Thyratrons

[bst]

Over the last couple of years, my compulsion to "rescue" large boxes of vacuum tubes from estate auctions and garage sales has led to the accumulation of many so-called "useless" tubes. In addition to the ubiquitous 6AL5s (those things seem to multiply like wire coat hangers), I've acquired numerous thyratrons.

Some are small miniatures, some are lovely ST-12 and ST-16 octals, and some are huge industrial units almost as big as my forearm. Most of these are NOS tubes in their original boxes, and it would be gratifying to find a practical use for these in tube audio projects.

I've heard of people experimenting with these as rectifiers and shunt regulators, and they could possibly be used as crowbar protection elements for output stages. In a wild flight of fancy, I've even considered that they could be used to build an all-tube SMPS.

From what I've read, however, these things are heater-current hogs, and prone to generating LOTS of noise. Is there any real, non-gimmicky use for thyratrons in a tube audio amp, or should I just get rid of them?

[AuroraB]

Thyratrons are the tube equivalent of a thyristor - usually used as switches in radars, pulse lasers etc. Unless you need a high speed kVolt switch, they have no use in audio, I'm afraid.....

[Enzo]

We used to use 2050 and 2D21 in Seeburg Juke Boxes. They are the tube equivalwent of a triac and used in the same way. In this case to energize a 120VAC relay coil.

I suppose you could use one to turn a speaker relay on. Or a fan controller.

I cannot imagine an actual practical use.

[wrenchone]

People use them to make relaxation oscillators for vacuum tube synthesizers. Apparently, they are pretty stable using the right circuit.

[Algar_emi]

The thyratron we used to have in our radar contained beryllium, a very toxic, nasty stuff if release in the air. We had warning sign all over the place at the radar site. Good thing we got rid of them. Now our new radar transmitter is using HV IGS modules to switch the HV to the magnetron. Much safer, if not more reliable. Be careful with these tubes!

[Sch3mat1c]

Most thyratrons are crummy switches, actually. Only hydrogen thyratrons are fast enough for pulsed radar service, with numbers like 20kV switched to 50A in under 0.1us.

Regular xenon or mercury thyratrons, for power rectification/switching duty, are much slower, typically several microseconds turn on, and deionization time around 1ms. Voltage drop is usually around 20V.

As for audio, you'll have quite some trouble building a PWM amp, although with hydrogen thyratrons you might get away with 50kHz with limited modulation (10-90% let's say; you'll never commutate reliably on the edges of conduction when one tube only has to fire for nanoseconds). Regular xenon and mercury thyratrons will do an excellent job at line frequency, and could be used to make a stabilized voltage source. Take this solid state circuit for example:

http://webpages.charter.net/dawill/I...e%20Supply.gif

You could replace the 5U4 (or something much beefier!) in your amp with a pair of thyratrons, triggered by a simple variable timer (this can be just a few tubes, even building it out with all the comparators as shown), and controlled by an error amplifier and voltage reference. The load must be choke-input, and since it's still full wave rectified line voltage, you still need as much filtering as ever. So it's not like an electronic regulator, where the voltage is stabilized with high bandwidth (out to 10kHz+), you need lots of filtering to get that clean. The DC value (low bandwidth, under 50Hz let's say) can be regulated as accurately as ever, though.

I wouldn't worry about RFI, at least after taking suitable precautions. Power thyratrons switch slowly (microseconds), so they'll make about as much noise as an ordinary diode will. If low-level circuits start buzzing, some hash chokes and Y-type capacitors can be added to the AC winding, and a series hash choke placed in front of the (fairly capacitive) iron-cored choke. RC or RL snubbers can be added at strategic points if more is necessary, and let's not forget screening around the thyratrons themselves.

Tim