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In the 1KD-5 amp there is a RCA jack in the Fil centertap line just
below the Bias Zener..The jack has a shorting plug ...Nothing in the
manual ,what is it`s purpose? Thanks Harold

Hi,
Looking at the schematic of this amp,
I believe you would normally leave the shorted phono plug
in J3 (note the shell of the J3 connector isn't grounded).

Should you want to calibrate the Grid or Plate current
meter reading in the amp, remove the plug and insert
an ammeter between center pin and shell.
Henry used trim pots to set the plate and grid
current readings (VR1 and VR2), so this would have been
a factory adjustment procedure.

Note that the HV supply is not returned directly to ground, but to R10
in the power supply (the grid current sensing resistor),
the top of which connects to terminal 2
and then continues to the metering circuit in the amp.
Since the 3-500Z tube grid is connected to ground,
grid current is measured indirectly. An old ARRL handbook
will show how this is done (See the 3-1000Z amplifier
metering in the 1963 Handbook, page 207, for instance).

73,
Ed Knobloch