Gates SA-94 program amp rack-up

Gates SA-94 four stage single ended program amps installed on a Gates SA-800 housing, with custom connector and switch panel. General Radio PSU. These are generally found in SA-40 and SA-50 consoles from the late 1940's-early 1950's. These are the UTC iron version, they were also done with a lesser Triad iron set. +/-85 dB total gain with 6J7 / 6C5 / interstage attenuator / 6SJ7 / paralleled SE 6SN7. I missed getting the whole console, but got these and the SA-800 PSU case right after it was all parted out by someone else. Primarily wired for mic preamp use, with every possible option I could cram in. All caps and resistors replaced.

• Mic input pad.
  Mic or hi-Z input switch; hi-Z skips the first two stages and puts input into the attenuator.
  Internal high/low gain switch that bypasses the 2nd stage 6C5; cuts gain to about 65 db.
  Polarity switch to address various input and gain configurations.
  Optional 500 ohm input and 5 ohm output wired to original terminal strips.

I've used these as mic preamps, DI amps for keyboards/bass/guitar, and 0.5 watt monitor amps. I purposefully left phantom power off of this since almost no condenser needs this kind of gain range.

I missed this one...

Once again amazing work!

did you build the power supply?

yeah i missed this one too....

bloody lovely ....what i would do to get my mits on a pair of those!!

could it be used as the summing amp to make a mixer??

best
Pete

I missed this first time around too! No idea how. I love how the back panel is jammed to the max and the front panel is as basic as you can get. Great job as ever.

How do these sound Doug?

Cheers,
Ruairi

These were used as console program amps, and would work fine for any 600 ohm mixer make-up. PSU is a stock General Radio type I re-capped. How do they sound; good - like you would expect with UTC LS series iron. I mainly use these for ribbon vocals with acoustic music and ambient stereo mics, drum overheads, etc. They work great in the lower gain Hi-Z input mode as DI amps or buss amps. They have been pressed into service for various rock instrument amp mics when I ran out of other options, but not ideal since gain is so high. Sounds fine, but always a funny feeling to see the gain control set at -42 dB. It would have been rare to find these outside of a console frame, but the rack frame they are in is the correct one. The most deluxe Gates console kept all amps in two six foot racks and the console was simply passive controller. These would not have been used in that type installation though, and very few of those packages would have ever found buyers, as it was $5000+ ($43K 2008) versus $1700ish ($15K 2008) for the standard console in 1950. If I'd kept it as a completely stock install, there would only be a blank center panel and all power and audio wiring would be to the terminal blocks....and I'd use the 5RU Gates PSU.

In my best chief Wiggum

thats a fine mic pre conversion you got there

Click to expand...

Don't know why I missed this too, but excellent work, Doug!

:thumb: :thumb: :thumb: :sam: :guinness:

Very nice Doug! :thumb:

Holy shit, gotta resurrect this thread! Wow, what a nice job.

Missed this thread too!
My first two radio stations had Gates SA40 consoles.
The first had an RCA BC3 console in the main control room, the SA40 was in production.
The second had 2 SA40s, one main control and one production.
The main control one was badly mangled by a succession of hack engineers who used plastic covered pot lamp cord for inside wiring modifications!
Needless to say, with the heat generated by all those tubes, the insulation dried up and fell off the copper conductors!
The first time, after the "engineers" were fired and I took over before a real engineer could arrive, I tilted back the console on it's hinge for routine maintenance and saw the safety chains so I assumed they were properly anchored.
They weren't.
The console went over on it's back, ripping off ALL the wires.
I sat there from midnight til 6am with a pair of headphones, putting it all back together with no schematics, no wiring diagram (the old guys never documented anything) and managed to have it working again by sign on.
The guy who used to be the engineer before being demoted to farm director said,
"Oh, you're supposed to prop it up with the garbage can."
The production console, however, was pristine as if just from the factory.
Wish I had grabbed that puppy when it was replaced a few years later.
These things sounded amazing!
Good on you Doug for rescuing some of the parts!

Hey guys, thanks.  Excellent story Mike!  I can see it all happening.  I don't really want to know the sound of an SA-40 hitting the floor upside down. 

I have an SA-40 frame with the other triad transformer version of these amps, but it has no external case.  I assume it came from one of the desk mount assemblies. 

Anybody have a schematic for this? I can't find the Gates SA-94 on the web.

Would be in an SA-40 or 50 manual if you can find one. The SA-134 is the easier thing, is three stage, and is posted here.  Output trans still tough; 25mA single ended with full bass response. 

Thanks - didn't find those manuals other than mail order copies. But the SA-70 and SA-134 schems that are available seem to have the same building blocks. Output on the SA-70 is easier - but less headroom without the paralleled 6sn7s I guess.

Think I posted the 3 stage SA-73(?) in the SA-70 thread.  Important to remember that it has the same output headroom, so it's effectively lower input headroom, really only good for low level signals.  I'm philosophically against high gain tube preamps that have to be run with interstage pots nearly off, all you are doing is throwing away a ton of system gain and compromising S/N unnecessarily.   

Also look at the 7X I posted, predecessor to the SA-134.  Same issue of a tough to source output transformer.