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I think when the amps were designed, each stage was considered separately, from input to output. Looking at the signal going coming out of the preamp, before the phase splitter, it's still pretty clean even at 12 with a normal enough electric guitar plugged in and strumming.
It's the power tubes that limit things, as the signal of the phase splitter gets clipped when the grids on the power tubes try to get driven positive, causing current to flow into the control grids.
I'm thinking the power tube output section of the early amp designs was more or less lifted straight from the RCA tube manual, while the preamp stages with the tone controls were designed in house, and the huge voltage swing coming from the preamp with it's less-lossy tone controls is to blame.
Tone control circuits in the tube manuals of the day were not as boosty as the fender ones, so if they were playing around with adapting the typical Bandaxall tone circuits and wound up with the Fender tone stack, they would find a surplus of gain on their hands.
Also I think some designs were adapted to use miniature 12ax7-sized tubes from older designs which used octal preamp tubes, like the 6SL7 for the preamp and 6SN7 for the phase splitter/driver. Both of those octal preamp types had less gain than the later miniature 9 pin types, so circuits designed to break up at say 10 would now break up way sooner.
Just a theory..
Or maybe the guys in Fullerton just loved distortion!
Oh I just read your original post again-
you're talking about the Deluxe-
It was more of a student amp or something, so they probably just did a great example of what i stated above, they just more or less slapped a preamp circuit from another amp designed to run the big tubes (like the 5c4 Super)onto the output using 6v6 tubes, which have less bias voltage so will clip way sooner..
they were selling fine so they left it alone.
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-Clint
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