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The AM-864U Broadcast Limiter

by WB4AIO on 23 February, 2010

The AM-864/U broadcast peak limiter

by Kevin Strom, WB4AIO

THE AM-864/U broadcast peak limiter, which I purchased “new surplus” from John Meshna and Company in about 1974, was my first foray into true broadcast audio processing for amateur radio. Like my TCS transmitter purchase from them, the unit was beautiful and flawless out of the box. It cost me $35. I sold it in the 90s (probably a mistake) for about $100. It now has acquired a “reputation” in the recording industry, and good ones sell for over a thousand dollars.

It’s 600 ohms transformer-coupled balanced in and out, with a simple all-vacuum-tube and all-balanced audio amplifier and peak rectification and gain reduction circuit. It was built in the 1950s by the Federal Television Corporation (some were built by other contractors, I am told) for use in AM and shortwave transmitters run by AFRTS, the Armed Forces Radio and Television Service. It came with a manual, the most hilarious aspect of which was its instructions on how to destroy it (“use axes, knives, machetes, flamethrowers, incendiary grenades” etc.) in case it fell into “enemy hands.”

AM-864/U destruction instructions, from the operator's manual (click for the full-size image). And yes, the "enemy us" typo is in the original!

I modified it for adjustable release time (a faster release was often desirable for amateur voice, even relatively high fidelity voice, to increase audio density) and improved low frequency response. The AM-864/U imparts an airy, floating quality to the audio, and gives the impression of emphasizing the upper midrange. It doesn’t control peaks as well as a CBS Labs Volumax or a Gates FET peak limiter, but it’s quite usable.

At that time, my main transmitter was a Hallicrafters BC-610E (a 250TH final modulated by a pair of 100THs).  I used a UTC Linear Standard hi-fi push-pull-plates to speaker transformer hooked up backwards to drive the grids of the 100THs and the Linear Standard was driven through a resistive loading pad by a Knight high fidelity amplifier using a pair of EL34s and negative feedback from the modulated B+ of the BC-610. It was clean audio. The AM-864 was fed by my Shure 556S microphone into a Heathkit tube-type cathode follower preamp/tone control and the AM-864 fed the Knight hi-fi amp.

I used this setup for several years and it was what I was using when I made the AM amateur bulletin transmissions in 1977 to build opposition to FCC docket 20777 (which would have effectively eliminated AM and other experimental modes). I know where some of that old gear went (everything I still had in my possession in 2006 was stolen by my then-wife in complicity with the FBI and Joint Terrorism Task Force), but I do wonder who is using my old AM-864 now.

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