Are vacuums tubes suitable for switching high current (~30amps) at high frequencies (~K-Mhz)?

Question

I was looking to build a system for high currents (say ~30 amps) and a friend suggested vacuum tubes. In theory, vacuum tubes seemed more robust for high current applications, but after looking around, and I found a bunch with relatively low current ratings (~10mA.)

I wanted to avoid mechanical relays if possible because they seemed a poor design for high speeds. I also preferred to use a CMOS type design--my setup is symmetric with respect to p-channel and n-channel MOSFETs (at least in my circuit simulator.) I noticed some vacuum tubes have a dual gate--which fits perfectly into my design. The question is whether they can handle these currents at high speeds.

My voltage should be quite low ~50V.

EDIT: I am designing an induction heater, but I am trying a different approach in which I treat the heating element and the object to be heated as a black box, and use the proper switching elements to counter whatever voltage the black box produces. That is the basic idea. There is obviously more to it (e.g. have the switch on only after a threshold voltage, and off beyond another threshold voltage that would damage the circuit.) Anyway, it was just something I wanted to try, and thus far, it seems like a relatively cheap project considering I can get a lot of parts from old stuff at yard sales, ebay, etc.

Answer 1

A very short answer with not too much detail goes like this:

• Tubes are good at medium to high voltage and low current.

• Transistors (especially MOSFETs) are good at low to medium voltage and high current.

Exceptions exist, but for the common and most used examples of said parts, the above... ahem... selection guide is a start.

For fast (> 10 kHz) switching applications based on tubes, you may want to look into (literature for) horizontal deflection circuits in old (pre-1970) TV sets.

Answer 2

Tubes exist which can be used to switch very high currents and voltages. You can do a search on hydrogen thyratron as one example. Thyratrons tend to be used in laser, radar and other relatively exotic applications, but have also been (ab)used by Tesla coil fans.

They work more like SCRs than conventional transistors. Once triggered, they remain in the 'on' state until something breaks the circuit.

Answer 3

In response to your goal of making an induction heater, wikipedia says that vacuum tubes were used in induction heaters until the early 1990s.

No, tubes are not great at switching 30A but this is not necessary as a transformer can (and was) used to drive the output coil.

http://en.wikipedia.org/wiki/Induction_heater

Answer 4

If you look at the RF power amplifiers used by radio and TV stations you will find tubes capable of handling kilowatts of power, so the short answer is that vacuum tubes can handle high power and high speed.

That having been said, that does not mean that they are the only way to go, or that they are appropriate for your application. You would need to describe in more detail what you are hoping to accomplish.

EDIT: Based upon the additional information you added to the question.

You state the application relates to inductive heating. Microwave ovens use vacuum tubes, specifically a cavity magnetron to heat the food. Microwave ovens operate at a frequency of 2.45 GHz, which would qualify as high speed by most standards.

So it is safe to conclude that vaccum tubes are in widespread use for dielectric heating.