Ask Amp Man: Supercharging a Super Reverb
Six ways to beef-up the breakup on the classic Fender combo.
Hey there from Argentina! Love your articles!
I have a 1966 Super Reverb amp, all original. I’m looking to mod it lightly to have early breakup (now it’s at about 7). I don’t like pedals, so wanted to ask you for advice.
Thanks so much,
Glad you’re diggin’ the column … in Argentina! And, might I say, nice amp. The Super Reverb is definitely one of my all-time favorites. It has great midrange content, which we all know (or should) is where a guitar belongs in the mix, because, well, a guitar is a midrange instrument. And at only 40 watts, it can be pushed into overdrive easier than higher-powered amps (although not as easily as, say, a Deluxe), and sometimes that’s the power range we need for smaller venues. That sounds like the situation you’re currently looking to solve, so let’s see if I can help you with some options. You mentioned you’d be open to modification, so let’s start with suggestions there, and then I’ll give you other options, too.
Mod 1: Install a master volume. Master volumes come in a few different styles and affect the signal differently. A standard type of master volume is one where the signal from the preamp is attenuated in the circuit prior to reaching the phase inverter. This can work fine in many instances where the preamp is designed to achieve distortion, and is the case with most mid- to hi-gain amplifiers. The drive is developed in the preamp and that signal is fed to the output stage via the master volume at whatever level necessary, with the output stage being used, for the most part, as a source of clean power.
There is another type of master volume—the simplest of all, actually—that acts to combine the two out-of-phase signals from the phase inverter before they reach the output tubes. And what happens when two out-of-phase signals are combined? For those who don’t know, they cancel out. Therefore, the signal being sent to the output tubes is shut down in varying degrees, from not-at-all to full.
Unfortunately, neither of these would work well for you, in my opinion, as there is not much in the way of overdrive that can be achieved in the preamp section of a typical Fender amp. What I would recommend is a master volume that actually changes the signal prior to the output tubes to simulate a bit more drive—and yes, they do exist. They’re called post-phase inverter master volumes, or PPI’s. There are two different types that, when reduced from maximum volume, begin to actually flatten out the peaks of the signal being fed to the output stage, which tends to mimic an overdriven output stage. This is one way to get more “apparent” overdrive from your amp at a lower volume. These master volume circuits can be found by you or your tech on the internet. I would suggest searching "The Trainwreck Pages" online for more background. This is an article written by the late Ken Fischer of Trainwreck Circuits in the mid 1980s. You’ll enjoy it.
The Brown Box was developed to run vintage amplifiers at the voltages that were present when the amp was designed, which in the U.S. was closer to 110 or 115 volts.
Mod 2: Convert to a quasi-triode mode. Have your tech either convert your amp, or install a switch, so you can run your output stage in a quasi-triode mode. This can reduce the output power by approximately 40 percent and generally causes the amp to have a somewhat smoother response. This can be done by simply disconnecting the screen grid voltage from pin 6, which is being fed to the screen grid of the tube via the 470-ohm resistor connected between pins 6 and 4 in a Fender amp.
Simply connect the pin 6 connection of that resistor to pin 3 of the socket, which is the plate connection of the tube. Connecting the plate to the screen grid through that 470-ohm resistor causes the tube to run in a quasi-triode mode. This can also be accomplished with a heavy duty DPDT switch so you can run the amp in either standard pentode more or triode mode for those smaller venues.
Non-mod 1: Swap rectifier tubes. If you’re running a 5AR4 rectifier tube in the amp, try a 5U4. While the 5U4 draws a bit more filament current than the 5AR4, Super Reverbs have vacillated between them over the years while using the same mains transformer, so this should not be an issue. As far as the difference, the 5U4 should lower the plate voltage by 10 to 15 volts, and it seems to compress a bit more. So, if you’re looking for merely a subtle change, this may be all you need.
(Left) If you’re running a 5AR4 rectifier tube in the amp, try a 5U4. Super Reverbs have vacillated between them over the years while using the same mains transformer. (Right) Removing the 6L6 output tubes, installing Yellow Jackets in the tube sockets, and inserting a pair of EL84 tubes into the adaptors reduces the output by 20 watts.
Non-mod 2: Install a Yellow Jacket tube converter. This allows you to convert the output stage from 6L6 tubes to 6BQ5s. You simply remove the 6L6 output tubes, install the Yellow Jackets in the output tube sockets, and insert a pair of 6BQ5 (EL84) output tubes into the adaptors. The output is now reduced to approximately 20 watts, but because the EL84 is the baby brother to the EL34, the tonal characteristics of the amp will change and have a bit more mids with less top and bottom. But hey, vive la différence.
Non-mod 3: Use an external attenuator. There are certainly plenty out there. Just be sure it’s impedance specific: You purchase a 2-ohm model for the Super Reverb.
Non-mod 4: Use an external voltage reduction unit. Try something like a Variac or a Brown Box. The Brown Box was developed to run vintage amplifiers at the voltages that were present when the amp was designed, which in the U.S. was closer to 110 or 115 volts, as opposed to the typical 120+ volts currently supplied. If you decide to use a Variac-style device, lowering the wall voltage 10 to 15 volts may get the results you’re looking for.
I hope that makes your Super Reverb super for all applications.