I dont think they even used ferrite parasitics on the finals either then.
They did. But as soon as the amplifier got run on high side for a while they would get hot, crack in half and pile up on the bottom cover.
Until the first time the bottom was removed. Then they're gone forever without a trace. Haven't found those rattling around in the bottom of a 300A in a very long time.
Sure would like to see a proper sequence of the engineering changes as they progressed.
Oldest ones had the tube sockets mounted to the metal chassis. That didn't last long, and the more-familiar circuit boards were where the tube sockets went, mounted on plastic spacers that get hot and shrink.
Older versions had an open-frame single-pole relay in line with the B+, mounted on the pcb with the HV filter caps. Then it got moved to the chassis behind the standby switch.
And then it went away forever. Used switched grid bias to put the tubes on standby after that.
Three relays mounted on the plug-in circuit board got changed around 1974 or 1975 to a soldered-in pc board with the antenna relay in a socket, located beneath the Load control. This version had the two R50 sealed relays that always go bad.
The factory schematic shows no separate input-matching circuit for Low side. But nearly all of the amplifiers I have seen include this coil and cap mounted to the rear of the High/Low switch.
The band coils definitely saw some revision, as the pics above demonstrate. We learned the hard way to remove the driver tank coil altogether and replace it with 5 turns of #12 wire.
Only the screen grids on the driver tubes got the positive voltage fed to them. This created a hazard to the 12-Volt circuits any time a driver tube got too hot and flashed over inside.
Like most manufactured products, the design would change every time they built another batch.
73
