1Ж42А. As a mixer/Frequency changer????

[ppppenguin]

I have come across some Russian methods (see some in the attachment). I show this as it also includes another use of the valve being used here. From the Russian circuit using the эм7 with transistors I have been playing with constant current feeds to the filament. Theoretically this should limit the flow via the filament circuit but it gets a bit thought intensive. The эм7 has a 1V 15mA or so filament. From this work I have been increasingly using constant current supplies as I use them often and the currents below 100mA are perhaps easier to handle than low voltages but it's not a real concern (although with my electrometer circuits a well regulated anti surge filament supply is useful).

I have bought some little transformers purely for filament supply although have occasionally in the past used capacitor tap off on normal secondary supplies to provide isolation (and scratched my head as to what might happen if a capacitor fails and I lose isolation). There are some standard methods of doing things but as you are now thinking - it's what is the best easy to implement and cheap method. Perhaps just using batteries with simple regulation? Really it is just a case of isolation of the filament.

How about a single NiMH cell with trickle charge via a capacitor. Need a schottky diode to drop the 200mV from the cell to the 1V heater. It's hardly a new idea. DEAC rechargeable batteries were used on some valve portable sets for this purpose.

If you're worried about the capacitor then put 2 in series and consider clamp diodes across the filament. 2x silicon in series will clamp at about 1.4V. Otherwise do something with schottky diodes to get closer. If you're using AC on the heater then use diodes in inverse parallel.