I've been working on an old stereo spring reverb project recently. It uses two single ended el84 pentode based amps driven by 12ax7 twin triode tubes and was from an old TV set. Both amps have treble, bass and loudness controls and are used to drive the spring tanks. I found the stereo amp in the back of a TV repair shop and one man who worked there told me to haul everything away that was out back of the shop. I picked up a number of tube based things, too. I added a mixer stage (mixes direct signal and reverb) and a spring tank preamp (on separate smaller box) to the stereo amp and converted it to solid state rectification which reduces the heater load by 1 amp without the 6CA4 rectifier tube in it. I figured by not using the 6CA4 I could power the heaters from two more 12ax7 tubes in the mixer and preamp stages. The plate supply voltage doesn't use much current for the 12ax7 tubes. One reason I put this together again is because it has a low output impedance and I wanted to try it with two Hammond necklace style spring tanks (pictures below) and should work with that. It works good with the more common spring tanks shown on top of the cabinet, but the output is low with the necklace tanks for some reason. It might be that I had the connections reversed. The patent shows the driver is 36 and pickup is 37, which I had reversed because coax was going to 36 and two wires connected to 37 and assumed the pickup connected to the coax because of better shielding. I'm going to put some RCA phono jacks on the necklace tanks and connect them to the modified amp again when I get a chance and see how it works. If the magnets in the tanks became demagnitized I could add small rare earth magents and see how it works.
I got the used cabinet from a music store.
https://patents.google.com/patent/US298 ... oq=2982819
I tried connecting the reverb system to an FM radio and my tubed stereo system and it works well with the straight spring tanks.
Note: I used an ammeter, voltmeter and capacitor checker to check the components and test the amp before and while powering the amp.