Unraveling the Mysteries of Amp Reverb

Troubleshooting, pan replacement, and tips on fine-tuning your amp's spring reverb for peak performance

By Phil O'Keefe

Recently, a question came up on the Harmony Central Effects forum from a member who was having issues with the reverb on his newly acquired vintage Fender Quad Reverb. The reverb was "too much", and he wanted to back it down, and was asking about ways to do so. Do you just remove one of the springs? Will a shorter tank give you shorter reverb? Actually, there's a bit more to it than that…

ACCUTRONICS AND HAMMOND (™)

The spring reverb circuit itself as we know it today was designed by Hammond engineer Alan Young in 1960. Hammond / Accutronics Spring Reverb Tanks have been used in countless amps ever since - including the Fender Twin Reverb and it's close relatives like the Quad Reverb, which is basically a silverface Twin with four 12" speakers instead of only two. The basic circuit is relatively straightforward, and consists of driver and recovery circuitry within the amp, the spring reverb tank (Figure 1) and a couple of wires connecting the remotely mounted tank with the amplifier electronics.

Figure 1: The Accutronics Type 4 reverb pan

The driver circuit takes the signal from the guitar amplifier's preamplifier and sends it to the reverb tank. Inside the tank, a small transducer converts the electrical energy into mechanical energy that vibrates the tank's springs. The reverb character is determined in part by the size, length and number of springs, with more springs, springs of different densities and different spring lengths adding to the reverb's complexity. The vibrating springs create the delays and reflections that simulate the way sound bounces around in a large reverberant room. At the end of the springs, a second transducer converts the mechanical energy of the spring vibrations back to electrical energy and that signal is set back to the amp's reverb recovery circuit. The reverb recovery circuit is fairly straightforward. It's basically just a preamplifier that brings the level from the tank's transducer back up and feeds it back into the amp, where it is blended in with the unprocessed signal.

The driver and recovery circuits can be either tube or transistor based, but in many tube amps, tubes are also used for the reverb. We're going to keep things simple this time and stick to only things that can be done without opening up the amplifier. Tube amps can hold and store potentially LETHAL voltages for some time after being unplugged, and they should never be opened up unless you know what you're doing in there.

While other companies also manufacture tanks, the spring reverb "pan" or "tank" you'll find in more amps than any other is made by Accutronics. Ampeg, Fender, Marshall, Peavey, Music Man, Crate and Mesa (among others) have all used these tanks. Originally a subsidiary of Hammond, Accutronics is now owned by Belton. For those who are interested in reading more about the development and history of the spring reverb and the companies involved, there is a very nice write-up on the Accutronics website.

TYPES

There are four main "types" of Accutronics reverb tanks, with various differences in the features of each one.

Type 1 is a economy model, with two springs in a short 9" (actually 9.25") tank.These are often found on inexpensive practice amps.

Type 4 is the original model and dates back to 1960. This is the most commonly found tank type in professional amplifiers. Type 4 is a long tank model.  Often referred to as a 17" tank , they actually measure 16.75" long. The Type 4 contains four counter-wound springs, arranged as two coupled pairs; in fact, unless you look closely, you might think there are only two long springs, when in fact each "spring" is actually made of two shorter springs connected together.

Type 8 is a three spring 9" (9.25") short tank. These are designed to provide high-quality reverb that approaches the quality level of the long tank units, but in a much shorter and more compact tank.

Type 9 is a 17" (16.75") tank with six springs arranged in three counter-wound coupled pairs, with the signal running in parallel. As with the Type 4, this tank appears to have only three springs until you look closely; when you do, you see that each long "spring" is actually comprised of two shorter springs linked together. The Type 9 is the top of the line model and usually can be found in sound reinforcement systems as well as pedal steel and keyboard amps.

READING THE CODES

Accutronics tanks use a sequence of seven numbers and letters that tell you various things about the tank itself, but unless you know the codes, it's gibberish. These codes are usually stamped on to the top of the tank itself and they tell you the type of tank, the input and output impedance, the grounding (if any) used by the connectors, the way the tank is supposed to be mounted and even the length of the reverb decay time.

Here are the codes and their meanings:

For Type 1 and Type 4 tanks:

1st digit: Reverb Type (Type 1, Type 4, etc.)

2nd digit: Input Impedance (A = 8 ohm, B = 150 ohm, C = 200 ohm, D = 250 ohm, E = 600 ohm, F = 1475 ohm)

3rd digit: Output Impedance (A = 500 ohm, B = 2250 ohm, C = 10000 ohm)

4th digit: Decay Time (1 = short 1.2 - 2.0 seconds, 2 = Medium 1.75 - 3.0 seconds, 3 = Long 2.75 - 4.0 seconds)

5th digit: Connectors (A = Input Grounded / Output Grounded, B = Input Grounded / Output Insulated, C = Input insulated / Output Grounded, D = Input Insulated / Output Insulated, E = No Outer Channel)

6th digit: Locking Devices (1 = No Lock - I've never seen a tank with a lock, so this seems superfluous)

7th digit: Mounting Plane (A = Horizontal, Open Side Up, B = Horizontal, Open Side Down, C = Vertical, Connectors Up, D =  Vertical, Connectors Down, E = On End, Input Up, F = On End, Output Up)

For Type 8 and Type 9 tanks:

1st digit: Reverb Type

2nd digit: Input Impedance (A = 10 ohm, B = 190 ohm, C = 240 ohm, D = 310 ohm, E = 800 ohm, F = 1925 ohm)

3rd digit: Output Impedance (A = 600 ohm, B = 2575 ohm, C = 12000 ohm)

4th digit: Decay Time (1 = short 1.2 - 2.0 seconds, 2 = Medium 1.75 - 3.0 seconds, 3 = Long 2.75 - 4.0 seconds)

5th digit:  Connectors (A = Input Grounded / Output Grounded, B = Input Grounded / Output Insulated, C = Input insulated / Output Grounded, D = Input Insulated / Output Insulated, E = No Outer Channel)

6th digit: Locking Devices (1 = No Lock)

7th digit: Mounting Plane (A = Horizontal, Open Side Up, B = Horizontal, Open Side Down, C = Vertical, Connectors Up, D =  Vertical, Connectors Down, E = On End, Input Up, F = On End, Output Up)

Returning to our example, the Quad and Twin Reverb (as well as many other Fender amplifiers) uses the Accutronics 4AB3C1B. The first digit tells us this is a Type 4 unit, so we'd refer to the Type 4 list for the rest of the information. The "A" indicates an 8 ohm input impedance, and the B in the third digit position tells us that the output impedance of the tank is 2250 ohm.

Note the "3" in the fourth digit position of the code. This tells us that the tank has a long decay time of between 2.75 - 4 seconds. Fender used long decay time tanks in the vast majority of their amps, but other decay times are also available. If you want to shorten the decay time, use either a 4AB2C1B (1.75 - 3 seconds) or a 4AB1C1B (1.2 - 2.0 second decay time) tank in your Twin.

TROUBLESHOOTING AND TRICKS

The original Hammond (™) reverb circuit is usually quite reliable, but sometimes things do go wrong. If the reverb on your amp isn't working, or suddenly quits, there are a few things you can check on yourself before hauling it in to your local amp repair technician.

The biggest issue you may come across is a missing tank. Finding a new tank online should be relatively easy - Accutronics sells replacement tanks direct via their website, but you'll need to know the tank number before calling or emailing them. When replacing a tank, it's important to have the right input and output impedance values or your reverb may not function properly. How can you find out what specific tank model number is needed for your particular amp? If you have the stock tank, the codes are stamped directly into the housing of the tank. If the tank is missing, you can check with the amp manufacturer; they will often have the information available on their website in a parts list or schematic for that particular amp. You can also try emailing them if you can't locate it. Of course, the Harmony Central forums are another helpful resource, and often a quick post will get you the information you seek.

Remember, as I mentioned above, if you have a tank with a "3" as the fourth digit of the code, it means it has a long decay time. Tanks with shorter decay times are also available, and if you want a shorter reverb time, just replace the tank. When ordering, the tank code will remain identical to your current code, except with a 2 (medium decay) or 1 (short decay) as the fourth digit of the code, depending on what you prefer.

Check the input and output cables. These sometimes get accidentally disconnected, and occasionally they suffer from shorts or opens that will prevent the reverb from working. You can test for shorts or opens with a digital multimeter set for continuity testing. If the cables are bad, replace them. Once you know the two reverb cables are good, check to make certain they're properly connected. Occasionally, they get reversed - either at the tank or at the connections at the back of the amp. Try reversing the cables at one location or the other and see if it "fixes" things. If not, the reverb driver and recovery tubes may need replacement.

If your reverb needs additional isolation, you can try "double bagging" it by using a reverb tank liner inside the reverb tank bag. (Figure 2) For situations where you're getting explosive crashes of reverb due to stage vibrations, placing the amp on an iso riser such as the Auralex GRAMMA can help considerably by decoupling the amp from the floor and separating it from the source of the shock and vibration.

Figure 2: The reverb tank's vinyl bag both protects the unit and helps isolate it from vibrations

Speaking of the reverb tank bag, in one case when I was troubleshooting a friend's amp, the reason the reverb wasn't working was that the tank had been installed in the bag improperly, and the bag had bunched up inside of the tank. This caused it to come into contact with the springs, which prevented them from vibrating freely.

One common complaint that I hear fairly often is that the reverb knob is too sensitive - many players never turn it up much past 3 because at higher settings it's just too "much." Some players also find it difficult to dial up just the right amount of reverb with such a limited useful range for the control knob. If either one of those applies to you, you can lessen the reverb knob sensitivity by replacing the reverb driver tube with one with a lower gain factor. Instead of a 12AX7 (gain factor of 100), try substituting a 12AT7 (gain factor of 60), 12AY7 (gain factor of 45) or even a 12AU7 (gain factor of 19). This will drive the reverb circuit less, and require higher settings on the knob.

While the sheer number of amps out there prevents me from telling you specifically which tube(s) are used in your specific amp for the reverb driver and reverb recovery, it's often one (or two) of the two small tubes. The one closest to the "big (power) tubes" is usually a phase inverter, and the one closest to that is often for the reverb. When in doubt, check your amp's manual for the specific tubes.

Phil O'Keefe is a multi-instrumentalist, recording engineer / producer and the Associate Editor of Harmony Central. He has engineered, produced and performed on countless recording sessions in a diverse range of styles, with artists such as Alien Ant Farm, Jules Day, Voodoo Glow Skulls, John McGill, Michael Knott and Alexa's Wish. He is a former featured monthly columnist for EQ magazine, and his articles and product reviews have also appeared in Keyboard, Electronic Musician and Guitar Player magazines.