Murphyv310Hi Lawrence.
I wonder if you could use a triode/pentode like an ECF80, use the pentode as a high impedance amp, then DC couple the Pentode Anode to the Triode grid. The only downside could be DC current but you could limit the current in the triode with a suitable anode load resistor with the anode de-coupled. In that way you may be able to have your matching TX directly connected to the cathode.
I find that many circuits will work and work well against all the theory, possibly a suck it and see scenario!
To make things worse Trev, I want to run the valves with 12 volts HT (12.6 volts LA battery) the nearest triode at that voltage for a cathode follower I can find is an ECC86:
http://www.r-type.org/pdfs/ecc86.pdf
Going back to that formula I posted earlier, Zo = 1/1/RK + 1/ra + gm I've plugged in some numbers for the ECC86 for an operating point with 0.25 volts -ve bias on the grid, I work it out to Zo = 55 ohms with a cathode resistor (RK) of about 80 ohms.
If that's correct then it would be a close enough match for the RF input impedance of the mixer (50) ohms
Can some kind soul check that for me?
Also in the above scenario if the RF port of the mixer was coupled to the cathode via a capacitor would that upset things within the mixer to any great extent as the transformers in the mixer are wound transmission line style.
Lawrence.
I wonder if you could use a triode/pentode like an ECF80, use the pentode as a high impedance amp, then DC couple the Pentode Anode to the Triode grid. The only downside could be DC current but you could limit the current in the triode with a suitable anode load resistor with the anode de-coupled. In that way you may be able to have your matching TX directly connected to the cathode.
I find that many circuits will work and work well against all the theory, possibly a suck it and see scenario!
To make things worse Trev, I want to run the valves with 12 volts HT (12.6 volts LA battery) the nearest triode at that voltage for a cathode follower I can find is an ECC86:
http://www.r-type.org/pdfs/ecc86.pdf
Going back to that formula I posted earlier, Zo = 1/1/RK + 1/ra + gm I've plugged in some numbers for the ECC86 for an operating point with 0.25 volts -ve bias on the grid, I work it out to Zo = 55 ohms with a cathode resistor (RK) of about 80 ohms.
If that's correct then it would be a close enough match for the RF input impedance of the mixer (50) ohms
Can some kind soul check that for me?
Also in the above scenario if the RF port of the mixer was coupled to the cathode via a capacitor would that upset things within the mixer to any great extent as the transformers in the mixer are wound transmission line style.
Lawrence.
