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Perhaps that relay was used in that application in an effort to reduce the voltage/current to operate it to low values, such as from a later-model (but older) transceiver with limited T/R relay switching capability. Also, if there is a power supply used in the amp for that relay, look at the output filter capacitor to get an idea of the maximum voltage that was applied to operate the relay.

If that relay really is a "Sensitive Plate Circuit Relay", you might find a miniature adjustment mounted on the armature by which the actual sensitivity was adjusted. Sometimes, that adjustment would have a very small indication plate on which the adjustment range was mentioned in milliamperes.

If you apply power from an external adjustable power supply to try to find what voltage/current used to actuate the relay, I would first apply that power through an external current-limiting resistor so as to limit the external voltage/current to something most likely used to actuate the relay; otherwise, you could burn out the relay coil very easily. Typically, such relays operated with just a few milliamperes but as much as 10 - 30 VDC. That "high" voltage was common because those relays were in the plate circuits of receiving-type tubes passing only a few mils at a hundred or more volts.

In the olden days, we hams used to use such relays to detect and protect the sensitive screen or control grids of tubes from the ravages of overcurrent. Less commonly, they could also be used to detect overcurrent of cathode or plate current by connecting them across resistive voltage dividers.

Steve,

K0XP

All, I have an old homebrew RF deck that the T/R rely is a Potter & Brumfield LM11-2500. I can't find what the coil voltage is. In the 1954 P&B catalog the LM series is titled?Sensitive Plate Circuit Relays and only list coil resistance and pull MA ratings. What was the typical application for a Sensitive Plate Circuit Relay, and what voltage energized the coil? Not sure why I am struggling to understand this but do not recognize the Sensitive Plate Circuit Relay application. Thanks Nick