzing at UHF and I simply cant see it I did wonder that
too. But I guessed that there would probably be signs, like more heat
in certain caps, and corona and such. But I cant discount it. I guess
I could test this with a filament lamp indicator or similar.

And as for ARVs concerns, quite right to inquire. The test design is
currently as screened as is practical, mostly boxed in an aluminium
box, but the base is off, which is where the grid circuitry is. I
think that at this level, there is negligable radiation from this side
of the circuit, but at the desired power, maybe there would be some.
However the base will be on as soon as I have it working properly, as
much for the sensitive electronics I have in the workshop as for
reduction of spurious emissions. The several computers I have have
demonstrated their sensitivity to RF at a low power level by all
deciding to wipe their hard disks when I used a new HF striking TIG
welder a while ago - a sad incident as I am sure anyone who has gone
through the same thing will agree.

When in final use, the output will be capacitvley coupled to a quartz
glass tube that passes through the aluminium case. Gasses at low
pressure will pass through this, be excited by the discharge and will
glow, and this glow is viewed through a wire mesh screen for
spectrographic uses.

I may need to add a second outer case depending - I am unsure yet
whether induced currents in the case will reradiate a significant
amount of RF or not, so we will have to wait and see.

As you say, one option would be to fix the frequency and then tune
with a cap - the main reason I dident shoot straight for that is that
in the UK you cant radiate anything at all, so irrespective of my
chosen frequency, I have to screen to a high degree of attenuation.
And now that I am begining to think that I need to include a variable
cap to tune the grid circuit to the tank (this is my latest hypothesis
why I am not getting the power I need), there seems to be no
disadvantage to fixing the frequency. So we will see.

Could anyone comment on that idea by the way? That perhaps, the
capacitance between the grid and plate or cathode is resonating with
the feedback coil, but is tuned to a different frequency to the main
tank, and so limiting my drive? I did wonder about changing the Q of
the tank to make the amp more broadband, which I think would reduce
the impact of such a difference in frequency. Also I could try adding
a tuning cap to the grid coil to tune it to the tank coil maybe.



On Jul 26, 7:58 pm, MarkAren wrote:
The APC is intended to be resistive (damps Q) at high freq and very
low reactance at operation freq. I should have mentioned that the 4R7
should be a carbon resistor (not wire wound).

I hope that the anode isolation inductor is mounted away, so can't
form part of the tuned circuit... What value is the inductor you are
using ?

Just for getting the anode tuning sorted out, have you considered
driving the grid with a crystal oscillator source and a suitable
buffer - this splits the problem in two. A few volts of drive should
be a good start, maybe a CMOS oscillator driving some paralleled
74AC04 buffers, that will give you 5V P-P drive into a 1:2 torridal
transformer, 10V P-P may be useful dive ?

Tube Power Oscillator not working properly
 

As long as the mH choke is mounted at 90 degrees from either of the
other inductors, you should be OK.

I am trying to think of ways to move you away from you current
stalemate, hence the suggestion about using an Xtal osc for drive. Do
you have access to a CB rig, ham rig, something that you can derive a
few watts drive from, just to see if the unit is tuning as expected ?

I am quite intrigued about your 12V supply and diode to the Grid. It
looks like the 33k should normally be at 0v and any -ve RF excursions
are clamped at -12v, is that correct/what you intend ? Might be worth
sticking a 100uA meter between the 33k and ground to allow you to
check for grid current.

I do like the idea from one of the other contributors to check for
jumps and hysteresis as you are cranking up Va while monitoring Ia.
The two should move smoothly together, jumps are a possible indicator
of instability.

I think 13.56Mhz may be the ISM band of choice if it is available in
the UK, why did you choose 25Mhz initially ?

Any chance of pictures of the current hook up ?

Have you considered grounded grid ? the cathode coupling loop might
only need one turn.

Is this a college/uni project ?

Regards,

Mark

On Jul 28, 12:10 am, wrote:
Anode choke is about 1.3mH, but its not mounted away, its pretty
close, about 2 inches, although its in the opposite plane to the tank
coil. Is that OK?

As for driving with a fixed generator first, it would be a good thing
to try I must admit - but its a big effort. I have no geny that will
go to 25MHz and to make one capable of providing the 20 watts or so I
would need would put me considerably back - I know shooting straight
for the goal in a case like this is harder of course, but seeing as I
have the thing running already at low power, I am hoping still...

As for the 'two grids' this seems to be bad documenting (the original
russian docs I think). There is definatly one grid - the tube has a
single grid between a pair of connections for the heater, one of which
is commoned with the cathode, and also a plate connection of course
(the heatsink). The docs showing 2 grids confused me but I assume that
its just that they couldent be bothered to redraw the picture! As I
say, theres simply not enough connections... Unless theres something I
dont know about here?

Re is it buzzing at UHF and I simply cant see it I did wonder that
too. But I guessed that there would probably be signs, like more heat
in certain caps, and corona and such. But I cant discount it. I guess
I could test this with a filament lamp indicator or similar.

And as for ARVs concerns, quite right to inquire. The test design is
currently as screened as is practical, mostly boxed in an aluminium
box, but the base is off, which is where the grid circuitry is. I
think that at this level, there is negligable radiation from this side
of the circuit, but at the desired power, maybe there would be some.
However the base will be on as soon as I have it working properly, as
much for the sensitive electronics I have in the workshop as for
reduction of spurious emissions. The several computers I have have
demonstrated their sensitivity to RF at a low power level by all
deciding to wipe their hard disks when I used a new HF striking TIG
welder a while ago - a sad incident as I am sure anyone who has gone
through the same thing will agree.

When in final use, the output will be capacitvley coupled to a quartz
glass tube that passes through the aluminium case. Gasses at low
pressure will pass through this, be excited by the discharge and will
glow, and this glow is viewed through a wire mesh screen for
spectrographic uses.

I may need to add a second outer case depending - I am unsure yet
whether induced currents in the case will reradiate a significant
amount of RF or not, so we will have to wait and see.

As you say, one option would be to fix the frequency and then tune
with a cap - the main reason I dident shoot straight for that is that
in the UK you cant radiate anything at all, so irrespective of my
chosen frequency, I have to screen to a high degree of attenuation.
And now that I am begining to think that I need to include a variable
cap to tune the grid circuit to the tank (this is my latest hypothesis
why I am not getting the power I need), there seems to be no
disadvantage to fixing the frequency. So we will see.

Could anyone comment on that idea by the way? That perhaps, the
capacitance between the grid and plate or cathode is resonating with
the feedback coil, but is tuned to a different frequency to the main
tank, and so limiting my drive? I did wonder about changing the Q of
the tank to make the amp more broadband, which I think would reduce
the impact of such a difference in frequency. Also I could try adding
a tuning cap to the grid coil to tune it to the tank coil maybe.

On Jul 26, 7:58 pm, MarkAren wrote:



The APC is intended to be resistive (damps Q) at high freq and very
low reactance at operation freq. I should have mentioned that the 4R7
As long as the

should be a carbon resistor (not wire wound).

I hope that the anode isolation inductor is mounted away, so can't
form part of the tuned circuit... What value is the inductor you are
using ?

Just for getting the anode tuning sorted out, have you considered
driving the grid with a crystal oscillator source and a suitable
buffer - this splits the problem in two. A few volts of drive should
be a good start, maybe a CMOS oscillator driving some paralleled
74AC04 buffers, that will give you 5V P-P drive into a 1:2 torridal
transformer, 10V P-P may be useful dive ?- Hide quoted text -

- Show quoted text -

Tube Power Oscillator not working properly
 

Sorry for the hiatus, I had an exam to submit over the weekend and
work is being a time hog at the moment.

Re something with a few watts of drive, I do have a couple of ham
friends, who might be able to provide this if I brought the project to
them. Only problem is that the power supplies et al are spread over
the workshop at the moment, but this might be a good route if I cant
get any further. It has occured to me that I might have broken the
tube, melted the grid or something, so this could prove the tube
works.

I have studied up on the tube capacitances and the interaction in the
circuit, and decided to rework the project into a different design -
either hartley or colpitts. Have come across a patent also that
describes exactly what I want to do (tube amp for plasma excitation),
and they use a modified hartley. But looking at the colpitts, I like
the way that the tube capacitances are in parallel with the tank caps.
They should swamp the tube values I think.

Re the 12V supply, the idea is that the resistor biases the grid (at
DC) to ground, the cap smooths the current through the resistor, and
as electrons flow into the grid and make it more negative, at 12V the
diode starts to conduct and clamps the DC grid bias at -12V. Of
course, the tube end of the grid circuit is being driven high and low
by the coil, so this shouldent clamp the grid voltages - just the grid
DC level (bias). Thats the idea anyway - might be missing something. I
have checked grid current and it peaks at about 50mA with the voltage
cranked up to about 2kV IIRC.

Re choosing 25MHz, I originally was shooting for 50MHz but had no joy
getting the circuit to work. It worked great at 5 MHz or so, so I felt
a compromise would be 25MHz. So thats where I am currently aiming for.
The higher the frequency the less reactance I will get from the 'load'
if I drive it capacitvley which is what I want to do (because of
striking the discharge, which is difficult to do inductivley). There
is a need to seal the plasma tube for certain experiments to raise the
pressure in it, and some of the substances I wish to excite are not
compatible with metals (an example is sulphur, which is a major part
of the experiments, and its performance is described in various
patents - its usually excited with microwaves in the current art, but
I want to use very small discharge chambers which make this hard) so I
cannot have any metal in the discharge envelope (I also have the kit
onhand to make these quartz glass capsules).

As you say I have heard of 13.56MHz also as a standard frequency for
this kind of work, but I am aiming higher for the above loading
reasons.

Now, as for grounding the grid, I have considered it, but have not
come across any designs of oscillators where the grid is grounded, and
I dont have enough confidence in my circuit redesign skills to work
out the details myself. I could of course work out the circuit, but
what Z will be where, and how is this related to that phasewise - an
example is the increased capacitance from the anode to the case (which
is now the grid). Just dont know.

I will get some photos of the rebuilt thing once its a colpitts, I am
also replacing the caps with some homemade ones made from heavy copper
foil, since I am a bit unsure of the quality of the russian doorknobs
I am currently using.

Finally, is this a uni project - well, not yet, but I am a physics
undergrad and am studying some related fields, so you could say its
related. It also relates to some harder core physics experiments I
want to start soon, with a cyclotron being one project, which I need
powerful RF drive for. Its all great fun - unless you cant get your
damn RF power house to work!

Current interest for this project relates to finding a discharge lamp
(high intensity) whos spectrum compliments the zooanthellae
photosynthesis absorbtion of marine corals. I could write a ream about
this but its off topic so I will leave it at that for the moment. But
its a means to an end currently - although I would like to have a
power oscillator capable of spanning 1 to 100 MHz at several hundred
watts for plenty of other experiments.

Thanks for the contributions so far. Hope to nail this one soon.

Tube Power Oscillator not working properly
 

wrote:

Hi,

I am trying to build a tube power oscillator...

snip

Although very incomplete and of low visual resolution, schematics
and cabling information for the V1's radio are at:

http://www.zenza.se/vw/v1_radio.html

Its power oscillator, running with 1kV on the plate, is at:

http://www.zenza.se/vw/gh%20bilder/gh512a_b.jpg

It appears to use interelectrode and stray capacitance to
resonate the tank; no component values are given ;(

Regards,

Michael