[Milsurplus] Navy MAK and the HY1269

[Ray Fantini]

So a WW2 Navy model MAK 25 watt HF AM transceiver followed me home from Dayton. Been looking into using it for a portable 3.885 transceiver at various field nets and the like with others who are using things like BC-611, pogo Stick and other WW2 AM transceivers. Think it's a good choice being it relatively small and low power and built to work with a short vertical antenna. In the process of building external Dc to DC converter power supply that will probably attach to the back of the radio out of sight and run everything from twelve volt DC source.
Lot of this is easy, being the filament strings are already configured for twelve volt operation, all the tubes in the receiver are twelve volt like 12SK7, 12A6 and the like and figure the receiver will work just fine at around 180 to 190 volts DC for the plates but the real question is what's going on in the transmitter.
The transmitter is a four channel crystal control using a 12A6 as the oscillator that drives a HY1269 PA tube, that tube is plate modulated y two more HY1269 tubes in parallel.
This is my first exposure to the HY1269 tube and it appears to me to be a quick heating power pentode rated at 30 watts, 750 volts max DC plate at a max current of 120 MA.
Just playing around with lighting up the filament string at eight or ten volts the filament appears to be real bright. Looking at the specs I can find it shows the tube can be used at six or twelve volts with a center taped filament. The filament, cathode and suppressor grid are all tied together internally in the tube. All three tubes have their suppressor filament center taps tied together decoupled to ground thru a capacitor but also a variable resistor is involved with that circuit. From what I can determine the whole mess was feed from the common DC input with a separate switch that powered the transmitter section by itself that brought in a relay to power up the filament string and the dynamotor for the high DC voltage was activated by the PTT line.
First question: How bright are the HY1269 tubes supposed to burn? They light up real bright, almost like light bulb filaments at ten volts and being that it's hardly the most common tube don't want to risk burning open any of them.
Second question: what should I plan for a plate voltage for the transmitter? I figure it's going to be around 400 DC or should I be higher? Maybe something like 500 or 650 DC being the tube is rated good to 750?
Attached is the only schematic I have been able to come up with and it's not very good. The key thing here is to build up an external power supply but not one that puts the tubes in danger, will also be looking at things like the voltage ratings of all the bypass capacitors and the plate coupling capacitor and thinking maybe the key maybe too go with half the rated voltages?
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