I have TELSTAR 6M that had a single 4cx250f 26 v heater version.
I replaced it with the Russian GS36B said to be very simular the the Eimac 4cx400 or SEVETLANA. It's a direct plugin.
The change over consisted of installing an extra 6.3 vac heater transformer on top of the original 28 vac transformer and wireing it to the heater leaving the original in so any tube in the family could be used just by adjusting the bias and wireing the heater per the tube.
The bias voltage is different but within the voltage available from the original neg bias supply.
The cooling sleeve had to be machined out on the ID to just set down on the 4cx400 anode. As it turned out the sleeve will be usable on any 4cx150 to 400 installed, for cooling.
The blower remains the same because the 4cx400 type anode does not restrict the air flow as much as the smaller 4cx250 type.
For blower flow info see the 2009 ARRL hand book that has a table for Dayton blowers and applications. Dayton has been aquired by GRAINGERS and now has different part numbers but the mounting and flow rates are about the same.
The amplifier has a plate supply for the original tube that was way over sized so handles the GS36B at full 400 + ma fine.
The plate supply unloaded is 2400 dc. Under SSB
loading with new filter caps oversized, the voltage hangs at no less than 2200 volts on peaks.
Screen supply, is about 310 vdc
Result is the GS36B will output 600 watts in tune. This is rated SSB
power for this tube. The original 250f output was at about 310 watts.
The drive power is less than 5 watts for more than 20 db gain.
I installed a 50 ohm 100 watt non inductive power absorbing resistor in the input circuit to have more control over the drive.
The intent is to attenuate the input still more so the driver radio can have enough dynamic range to take advantage of a later ALC circuit to be installed.
I installed an inrush control in the AC plate supply as well.
The power transformer is only 120ac.
I use a heavey AC main supply for it and my AL80B by splitting the phase and outlets so both 240 and 120 are available for use one at a time.
This amplifier physically is very close to the same size as an AL80B and nearly as heavey.
If anyone has a circuit drawing I would be most happy to have a copy.
Otherwise the circuit is straight forward for this type of grid driven amplifier but would be nice to have something to look at without doing a lot of tracing and scratching on paper to see the actual hookup, in the future for service and mods.
Bottom line on performance is it opens the band sooner and hangs on later when it closes, off a 5 element home built wide spaced beam.
I hope this info is of some value as I have not yet found any info about this amplifier on the internet or elsewhere.