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While overload relays operating in the region of 100 to 500 mA are readily available, it may be difficult to locate one suited for use in the screen-grid circuit of an r.f. amplifier which employs a tube such as the 4X250B. The requirements in this case call for a relay that will disconnect screen voltage at a screen current of 50 mA or somewhat less.
Use of an inexpensive surplus relay of the dual-winding type provides a simple solution to the problem. Fig. 3 is the diagram of a relay-type protective circuit that was whipped up and placed in operation in less than one evening. Although no new principle is involved, the system does provide dependable protection for those expensive pentode and tetrode tubes which ruin so easily because of excessive screen dissipation.
Fig. 1. Diagram of W1FYN's screen-grid protective circuit.
K1 Surplus relay; see text.
R1,R2 See text.
S1 S.p.s.t. toggle switch.
The s.p.d.t. surplus relay has a pair of 200 ohm windings on a common core. One of these windings is connected in series with the screen-grid lead, the normally-closed contacts of the relay and the line from the screen supply. This path between supply and screen grid remains closed until a predetermined value of screen current activates the relay and opens the normally-closed contacts. When the relay is tripped by overload current, voltage is transferred to the second winding which now receives voltage through the normally-open contacts and R1. The second coil will hold the screen circuit "open" until the relay is re-triggered by the opening of reset switch S1.
R2 is a sensitivity control for the relay and, in the original circuit, is adjusted so that the relay kicks over at screen current in excess of 35 mA. Of course, any abnormal operating condition that usually causes excessive screen current will operate the relay.
Reverse the leads to one of the windings if the relay chatters or fails to hold when activated by screen current. If the relay has a spare set of contacts, these may be used to control a "screen-on - screen-off " pilot lamp.
I.S. Simpson, WJFYN.