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John L Stewart John L Stewart is offline
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Smile Simple NFB Simulations for a FB Pair

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
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Last edited by John L Stewart : November 26th 12 at 06:19 PM Reason: My last statement is incorrect. The CFB in the OP stage will improve the PSRR.
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patrick-turner patrick-turner is offline
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Default Simple NFB Simulations for a FB Pair

On Monday, 26 November 2012 12:31:56 UTC+11, John L Stewart wrote:
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.


Its shunt FB, with Ra of 6SJ7 and any other parallel R forming R1, with R2 being the 560k. Pages 333 and 334 of RDH4 explain it further.

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

I tried various forms of the shunt FB as you mention in SE amps in old radios where GNFB may have been difficult. I also tried it for a PP hi-fi amp, but I found later the CFB from OPT worked better, with some GNFB.

But any PP UL amp with plain old UL OPT may be converted to local CFB while using the existing PSU. One finds a CT choke with no air gap, able to take Ia of each tube, and able to take about 100Vrms across winding without saturating at above say 20Hz - not hard to make, or could be a small old OPT primary. THEN, each cathode is bypassed with 470uF to a screen tap, with screens taken to a fixed Eg2. Phasing needs doing right, but then also sec load is reflected to tubes as a higher RLa-a load. Many things are possible with existing OPTs without needing any speacial OPT IF there is a tap along a primary winding somewhere. Such things suit multigrid tubes. Triodes don't need to be anything except a triode. So, in old radios, I remove tube rectifier and use Si diodes to boost B+, and then instal a 6L6 or EL34 in triode to replace the 6V6. Voila, much better sound, and usually slightly more PO.

Patrick Turner.
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