Tuning & Loading Meter Circuit
In operation, I could not obtain a useful meter-reading from the TUNING & LOADING meter position. Digging further, CR101A (a 1N34 diode) was completely shorted. Under this condition, the TUNE/LOAD meter gave about a 1/3-scale reading, with only a tiny response to adjusting Tune and Load controls. I tuned the amplifier for use by observing Screen Current.
I replaced both CR101A and CR101B with 1N5711 Schottky hot-carrier diodes. Capacitor C112 and resistor R105 both required adjusting (per 1976 mnaual, page 4-2, section 4.5). To adjust R105 safely, I pushed a foot of cable-jacket onto the shaft, then jammed a medium-length screwdriver into the other end of the cable-jacket - making a flexible adjuster which also kept my fingers out of the compartment.
With the change to a 4CX1500B tube, the elevated screen-supply, change in diode-specifics, and aged surrounding components, I could not quite dial-in a "perfect" 0-reading on the meter, at resonance - there's no adjustment-travel left on either C112 or R105.
Satisfied that the circuit is now generally functional, I continue to tune by watching Screen Current, hitting 0mA at whatever power-level I use.
(Thanks to K5MGR / Mike Langner for his generosity in sending me the diodes!!!)