This was my thought as well. There were no pedals. There was no reverb quite yet. Leo got his from the Hammond Organ company a bit later. Delays and echoes, in the form of tape machines, oil cans, etc were unwieldy, yet very very cool.I am wondering if Trem/Vibrato was built in to amps because it was new and cool. There were no other effects at the time. From the second half of 50's to early 60's vib/trem was probably all there was. No? Reverb was not in guitar sounds yet. Nobody really wanted dirt then. It just developed as pop music (ie. Stone and Beatles) started turing up amps. Pop songs with dirt became very popular. Then dirt/fuzz pedals came along. Univox, EHX LBP-1 ect. Then came flange, delay, chorus, phase and tons more. In the late 50's were there any other neat effect options to add to amps? I'm not sure. Just assuming trem/vibe was the **** man!
I don't remember if I ever figured this out. Maybe you guys know: why do only the smaller Fender amps have the bias vary tremolo ('vibrato')? I used to think it was something to do with the wattage, but Allen's whole amp line, even the large amps, have luscious bias vary trem. Was it simply tube economics? The smaller amps had an extra triode available that the larger amps did not?
@theprofessor, you mentioned your trem pedal using an opto circuit. Yep. If it's a pedal, it's either opto, or purely a digital recreation. Bias vary (obviously) and Harmonic both require actual tubes!