I've been tweaking with the reverb in my solid-state Randall head and it still doesn't sound quite right. Built a tube head which is really fun but doesn't have reverb. Not really a pedal guy so I thought it would be cool to build a spring reverb into the 4x12 speaker cab. That way any amp I plug into it will have reverb. I have a long 3-spring MOD reverb tank (9EB2C1B) that I bought for the Randall [and didn't use because of mismatched input impedance]. Since the input transducer basically works like a speaker, simply hook it parallel with the speakers in the cab, no need for an additional driver circuit. The 800 Ohm input on this tank is actually a pretty good match for this. All it needs is a couple passive components to filter and attenuate the signal. I measured the DCR (76 Ohms) and inductance (137 mH) of the input transducer, plotted it out in Excel, then fiddled with R and C values until the high and low cutoffs were about right [based on the reverb driver info on the Valve Wizard site]. The following pic shows the response with a 0.47 uF cap and 1.5k resistor in series with the driver coil. No need for a parallel capacitor as the inductance of the coil provided the high frequency roll-off. I actually guesstimated 680 Ohms for the series resistor but through experimenting found the 1.5k to sound better. I'm still playing around with it, haven't settled on anything yet. Of course, it does need an amplifier for the recovery. Right now I just have the tank sitting on top of the head, a Y-adapter on the speaker lead to drive the input, then the output of the tank is run to a separate amp. It works surprisingly well and already sounds better than the reverb in the Randall head [which has a nearly identical reverb tank]. The plan is to replace one of the 8 Ohm speakers with a 16 Ohm so that the cab can be wired up like a 3x12, then add an amp in the cab to drive the fourth speaker from the reverb tank output. I'd like to find a slick little Class-D amp so I won't have to worry about heat in the closed-back cab.