Automatic Volume Control

[IBreedBassetts9C]

great video! curious about one thing, however -- at around 2:20 you explain how when the top of the secondary transformer is + the bottom is negative and that negative charge feeds into the .05 mF cap. Since there is an AC signal going thru the secondary on the second IF coil won't the bottom of the coil turn positive immediately afterwards and induce the same capacitor to discharge through it? Am I making sense? Thanks

[AllAmericanFiveRadio]

@IBreedBassetts9C

The diode in the 12SQ7, pin4 and the cathode, changes it to DC. Current can only flow when the plate, pin4, is positive. When the top of the coil is negative there is no current flow, so that part is discarded.

[1stsmosh]

that schem u showed, starting off, looks like a SUPERHET :)

[AllAmericanFiveRadio]

Yep it is a superheterodyne.

[1stsmosh]

thnx :), can this be tuned to pick up low signal 675 kHz?

[AllAmericanFiveRadio]

Sure.

[windoes98se]

i never knew there was such a thing as a triode?

[BeanieandCecil]

I always found the AVC circuits a bit confusing when trying to align my radios. Thanks very much. This helps me understand it better. I've read that this circuit should be disabled during an alignment. Is that necessary and how would that be done, for instance, on this circuit?

[AllAmericanFiveRadio]

To disable the AVC you use the smallest usable signal possible. You don't want the .05uF 12.11 capacitor, the one at the bottom to change up. If it charges up and it will be negative, that charge will lower the volume making it harder or impossible to find the I.F. peaks. Thanks!

[eufaula1]

Great video!! That gives me some ideas about a Philco that I am working on. It's a 41-258, and uses a similar arrangment for it's AVC. Thanks.

[AllAmericanFiveRadio]

Thanks. I have found that the AVC circuits in the early radios are more similar than they are different between manufacturers. I hope the video will help!

[AllAmericanFiveRadio]

Thanks.

Not yet, but you can download my book for my YouTube Channel.

[raymondleeleggs]

I always thought AVC was for cassette decks

[AllAmericanFiveRadio]

AVC is useful for tape recorders, for recording it would be a Automatic Level Control. Very similar ideas. When things get quiet the gain increases. The AVC in radios is evening out the volume of strong and weak stations.

[bonecrime]

Seeing this video reminds me that I do infact have something about AVC hiding in a remote part of my mind. Must have read about it in school och somwhere on the net many years ago. Thanks for this awesome hands-on demo! Ingenius invention I must say!

[bonecrime]

Typo! och = or ... don't know how I managed that.

[AllAmericanFiveRadio]

Thanks, it is a very interesting and useful circuit. Important to know about when aligning radios.

[CameramanLink]

Thanks for adding this video! It really helped me understand how the AVC really works. It's nice to understand why and how things work after I build them. Very good demonstration.

[AllAmericanFiveRadio]

Thanks, I think the AVC is one of the more interesting circuits.

[jstrunck]

AA5,

Awesome AVC operation vid! Now, I know how it REALLY works! You can only get so much from a book!

Take Care,

John Strunck

[AllAmericanFiveRadio]

Thanks John. I always thought the AVC was interesting and of course very useful.

[radioam232]

Very useful and instructing video for people like me, more or less coming out of the transistor age. Beautiful to see how sensitive these tubes react. Also unique to see that the AVR voltage goes back to the first tube and drives a grid there, in many transistor radio's only the IF stage is regulated in AVR.

[AllAmericanFiveRadio]

Thanks! It is very interesting how similar the two circuits are, and also interesting what the differences are. But they both work very well.

[jefferyb304]

Kewl video. I know FM is at least slightly off topic here. I live in an area where FM stations are abundant. I seem to notice that the more distant stations( some 60-80 driving miles away ) will come in without distortion of any kind but a weakend volume. I have some stations in the front door that are high powered. Tune into one of those with out turning the volume down, and its like WHOA NELLIE !

This seems especially so with the Magnavox console stereo that you commented on a few days ago

[AllAmericanFiveRadio]

I'll have to look at the Magnavox diagram to see what to check. And Thanks.