Ignoring some minor (0.1% or 0.01dB) servo-insert details. and possible fraction-dB error at very high gain:

The gain is IIG * (1+Rf1/Rs1) * (1+Rf2/Rs2) * OIG, where

IIG= Input Iron Gain, about 1:2 or +6dB for JT-16

OOG= Output Iron Gain, about 1:0.9 or -1dB for JT-11 loaded in 600 ohms

Conditions:

Rf+Rs should not be "too low", where 75 ohms is way-low and 600-1,000 ohms is more comfortable.

Rf1||Rs1 should be "low" compared to the 600~800 ohm source impedance at JT-16. (A similar issue arises for Rf2||Rs2; actually Rf2||Rs2 can be higher than Rf1||Rs1 by about 1+Rf1/Rs1 so it tends to be no problem.)

(1+Rf1/Rs1) and (1+Rf2/Rs2) should not be less than 2 (I think) to keep the 990 stable.

In the published plan, Rf1+Rs1 is 2,253 ohms, worst-case Rf||Rs is 563 ohms. A somewhat lower impedance might be measurably better, but pot-values are limited, and anyway the gain at this worst-case condition is so low that noise is moot.

Gain: (1+Rf1/Rs1) is minimum 1+1,210/1,043= 2.16 = 6.7dB, maximum gain is 1+2,210/43.2= 52.15 = 34.3dB, and both nets have the same gain.

So suggested minimum gain is 6dB+6.7dB+6.7dB+(-1dB)= 18.4dB, max is 6dB+34.3dB+34.3dB+(-1dB)= 76.7dB

Taking IIG and OIG (transformers) as fixed gain, the two amps give gain of 12.4dB to 70.7dB total, 6.2dB to 35.3dB each.

Total range of gain is 76.7dB-18.4dB= 58.3dB

>

*I'm thinking 31 steps.*Yo can get a 31-throw 2-pole switch?

I assume 31 switch positions which is 30 steps. 58.3dB/30= 1.9433dB per step. I'll assume you like "2". And that is really 1dB per step in each stage.

If you want a dead-simple calculation, find a

"Switched potentiometer" calculator, tell it you want a total resistance of 2K or so, and 1dB steps from -6dB to -35dB. (Or 0dB to -35dB and throw out the top settings.) (For the above calc, tell it 36 steps and -36dB, fixed slope.) Normalized to 2K ohms: 591 to FB, 122, 109, 97, 86.6,... 5.46, 4.87, 4.34 to ground. (No, that dang Java App won't let you cut/copy the values to text... I'll let your fingers do the transcribing.)