It was a common practice in a lot of the last generation of Harris Broadcast tube transmitters to install the plate current meter in the negative side of the plate supply. They would install a ten or fifteen volt Zener Diode across the shunt so if the resistor opened the diode would short and protect the plate current meter. After a bad lightning strike it was not uncommon to find the transmitter on the air, putting out full power with no issue but the plate current meter reading zero.

Ray F/KA3EKH

From: [email protected] <[email protected]> On Behalf Of B. Smith
Sent: Sunday, April 3, 2022 7:40 PM
To: Military Radio Collectors Association <[email protected]>
Subject: [MRCA] ART-13 Junque Box Power Supply

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The ART-13 plate current meter circuit requires a meter "shunt" to be installed in your power supply and the voltage taken from this shunt is used to drive the  meter.  Usual this is a resistor around 12 to 15 ohms.
When constructing this "shunt" try using two or more  resistors in parallel and avoid just using a single resistor. By using a pair of resistors if one fails you will still have a DC return to chassis ground and your meter in this case will not be accurate but the power supply will be safe and still function. BTW the meter scale is not very accurate anyway in the plate current function. Full scale is around 300 ma.
Z