Federal AM 864 Compressor MODIFICATIONS questions

[Bonsaimaster]

Dear Friends,

I am inheriting a Federal AM 864/U limiter. I think it is in good condition. I am looking to make modifications to the unit. I have searched all the threads but still a bit confused. Therefore, I am ask you all to help with the information I have obtained. Maybe this will be a resource to others trying to do their own modifications. I am going to try to keep the modifications compartmentalized as this may be easier to follow. Please correct or enlighten me on any of these modifications. I will keep updating the post with the new information.

1) INPUT ATTENUATOR
Replace the 500k linear pot with a 500k log pot.

2) INPUT ADDITION
Add a 30dB "H" pad before 600 Ohm input

3) ADD ATTACK CONTROL
Replace R11 (47k) with 25K log pot and 22k resistor in series
(Are there any suggestions for a stepped switch? resistor values? or just a 12 step linear switch?)

4) ADD RELEASE CONTROL
1 Meg log pot in series with R1
(Are there any suggestions for a stepped switch? resistor values? or just a 12 step linear switch?)

5) METER ZERO
Add 500 ohm in series with R12 (1.8k)
Place on back of unit

6) ADD OUTPUT
1k Log Pot as part of the H-pad

7) THRESHOLD LIMITER (RATIO)
Replace 500k pot with 100 K log pot plus a 400K resistor in series
Move to front of unit
(Are there any suggestions for a stepped switch? resistor values? or just a 12 step linear switch?)

8) CURRENT CONTROL (THRESHOLD)
Replace 1k pot with 1k log pot with a 1k3 resistor strapped across legs 1 and 3. (resulting in ~ 600 R total resistance).
Move to front of unit
(Are there any suggestions for a stepped switch? resistor values? and corresponding ratio values?)

8) Things I am not sure about

-Disconnect jumpers for LO 600 Ohm input (Not sure what jumpers for LO 600 ohm input mean. Necessary? )

-Place a 680 Ohm resistor across output terminals (Do I do this if adding the H-pad ?)

Thank you for being patient with me. There maybe some misunderstanding but this is what I have deduced by reading the forum posts. If there are any other suggests please share.

Bonsaimaster

 

[scott2000]

I honestly can't recall but maybe bernbrue and Zayance's NU Federal project had some of the popular modifications implemented and you could pick a few things from it...

 

[HerbertR]

As Scott says - use the Nu Federal 864 scheme from Zayance and Bernbrue. All the mods are in there. The Nu has an important H pad on the input. This depends a lot on the tubes used to get the current control/threshold and ratio/threshold limit potentiometers into a good working range. I've had tubes need 6-24dB of attenuation or easily overloaded the circuit to the point of "motorboating". The input potentiometer after the input transformer, before the Mu Tubes, on the Nu Federal is a dual 500K logarithmic. An output Transformer termination before an output pad makes no sense in my opinion. But I can talk only about the Nu federal here

I bet the original AM864 sounds like it was made for heaven, and with these mods...

 

[thomasdf]

Some years ago I had a Federal in my studio and built one from scratch, for fun. These are very nice sounding unit, but they are getting expensive and sought after so maybe I'd think twice before drilling a bunch of holes in it. The vintage unit was not mine and the owner had asked me to modify it for output attenuator (mandatory ! as they have TONS of gain I used a fixed pad + bourns variable T Pad) and a simple two position switch with two release positions, faster than stock. In my opinion, it was just what the unit needed. I played around with threshold and ratio but did not find much use for this.
I like the big naked front panel and have never been into swiss army knife pieces of equipment .. so YMMV !
You can croc clip a cap a replace C1, see where it gets you before the infamous motorboating

 

[Bonsaimaster]

Totally agree. Looking to just get a new panel and keep the old panel if I ever want to sell it.

 

[DaveP]

Why replace linear pots with log? Log pots are just several sections of linear track joined together. A good quality linear conductive plastic pot will give you minimum noise.
best
DaveP