Federal Limiter AM864 thread

[cannikin]

Since I'm going to start building the AM864 Limiter, I figured I'd list all the different threads to centralize the info. (maybe this can be the AM864 Meta)

The Schematic:
http://www.waltzingbear.com/Schematics/Misc/AM-864.htm

Federal AM864/u mods and MAXSON Dept. of commerce:
http://www.groupdiy.com/index.php?topic=4465&highlight=federal

Federal Am864/U limiter attenuator problem:
http://www.groupdiy.com/index.php?topic=5828&highlight=federal

My New Project?? - Collins Limiter:
http://www.groupdiy.com/index.php?topic=11555&highlight=federal

6L7 vari-mu compressor design:
http://www.groupdiy.com/index.php?topic=10097&highlight=federal

Input tran ratio for Federal AM864/U limiter?:
http://www.groupdiy.com/index.php?topic=5994&highlight=federal

Federal AM864 troubleshooting help:
http://www.groupdiy.com/index.php?topic=7517&highlight=federal

AM864 repro?
http://www.groupdiy.com/index.php?topic=10326&highlight=am864

lots of HEATER current:
http://www.groupdiy.com/index.php?topic=2080&highlight=am864

Pre amp & Collins AM864 compressor combined, ideas need:
http://www.groupdiy.com/index.php?topic=5611&highlight=am864

PAD for the Federal
http://www.groupdiy.com/index.php?topic=29373

If I missed any threads about the AM864, please post them. :grin:

 

[cannikin]

manual Posted on page 14 of this thread

 

[dramadisease]

thanks for compiling all the relavent data!

 

[NewYorkDave]

Just wanted to point out that the 6SK7 plate resistors--12K in the schematic linked above--were 47K in some versions. Ask Soundguy which values were used in his limiters, 'cause his sound killer.

A relevant outside link that includes an alternate version of the schematic:

http://www.montagar.com/~patj/ftc.htm

 

[tubemonkey35]

I own two of these units; they are fantastic.

 

[kiira]

This looks pretty easy to build. The tubes are $4.50 NOS, the 6sn7 may be more. I laready have the output xformer ha ha (the edcor I ordered by mistake) and a nice little enclosure I scavenged at the shop - former multi-serial port box.

This should be an am864 meta. Thanks.

Kiira

 

[Rob Flinn]

good work David, May well have a go at this oone myself (when I finish the rest of the half finished projects I have)

 

[audioforge]

good idea canikin to do this thread. i was about suggest it in your drawing board 'one.
tubemonkey ,maybe you can help...
i ve read that there are two versions and one with "extended" frequency band. on a pic i have the stanley input traffo is only rated for 100-5000~.(can t read the output on the pic..)
could you tell us how are labelled yours.
audioforge

 

[tk@halmi]

I think I am going the low cost route with this one. I already have the Edcor WSM600/10K for the input and the XSM10K/600 for the output.

I do need a recommendation for the power transformer though. If I used a HEXFRED rectifier and eliminated the need for a 5V tap the Hammond T269EX may be a workable solution (380VCT@65ma, 6.3V@2A).

I can't believe people are paying $800+ for these things. You can DIY one with higher quality components for $250. That includes a decent VU meter and a pro looking front panel.

Tamas

 

[soundguy]

mine have diffferent transformer sets and sound similar. Granted, the transformers might be similar souding, dont know for sure, but Im inclined to guess that the tone of this thing is in the circuit, not entirely the transformers.

This could be one of the easiest things to build ever, completely silly to pay that kind of money now. Shit, it even makes more sense to build a stalevel from scratch when you consider all the mods you have to do to those things stock...

dave

 

[PRR]

> A relevant outside link

Some errors there.

The text says gain reduction is done with G2 voltage; in fact it is G1 voltage like most other limiters.

The schematic is missing a critical dot where V2p8, R4, C3, and C5 come together.

The detector is half-wave, and will give inconsistent peaking on asymmetrical waves. It is slow and that can't be changed. Neither is a killer fault if the sound is right; if you want a different sound, considerable rework would be needed.

People insist on using DC heat.... here is a Classic where the heat is not only AC, but (apparently) unbalanced. You can't say it has too much filter cap either.

 

[NewYorkDave]

[quote author="PRR"]
The detector is half-wave, and will give inconsistent peaking on asymmetrical waves. It is slow and that can't be changed. [/quote]

That's what stuck out to me when I first saw the circuit some time ago. As I remarked in another thread, I had seen the schematic and dismissed it as a flawed circuit until I paid Soundguy a visit one day and heard his AM-864/Us. I only heard them on a full mix but they sounded really good to me.

It's interesting that the designer(s) went for half-wave rectification in a compressor that's presumably meant for voice transmission!

People insist on using DC heat.... here is a Classic where the heat is not only AC, but (apparently) unbalanced.

I attribute the knee-jerk insistence for DC heaters (and regulated B+) to the audiophiles, the same guys who have to clutter up a perfectly nice, simple tube circuit with BJT current sources, MOSFET source followers, bias servos and other crap...

In the classic old gear, the stuff everyone fiends after today, unregulated B+ was the norm and you only found DC heaters used in the lowest level stages, if at all. And they didn't have an effective way to regulate such a low voltage, so the normal way was to use a full-wave bridge, a couple of big filters caps and a dropping resistor (fixed or variable). Your heater voltage would rise if you pulled a tube!

 

[tubemonkey35]

I will look at the tranny's on both of mine and report back. Are there any other questions while I have the tops off? Does anyone want to know something from the inside?

 

[Rob Flinn]

tube

Any chance of some detailed pictures ???

 

[cannikin]

Edit :thumb:

 

[soundguy]

anyone in the NY area is welcome to stop by my studio and check out my federals, I have a pair each a few years apart. Nobody reading this forum should be buying these for what they are selling for now, you can build one in an afternoon...

dave

 

[cannikin]

 

[audioforge]

"The more I look at the schematic, all I think is: "Wow, too easy"
that s why i suggested this one..(and cos i m interested building one too :wink: ).
"I will look at the tranny's on both of mine and report back. Are there any other questions while I have the tops off? Does anyone want to know something from the inside".
thanks a lot tubemonkey35. i have some more question about R5 input attenuator. is it a dual 500k log pot???? could you measure it?
then r8 pot(1K1W) seems much bigger on the inside pic i have than the 500k pot(R10). could you tell us more (AB or clarostat etc.. type ref.)
the big metal can should be C1 and i think it s soundguy said it s a MICA 1uf. what will be a good substitute..
as for pictures http://www.montagar.com/~patj/ftc.htm will give you ideas to do a layout and organizes your unit...
some voltages mesures will be great at the power traff out around the 5y3 rectifier before and after the choke too...
then only for "cosmetic" thoughts vu meter dimensions type (brand and sensitivity fs etc...) and diameter of the big knob....
hope it is not too much asking...
audioforge

 

[synthi]

:idea: Nice project!

I think we could have some cheap diy vari-mus after all!

Could anyone send me the PDF by email??

Just push the email button at the bottom!
:grin:

Thanks,

Synthi.

 

[adrianh]

Yes blame me.
I have a scanned 66 page manual.
I have only cleaned up 22 pages.

pester me by email
tell me to get busy.

Yes I will share.
That was my goal over a year ago.