I'm working on a rare Marconi Challenger Transmitter. Output is 1.2KW from 4 off 4CX350a's in parallel. As only 12 of these transmitters were sold documentation is hard to find. I have an operator's manual which would normally give the circuit diagrams, however the crucial pages covering the final stages are missing! I have deduced that the valves are shot as I can't set up the initial conditions: 100mA Ia with Vg1 between -20 and -27V. If I attempt to tune up they overheat and the HT trips out (correctly!). I get about 150 - 200Watts of RF before the HT trips. I get no increase in Ia when tuning and the drive, as shown on the meter barely registers. I have a new set of 4CX350s on order, so now I come to the crux of my enquiry - before plugging in these expensive new valves I want to ensure that all other conditions are correct. (HT is 2.2KV, Screens are a solid 400V, G1 Bias, nominally -27v, adjustable to -18v) I am particularly confused over the drive requirements. Briefly, the four valves are in parallel in grounded cathode configuration ( I do not have the manufacturer's circuit, I've traced it out manually). There is a synthesizer/Mixer/amplifier stage which works correctly and supplies 6Vpp, CW or SSB out to a Drive amplifier. The drive amplifier then raises supplies the necessary drive for the valves (circuit attached). Despite the 4cx350s being faulty I would have expected to to more drive from this amplifier. The active device is a CTC BAL000010 dual npn transistor in a 744a-01 package. I can't find a datasheet for this transistor, but it is probably similar to MRF392/393 which are 100Watt devices. It is mounted on a very hefty heatsink. The circuit looks like a 100Watt amp, I note that L4 in the 24V collector feed is rated at 3Amps. What I don't really understand is that the 4CX350 datasheet states a Peak RF Grid voltage of 21 volts and a Grid current (Max) of 2mA which does not, to me, equate to 100Watts. Assuming a Grid impedence in the order of 1000Ohms only about 4 Watts woiuld be required. Other sources that I have found suggest that only "a whiff of RF" is needed to drive these valves to full power, others seem to suggest that substatially more may be required. I've successfully interfaced solid state and valve amplifiers in the past (much lower power though) and my understanding is that voltage swing is more important for valve amplifers than outright power owing to the high input impedence at G1. I hope this is not too much of a ramble, can someone explain, how much voltage is required at the Control Grid, and how much power please? Many Thanks - Michael.