[John L Stewart]

There is often AF signal down to about 20Hz in transmitted AM signals.
Old radio OPTs often saturate at 80Hz, full power, and speakers don't
go low so depending on the radio one has to ensure the CR couplings
have poles which are a lot higher than what's used in a hi-fi amp. The
typical AM radio might have a 6AV6 triode-diode first AF tube coupled
to the 6V6 output and NO GNFB whatsoever. Use of any FB at all often
makes oscillations happen because of very poor OL bandwidth. And
because AF is limited by the sideband cutting, ie, the AF BW is
defined by the total response of 5 tuned circuits then the rising
speaker AF output without GNFB gives a wanted boost to F between 1kHz
and 3 kHz before the AF drops away at 20dB per octave. But if the IFTs
are well made and some other tricks are used to reduce selectivity
while maitainining skirt selectivity over 30kHz away from some other
station then audio BW can be 8kHz and then a flat amp with NFB makes
good sense. If proper rules are followed as one does for a hi-fi amp
you can get AM that is excellent sound and none of the VLF
instabilities being cited here.


I had no problems at all with the application of NFB in several AA5s, Etc. Here are the results for one of them along with schema.

12J5 50L6 FB Amplifier Results

Frequency Response from G1 of 12J5

Level 0.25 Watt 0.50 Watt

-1 db 22.5 Hz 34 KHz 30 Hz 23 KHz
-2 db 18.5 Hz 39.5 KHz 26 Hz 26.5 KHz
-3 Hz -- 44 KHz 20 Hz 31 KHz

Square wave shewed ringing at 133 KHz. Removed by the 1K8 / 5 nF network.

Feedback is 21.4 db.

Original circuit measured 1.5 % THD at 0.25 Watts & 5 % at about 1.2 Watts.

Modified circuit measured less than 0.5 % THD at 0.25 Watts & 5 % at 1.6 Watts.

No data on DF.

Started out with a tabletop AA6. The RF stage helps a lot. All done around 1960.
For this one I pulled the 35Z5 rectifier & replaced it with a silicon diode & 33R resistor.
With the extra heater volts I replaced the 35L6 with a 50L6 output stage.
In the socket vacated by the 35Z5 rectifier I wired a 12J5 RC coupled stage to form a FB Pair with the 50L6. NFB taken from the original OPT 3-4 ohm tap back to the 12J5 cathode.
Still running in the barn at the lake driving a 12 inch speaker in a large ported box. Sounds great!!

Cheers to all, John