[John L Stewart]

Tried some simple nfb amplifier simulations today based on 6V6 OP driven by a 6SJ7.

The amplifier uses the following settings-

6SJ7 G = 1.0 ma/volt
rp = One M
Rl = 270 K
Rg = 560 K Following grid resistor

6V6 G = 4.1 ma/Volt
rp = 50 K
Rl = 5 K

The first check was with a 560 K NFB path from the 6V6 plate to the 6SJ7 plate, a simple form of local feedback.

NFB = 16.13 db DF = 4.54 Looks useful & easily done

The second check tried NFB from the output transformer secondary of an OPT of 5K to 8R. Results would be similar for say something like a 35L6 driving a 4R speaker.

NFB = 4.66 db DF = 0.95 (Unity Damping?) Better than none!
These numbers agree well with a simple mod I did to a car radio about 50 years ago.

A third check tried NFB from the OPT secondary again, but this time the OPT was 5K to 200R (20%).

NFB = 13.19 db DF = 5.00 Looks good but needs a special OPT

All have the effect of lowering the apparent plate resistance. More will result in sensitivity to power supply disturbances, just as with a triode.

My thoughts anyway, John

Here are the real PSRR numbers-

For the bare 6V6 in the example working into a 5K/8R or 5K/200R OPT the PSRR is 20.8 db. If the plate to plate NFB is used the PSRR drops to 1.73 db.

For the 6V6 working into the 5K/8R with CFB from the 8R wdg. the PSRR is 25.8 db. If the P-P NFB is added the PSRR drops to 2.88 db.

For the 6V6 working into the 5K/200R with CFB from the 200R wdg. the PSRR is 35.5 db. If the P-P NFB is added the PSRR drops to 6.40 db.

For the last while I've traded in my soldering iron & calculations for a chain saw & axe. So not much electronics gets done here, with or without errors.

Cheers to all, John