Federal Limiter AM864 thread

[johnnytoobad]

you are right about linking with insert and switch between the CT and R1/C1 (thanx for making me realise that my first idea was very stupid, because it was straight on 0), but maybe i was thinking of a switch (with something written like "comrade" or "slave" meaning a slave/master relationship opening circuit between link-jack and R1/C1, or not opening the switch, on any of the pair, a kind of mutual acting/reacting relationship, settings and inputs of both would affects both compressors... but i am not really sure we can mix control signal that way, maybe with an additional kind of "cv mixer"... i'll try first the easier way... and if it doesn't work i'll start thinking of a  "cv" mixer external or internal,but let's see easier first.
i got almost all parts, i am waiting next month to order the front panel from schaeffer, i'm still working on lay out, i'll probably use a kind of modular architecture inside of it, i'll use a 5U 19" rack panel and box is a hammond chassis bought  to build an amp for a friend that never came out of paper and discussions, so chassis is hammond 1441-28 16"/8"/3", tubes and trnasformers are out on the back of the box.
for input transfo, i got sowter transformers and for OT a pair of UTC  HA134 (cheap on ebay... it happens sometimes), that made me go to 6V6s PP.
here is picture of the modified schematic i'm working on...

 

[mushy]

There's also this: http://www.groupdiy.com/index.php?topic=8856.msg329485#msg329485

It depends if you want independent attack and release control. As mentioned in that thread, though, tube matching is really important. 

 

[johnnytoobad]

thax for the link, i already tried to read this, but link seemed broken...
what about the external sidechain, will i need a hotter input? or as drown on ths schematic seems good?
some of you did it on yours?

 

[mushy]

Thats the gist of the previously linked thread. The sidechain insert point is at the ct. Other than that, I yeild the floor to those smarter than me. I find that I always have to adjust sidechain levels but I'm not sure in this case if youll have balanced/unbalanced issues or anything. 

 

[pH]

Hey Cannikin, I win!  Got my Federal hum-free tonight by playing with grounding and getting the orientation of the input transformer right.

Now if I could only find a meter that would work...

 

[johnnytoobad]

I think it is easier and it might be just as much effective to just link with unbalanced jack one input transformer CT to the other and add switch between that and C1/R1, to choose which one you want as master or slave, if both are masters both settings should influence both compressions...
What about modulating compression with an output signal? did somebody tried this mod on hhis/hers? (for ex. compressing bass with bassdrum...)
About going full-wave rectifying, i am finally planning to use 2 6SQ7, C5 will have 2 values and a switch if i want compression to react to low frequencies or not.
for external compression control i will  go for unbalanced jack input, i need a phase splitter, there is 2 possibilities, a 6sl7 schmidt phase splitter that is both a phase splitter and amplifying or a transformer for phase splitting and impedance matching and a 6sl7 for amplifying signal, but do i really care about Z matching? this doesn't need audio high end quality... schmitted seems cheaper. i really need yoyr advices.
How would you add an unbalanced to main input? am i right with signal on one side of the primary and ground to the other side of input transformer?
is it really hot enough to use for preamp? so what about adding phase inverter, low cut and phantom power (i got an unused 50v/0,05Amp tap, usually for bias, on the power transformer that would be great for it)?

 

[gemini86]

...looking over the data sheets now, I'm still not certain:

Is the 6Q7 a valid substitute for 6SQ7 in this circuit?

6Q7
http://tubedata.itchurch.org/sheets/021/6/6Q7.pdf

6SQ7
http://tubedata.itchurch.org/sheets/049/6/6SQ7.pdf

Thanks

 

[emrr]

I had a real one in last week.  The owner was tearing his hair out on the restoration.  He finally killed the vast majority of hum with tube selection.  At one point he had tubes in that were hum free with no comp, but hummed on comp release.  That points to mismatch of tube curves. 

We ran some test plots, and there were significant 60 and 120 spikes at idle.  Driving a lot of compression covered the hum.  If you put a pad in front of the unit, the hum shot way up; a 40 dB line pad put 60 Hz at +15 over signal.  So there's a fairly narrow noise window in the first place, and this is on a device without a whole lot of gain.  I'd go with proper filament AC center tap method or even DC filament here.  With such a small amount of B+ filtering, I would probably experiment with a full PSU rework, knowing that it may also kill some of what's desirable if taken too far.    This is the classic simple house of cards. 

This one sounded cool.  I see why people love them on bass.  The attack is so slow that all transients get through.  More apparent bass largeness than is actually there on a plot.  Without compression, there's significant loss of bass.  Compression pushes bass up closer to flat, but not all the way, and also kills treble at the same time.    Seems like the kind of box you want to keep the needle reasonably slammed on at all times. 

 

[mushy]

Also why they are a popular choice for parallel compression. All that goodness left in the middle.

 

[emrr]

It's worth noting that the one I heard would be considered quite hi-fi compared to the various Dept. of Commerce limiters, and they seem to somewhat get lumped together in a lot of people's minds. 

 

[DerEber]

Hello all!

I finaly nearly finished my Federal after all this years. I am very happy as I feard all the hum problems reported here, but mine is dead quiet!

Only this motorboating!!
I tried lowering my B+ as others did sucessfully here but had no luck. I came down from 210V to 160V. Still motorboating.
Now my aim is to fully understand whats happening exactly to be able to solve the problem.
Hopefully some of you can give me some insight here.
Fist it´s best to discribe the problem that occures.
The schematic link is on the first page here.
A rather deep clicking sound simmilar to a just started tractor uccurs at following circumstances.
Only if there is no or a small input signal is applyed. So only when there is no or small gain reduction. Motorboating starts if one rotated R10 towards C5.
Motorboarting starts earlyer (pot position) at my faster speed mods. (smaller C1 R1)
It disappears when disconnecting C5.
I tryed some things and it worked much better when lowering C6 to 0.003. I would have been happy, but now the compressor startes to ignore low frequencys starting at about 200Hz. This is not my dream solution for a slow limiter like this, wich will manly be used on Bass  ???
When I rise C6 to 0,47uF I get no motorboating, but when I apply some input I only get a slowly pulsing signal.
I am a bit of ideas now what to do next.
I´d need to lower the gain of V4???
Also the datasheet of 6sql is telling me that diode biasing is not sutable fot this tube. What do you think of giving some exrta bias Voltage to the grind of V4.

please release me of this motorboating  ;D

I´ll send some pictured when I am done!

 

[emrr]

Does sound like it's centered around V4.  I'd pull V4 out completely, take the B+ back up to the stock value, and be sure it functions correctly as a fixed gain amplifier first.  Do you have other 6SQ7's to try? 

 

[DerEber]

Yes it seems to be V4. when pulling V4 it works fine without reduction.
I´ll have another tube, but they are only russian substitudes and it don´t work either.
So maybe better get the real stuff.

 

[emrr]

Metal tube with pin 1 grounded?

 

[DerEber]

Yes. They sold it to me as 6sq7.
It´s russian. Now it comes in to my mind that it manybe is an 6sq7....
I just cant read it. 

 

[DerEber]

Hi,
got some new 6SQ7 Tubes yesterday. This time glass type. Still no luck.
I am thinking about adding some series resistor somewere. But where and to what extend?
I tried 12K next to C6 but no change at all! 

Btw. I too swapped out the 12K R3/R4 to 47K as in former schematic. I couldn´t meassure any difference with my scope.
No "knee" for me.
Mine is a pretty hard Limiter, no matter what I do. Even driving it with +16db extra doesn´t change the output at all. Besides some arrising distortion. "Threshold" is doing what it claimes and "Current" is somewhat changing overall gain. I feel like "Current" will mostly stay at the same position (for 2,5V) for me.
So if someone else is thinking about not putting one of the Pots in front I´d reccomend leaving the "Current" pot. "Threshold is very usefull.
I am having a 1:1 at the Input and a 12db Pad at the output and it seems like beeing reasonable for me. I am having about 16db gain left.

best Greetings,
Stephan

 

[nielsk]

anyone have a schematic w/ voltages on it?

 

[gemini86]

http://www.preservationsound.com/wp-content/uploads/2014/02/Federal-AM-864-U-Manual-copy.pdf

Page 42 has voltages at all tube pins and most points in the circuit. I had at one point transferred all the voltages to the schem, but I've since deleted my entire schematics/resources cache while installing a new SSD into my PC. Still not very happy about that.

 

[nielsk]

Thanks for that, very sorry to hear of your loss...

 

[thomasdf]

Hi folks

I am working on building a turret board Federal with a few simple mods:

1) I will probably rework the PSU in order to get rid of the rectifier tube and to have a cleaner steadier hi voltage, and a DC heater power supply.  Do you have any advice / schematics for a similar PSU?
What tensions should I get? 6.3 VDC for the heater, but what about the high voltage?
Will I still need L1?

2) Also, from what I understood, the output bridge rectifier induces some distortion to the signal. What if I place the rectifier after the GR / Output level switch? This way, the rectifier would be "in the circuit" only when metering the output level. Anyway, this is a detail I guess. Maybe that since the circuit is open in GR mode the rectifier does not even interfere with the output signal?

3) I am wondering too if I should replace R13 and R14 with a dual 500K pot in order to have a pre output-stage pad as mentionned in a post somewhere in that thread. Any feedback on that? Anybody tried that? Is it of any use?

4) Last but not least, I want to add a rotary switch to control the release time by switching 3 or 4 different resistors (instead of using a fixed 2MOhms R1). I read that you could go down to 1M without any motorboating occuring... But only a few tests will tell I guess!

Thanks for reading, and for your help! Any comment is welcome!

Thomas