Isn't this essentially the same idea behind the BC-610? It ran a single 250TH and used only one half of the jackbar. Mod-U-Lator, Mike(y) W3SLK
The BC-610 uses "plate" neutralisation. That means the plate tank circuit is balanced just like a pushpull amplifier, but the grid tank is single-ended. You have to have either a split plate tank or split grid tank, to get the out-of-phase rf needed to feed back to the grid through the neutralising capacitor, to counter balance the in-phase rf that is fed back through the tube's grid-plate capacitance.
With single-ended, the tank doesn't have to be balanced with a split stator capacitor. The tap can be off to one side and the capacitor can be single-section (with the frame and rotor ungrounded), with an appropriate adjustment to the neutralising capacitor. But it is much easier to make the neutralisation adjustment hold over several bands by using a balanced split tank circuit. Plate neutralisation works better than grid neutralisation. It is difficult to get 100% complete neutralisation with grid (sometimes called Rice) neutralisation.
Many triodes, like the 250TH and even the 304TL have very low plate-to-ground capacitance, in the case of these tubes, a fraction of a picofarad. The plate doesn't have enough capacitance to unbalance the tank circuit using a variable capacitor with anywhere between 25 and 400 pf per section. OTOH, the grid to ground capacitance is quite large and tends to upset the balance, especially on higher frequency bands, where the tank capacitor is set near minimum.
Some triodes, like the 211 and 833A have higher plate-to-ground capacitance, which may tend to unbalance the circuit near the minimum-capacity setting. In my HF-300 rig, I use a 211 to drive the final. It is plate neutralised with a split tank circuit, link coupled to the balanced grid tank in the pushpull amplifier. I used a small variable capacitor at the opposite end of the balanced 211 plate tank circuit to ground, to balance out the plate capacitance of the tube. I kept re-adjusting it until the same neutralisation setting would hold up on 20m and 160m. I haven't used the rig on 20m since the early 1970's, but that balancing capacitor setting has remained locked down, and I have never had to re-adjust it upon replacing a 211 driver tube, having gone through several in the 35 years or so that I have used the transmitter, plus I have swapped out numerous spares to help prevent tubes from taking gas while lying unused on the shelf.
Don k4kyv
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