> The compressor it was originally based on - was that a Siemens? Model number?
Do we know it was based on some specific predecessor?
While there was a large body of other limiters, and this probably grew out of dissatisfaction with all the others(?), presuming it was "based on" something else might be insulting (or illuminating). I'd phrase that question carefully. Maybe lead-in with a survey of what other limiters were around at the time, and which ones he liked or found fault with.
And it seems somewhat unlikely he would have had a Siemens. I could be VERY wrong. But generally, audio gear did not cross the ocean much in those days. The USA had RCA and Altec and UREI and lots of other home teams, founded when radio and talking-movies were BIG money. The UK and Germany and other Euro countries had their own broadcast and recording gear, some of it very fine and not stuck in the US ruts. But distance and communications and regulations and voltages and martini lunches stood between them and the US market.
> Why is there a tapped pot on the control amp input ? Is it essential that it is tapped?
I'd like to know more too. But with a plain linear pot, the "5" position would be -6dB. With the tap and resistor, the "5" position is -15dB. A common "audio taper" would be -20dB. So if you don't need strict calibration, you could use an "audio" pot directly, or a linear pot but be cramped in the lower end of rotation. And maybe he could not get an audio taper pot of the requisite quality and matching (still hard today). An alternative would be a Daven/Shalco stepped attenuator ahead of the control amp input transformer, though that loads the vari-gain stage and is expensive. Or he could have custom-ordered a 180K Daven at frightful cost. And maybe you need better than 2dB or 1.5dB resolution when cutting disk levels.
> Why is the power supply regulated in some parts & not others?
If unregulated, the threshold voltages vary with line voltage. The vari-gain plate voltage and the 12AX7 voltages are all fixed.
But some stages don't need to be regulated. The 10 Watt output tubes sure don't need regulation: they make what they are told to make and ignore variation in supply. And the grid-bias on those tubes can be unregulated. If line voltage rose very high, letting grid voltage go negative keeps the plate dissipation from soaring. Also the control amp does not care about "crossover distortion", it only has to reproduce the peaks cleanly.
What still has me puzzled or amazed is: the vari-gain cathode bias and the back-bias on the rectifiers are both unregulated. They do track. I assume they actually cancel across the vari-gain tubes' grid and cathode, and over the likely range of wall voltages their exact value does not matter much. That is a very clever design observation, one I would not make when today I could slap a 7912 neg-reg in there faster than I could think about the real situation.
> why did he design it in the first place? I'm sure it wasnt for bass guitar
Rock on, man!
The stereo version is VERY clearly sold as a disk-cutting limiter. There was a mono before that, and again the fast time constants match disk-cutting problems. A radio limiter does not have to be so fast; overmodulation won't hurt the transmitter, you just need to keep the splat so short that nobody complains. In disk-cutting, overmodulation runs the cutter into the next groove, ruining the cut. And the time constant will catch anything you can get from a tape or disk source. The other media that needed fast limiting was RCA's film-sound ribbon shutter, but RCA made limiters for that (I have heard they were forced to develop limiting so they did not have to use WE-patent film-sound).
