The original Drake L4 (not L4B) used Eimac 3-400z (pair). There is a single design 3-400z amp in the ARRL handbook for 1966 or 1967. The 3-400 is slightly shorter than the 3-500, hence L4 owners have to watch carefully plate cap placement and make sure enough clearance exists to the top cover.
Good point. The original L4 used Eimac flush mounted sockets, so the shorter 3-400Z was a non issue. With a taller 3-500Z plugged in, it could then be a height issue. The L4B that came out in 1969, used recessed Johnson sockets, so the taller 3-500Z was a non issue.
The Johnson sockets HAVE to be recessed, since the airflow is via the space on all 4 x sides of the recessed socket. On the Eimac sockets (black plastic ones), the airflow is through the center of the socket.
On a side note, when using the flush mounted Eimac METAL sockets (SK-400 for the 3-400Z / 3-500Z / 4-400A/B/C)...and esp the SK-500 (4-1000), folks would remove the pin assy, then slice the entire bottom off the metal socket assy. That would increase the airflow by a significant amount, by reducing back pressure. Another trick back then was to cut out every 2nd hole (with tin snips) on the metal shell on 4-1000's, and also 4-400 tubes. But you still had all the airflow having to initially flow through the holes in the ceramic base of the tube.
I notice on all 4 of my L4B amplifiers, with the top lid off (and HV interlock temp defeated), the amp is de-tuned, esp on 20-10m. The top steel lid adds more stray C, when installed.
