Federal Limiter AM864 thread

[Linus]

I have an original Federal Tube Limiter AM-864/u for sale. It's in original condition and will need to be gone over before you use it. But if you are able to build a clone, you shouldn't have any difficulty with that. LOL.

I have it listed on eBay for $1650 with a "best offer" option.  I imagine the cost of all the part of a clone is pretty close to that amount. Considering how high these usually sell I think Inhave it priced fairly.

Thanks.

 

[Deepdark]

I have an original Federal Tube Limiter AM-864/u for sale. It's in original condition and will need to be gone over before you use it. But if you are able to build a clone, you shouldn't have any difficulty with that. LOL.

I have it listed on eBay for $1650 with a "best offer" option.  I imagine the cost of all the part of a clone is pretty close to that amount. Considering how high these usually sell I think Inhave it priced fairly.

Thanks.

Building a clone is pretty cheap. Under 1000$ for sure. I built one few months ago and it didn't cost more than 900$

 

[dwaterhouse]

Hi all. Rather new to this place. I've been perusing this topic as I've picked up a Federal by chance. My question might be rather naive, but this seems like the place to ask if I'm going to get an answer!

A little info on the Federal:

Seems to be original, no tag on front. When testing to see if signal passes, I only get low distorted sound with lots of hum. I've tried replacing filter caps with the exception of the can capacitor, no hum reduction. I recently got brave and decided to test all of the voltages and resistances at the tubes. I've been finding that there are some discrepancies with what the manual is asking for, but here is what I've got:

V1:
1 ) X
2 ) X
3) X
4 ) - .978V / .788M (Manual calls for -1.3V/ 2M)
5) X
6) 9V / 1K (Manual calls for 118V)
7) 6.5AC
8) 146V / 28K (Manual calls for 155V / 60K)

V2:
1 ) X
2 ) X
3) X
4 ) - .978V / .788M (Manual calls for -1.3V/ 2M)
5) X
6) 9V / 1K (Manual calls for 118V)
7) 6.5AC
8) 146V / 28K (Manual calls for 155V / 60K)

V3:
1 ) X / 470K
2 ) 149V / 15.5K (Manual calls for 185V / 16.5K)
3) 3.3V / 249
4 ) 470K
5) 148V / 15K (Manual calls for 185V / 16.5K)
6) 3.3V / 249
7) X
8) 0AC? (Should be 6.3 AC)

V4:
1 ) X
2 ) X / 245K
3) 2.3V / 260
4 ) -.864V / 1M (calls for -1.42 / 2M)
5) -.864V / 1M (calls for -1.42 / 2M)
6) 130.5V / 44K (calls for 140V)
7) 6.5AC
8) X

V5:
1) X
2) 159V (calls for 180V)
3)X
4)166AC / 182 (calls for 175AC / 220)
5)X
6)166AC / 182 (calls for 175AC / 220)
7)X
8)160V (calls for 180V)

I know thats a lot to put out there, but if anyone has any direction they could put me in, I would be super grateful! I've just sort of come to the end of my ability to diagnose here and I've probably taken on a bit more than I can chew. Added a picture if that helps!

Thanks!
D

 

[Joechris]

Hi, I think you should replace the can capacitor in the power supply. .. something is def wrong with the heater around V3.
The unit looks original to me..nice.

best

 

[dwaterhouse]

So I've replaced the can cap and the hum is gone! I get signal through that's clean and undistorted! Though I don't think any gain reduction is happening. On the V3 puzzle, it seems that the 6.3VAC is pin 7 on every other tube, yet the schematic says 8. I measured 6.5VAC at pin 7 on V3 which seems to follow suit with the rest of the tubes and pin 8 is tied to ground.

Thoughts?

Hi, I think you should replace the can capacitor in the power supply. .. something is def wrong with the heater around V3.
The unit looks original to me..nice.

best

 

[Joechris]

Have you tried to adjust the current pot and trehold pot on the back? Mabye a new 6SQ7 (V4 ) tube..?
V3 is working now.

 

[Matt C]

I've been collecting parts to build one of these units, and while studying the schematic I'm finding a few things that I can't make sense of.  Anyone have any insight into the following?:

- The manual states that the 6SK7 screens sit at 118Vdc.  To my eyes it looks like they should be at 12.5Vdc (which seems unusually low), coming from that simple PSU voltage divider/bleeder made up of R6 and R8.  What is going on here?

- Around the 6SQ7 triode section, again my understanding/calculations disagree with the voltages listed in the manual.  The cathode is at +2.5Vdc, coming from R8.  The bottom of the 500k pot (R10) feeding the grid is connected straight to the cathode, not to ground.  This would lead me to believe that the Vgk is 0V because that pot shouldn't be pulling any DC, but according to the manual there is a 0.6V drop across it. Not a lot of current, but it's something.  I suppose it must be grid leak biasing, but I thought that usually required a bigger grid resistor.

- Similarly, the grids of the 6SK7 sections are listed as sitting at -1.3Vdc.  Is this another case of grid leak bias?  or is the diode section of the 6SQ7 conducting just a little bit while idle?

 

[Wildfara]

Probably gonna be my next project as well...I have a fair bit of the parts around...Found a sweet old Triplett meter for it and the tubes...I have a Sowter 1160S in a UA610 I built that I think will work for the OPT temporarily...till I have more money...easy enough to swap around. I don't think the mains tranny is super critical. Got plenty around

 

[vinyvamos]

Adding info/discussion to an old thread here, rather than start a disjointed new one...

Have any of you actually measured/analyzed the attack and release times on the AM864 circuit? I have one on the bench currently and I simply cannot justify fitting the attack pot to my front panel, as it simply does damn-all! I have tried so many different values, pre-charge/attack caps or whatever you wanna call it, and the sidechain seems to just not want to have a controlled attack. Or at least not controllable with any perceivable change.

Right now I have 22K for R11, followed by a 50K pot for attack, then instead of R1 I have a 150K resistor with 2M pot in series. This 2M pot gives me decent control over release time.

I see that the Nu-Federal PCB/Design uses 22K for R11, 25K release pot, and R1 is made up of 1M pot plus 1M resistor in series. This is all well and good but I have yet to find any evidence of actual attack and release times... Are we all just fitting this attack pot for the sake of it? Hmmm

- The manual states that the 6SK7 screens sit at 118Vdc.  To my eyes it looks like they should be at 12.5Vdc (which seems unusually low), coming from that simple PSU voltage divider/bleeder made up of R6 and R8.  What is going on here?

Bit late quoting this now Matt C but why not I guess... Yes I also noticed this 118V versus the low V in the actual circuit. My assumption was that it was a typo, missing a decimal point and should have been 11.8V in the manual. This is an extremely low Screen voltage compared to say the BA-6A which uses same tubes, but then the plate loads in the federal are lower at 12K compared to about 22K in the BA-6A. BA-6A has reg.150V thru 5K to screens. Maybe this is just what gives the federal it's sound??

Similarly, the grids of the 6SK7 sections are listed as sitting at -1.3Vdc.  Is this another case of grid leak bias?  or is the diode section of the 6SQ7 conducting just a little bit while idle?

Yes, well mine are measuring just -800mV, with and without the 6SQ7 fitted in fact. And in order to completely turn off G.R. in mine I need to apply a set minus voltage to the sidechain, otherwise the 6SK7's still drop their conduction and compress with high level input. and that is with AND without 6SQ7 fitted! But with the set bias voltage she distorts suuuuuper nicely!

 

[beezer4]

Hi folks!
I wanted to start by saying how great it is to have this site and all these threads as a resources. I know it's cheesy, but as someone learning DIY audio gear and teaching myself, this site is insanely helpful.

I completed my first tube build, a Federal AM-864, and wanted to share some pics and deets. I followed a lot of what Chris posted on his site: Federal AM-864 Limiter Clone: Build Notes: update 1 – Preservation Sound If you haven't checked how Preservation Sound, please do.

It sounds super dope and has a pretty low noise floor. I do get a little bit of motor-boating with the threshold and current pots set to the extreme, but honestly it's not a setting that's useable anyway. I was really surprised by the bass response and how nice bass guitar or a mono drum OH sound through the unit.

I need to clean up the wiring a bit more for my next build, but was pretty happy with the layout and low noise.

Here are a few pics:

 

[beezer4]

I’ve built a few 864s successfully now. These are great sounding compressors and fairly easy to knock out.

I just wrapped up a 2U version and wanted to share some pics.

 

[vintagelove]

I’ve built a few 864s successfully now. These are great sounding compressors and fairly easy to knock out.

I just wrapped up a 2U version and wanted to share some pics. View attachment 92056View attachment 92057View attachment 92058

Beautiful simple layout. Nicely done. Is it your own design?

 

[ReDolby]

Layout looks great. The front panel is nicely symmetrical (and colorful) and the knobs must result in a great tactile feel. Well done!

-Tim

 

[Tubetec]

Nice work Beezer . I have a shelved AM864 project I must get back to one of these days

 

[beezer4]

Beautiful simple layout. Nicely done. Is it your own design?

I followed pretty closely to the original schematic. Primary differences are leveraging a solid state rectifier for B+ rather than tube and additional filtering via the JJ cap can / tweaking R1 and C1 for better release time / added an output level control with a Bourns 600 ohm T-Pad / adjustment to R2 for the VU meter.