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EHT HF/VHF Triode VFO Concept

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Proud Mary

Fri Dec 05 2008, 12:31PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

This thread is for people who would like to put some ideas into the development of a high voltage RF oscillator using an EHT triode valve.

Background: I have a box of PD500* shunt stabilizer triodes (nominally 25kV/1.5mA) and thought to build an EHT HF (or even VHF) VFO to see what it might do.

Questions: What sort of oscillator? As EHT vacuum variable capacitors are way outside the budget, this seems to leave variable inductance tuning - something like a ferrite rod or dust-iron core on a screw drive - as one easy option I can see, but there must surely be others.

The easiest configuration would seem to be a tuned grid with an anode feedback winding, but this is not a very stable circuit.

Miller effect oscillator perhaps? Input gentlemen please!

*Similar to the American 6BK4B TV EHT shunt triode.

GeordieBoy

Fri Dec 05 2008, 01:03PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

> this seems to leave variable inductance tuning - something like a ferrite rod or dust-iron core on a screw drive - as one easy option I can see, but there must surely be others.

The variometer arrangement would let you vary the inductance without incurring core losses.

Proud Mary

Fri Dec 05 2008, 01:41PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Here is the PD500 data sheet:

X-ray enthusiasts sometimes run PD500 at 50-60kV - though they don't tend to last long above 50kV/2mA when they are pulling 100W - 60W more than the design centre rating.

With a Class-C conduction angle, it might be possible to generate 70W or so of 50kV RMS RF at HF and even VHF frequencies.

Two-valve oscillators are also an option, as I have a number of PD500s.

I will look into the variometer idea, though I had always thought of variometers as being LF/MF devices.

Wolfram

Fri Dec 05 2008, 01:46PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

What is your budget? 25kV vacuum variable capacitors seem to go for around 60 dollars for a 5-100pF unit and 50 dollars for a 3-50pF unit on eBay. Is capacitance in a usable range for this project?

Anders M.

Steve Conner

Fri Dec 05 2008, 01:59PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The required capacitance is tiny, so you won't need a vacuum cap. Something like two small metal discs hanging in the air would do.

See our recent thread on characteristic impedance for an idea of why the required capacitance is tiny, and why it won't work at VHF.

Hint #1: What is the characteristic impedance of an inductor resonated by just its stray capacitance? How high is it practical to get this?
Hint #2: What load impedance does the extremely high voltage and low current PD500 triode require?

I'll leave this to our other readers to debate, while I get back to my day job of hitting XScales with a mallet.

Proud Mary

Fri Dec 05 2008, 04:13PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Steve McConner wrote ...

Hint #2: What load impedance does the extremely high voltage and low current PD500 triode require?

Good point!

Proud Mary

Fri Dec 05 2008, 06:00PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Anders M. wrote ...

What is your budget? 25kV vacuum variable capacitors seem to go for around 60 dollars for a 5-100pF unit and 50 dollars for a 3-50pF unit on eBay.

Anders, this little project is only meant as a curious amusement, a bit of fun that I can put together from parts I already have to hand.

If I find that the EHT oscillator has some unusual, exotic , or enjoyable properties worth looking into more, then I might consider buying some expensive parts for it.

As to whether $60 is a lot of money or not depends on where you are in life. It is a lot out of an Old Age Pension in Britain today!

Hazmatt_(The Underdog)

Sat Dec 06 2008, 09:04AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

I would be curious as to how stable you want your output over your sweep range. I know this may be totally impractical, but if you had 3 of those variable caps and stepper motors, I would build a phase shift oscillator. After that I would get an old scope to monitor the output and have a controller adjusting things real time to keep it within your desired range, or sweeping.

Another Idea, perhaps much better, just get a power oscillator going and feed that to your power tube in class C for Hipot output. That's what i'm doing in the near future with 4 6L6's.

Proud Mary

Sat Dec 06 2008, 12:05PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Thanks for your ideas Hazmatt - though I think I shall limit my plan to a single variable capacitor - or none at all.

One possible scheme would be to use two of the valves in a Franklin oscillator - which is a kind of flip-flop where the rate is determined by a single LC tuned circuit.

The idea is to have a bit of HV fun with some cheap parts which I have to hand, developing some circuits in a creative way, rather than embark on some grand impractical design!

Steve Conner

Sat Dec 06 2008, 12:57PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I think the simplest oscillator you could make with these would be a shunt-fed Tesla coil. You'd connect the tube anode to the top of the resonator, and feed something like 25kV DC (from TV flybacks?) to the base. To get voltage gain, you could tap the anode down from the top, maybe halfway up the resonator or something. Grids could be driven from a feedback coil, like in a normal VTTC.

It would probably be limited to 10s or 100s of kHz in order to keep the Z0 of the resonator high enough. Connecting 10 or 20 tubes in parallel would help of course

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