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"The Secret of the 1612" RCA VitaVox Schematic

Posted: Thu Sep 17, 2015 4:58 pm
by mediatechnology
“The Secret of the Sixteen Twelve,” Oliver Berliner, Radio World, 8/21/2015
Around 1940, when radio was still in its youth, a company proclaiming itself “the fountainhead of modern tube development” introduced a unique electron tube. It was designed to perform multiple functions in a consumer receiver but was eminently qualified for use in broadcast “speech input” and sound reinforcement equipment. The characteristic that set it apart was that it was a pentagrid.

Designated the 6L7, this octal-based tube boasting five grids would, in souped-up form, qualify it for admission to the Radio Corporation of America’s elite broadcast group under the type number 1612.
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RCA VitaVox Schematic Using the 6L7/1612

https://www.radioworld.com/news-and-bus ... een-twelve

This may be the only example that I recall seeing of a non-VCA-based feedforward compressor/expander.

Re: "The Secret of the 1612" RCA VitaVox Schematic

Posted: Fri Sep 18, 2015 7:33 am
by emrr
I remember that one from the books. I think I can come up with a few more FF types....

Re: "The Secret of the 1612" RCA VitaVox Schematic

Posted: Fri Sep 18, 2015 8:39 am
by mediatechnology
I'm glad that there are more examples of feedforward from the tube era.
Any classics?

I thought the expand function on the VitaVox is cool.
People talk about the luxury of having transistors with multiple emitters or collectors on ICs and here we have an example of a five input tube.

Re: "The Secret of the 1612" RCA VitaVox Schematic

Posted: Fri Sep 18, 2015 12:52 pm
by emrr
According to RDHB4 the 1612 is superior to the 6K7/6SK7 allowing greater input level before distortion, yet only a handful of broadcast limiters used it. RCA didn't use it other than in single ended PA grade apps.

Re: "The Secret of the 1612" RCA VitaVox Schematic

Posted: Fri Sep 25, 2015 11:50 am
by emrr
Here's an expander from RDH3

Image

Re: "The Secret of the 1612" RCA VitaVox Schematic

Posted: Tue Oct 13, 2015 1:34 pm
by mediatechnology
What a simple circuit. Very cool.
Thanks for posting that Doug.

Interesting to see a schematic where "all resistors are in megOhms."

Re: "The Secret of the 1612" RCA VitaVox Schematic

Posted: Wed Oct 14, 2015 7:25 pm
by brianroth
IIRC, there were newer pentagrid bottles in 9 (?) pin miniature. I always wondered about using them in a "new" limiter circuit....

Bri

Re: "The Secret of the 1612" RCA VitaVox Schematic

Posted: Sat Jan 23, 2016 3:51 pm
by emrr
Just came across a more fleshed-out version of the RDH3 example, from RCA Broadcast News 12/35, article about the 'dynamic amplifier'. This expander was used in the RCA D-22 radio to extend dynamic range.

Re: "The Secret of the 1612" RCA VitaVox Schematic

Posted: Sat Jan 23, 2016 4:48 pm
by mediatechnology
This expander was used in the RCA D-22 radio to extend dynamic range.
Very cool. The listening environment for radio was quite different then.
I did an image search of the D-22 and it looks like it may have had a cutting lathe/transcriber.
I had a Silvertone from that same era with a second arm that was used for transcribing.
The D-22 must have been the ultimate 1935 home audio system.

Re: "The Secret of the 1612" RCA VitaVox Schematic

Posted: Sat Jan 23, 2016 5:27 pm
by emrr
It looks top of the line, probably not many sold. Glad I noticed it.