Gates SA series preamp racks

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emrr

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Recently rearranged a couple of Gates SA series preamp racks I’ve been using a long time, this was spurred by the desire to take these out on an Avett Brothers remote recording job.  The SA-70’s were in a frankensteined SA-700 rack case, on which someone hacked the rear to be SA-800 sized, and had no front panel controls or connectors.  The SA-94’s were in an SA-800 case that was a little more cosmetically challenged, so I swapped cases with the new control escutcheons hiding more scrapes.  It took a number of years to find 5 of of the original Gates escutcheons. 

The SA-94 rack was previously covered here:
https://groupdiy.com/index.php?topic=28450.msg343899#msg343899


First up is the impressive hacking of the SA-700 case.  A bit of custom metalwork with welding, and careful cutting of the case. 

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Re-racked SA-94’s

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SA-70 rack rearrangement

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The connector panel and spacer had to be odd sized, and barely fit.  The answer was to cut down a piece of L bar.

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Interstage gain controls are moved off module onto the front panel, using Daven pots.  I’ve done this before with standard pots on other re-racks, and have to say response at intermediate gain settings is much better with the Davens.  Standard pots start to show some treble roll-off, but there’s no change here at all. 

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I burned in a new set of 6J7’s and 6C5’s, then looked at gain/response/noise compared to the tubes that have been in there since the early 2000’s, and found no real degradation in the used tubes.  Current was slightly lower, but that was it. 

I listened to a number of PSU’s I have here, mostly rebuilt tube regulated types, and didn’t find any appreciable noise difference.  The SA-94’s are still on the General Radio, and for now the SA-70’s are still on a Heathkit.  I have a Gates SA-40 console PSU (CLCLC) which would be the right matched piece that I need to try after a recap, it would add another 5RU though.  In practice these are fine with AC filaments.  This rig went out on a remote recording job which was acoustic duo, mix of condensers and ribbons.



 

dmp

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Beautiful work!

Are the Daven pots 100ka like shown on the schematic? Why are they better than 'standard' pots at avoiding treble loss?
 

emrr

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dmp said:
Beautiful work!

Are the Daven pots 100ka like shown on the schematic? Why are they better than 'standard' pots at avoiding treble loss?

Yes, 100K.  I have to guess capacitive effects are lower with larger spacing and discrete resistances. 
 

ruffrecords

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dmp said:
Beautiful work!

Are the Daven pots 100ka like shown on the schematic? Why are they better than 'standard' pots at avoiding treble loss?

The most likely cause of treble loss is capacitance between the wiper and ground. A 100K pot driven form a very low source impedance has a maximum source impedance of 25K. For a 3dB drop at 20KHz you need 318pF of capacitance with 25K ohms. Most tube input stages will be a bit less than this even allowing for Miller capacitance raising it to 100pF. So as long as you don't have long high capacitance screened cable from the grid to the next stage you should be good up to 60KHz.

So the big question is what source impedance drives these pots?

Cheers

Ian
 

emrr

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ruffrecords said:
So the big question is what source impedance drives these pots?

https://groupdiy.com/index.php?topic=44868.msg597855#msg597855

The Daven with 20 total inches of screened cable is better than the onboard standard pot with no screened cable length. 

The SA-94 is an SA-70 fixed gain two stage preamp that feeds a 250K Daven rather than an output transformer, and that drives a 2 stage output amp.
 

ruffrecords

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EmRR said:
The SA-94 is an SA-70 fixed gain two stage preamp that feeds a 250K Daven rather than an output transformer, and that drives a 2 stage output amp.

Is the input stage of the output amp a triode or pentode? (can't seem to find schematics anywhere)

Cheers

Ian
 

emrr

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ruffrecords said:
Is the input stage of the output amp a triode or pentode? (can't seem to find schematics anywhere)

Cheers

Ian

6SJ7 pentode on the SA-94...which is an aside from the 100K discussion in the SA-70......
 

ruffrecords

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EmRR said:
6SJ7 pentode on the SA-94...which is an aside from the 100K discussion in the SA-70......

But if the pot is driving a pentode stage there is no Miller capacitance at the input.

Cheers

Ian
 

emrr

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ruffrecords said:
But if the pot is driving a pentode stage there is no Miller capacitance at the input.

Cheers

Ian

There are two amp types here.   

100K is SA-70, feeding triode stage with NFB across it.  That's the one I brought up.

It is interesting in relation to the SA-94, 250K in the consoles with 2 feet of screened cable feeding a pentode.  The previous iteration of that basic Gates amp used a triode instead of a pentode after the gain control, but with a standard pot mounted between the stages, short run. 

 
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