Adding spring reverb to a speaker cab.

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.

 

The 16 Ohm speaker showed up in record time. I got it installed and rewired it so that three speakers are driven from the standard speaker input on the back of the cab. Also mounted the reverb tank and wired it's input to the main speakers. The output of the reverb tank and the fourth speaker are each wired to a 1/4" jack on the side of the cab. This way I can experiment with different amps and EQ before settling on one to mount inside the cab.



Before mounting the reverb tank I added the input filter. I went with a 0.47uF cap and 1.5k resistor in series and ended up adding a 0.1uF cap parallel to the driver coil. That took out the "zingy" metallic overtones.

 

It was getting some feedback when the reverb was turned up. To remedy that, I unscrewed the tank from the floor, wrapped it in a couple layers of rebond foam (carpet padding) then tied it down with a Bungee.



Now, it'll go all the way up to cavernous dreamscape levels of reverb without any feedback.

 

After auditioning several amps from 1 Watt to 100 Watts I finally settled on the VHT Micro 6.



The 10 Watt amp actually gave the best, strongest reverb but was a bit noisier than the Micro 6. They call this a 6 Watt amp but it sounds a whole lot more like the 1 Watt amp than the 10 Watt amp. It can't do the over-the-top dreamscape soaking reverb but it's quiet, small, simple, and runs cool. And lets be honest, the mega reverb is a fun novelty but not entirely useful. It's kind of funny, though, that this entire project was motivated by NOT wanting to use a pedal and I ended up using a pedal amp.

The reverb tank output was wired through a 250k audio pot to give external intensity adjustment.





A power jack was added to the back panel to power the amp.



I tried a 9 Volt 1000 mA power supply but the manual said that it could handle 12 Volts. With it hooked to a bench power supply I could see that it was pulling up to 175 mA at 12 Volts. Found a 12 Volt 2000 mA wall wart but it was noisy as heck. Dug around some more and found a massively over-rated 12 Volt 3.7 Amp brick [from a Nintendo Wii].

 

Here's a sound clip. https://www.gofastforless.com/junk/ReverbCab1.mp3

To do this with a single-speaker cab you'd need a dummy load and "PA" amp to mix the wet and dry signals. Having multiple speakers greatly simplifies it. The guitar amp directly drives one (or more) speakers as well as the reverb tank. The output of the reverb tank is amplified and sent to it's own speaker.

The recovery amp is nothing special. Pretty much any guitar/mic amp will work. On most of the amps I tried, the tone/treble was turned all the way down. This was done, mostly, to reduce background noise. The effect on the actual reverb signal was almost unnoticeable since the signal going into the tank is so heavily filtered.

I still need to find a better power supply for the Micro 6. The Wii brick does cause a little bit of a hum. The 9V "pedal" power supply also had a bit of a hum but at a much higher pitch. I plugged in the bench power supply again and it is dead quiet. Just for fun, while the bench power supply was hooked up, I drove the cab with the 100 Watt Randall head. The more I turned up the guitar amp, the more current the recovery amp drew which leads me to believe the the input attenuator is effectively preventing the driver coil from saturating. At one point, with the Randall cranked to the max, I saw the current draw of the Micro 6 exceed 900 mA. The digital display on the bench power supply is slow to react so I imagine it may have shot higher. Can anyone recommend a QUIET 12 Volt power supply, minimum 1000 mA?

 

Bingo! Thank You for the suggestion.

@ArcticWhite stopped by this weekend and gave me an inductor. The impedance bridge said it was 3.7 mH. I robbed a 4700 uF capacitor from a circuit board the neighbor gave me and made up this LC filter cord.



Was disappointed when it didn't quiet down the Wii power brick. Then I remembered the 9V power adapter with the high-pitched noise. Badda-bing badda-boom, with the 9V adapter and LC filter it's as quiet as the bench power supply, if not quieter.