2 12AX7s doing volume modulation, with controls for speed and depth of modulation as well as a "shape" control that blends between different LFO waveforms. This is an older 3 knob version, the current production model adds a "rhythm" control and changes the tubes to 12AU7.
I'm taking a look at this for a friend, apparently it was used with the wrong power supply and is now dead.
After opening it up, I'm surprised to see it's a mixed analog/digital design. There is a 8 pin SOIC PIC 12F5091 microcontroller acting as a switch-mode power supply controller and a second larger PIC 18F2620 reading the pots, switch, driving the LEDs and bypass relay and generating the LFO. Nice sealed Omeg pots for all controls. Switching FET is IRF740AS. There is also an IRF FL210 FET, I'm not sure what that one is doing as I didn't trace it out. The SOT23 transistors appear to be FMMT717 and FMMT617.
After powering the pedal with the correct 12V DC the 78L05A 5V regulator was not working correctly (high output voltage). I tried replacing it with a 78L05 in a TO-92 package but eventually found that the 1206 R470 resistor in series with the power jack was burned and was reading high (Mohms), limiting current to the regulator. A 1206 resistor is usually rated for about 1/4 W so I replaced it with a 470 ohm through-hole 1/4 W resistor. Still doesn't work. Whoops, "R470" means 0.47 ohm, not 470 ohm. I swapped in a 1.2 ohm resistor (closest value at hand) and it fired up.
It sounds very good, and gives a great variety of authentic amp-tremolo sounds. It doesn't really go beyond this into more extreme territory or do industrial-style hard chopping. There might be a slight volume drop, but that could be something inherent to all tremolo effects. Personally I don't know if I like tremolo enough to need an accurate tube version, but I guess I'm not really the target audience here. I didn't take apart the optocouplers as this is not my pedal but I am fairly sure they are just neon bulbs and cadmium sulfide photocells heatshrinked together.
There were a few things I didn't like. A microcontroller as a SMPS controller doesn't sit well with me. This increases development time and is another part that needs to be programmed before assembly, all for a controller that will have slower loop response than a dedicated controller IC, which could also cost less. It's also annoying from a repair point of view, if the microcontrollers are dead they can't be replaced without access to the firmware. I find this acceptable for the larger controller (but manufacturers, please provide people with binaries so they can fix their stuff!) but I can't see any reason why that 8 pin PIC needs to be programmable.
The SMPS provides the high voltage supply for the tubes, the 12AX7 cathodes are sitting at 251V. However there are no high-voltage warning on either side of the PCB, it's pretty easy to open the thing and touch a capacitor charged to a couple of hundred volts (ask me how I know this). The PCB is also unusually difficult to solder to and seems to suck heat away. I don't think there are any internal layers so I guess the copper might be extra thick? I can't see any reason why this would be needed. Finally, I feel that something in this class should be bulletproof in terms of surviving the wrong power supply. I'm not sure what was used when this thing died (probably an AC supply) but I have seen much cheaper pedals survive quite a bit of abuse without burning anything out.
I never had the correct power supply, but it needs a little over 600mA at 12V, maybe more if the tubes are starting off cold.
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