Electronics > RF, Microwave, Ham Radio

Looking for advice to create/implement "grid driver" for vacuum tube RF PA

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wb0gaz:
I have a need to implement a gate driver-style circuit that will match 3V logic level (square wave) signal to the voltage and charge/capacitance of a vacuum tube (specifically, a triode or pentode) used in common cathode (i.e., grid driven) configuration as an RF power switch (it will end up being an RF generator, whose output is dictated by logic square wave from a simple synthesizer IC.)

This is an initial exploration (so I'm looking for general concepts and possible resources) as the tube and operating conditions have not been selected. Operation will be up to 50 MHz, input (grid to all other element) capacitance probably tens of pF range, and there may be some grid current drawn during positive-going part of the cycle (probably no more than tens of milliamps.) RF voltage as "seen" at the grid would be as much as a few hundred volts peak-to-peak.

The (likely) required RF voltage seems to make an IC gate driver impractical (typical FET gate driver is called on to drive at most tens of volts peak-to-peak, but with a much higher gate capacitance/charge and no expected gate current), but I may be searching improperly.

Grateful for any suggestions, ideas as to where to look for readings or circuit examples, and of course any requests for clarification of problem statement.

Thank you!

Andy Chee:
Maybe you can adapt this circuit topology to suit your application?

https://sound-au.com/valves/design.html#s10

Roehrenonkel:
Hi wb0gaz,
 
maybe a step-up transformer would work (in a limited frquency-range) for your application.
What tube (hundreds of volt grid-drive) =8-O have you in mind?
What anode-voltage? Kilovolts even?

Take care, good luck

wb0gaz:
Thanks for the replies!

Andy - the linked article, while focusing I think on audio applications, is worth studying - PNP/NPN complimentary pair is what I've seen in some gate driver schematics, but I wasn't at all sure how to drive the pair when originating from the 3.3V synthesizer output. There are presumably "dead time" issues that need to be addressed, so that the two output devices don't try to be in conduction or transition at the same time, and the device selection would need to be revisited due to the ~1000-10000X higher operating frequency.

Roehrenonkel - you hit on a "unstated requirement" error in the original posting - I would want the grid driver to be functional across a fairly wide spectrum (at least 1.8 MHz to 50 MHz - single tone), so that would seem to rule out tuned circuit elements in the grid driver signal path. As for device(s) being driven - at the outset thinking 6146(s) or sweep tube(s) (single ended, parallel as needed, not push-pull), so 600-1100V on plate(s), with attendant fixed regulated, current limited screen voltage as needed.

In a nutshell, I'm interested in whether a single-ended FET "class E" type output stage - imagine a IRF510 switching FET being fed by a garden-variety gate driver at RF - could be implemented with vacuum tube(s), provided that a suitable gate driver (grid driver) could be realized.

Again, thanks for both replies - both very helpful and definitely press the idea forward!

Dave

mag_therm:
Hi wb0gaz,
I have a homebrew Linear amp 45Watt for Ft8 , up to 14MHz
It uses high voltage SiC Mosfets with a drain voltage supply of 200V.
The drive boards are surface mount wideband amplifiers using "upside down " complementary BJT pairs like Fig9 in Andy's link
The RF circuit is more minimal than Fig9

I am presently building another set of boards slightly modified for 21 MHz  with intention of using sweep tubes in place of the Mosfets.
While hoping to keep the amplifier wideband (untuned), at >= 21 MHz neutralization of Ccb may be necessary on the driver output pair.

Looking at 6JS6 which are low cost compared the the similar, more popular 6JZ6, I see that Compactron tube bases may be coming hard to find.
I think with transistor drive it is a good idea to use tubes with top-cap anodes so the drive boards are physically spaced from the output.
6JS6 data sheets show a grid Eg of -30V for near cutoff, so peak grid drive would be about 28V pk to avoid grid current.

My circuit is push pull . One reason is the FCC out-of-band limit of -43dBc; P-P has inherent reduction of the 2nd harmonic.
I have adjustable bias for each side which allows trimming to minimize the 2nd Harmonic.
If the wideband amplifier inherently has high 2nd harmonics, the output filter becomes complicated and more lossy.
I presently have a surface mount double Tee constant K output filter which is not intended to attenuate the 2nd, it does mainly the 3rd and 5th.

Happy Toobing...
regards
KE8UZF

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