6GK5/300B

[audiowize]

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)

 

[Eli Duttman]

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.

 

[audiowize]

Ah, yes, that's what was departing from my mind.

I guess it's time to consider differential EF86's!

 

[Eli Duttman]

The EF86 can be as bad a wimp as the 12AX7. You need high gm to deal with the 300B's Miller capacitance. A differential 6AC7 pair could be the answer. DC heaters and regulated g2 B+, please. Place "O" ring dampers on the metal envelope too.

 

[audiowize]

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.

 

[Eli Duttman]

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.

 

[audiowize]

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...