Project Update:The metering was lacking, so installed plate current, screen current, grid leak voltage and plate voltage on a Plexiglas mount. Since all meters are floating at high voltage, it required extra precautions.
I increased the HV to 1600V and now seeing about
250 watts of carrier and 1250 watts peak output. Still not satisfied with that, but it's getting better. Not too bad for a tiny tube rig, I thought.
Still using used tubes - one of the old 6146Bs arced over and made a minor light show, but it was replaced by another soldier and works FB.
Take a look at the nice square and saturated PDM pulse coming out of the high level PDM modulator! Bet that's running near 90% eff. I needed to increase the 6LF6 screen voltage up to 90V to get it that smooth. The peak power went up a bit too, so I'm thinking that maybe I need some positive bias, not sure yet.
The RF final parameters look good on the meters. The HV meter reads high and needs more R.
When finished, for safety I will probably enclose the whole rig in a Plexiglas cabinet.
Next, I'm trying to come up with a shutdown circuit. Not sure yet. It's tough to do cuz of the floating HV circuitry. For now, I have a 1.5A fuse with a 50K resistor across it in the B+ line.
So far the rig seems very stable. I even brought it up to 2KV for a moment to see what happens. Stunts like that will be my undoing, I'm sure...
Still thinking of Steve's suggestion to replace the 6146Bs with sweep tubes. (6LF6s or 6DQ5s) They have 2.5A filaments and could increase the output power. Even though the 6146s have a higher plate dissipation.
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