• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.


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I was considering building a 300B push-pull amp with a pair of 6GK5's set up as a LTP with a CCS and a negative rail.

My operating point was going to be 350V P-K, 71V bias, and ~80mA per tube into a 5K P-P transformer.

For the 6GK5, I figured I'd stay right at the 200V limit for plate voltage and run 2V of bias for around 8mA of driver current per tube with a 30K load resistor.

I feel like there is a box that I haven't checked yet... (TIA for the help)
You might need a line stage that has some gain. Remember, you will get less than 1/2 of the 6GK5's μ out of the the LTP. A quite significant drive voltage is needed, for 300B "finals". Poindexter drives PP EL34s with a 6GK5 LTP, from a CDP and no line stage. I suspect that's about as far as is possible. Check the 300B Operating Conditions Chart out. Notice things like -80+ V on the grid. :yes:
Man, I have a million 6AC7's sitting in my stash. I'd always get half excited to come across them, thinking they were 6CA7's.

So maybe 300V P/K, 150V SG, ~10mA per tube, and a 20K load per tube? It still leaves me with 2V of bias, but the LTP gain should be over 50 at this point.
There is a fly in the ointment. True pentode operation requires that g2 be at the same AC potential as the cathode. Therefore, the regulated g2 B+ supply must "float"! Complete isolation from the main B+ supply is essential. Use a separate, low current, "120" VAC winding; bridge rectifier; filter cap.; and LR8 IC. The - side of the resulting rail connects to the LTP cathodes and the + side to the screen grids of the pentode LTP.
Eh, that's a pretty small fly. I can use one of those $17 Allied transformers to get the job done. That actually helps me a little bit, as I'd like to put a voltage tripler on my 12V heater winding on the main PT to produce my negative rail, and the Allied has just enough heater current to warm up a pair of 6AC7's on DC.

That LR8 is almost too easy...
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