04-02-2017, 09:54 AM
Hi.
Well I finished the single valve TX some time ago and built it into a little tin, I didn't cannibalise the original breadboard project, I stated afresh.
Since then I have taken a different approach.
I've now gone back to three valves, I bought some miniature Russian Triodes that came midway through January, they are fairly rare EM-7 valves, they have a 1 volt filament and run on very low HT, the gain is about 2 times max. I decided to try this as an audio amp and although the gain is limited it drives the "modulator" 1ж42а very well without distortion. I also tried an EM-7 as an RF buffer without any success.
I then decided to try a different Rod Pentode the 1ж18b as an RF output valve. This makes a huge difference to the linearity of the project and also increases the range. I'm using 72 volts on the 1ж18b anode and have fed the G2 via 47k decoupled by a 0.0022uf cap, Anode current is 1.5ma so the life from the 8 PP3 batteries should be good.
The EM-7 needs normal grid bias so this is fed from a standard AA battery via a 4.7m resistor. Filaments are from a 1.2v Ni-Mh battery with limiting resistors for each valve, I'm keeping the filament for smaller valves just about 1v and the 1ж18b at just under 1.2v.
It will be interesting to see how the life is on these valves over-time. On a long wire and the tank set up properly I can just about get a quarter of a mile but a short wire of a few feet in the shed gives me sufficient range to cover the shed, house and about 20 meters out with the front of the house.
The Ni-MH AA battery I am using for the filaments last for about 36 hours, its far from new though.
I'll draw up the circuit hopefully today and post it here.
Well I finished the single valve TX some time ago and built it into a little tin, I didn't cannibalise the original breadboard project, I stated afresh.
Since then I have taken a different approach.
I've now gone back to three valves, I bought some miniature Russian Triodes that came midway through January, they are fairly rare EM-7 valves, they have a 1 volt filament and run on very low HT, the gain is about 2 times max. I decided to try this as an audio amp and although the gain is limited it drives the "modulator" 1ж42а very well without distortion. I also tried an EM-7 as an RF buffer without any success.
I then decided to try a different Rod Pentode the 1ж18b as an RF output valve. This makes a huge difference to the linearity of the project and also increases the range. I'm using 72 volts on the 1ж18b anode and have fed the G2 via 47k decoupled by a 0.0022uf cap, Anode current is 1.5ma so the life from the 8 PP3 batteries should be good.
The EM-7 needs normal grid bias so this is fed from a standard AA battery via a 4.7m resistor. Filaments are from a 1.2v Ni-Mh battery with limiting resistors for each valve, I'm keeping the filament for smaller valves just about 1v and the 1ж18b at just under 1.2v.
It will be interesting to see how the life is on these valves over-time. On a long wire and the tank set up properly I can just about get a quarter of a mile but a short wire of a few feet in the shed gives me sufficient range to cover the shed, house and about 20 meters out with the front of the house.
The Ni-MH AA battery I am using for the filaments last for about 36 hours, its far from new though.
I'll draw up the circuit hopefully today and post it here.
