Gates SA-70 Western Electric 141-A mash-up and tweak
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Hi Doug,
i have some questions regarding this interesting WE 141-A Mash-up you posted. There is this feedback path with two caps and two resistors you used to shape input transformer's response; i searched RDH 3 and 4 where i could only find clear explanation up to 0,068uF cap and 56k resistor. 250pF cap and the other 56k resistor look like another filter, but i don't really understand the whole picture with this two components added. Can you please explain a bit more about it?
Another thing i'm wondering about is if what you say "feedback path" is ussual NFB with some filtering properties. There is not much written about anode to anode NFB in those two books beside what i could find.
In more ussual NFB from anode to cathode of previous stage resistor values aren't as low as yours, this is why i would like to get better explanation and understanding. If this is NFB with filtering properties, how much of NFB there is (more or less) in dB? I know years have passed since you had preamp at home, i hope you can at least partially reply to questions like this one.
I have a few Cinemag CMMI-7's and some other good input transformers which could be used to build this. A friend gave me a very nice case to recycle, i have power transformer and most of components. But beside having material i would like to understand this circuit a little better.
Btw, Lassoharp kindly replied to my questions and said very good things about this preamp. It looks very interesting to me because it lacks OT and have interstage volume control. IMO it deserves more atteniton, it's nice to see something like this that is not just another copy. Can't wait to hear replies and build it!
Cheers,
Dejan
i have some questions regarding this interesting WE 141-A Mash-up you posted. There is this feedback path with two caps and two resistors you used to shape input transformer's response; i searched RDH 3 and 4 where i could only find clear explanation up to 0,068uF cap and 56k resistor. 250pF cap and the other 56k resistor look like another filter, but i don't really understand the whole picture with this two components added. Can you please explain a bit more about it?
Another thing i'm wondering about is if what you say "feedback path" is ussual NFB with some filtering properties. There is not much written about anode to anode NFB in those two books beside what i could find.
In more ussual NFB from anode to cathode of previous stage resistor values aren't as low as yours, this is why i would like to get better explanation and understanding. If this is NFB with filtering properties, how much of NFB there is (more or less) in dB? I know years have passed since you had preamp at home, i hope you can at least partially reply to questions like this one.
I have a few Cinemag CMMI-7's and some other good input transformers which could be used to build this. A friend gave me a very nice case to recycle, i have power transformer and most of components. But beside having material i would like to understand this circuit a little better.
Btw, Lassoharp kindly replied to my questions and said very good things about this preamp. It looks very interesting to me because it lacks OT and have interstage volume control. IMO it deserves more atteniton, it's nice to see something like this that is not just another copy. Can't wait to hear replies and build it!
Cheers,
Dejan
According to the Gates SA-70 manual, there is 8 dB NFB there, and it should be similar here. The 1st 2 stages in my mash-up are essentially the Gates circuit, with the biggest difference being that stage 2 is loaded much more lightly, and should have more voltage gain than it would in the Gates amp. So maybe a hair difference in the NFB loop, which is virtually local FB around V2, just including the previous interstage cap and attenuator in the loop.
An additional note is that these first 2 stages exist with similar lighter loading in the SA-94 program amp, where they are the 1st 2 of 4 stages, and feed a 250K gain control. The anode to anode NFB is the same there, but gain is fixed as if at the top of the SA-70 volume control, which is simply a fixed resistor.
Back to the FB loop, and how it came to be in the SA-70, originally it was a simple 100K R between anodes, with the added benefit of slightly raising V1 plate voltage with the additional B+ path, and the interstage cap as the only tailoring element between V2 anode and grid. There I quote from a published article written by a Gates engineer, exploring various methods of NFB application in the SA-70. Over time, they added the various response tailoring elements. I cannot think of another commercial example of anode to anode FB similar to this.
With modern transformers, you may not need any tailoring of FB at all. You may like the sound without that FB path at all. I never tried it at the time. Now, having much better test tools, I would explore all of those possibilities if I were to revisit. I did rebuild an SA-70 using 37 and 76 (i think) tubes, and left the NFB out completely, and used it a lot for awhile. Lots of possibilities.
An additional note is that these first 2 stages exist with similar lighter loading in the SA-94 program amp, where they are the 1st 2 of 4 stages, and feed a 250K gain control. The anode to anode NFB is the same there, but gain is fixed as if at the top of the SA-70 volume control, which is simply a fixed resistor.
Back to the FB loop, and how it came to be in the SA-70, originally it was a simple 100K R between anodes, with the added benefit of slightly raising V1 plate voltage with the additional B+ path, and the interstage cap as the only tailoring element between V2 anode and grid. There I quote from a published article written by a Gates engineer, exploring various methods of NFB application in the SA-70. Over time, they added the various response tailoring elements. I cannot think of another commercial example of anode to anode FB similar to this.
With modern transformers, you may not need any tailoring of FB at all. You may like the sound without that FB path at all. I never tried it at the time. Now, having much better test tools, I would explore all of those possibilities if I were to revisit. I did rebuild an SA-70 using 37 and 76 (i think) tubes, and left the NFB out completely, and used it a lot for awhile. Lots of possibilities.
We've talked about this in several Gates threads around here. It's basic single stage plate to grid NFB, if you look closely. I can confirm from simple act of disconnecting, and watching gain change, that it is indeed negative. Being 8dB (again, quoting from Gates manual, also posted here), it's hardly enough to make the volume control behave in the crazy manner you find with 20+ dB NFB.
Jean Clochet
Well-known member
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Jean Clochet
Well-known member
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Thanks for reply Doug. I had to redraw SA70's schematic to understand it better. The way Gates draw them is a little confusing to me, specially with this unussual network around 2nd stage anode. You are correct, only 8dB of NFB going on, i knew it should be low if interstage pot is inside it. There is also this interesting note in the manual about the heater voltage; Gates engineers could get it as low as 3V (yes, 3V)without problems, only heating time was longer. I couldn't notice any difference at 4,9V in another preamp either.
I couldn't find schematic for SA-94. if you have it, would you please upload it? I'm also very interested in reading article about various methods of NFB application in SA70.
I couldn't find schematic for SA-94. if you have it, would you please upload it? I'm also very interested in reading article about various methods of NFB application in SA70.
