> mix control to blend clean (bypass) signal with the vibrulated signal
Reverb is different. The sound coming out of the spring has ZERO original sound. It's like the tree that falls in the forest, makes no sound, yet there are these big THUDS coming back from the hills... un-natural.
Tremolo is usually made with a "straight amplifier" except some normally-fixed part (bias or a resistor) is made voltage-variable so the gain is not constant, wobbles up and down.
Then if you dial that varying voltage down to zero, you are back to a "straight amplifier" with constant gain. AA764 w/Trem
and AA764 no-Trem
are the SAME plan, except w/Trem has an oscillator and a buffer. The buffered wobble is dumped via a pot into the second stage of the no-Trem amplifier.
If this pot is full-down, that 2nd stage works normal (insignificant bias change due to added 25K in cathode; also minor change in cathode cap). It IS a AA764 no-Trem.
If full-up, +/-1mA is dumped into 2nd stage. Since 2nd stage runs at 1mA normal, +/-1mA forces it to 2mA (higher gain) and to zero mA (zero gain). This is done at the tremolo rate. The sound tremolates.
So Tremolo usually does not need/want a MIX system. It just dials-back the wobble to nothing, leaving a simple amplifier stage.
The "problem" is that this type of simple not-balanced gain control has HUGE 10Hz output. Maybe 100V at tremolo rate mixed with 10V of signal. You can't hear the 10Hz directly, but the 100V will distress the next stage. AA764 uses 0.02u+220K grid-filter into 6V6 which reduces stuff below 40Hz, but you may still have 25V of subsonic slam. Apparently the cheap OT further reduces it so that the cheap speaker did not slap (too bad).
Hmmm... a 0.1u cap from that 2nd stage into a Fender Reverb transformer will shed a lot of subsonic AND reduce that whopping 20V signal down to 0.3V, hot-pickup level instead of smoking-amp level.