> why not use a 6SQ7 in the sidechain
Makes things worse.
6SQ7 (half a 12AX7 only lamer) has higher output impedance than 12AT7. Actually 5 times higher.
That means C1 will charge slower.
> as in Federal Limiter?
The Federal is also slow. Even slower than the 1954. It was never made to be great audio.
That's not bad. I was taught to avoid clipping more than a few mS. Some systems are hyper-sensitive and need 50uS. But clipping is widely used for "sound". You can't put drumset in a mix without violent peak reduction, which pretty much means clipping, and often quite a lot of it. Fortunately a ADC accepts overdrive without telling you about the lost bits, no rail-bounce/stick such as some analog channels do.
If you want "FAST" audio limiting, with the usual hold cap values, you need under 1K source impedance. And since GR voltage needs to be larger than peak signal voltage on the vari-tube grid, you want, 5, 10, 20, even 40 Volts behind that 1K. You want a 0.1 Watt to 1 Watt Power Amplifier.
Several of the Classics used the existing 0.1 Watt line output stage to also drive the rectifier. This makes some control difficult; also tends to load-down the peaks. Both flaws are very acceptable, either in safety limiting or limiting for effect.
Sand-state devices dump current tons better than tubes: faster attack. Gain is cheap too: flatter limiting. If you wanna "make it better", easy: go sand. Heck, put a DSP in the sidechain and compute any dynamic response you like. (Also program it to self-test and compensate tube unbalance.)
lesser stepdown in the output transformer
To a point. 10K instead of 600 would give 4 times the level, except many of our inputs really are 10K so we could lose half of that, get only double the level.
The real question is: what is the output power? To get 20dB compression (surely more than TV commercials merit) from the operating point they seem to have selected, 6SK7 current has to fall to about 0.2mA. Depending on the load, that's not much over a milliWatt, well short of traditional Line Levels which reference 1mW and go up from there. No doubt this beast can jam a ADC to max, but a lot of engineers are not happy unless levels are HOT, even though they end up padding-down.
Getting a limiter to just "work good", control audio with minimal damage, is harder than about any other "basic" audio design. Doing it well in tubes is just awful hard. Fortunately most modern music needs "color" more than accuracy.
