EMI RS124 questions and "Superfuse"

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valveandsound

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Hello everyone, first time poster here. I repair studio and musical equipment in Dallas Texas and build the occasional piece of studio gear.

I am currently in the middle of converting an Altec 438 into the venerable RS124 (big thanks to Winston O'boogie for kindly sharing that documentation) and I have a few questions about the super fuse mod:

The RS124 has fixed attack settings - am I safe to assume the 33K in series with the 6AL tube anodes sets the attack on this circuit? Would a 250K linear pot in series with a 10K resistor work well to make a variable attack control?


Regarding the super fuse control on the Chandler reissue:

Chandler seems tight lipped on what this does. However, from the descriptions of it and looking at a couple of gut shots I can see that it looks like a switch between the release control and something. My guess is this either shorts the release switch, or switches in a much lower value resistor in parallel with the release resistor value. I don't think it actually shorts the control because that would in effect short the signal passing through the side chain.

Does anyone have any info specifically on the super switch?

Thanks in advance.

Schematic attached


 

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  • Abbey Road Studios RS124 Schematic-D1-2.jpg
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emrr

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Is 'super fuse' the hold feature?  WTH they come up with these dumb-ass cartoon names.....

If so, it lifts the release resistor. 
 

valveandsound

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emrr said:
Is 'super fuse' the hold feature?  WTH they come up with these dumb-ass cartoon names.....

If so, it lifts the release resistor.

No, i believe the hold feature is just exactly what you described (typically the "in between" positions on Recovery switch"

and yes, I agree the name is dumb. I think its taken from a setting on the plugin which makes the attack very aggressive.
 

emrr

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valveandsound said:
am I safe to assume the 33K in series with the 6AL tube anodes sets the attack on this circuit? Would a 250K linear pot in series with a 10K resistor work well to make a variable attack control?

Many here have confirmed the existing value is about as fast as it reasonably goes before misbehavior.  You should see what the longest useful time is and tailor any variable. 
 

valveandsound

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emrr said:
Many here have confirmed the existing value is about as fast as it reasonably goes before misbehavior.  You should see what the longest useful time is and tailor any variable.

Thanks Doug.

From my understanding of how the sidechain works on these Vari-mu designs, the .5uf cap (in the sidechain) discharge rate is what dictates the release function. The recovery control essentially switches in different values to ground which set how fast it discharges. At fastest settings this is a 1Meg value, roughly what? .5 sec release time. IIRC the max release setting is something like 6 seconds (!?) So if you wanted the release to be faster then heavily reducing the 1M value or switching in a much smalller value in parallel (say, maybe 50K? should get you that aggressive release. At least thats what my logic says.
 

emrr

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Right on the effect.  Note as release gets faster, threshold effectively goes up because the cap never charges completely, so at a certain speed you will start clipping the amp more than you will get compression. 
 

emrr

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It could be a fast release to cause purposeful misbehavior, it could be wired there to throw everyone off the scent, and be an unbalancing of the detection like the Federal.  It could be something else. 
 

valveandsound

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FWIW I spent a lot of time on this superfuse setting, and here is where I ended up:

At the fastest setting, the release control has a 1Meg value. I ended up adding a switch that adds a 500K trimmer resistor in parallel with this and tweaked it until I was able to approximate how it sounds with several YouTube demos of the "superfuse" mode. This seems to do the trick. The added benefit is that if you dial the trimmer to zero the toggle can be made to simply turn off the sidechain and use the compressor as a Preamp.

Hope this helps others who have been curious as to this feature!
 

UffeK

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valveandsound said:
FWIW I spent a lot of time on this superfuse setting, and here is where I ended up:

At the fastest setting, the release control has a 1Meg value. I ended up adding a switch that adds a 500K trimmer resistor in parallel with this and tweaked it until I was able to approximate how it sounds with several YouTube demos of the "superfuse" mode. This seems to do the trick. The added benefit is that if you dial the trimmer to zero the toggle can be made to simply turn off the sidechain and use the compressor as a Preamp.

Hope this helps others who have been curious as to this feature!

Thanks for sharing this.
It certainly look like some sort of short of the recovery through a fixed resistance  at the superfuse switch.
What value did you end up with on the 500k pot (the sweet spot)?
Also the hold positions of the switch seem to be chained and in with the superfuse also.
 

Winston OBoogie

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Yeah, I bet that's about the right deal with what that thing is, nice work valveandsound 👍
It def won't be the result of sound engineering principles, more of a "let's f**k up the tone" kind of thing. 

The other stuff re. replicating certain A.R. serial numbers on the new attack feature is pretty easy to figure out and mostly smoke and mirrors to downplay the authenticity of the schematic I posted. 

But the schematic tells the story just fine.

Thus endeth my usual annual post, hope all are well,

Hinson O'Bugger
 

letterbeacon

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valveandsound said:
The RS124 has fixed attack settings - am I safe to assume the 33K in series with the 6AL tube anodes sets the attack on this circuit? Would a 250K linear pot in series with a 10K resistor work well to make a variable attack control?

Did you try this and did it work as expected?
 

rickc

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yes, varying the 33k resistor changes the attack time, HOWEVER if you make the attack time too short, the circuit becomes unstable. Yes, you can vary it to some extent.
 

rankot

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Does anyone have complete schematics for RS124, or docs for converting Altec 438 into RS124?
 

Winston OBoogie

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The only thing missing off that document is where it shows there was a pre output attenuator signal which was available and brought out to the terminal strip.

Everything you need to know is shown.
 

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