Thyratron Vs. SCR - Page 1
Forum section: electronics / beginners

author: bullet308
author_rank: Frequent Contributor
timestamp: September 09, 2012, 04:06:24 am
content: Hi:Would somebody please school me on some of the ins and outs of replacing a thyratron with a SCR (or some other solid-state solution). Apparently they are used to do essentially the same thing. It couldn't be as simple as swapping one of the same value in for the outgoing thyratron, could it?Old Soviet Geiger counters are kinda' trippy sometimes...TIA

author: vk6zgo
author_rank: Super Contributor
timestamp: September 10, 2012, 08:46:07 am
content: Back in 1959,the lecturer showed us a Thyratron,& said "Don't worry too much about it,you won't see that many of them!"He was pretty right,I probably saw one or two of them in the next few years,then none till around 2001,when I had to fix a Scientific Instrument---Turns out the Uni bought it in 1959,& had a bundle of Thyratrons & 6V6GTs for it.My point is,they are rare,& have been for years,so a schematic diagram would be nice!

author: bullet308
author_rank: Frequent Contributor
timestamp: September 10, 2012, 10:24:37 am
content: Well, yeah, schematics are available but...how good is your Russian? :-)The portion of the circuit employing the thyratrons is in the fourth attachment. They are located in the probe housing and regulate the current to the two Geiger tubes located there.An excerpt from the manual on the theory of operation:"5.2. Principles of operatingHigh Voltage CircuitThe high voltage is generated by an oscillator, V6 which provides pulses to the primary windings of step-up transformer T1. Across the secondary winding of T1, high voltage pulses are created via transformer action. These pulses are rectified by diode D7 and filtered by C12, R26 and C10. Ionization tube V4 drops a certain amount of DC voltage and applies the remainder to the gate of feedback regulator V5. V5 in turn controls the strength of oscillations in V6, culminating in a closed loop electronic regulator. The final DC voltage (390-400V negative) is applied to the GM tube card in the probe head via an attached 5 conductor cable.Probe HeadThe probe contains the 2 Geiger Mueller tubes and Thyratrons.There is a high range GM tube and a low range GM tube. These make pulses from the incoming radiation, and those pulses are transmitted back to the main unit via the 5 conductor cable.Main UnitIn the main unit are housed the battery compartment, HV section, metering unit and range switch. Calibration resistors R-20 through R-25 allow precise calibration by a laboratory."More Info:http://radiation.fallout.cz/htm/dp-5v.htmhttp://www.civildefensemuseum.com/southrad/russian-dp5v.htmlhttp://commons.wikimedia.org/wiki/Category:DP-5V

author: SeanB
author_rank: Super Contributor
timestamp: September 10, 2012, 03:46:01 pm
content: So what is wrong with it? Looks like V4 is a simple neon stabiliser, replaceable by a selected series string of zener diodes. V1 and V2 look like they are used as a coincidence detector to combine the outputs of the tubes and provide voltage amplification to get a constant voltage and shape pulse to run the integrator in the meter.

author: bullet308
author_rank: Frequent Contributor
timestamp: September 10, 2012, 04:26:04 pm
content: Nothing is wrong with it: it works very well in the finest Soviet tradition, except I am reworking the contents of the probe into another package and adding a couple of alpha-sensitive pancake tubes and I need things to get smaller if possible.V4 is a voltage regulator, that much is certain, and I dont need to replace that one.On the coincidence detector, probably not in this case. IIRC, the high-level tube is powered full-time and the larger BG tube is switched in and out depending on the range. Its just two separate circuits. Probably has a lot more to do with voltage to the tubes.

author: bullet308
author_rank: Frequent Contributor
timestamp: September 11, 2012, 02:07:22 am
content: A few pics of the board inside the probe: