Allison Gain Brain, FET's etc.

[barclaycon]

Hi. Does anyone have experience of fixing these?
I have got one that needs a new gain control FET (Q12).
It is a 2N5486. The manual says that the criteria for this is Vgs (off) 4.0 to 4.5v. I guess this means buying a load and testing them - but what is the criteria for this.
I was thinking of asking Paul Buff. I notice that he is running a lighting company now.
Any help would be appreciated.

 

[drunton]

Hey, I have fixed several of these, but have never actually seen the FET go bad. It was always been one of the electrolytic caps going bad.

Of course, that said, I have several on the bench that need to get a re-cap first and a few LED's. Maybe I will hit some FET's needing replacement. Should be a standard Vgs(off) test - using a curve tracer. Unless you are stereo paring them, you might not notice the difference in FETs. There is no built in stereo link anyway, just dual mono.

I'm still learning, let me know what you find.

 

[joe-electro]

I'd put a socket in and test a bunch of different FET's, then choose the one you like the sound of the best. Ears are the best test equipment anyway :grin:

 

[matthias]

bf245c and mpf102 comes quite close

 

[barclaycon]

Thanks for that.
I've just bought a batch of 50 x 2N5486
I'm going to borrow a curve tracer and see if I can get some matches.

 

[emrr]

you'll probably have to match the metering FET also.

 

[barclaycon]

Funnily enough Doug, the metering FET has a different spec. and is shown as having a Vgs of 2.3 to 2.9v (off) whereas the control FET is shown as 4.0 to 4.5v (off).

 

[emrr]

Ah, thought I remembered them being the same. I had to replace some metering FET's once, and it threw the metering off. Took some work to get it correct.

 

[matthias]

do you have a schematic?

 

[barclaycon]

Yes. I have the schematics. Both for the Kepex and the Gain Brain.
Kepex racks used to be a standard feature in studios when I started.
It was the only usable noise gate.
Remember the first version that used neons ? That was a wake-up for any tape op swapping modules over and getting a 110v. belt!
I think the first versions of both the Kepex and Gain Brain were much better than the second VCA-type version - even though relatively speaking they were noisy. The Gain Brain is still great for getting real 'impact' on things like tom toms. I'm going to retro fit them into my Scamp rack.
Just trying to get them into tip-top condition.

 

[matthias]

so the old gainbrain v1 is a fet limiter/compressor?

I'd really like to see a schematic...

 

[barclaycon]

Yes. It's more a limiter than a compressor. The ratio is something 40:1. But it has a Peak/RMS response control. It is extremely fast and was probably designed for catching peaks more than anything.
Like I said, studios like Trident used to use them on great records like Supertramp and Elton John. You get a high-impact sound that is completely levelled.
I don't have a scanner to scan the circuit in, but if you can't find the schematic elsewhere I will send it to you.

 

[drunton]

Yes, these are very fast, but the cool feature is the Peak/RMS detect control. It is not a switch but a fully variable control. This allows you to essentially dial in the attack. There is a separate release control.

Combine the original kepex and the original gain brain and you have full control over drums sounds. For kick you can dial in the snap or the wump and on snares control the head/strainer relationship - very nice - the kind of things that just don't show up on software comps.

They are strictly unbalanced, but fully discrete class-A. IMHO, Add some transformers and you have a poor man's 1176. They used to be about $30-50 each a real steal, but I recently sold a pair for $160 each.

I'll also see about scanning the paper copy of the schematic that I have - and post a link.

 

[barclaycon]

Yes, absolutely drunton.
The Kepex, Gain Brain setup was a classic combination used on a countless records for a discrete, punchy snare sound.
However, I disagree with you about the poor man's 1176. Even with transformers they would sound nothing like one.

 

[matthias]

I just bought a pair of gain brain II
but that one contains the valley vca.

I searched a lot for a schematic but had no luck...

if I understand everything right, there is a

gainbrain made by allison research
gainbrain made by valley people
gainbrain 2 made by valley people (several revisions)



http://www.actionfigure8.com/photoalbum/valleypeople/images/114-1446_IMG.jpg
http://www.audiobau.net/fileadmin/images/tx_qycontents/Gainbrain_gerackt.JPG
http://www.gearslutz.com/board/attachment.php?attachmentid=20403&stc=1&d=1148689651
http://www.proaudioeurope.com/images/products/outboard/UV/valleypeople_gain_brain.jpg
http://www.gearslutz.com/board/attachment.php?attachmentid=31987&stc=1&d=1172744632

 

[barclaycon]

Well the originals were Kepex and Gain Brain made by Allison Research and designed to fit in their rack. They were FET-type units.
Paul Buff who designed them (and whose first wife was named Allison) then teamed up with someone else and formed Valley Audio.
He had subsequently developed the Valley People VCA - I think from work done on automation systems such as Little Helper and the Allison mix system (horrible systems!) and when they relaunched Kepex and Gain Brain under the Valley banner (and in a new rack format) they called them Kepex 2 and Gain Brain 2.
As you say, these units used the Valley VCA (EGC 101 I believe).
I feel I am becoming a historian!

 

[drunton]

I don't think there was ever a Gain Brain I made by Valley, just Allison.

There is a Gain Brain 2 schematic as part of the operations manual over at the galaxy audio page in the manuals section. I think its here, but for some reason my browser is timing out on the page: http://www.galaxyaudio.com/Manuals.html

I like the GB2's also from a color perspective. I think of it this way - if you want color then go outside of the box, transparent use a plug-in, but that's just my experience.

I do find that I like my "go to" Allison version on all the same things I would use an 1176 on - but I only have a silver face 1176 and the "go to" Gain Brain has jensen iron on the front and the output drives a 990C pushing a Neve L02567 which adds a bit more color ( http://www.creativeion.com/authenticaudio/Studio_Projects/api.html ).

I find a good balance with the 1176 on Vox, the GB on Bass, the GB2's on kick/snare and an RNLA mixed a little in parallel on the drum bus.

YMMV, of course... Dave

 

[barclaycon]

Great stuff Dave.
I enjoyed looking at your web pages and all that customised gear.
Very well done.
I love seeing creativity like that.

 

[drunton]

Thanks for the complements - it is a labour of love. Racked some stuff for somebody once, it was fun and paid well, but it was hard to put it in the box to mail away.

 

[matthias]

isn't there anybody who has a schematic (pdf) of the allison research gainbrain ?