Thyratrons are a class of electron tube that are gas filled, and specifically made to switch either ON or Off. They have been replaced by modern thyristors, such as TRIACs and SCRs. They fall into two groups: (1) Cold cathode, often referred to as gas triggers, and (2) hot cathode, just referred to as thyratrons.

Hot Cathode Thyratrons:



Above is a sampling of small to medium sized thyratrons. These can pass significantly more current than vacuum tubes, but the larger ones require significant filament power. Below is a table of specifications:



Unlike TRIACs, thyratrons can only conduct in one direction, and act more like SCRs. One of the key limiting specifications is plate current, where both Peak and Average values are given. For devices intended for continuous current flow, as opposed to pulse operation, the peak value is usually not more than 10x the average rating. The following is a table of loads that can be driven by common thyratrons:


On the right are nominal power values, if the load (such as a light bulb) was hooked up directly to a full wave AC source. Its nominal Hot resistance is determined, and entered on the table. Note: this is the Hot resistance, and will be several times what is measured with an ohmmeter. This represents the true load, when the load, is operating at a raised temperature. From that resistance the peak current, and therefore the average current (1/pi of the peak), can be determined. In the comment field, either the part number of a specific thyratron, or the load type, is given.

Thyratron Color Organ

The tube circuit, that made a bad example, that carried on to the state solid circuits in the 60’s and 70’s. No TRUE phase control ! And all this time I’d been blaming it on the Hippies ! (Click on magazine cover to download PDF article – 1.7MB):


The circuit, in the article, above, makes a half hearted attempt at phase control. The thyratrons used are directly heated, and the AC fluctuation filament windings, are used for rough phasing. The problem is that only works for a quarter of the wave. plus the transformers used, are underated, and deliver inadequate current, which you should never do with thyratrons. Especially, with thyratrons passing a significant amount of power.

A Proper Color Organ Circuit:



(Click on drawing, above, to enlarge)

This circuit works similarly, to those I designed for modern TRIACs. The AC line is used to generate a synchronized ramp pulse, which rises continuously, during the entire active half wave, where the thyratron may switch ON. The earlier in the wave, the brighter, the driven lamp load. This ramp is compared to the audio signals “envelope”. The V2B triode stage, is hooked up as a voltage comparator. When the audio signal (VA) is driven more negative than the ramp (VR), V2B, will turn OFF, and its plate voltage will go positive. This will turn ON the thyratron. V2B is ON most of the time, presenting a negative voltage to the thyratrons control grid, keeping it OFF. The louder the audio, the more negative, will VA be, it will cross VR earlier, in time, increasing the thyratons ON time.

Auxiliary info, for multiple channels and power supply. (click on drawing to enlarge):


This proves, that a proper color organ circuit could have been made, well inside the tube era. The tubes, used in this circuit, are all miniatures, which first came out in 1940. But thyratrons came into existence early in the tube era, so this circuit could have been made as early as the late 20’s.

(Click on drawing, below, to see video of the circuit in action):


Unlike modern thyristors, hot cathode thyratrons have very sensitive grid triggers. Known as critical grid current, its a very low value. Typically under 1 uA, and rarely over 10uA. SCRs usually need a few 100uAs, and TRIACs, 3mA at the low end, and 50mA being quite common. That means, this circuit, can be used with just about any thyratron. Even a big 5559 ! Only adequate filament power needs to be addressed.

Sweep Generator Application:

Note, in the organ app, above, a common triode was used to generate sawtooth ramp waveform. In the tube era, however, thyratrons were used for this purpose. Especially, in oscilloscopes. Thyratrons like the 2.5V 2B4/885/128A, and 6V 6Q5/884. Below is the typical sweep circuit from an RCA 884 datasheet:

(click on drawing to enlarge):


Here is a demo circuit, that trips a thyratron with a simple neon relax oscillator:

(click on drawing to enlarge)


This demo circuit is powered from a 12V supply, and draws a tad over 1/2 amp. A royer oscillator is used to primarily generate the filament voltage, and oscillates at near 50KHz. The primary has a bit of inductive “kit”, an that’s pumped thru a multiplier ladder to generate ~55V for the thyratron anode supply, and ~140V for the neon relax oscillator.

The demo tube (V1) has its cathode biased up to +6V. This makes the grid nominally rest -6V below the cathode. When the neon flashes, the current spikes the grid voltage positive, and initiate ionization. C14 then dumps its current thru the tube causing a flash, and pull the anode voltage low. Once, below ionization voltage, conduction extinguishes, and C14 ramps up again, waiting for the next grid pulse, and repeat the process.


(Click on photo, above, to enlarge):

Dinky Thyratrons:

Thyratrons come in all sizes. Not all, are large devices for driving heavy loads. Some a quite small, intended to pass just enough current to drive a small relay, or some odd application, such a noise generator (6D4). Here’s a photo of a view:

(Click on photo to enlarge):


The 5663 and 6D4 can drive currently available general purpose relays. Many draw a tad less than 20mA. The Raytheon RK61 is an odd ball. Its intended to be battery powered, and can only drive a 1.5mA load, maximum. There use to be completely electromechanical relays, in the past, that could be engaged by such a low current. In the modern era, due to semiconductors, the need for such devices are no longer needed, since its child’s play in silicon.