I started laying out the RF deck for my 4-65A transmitter tonight. Before cutting metal, I set parts on the top and tried to figure out how I might place them all.
The idea is (as you might see) to make sure that in the future, I can swap the socket, the filament transformer, and the 4-65A out for a 4-125A later on if I get into that position. The cutout for the socket and the filament transformer is identical, so chassis rework would be minimal.
I have this single capacitor that's 114pF, and spaced 0.070" between the rotor and stator, and I have a capacitor that will take up the entire section marked off there and is 120pF in two sections, with 0.180" between rotor and stator plates. Either cap has enough capacity for 80 to 10m with the 4-65A or 4-125A.
As for the loading capacitor, I have a receiving type variable that's a 365pF+365pF dual section. That is only 30pF more than the needed amount for an 80M match to 50 ohms.
Looks like 160M is not feasible without switching in capacitors on both variables AND inductance. That's alright. I can be satisfied with 80M-40M-20M-10M.
Here's what I am proposing for a general layout-things are fairly tight on the top deck. I moved the 4-65A tube from dead-center over a bit, centered up on the X. I can jog it forward a little, towards the front of the chassis, to make more room for the plate choke.
You see on the far right hand side, I have cordoned off a section for the 2E26 driver. Input to the driver's grid will be a passive 50R input, and will be driven by a DDS-generated source. The output of the 2E26 only needs to be around 5W. I figure a switched-parallel plate tank for the 2E26, with the passive grid, would be enough to do the job. I plan to make the parallel L-C tanks around a pair of switch wafers.
Does anyone see any major, layout-changing failures in this plan?