Federal AM864 - My build

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Deepdark

Well-known member
Joined
May 19, 2013
Messages
1,321
Location
Quebec, Canada
Hi guys

Here is my Federal AM864 build. I think it could be a good idea to list all issues I got and what were the solutions, so if someone encounter the same ones, there will be a resource for solutions, since a lot of am864 thread have some lost informations, outdated, etc.

So, here are the specs:

Input tranny: Edcor XSM 600/10K
Output tranny: Edcor XPP1 600/10K (push pull)
Power tranny: Edcor XPWR012
Meter* Hairball audio 8020
Case: Collective case blank case

Layout: Nearly identical to the one we found Inside the manual. I interconnected some elements on the board, instead of wiring everything to the tubes. R6 is on the main board, at the other extremity of the turret board. Caps are all Panasonic polypropylene.

PSU: Diode rectifier. I replaced the entire psu section with a RC topology, as linked below. Super quiet, lot of filtering.

Heaters: Ac heaters. The power transformer has a center tap so I strap that to ground. I run both wires to heaters pin, so basically, I didn't followed the schematic. I did it the classical way. No hum. Tightly twist and far from B+ and all other circuit wires

Ground: I made a little ground buss, taking all ground connections of the audio circuit in a hierarchical maner, then strap the ground buss to the psu 0v, and then straight to the main bolt. Shields (6SK7's and 6SQ7, xlr's shielded wires) are short, strap on their respective mounting bolts.

Motorboating: I had motorboating that was related with filtering. I had to add another 47uf filter cap on the psu and rerout a little my wires to get rid of it. The only remaining motorboating is related to the 6SK7. They are pretty well balance, but one can make a balance pot. The easiest and most effective way is to play a little with the curent ctrl. I ended up setting my curent ctrl to about 2.5v, and play with threshold to achieve what I want. I set it to about 10db of gain reduction.

Meter: I read over the internet that it needed a 2ma fs (DCma), type of meter. Once searching over some original unit's pictures, the meter were rated 1.010ma fs. What I did, is taking a Hairball standard vu meter. It's basically rated 200μA, so 1/5th of the original spec (speaking of 1.010ma fs). R2 is set to get about 1.2 vdrop dc across that resistor, with the proper meter strap in ||. With mine, I got about 0.28 vdrop, so the needle sit really low on the scale. I basically change the 240ohm resistor for one that is about 5 times it's value. I ended up using 909R instead of 240ohm, and the needle sat nearly 0. I trim the little bolt on front of the meter and Voila!. the 0 has been adjusted to my 2.5v curent ctrl. if you change that setting, you have to recalibrat the meter. Does it change anything to the circuit? No. no motorboating, no anything else bad.

Schematic: I used the one with 12K plate reistors (R3/R4)

Voltages Values: Basically, if you use the manual to validate your voltages, beware of the fact they were measured with 47K plate résistors, and calibrate as speak in the manual. If you use 12K plate resstors, then your plate voltages will not be 155v but will sit higher, about 185v (if you search over the other thread, lot of peoples get those values). V1 and 2 pins 4 are about -0.92v, instead of -1.3. I didn't noticed any issues with that value. I know some guys were aware of that value, not being the same as the one found on the manual. Other than that, voltages are pretty spot on. My B= is 191v. It runs good.
 

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You're welcome. Since the others thread are old and some informations has just gone, I tink it could be nice to report back issues and solutions. A kind of up to date thread :) Good luck with yours. It's a nice compressor once calibrated.
 
Hello!
Be getting the parts out of my boxes, and I intend to start my build soon.
Again, thanks for theses informations, I will buy the Edcor PSU transformer soon.
Just, one last question, how do you add a balance pot to the output amp?
Merci d'avance!
Thomas
 
I didn't have to make a pot balance. but if you need one, you can use a 100 ohm pot between the cathodes 3/6, change R9 to about 190R and connect to the weeper of the pot. It will give you your 240ohm at each cathode, and you'll be able to fine tune depending of your tubes
 
This looks great Deepark!

It is P2P,do you have layout for it!?

It is very similar to RS124 and Altec 436c!!!!


But looks amazing.....

Thumb,thumb,thumb

 
It's alive!!

I have prototyped it in a guitar amp chassis, and it didn't caught fire when I turned it on. It actually works, but the meter is way off... I'll see how I can fix that. Also, the current control and ratio know do not seem to have as much effect as in the original unit (I have one for comparison here). But you are right, no motorboating whatsoever.
I built it with vintage input and output transformers, big irons from a General Electrics Uni Level, and it has a very nice whooly color to it. I think I might even prefer it to the old one... but I need to do further adjustments and tests.
I'll add a faster release time too.

My input knob is "scratchy" even tho brand new, like if there was so DC going thru it... Same for you?

I am very happy, thanks a WHOLE LOT for your PSU design and informations.

@iprovlek, the P2P layout is in the original manual, it's very easy to follow it or tweak it to your needs.
 
I have racked and used my 864 build...it is an agressive sounding unit...very hot in needing a 15db inpuut pad...iron are old Merits...unshielded I found a piece of steel to shield input xformer. I am still getting to know it. 1ma gr meter and matching 'audio volts'...I like the BC 730 metering scheme but audio volts meter dimes when unit is sounding good...need to explore my control voltage as it is high compared to manual specs....still sounds good though not liquid as I was hoping for....optical build in near future
 

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I have racked and used my 864 build...it is an agressive sounding unit...very hot in needing a 15db inpuut pad...iron are old Merits...unshielded I found a piece of steel to shield input xformer. I am still getting to know it. 1ma gr meter and matching 'audio volts'...I like the BC 730 metering scheme but audio volts meter dimes when unit is sounding good...need to explore my control voltage as it is high compared to manual specs....still sounds good though not liquid as I was hoping for....optical build in near future
Amazing front panel, i like the color, is it raw iron? Or paint? It looks a lot like vintage military gear!
How fast can go the attack?
 
Thx guys for your kind comments...the front panel with support sides is from the salvage store. I bought many old test carcasses and saved them for projects and iron and tubes that had any relevance to audio...the owner past away 15yrs back and was a spirit guide. The panel already had the meter holes.it was likely a military test gear that I gutted....I liked the scratched up vintage look so just kept it as crinkle finish charcoal dk gray steel.
I HAVE NEVER HEARD AN 864U, SO I DON'T KNOW HOW CLOSE I CAME, but it has an edgy in your face compression sound. Puts a force field around the vocal and makes it cut through a mix. Very late 50s/early 60s vocal sound...definitely NOT an LA2A...
 
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