Federal AM864/U compressor

[joel hamilton]

Man, it feels like everyone got hip to these quick....

[Osumosan]

I wonder why, Joel...

[joel hamilton]

heh... I know... Me and Garges certainly didnt keep our mouths shut.
I think it is because Cgarges had like 29 of them to sell...

They really are great though. Has become my go to bass and sometimes vocal compressor.

[honkyjonk]

Are you guys strapping a resistor across the output to lower the output signal a bit on these babies?

Anyway, I did, now I can't remember the value of the resistor, but I think I need to lower it even more. It overloads my digital HD recorder before I want it to.

[cgarges]

Yeah, Joel, you spent entirely too much money on yours.

I still have five of them. Don't email me about selling unless you want to give me an absurd amount of money for them. I sill need to do some work on a few of them.

Chris Garges
Charlotte, NC

[joel hamilton]

Hopefully I can get the guy that set mine up (perfectly, I might add) to post here about what he did!

[tubemonkey35]

I still have both of my pristine ones. We should thank M. Brauer turning most of us on to them.

honkyJonk: I have a 620 (Blue, Red, Brown) Ohm +/-5% 1/2 watt strapped on mine.

[soundguy]

putting a single resistor on the output is only going to bridge impedances more gracefully, if you have too much level coming out of the box (which you shouldnt if your system is properly cal'd for +4dBu) then you should really build a pad, I think H pads sound the best, they are balanced. You could also buy a decade attenuator box and plug the output into that so you can dial in the exact output level. Decades are really handy to have around if you are dealing with old shit like this and you can find them on ebay entirely reasonable for what you'll get out of them.

dave

[tubemonkey35]

hmmm... Mine works great the way it is with the resistor in place. However maybe it would be nice to have an output volume control on this box; like a buile in decade box.

[soundguy]

as it should. That single resistor isnt going to pad level if you need a big pad in there, it just presents the proper load on the transformers secondary for the rigth frequency response, that output transoformer was meant to look at a 600 ohm load. If the output is overloading a digital input, the unit needs a pad, looking at a modern 10K load is likely not enough to throw things off that much. Personally, Ive found out of all the old tube junk I have used, most of it was designed for totally different operating standards, the 864 matches thhe modern +4dBu standard so if you are having trouble interfacing with a digital system my guess is that there is either something wrong with the limiter or the digitial system needs to be calibrated. In a pinch a pad on the output would work however my experience with these things really points to something else.

Put a 1K tone into that 864 and measure the output with an AC voltmeter, set it up so you see 1.2ish volts on the output then stick that into whatever digital system you are using, if you are overloading you need to recal you system, 1.2v AC is +4dBu or your common 0dB on a VU meter. The 864 as is should put out 1.2vac if you have everything set right. If you cant get the output near 1.2vac then you can try a pad, a decade is a variable attenuator so you can dial in the exact amount for the pad and its easy to find a nice decade with daven attenuators which if you dont set the thing on fire will more than probably outlast you and your kids, those things are built like tanks.

dave

[dhenderson]

There should be no need smother the Federal with H-pads, though as a last resort that would be the way to go. This isn't like a Gates SA39B, designed to drive a transmitter at +26 with 2 6V6's driving the output. It's a mildly tuned +4 unit. The key is to get the threshold set properly, and the thing called "Threshold" on the back is actually a ratio control (and extreme clockwise settings will produce pretty wild overshoot, or "negative" compression values as per Mr. Gerzon). The think called "Current control" is actually threshold, and if you set it to read 0.7 VDC at the test point on the rear (no signal and all other controls set counterclockwise), you should start hitting compression at about 0dBu. As you wind on <the thing called "Threshold" which is actually ratio>,to about 2/3 CW, the threshold point appears to drop into the realm of-10dBu or so. So then you can actually use the input control to get the sound you want without padding anything out. I have to say it would probably be the most fun to pull both these controls out to the front panel -- it appears that you can't actually damage the thing even with ridiculous settings (where you would think that the rectifier/triode tube would just catch fire). Pull the test point to front as well and you can calibrate the thing in 2 seconds. Just an idea...

[luckybastard]

what are the going rate on these things and where can you find them?

[cgarges]

what are the going rate on these things and where can you find them?

TWELVE MILLION DOLLARS! Bwah ha ha ha ha ha.

They were designed in the fifties. Ebay.

I'm sure the rate will now go up.

Chris Garges
Charlotte, NC

[joel hamilton]

what are the going rate on these things and where can you find them?

Just look around at all the usual suspects.... Old radio shops, attics, basements, estate sales, ebay, CGarges house....

[cgarges]

Old radio shops, attics, basements, estate sales, ebay, CGarges house....

I was going to say that they're all at the studio but I just looked to my right and there's one here. And the earlier Transformer Corporation version, too.

Chris Garges
Charlotte, NC