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Rf Power supply @ 4kV

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Ash Small

Thu Mar 14 2013, 10:48PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Noah Hoppis wrote ...

I though an amp would be easier seeing as one could get a RF signal generator at a ham fest for ~20$. I also though that most generators didn't supply both sides of the sine, above and below 0v. would the Hartley provide variability as well? I need to be able to tune the frequency.

As far as I'm aware, all oscillators of this type pruduce an AC sine wave. Maybe this point needs verifying by someone else?

I'm certain you can tune the Hartley oscillator for almost any frequency, but I'm aware that Colpitts oscillators have advantages at some ranges (I have a signal generator with both in, one for the lower ranges, and one for the higher ranges. I'll have to check which is which)

I don't think you need a 1kW supply, though.

The Hartley oscillator above provides everything you asked for at the beginning of this thread, and was designed for a plasma source, so I'm guessing it should be suitable for a cyclotron.

I'm no expert in these matters, as you know, but I'm hoping to learn a lot from this thread too.

I'm hoping others will comment with their opinions. You could use an amp (the Rutgers cyclotron uses a 100W amp), which would work fine, as long as the amp was suitable, but trying to design something from scratch without an EE degree sounds like hard work to me (I can design matching circuits now, thanks to this forum, and I understand the basics of RF amps, and Hartley and Colpitts oscillators, but I'd still want to adapt an existing circuit, rather than starting from scratch, unless I had no choice.)

BTW, the Rutgers matching circuit (for the 9" cyclotron) is a work of art, due to it's simplicity. It is tuned by compressing the 8 turn secondary coil, thus changing it's inductance.

As I said before, I don't think you actually need an RF supply anywhere near as powerful as the Hartley oscillator I linked to.

EDIT: I think, from memory, you could probably use a GU5B, I'm not familiar with the other one you mentioned, but I think, with some modification to the Hartley circuit, you could use a much lower powered (ie, much cheaper) tube. I don't think you need anywhere near 1kW.

I think you should compare these with RF amps, though,

Proud Mary

Fri Mar 15 2013, 10:54AM

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

I have looked at the RF sections of two small cyclotrons, and found both of them configured as follows:

The RF signal was producted by a function generator feeding a 100W RF power amp with output variable across 1 - 20 MHz. This simplifies construction enormously, because the design and construction of a stable RF source variable/selectable over a wide frequency range is a world of its own, with its own people in it.

The designs I looked at both used an off-the-shelf 100W RF amp module, but if you wanted to save money you could build a suitable unit with valves - two 807 beam tetrodes in parallel driven by a 6V6 buffer would be easy and inexpensive to do, providing you can provide the 600V/200 mA anode supply. You will find numerous circuits online if you google HF power amplifier

I would not advise using triodes in the power amplifier, as they have less gain and are much less easy to control than tetrodes or pentodes. I would also avoid directly heated valves (such as 811A) , and choose indirectly heated ones (such as 807) where the cathode is electrically separate from the heaters inside it. Indirectly heated valves don't need their heater supplies to be de-coupled by RF chokes, making it much easier to operate them over a wide frequency range without risk of instability due to choke resonances, and so on.

Noah Hoppis

Fri Mar 15 2013, 11:20PM

Registered Member #10072 Joined: Thu Feb 14 2013, 05:12AM
Location: seattle wa
Posts: 21

Proud Mary you got the nail on the head! I've seen plenty of DIY amps, and I can get a signal source, but I still don't really know what voltage a cyclotron is at. I assumed it was in the low kV. If anyone knows it would be very helpful, as well as any good amp designs for that V, at roughly 1-20 MHz please say so because I don't really know much about the RF for a cyclotron except that it needs between ~1-20 MHz and a large(r) voltage.

Proud Mary

Sat Mar 16 2013, 09:25AM

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

Noah Hoppis wrote ...

Proud Mary you got the nail on the head! I've seen plenty of DIY amps, and I can get a signal source, but I still don't really know what voltage a cyclotron is at. I assumed it was in the low kV. If anyone knows it would be very helpful, as well as any good amp designs for that V, at roughly 1-20 MHz please say so because I don't really know much about the RF for a cyclotron except that it needs between ~1-20 MHz and a large(r) voltage.

The Dee - considered as a lumped constants tuned cricuit - will have a high Q and high impedance, so a high voltage will be developed on it when it is excited at radio frequency. This voltage will increase in proportion to the square root of the RF power applied.

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