Above contains the schematic of my little 4 tube Majestic as received, and then after I modified it. Things like the negative feedback in the audio circuit aren't necessary if all you want to do is add AVC. I used a 68 ohm resistor in series with the plate circuit of the output to decrease hum. I would have used an even 100, but had a 68 at hand, and tried it first with good results. I have no idea what the original filter cap values were, since no schematic can be found at this point, and no values were marked on the crusty filter unit. I just used 47 MFD units at hand, and was not at all satisfied with the results. The audio had lots of hum. With the addition of the 68 ohm resistor the audio is very acceptable. With the radio as-received, it left a lot to be desired. It was really terrible.
The 5K filter resistor in the rest of the radio's B+ circuit was reduced to 800 ohms for more gain, after AVC was added. Otherwise such a change isn't necessary. What I found was that when the AVC circuit reduced RF gain so that there was no distortion in the audio with strong stations (overloading), full volume was a bit less than desirable, since the radio uses only 3 amplifier tubes. It is missing the 4th one found in more modern sets (1st audio--usually a triode). Decreasing the B+ filter resistor to 800 ohms increased possible gain without increasing hum. Also, notice how the detector's screen grid originally obtained positive voltage by being connected to the output tube's cathode (probably to save on extra components--this radio was the bare minimum). I connected it instead to B+ through a 2.2 meg resistor, with a .1 MFD filter capacitor. The detector can now amplify a bit more without distortion.
For larger radios with more amplification stages (such as my Majestic 180), modifications to the power circuit for extra gain won't be necessary when AVC is added. The modifications will only be necessary with little 4 tube sets. With a 4 tube set, not as much gain is available. With a manual volume control in the antenna circuit, gain can be increased to produce a very loud signal. Some distortion will be present, but distortion is generally more acceptable at those louder volumes. When the antenna control is turned down, the distortion will go away, and audio will sound pleasant at the lower volumes (until, of course, the detector enters the square law region). With an AVC circuit, you want to maintain the RF circuits at a gain level that will give good volume. However, if not enough gain is present in the audio section, and the RF circuits are maintained at a gain level that produces a bit of distortion to compensate for the lack of gain in the audio section, the distortion will still be heard even when the audio section is turned down (since the volume control is in the audio section, not in the RF section), and distortion at lower volumes generally isn't acceptable. That said, you want to maximize the gain of all circuits so that good gain can be had without distortion.
Another modification that I found helped gain, though I didn't make it permanent yet, was to decrease the output cathode resistor to 220 ohms instead of 600. I was pretty satisfied with the audio at 600, though, so I'll probably keep the resistor at 600.
I used a .01 MFD cap in the AVC filter circuit because larger values were too slow.
I also wrote a bit more below the modified schematic, which you can read.
T.
T.
The AVC circuit really only consists of putting the volume control in the audio circuit (and increasing it to a 1 meg unit), and it could even be put in place of the 2 meg grid bias resistor if you wanted (grid goes to wiper). The other components are two diodes, a 52 pF capacitor, a 2.2 meg resistor, and a .01 MFD capacitor. Build the AVC circuit and then lift the antenna secondary from B-. Connect it instead to the AVC circuit. Keep the .01 MFD capacitor close to where the AVC connects to the antenna coil, or else the AVC circuit will pick up radiation from successive circuits, and will oscillate.
T.
marv
:Also, if my modifications intimidate you, keep in mind that I tailored the audio to my liking. I'm not really fond of an unaltered single ended amplifier. I added the negative feedback circuit, which reduces gain somewhat. I decreased the output audio filter to .0047 MFD, which increases output somewhat, but I also added a .0005 MFD cap at the grid of the output, which reduces gain (and makes the audio a lot nicer). If I was to leave the output stage original with only the .01 MFD tone cap across the output transformer, there'd be a lot of gain, and modifications elsewhere wouldn't be necessary.
:
:The AVC circuit really only consists of putting the volume control in the audio circuit (and increasing it to a 1 meg unit), and it could even be put in place of the 2 meg grid bias resistor if you wanted (grid goes to wiper). The other components are two diodes, a 52 pF capacitor, a 2.2 meg resistor, and a .01 MFD capacitor. Build the AVC circuit and then lift the antenna secondary from B-. Connect it instead to the AVC circuit. Keep the .01 MFD capacitor close to where the AVC connects to the antenna coil, or else the AVC circuit will pick up radiation from successive circuits, and will oscillate.
:
:
:T.
T.