tube preamp - RCA/ GATES inspiration

[dmp]

I've been planning to build a two channel tube preamp with UTC transformers and an RCA / Gates type circuit for some time. I built a RCA BA2 that I really like.
The coil audio version of the SA-70 has an output control ( between the tubes, so replaces R5) and a feedback (a pot in line with R8 I suppose), which is a cool feature.. In the demo I heard, the feedback doesn't seem to change the volume - just the tone. I don't understand how that would be accomplished? But they claim 42db-54db of gain so maybe that is the feedback sweep?
Ideally I'd like to build something that could go up to 60 db of gain. Suggestions?

 

[emrr]

Lassoharp did a BA-2 inspired 3 tube circuit that might still be viewable.  On the Gates, 42-54 would be right from stock to no feedback.  I'd guess they did gain compensation so the tonal change could be heard.  I had an SA-70 for awhile I rebuilt with 4 and 5 pin 1930's tubes and no feedback, it worked and sounded well, no drastic difference.  Transformers are always the majority of the color until overload. 

 

[dmp]

Yup, I've seen his build. And I've been away from building for a while so maybe I should review some posts.
But the question I am stuck on is if a third tube stage is added, should it be switched out at low gain to get a better noise performance? Or run through all the stages at all times and attenuate down for lower gain even if you might start hearing the noise floor?
Thanks for the observations on the Gates gain.

 

[emrr]

I don't find it unreasonable to suggest entirely removing a stage to lower gain though you rarely see it.  Good layout and intelligent switching could make it happen.  There are WE and RCA amps in the 1930's that have all sorts of strapping options to remove stages, add pads between stages, etc, to arrive at a particular max gain (and overall headroom) figure.  I've a layout on paper for one using grid cap tubes that allows you to move the grid cap to a later stage tube and lower overall gain.  It could be done with relays, maybe. 

Though for the most part the classic method that works just fine is a pot after stage one, knowing most noise is in stage one.  I've yet to see an old 3 stage tube preamp of that structure that had any discernible noise with the gain pot all the way down, meaning 2 stages and output transformer are always pretty quiet if decently designed and laid out. 

There are later Gates program amps that use dual volume pots between stages 1/2 and 2/3 to minimize noise, and so far I've found it does a better job than skipping a stage for lower gain.  Those amps all use pentode front ends, so overall it's higher gain stages they are trying to wrangle.