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Ask Amp Man

Our much-loved former columnist Jeff Bober returns to explain how to check and reset the bias of your amp’s output tubes—and delivers some potentially shocking warnings about a few common but dangerous techniques.

Hello again, Premier Guitar readers! It’s your old bud Jeff here, author of the once popular Ask Amp Man column. Editorial Director Ted Drozdowski asked me if I would be interested in writing about bias, and, of course, I said, “Sure, I know a thing or two about that!” So here I am, temporarily returning to these pages. Now, let’s get started.

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How to use a Super Reverb, with a slew of pedals quietly looped in, for late-night practice.

Hello Amp Man readers. This month I’ve picked two interesting questions, and since the first references a Fender Super Reverb, I’m going to answer both based on that amp. So read on!

Question #1

Okay, let’s get started with question #1. As far as “would I want a wire connected between a Super Reverb and my ears?” Well, that’s for you to decide, but if you choose “yes,” here’s how to do it.

Fig. 1 is a simple circuit I came up with to allow you to play your amp through your headphones without waking the kids or disturbing your neighbors at 2 a.m. So you can understand what’s happening, I’ll go through it and describe the function of each component.

Fig. 1: This diagram illustrates the modifications required to add a headphone out to a Fender Super Reverb.

Jack 1 is the input from your amplifier’s speaker output, at left in Fig. 1. The resistance value should be as close to the output impedance of your amp as possible. For a Super Reverb, that would be 2 ohms. If it’s not possible to get the exact resistance, it’s okay to go up in value, but not down. This resistor will be replacing the speakers in your amp, which need to be disconnected, so it will be absorbing your amp’s full output power. I recommend that the resistor’s power-handling capability be at least double your amp’s output power. A Super is rated at about 40 to 45 watts, so I recommend at least a 100-watt resistor here. Also, if you’re using the large, gold anodized aluminum resistors, they get very hot and need to be mounted to properly dissipate the heat. If you’re building this inside a Bud box or something similar, mount the resistor to the box and be sure to install feet on the bottom of the box so that the heat doesn’t damage what the box is sitting on—like possibly your amplifier. It would also be a good idea to vent the box.

Next, the signal goes to the volume pot. A 1k-ohm, half-watt or higher, linear pot works fine here. A 22-ohm 1-watt resistor gets connected to the pot’s counter-clockwise arm to provide a little isolation from the amp chassis and add a bit of a signal drop.

Then the signal from the pot’s wiper gets connected to the headphone output jack through a 100-ohm resistor. For testing, I used a set of Tascam headphones with a 32-ohm impedance, which is pretty standard today, and the 100-ohm resistor worked fine and provided plenty of volume. If you need a bit more level, decrease the value of this resistor. Also, the output jack should be insulated from the chassis because we’re trying to maintain a bit of isolation from input to output. A typical “British-style” jack works fine here. Just make sure it’s stereo and don’t forget to connect the tip and ring connections together. The last component you see is a 5 µF 50V non-polarized cap. Since a resistive load on an amplifier causes it to react differently from a speaker load, amps tend to sound more “spiky” with a resistive load, so this capacitor helps smooth out some of the brittleness of the sound in the headphones.

There you have it—a way to play your amp silently.

Question #2

Dear Premier Guitar,

I’ve grown to appreciate your DIY pieces, and they’re well written to the targeted reader. I would like to get your take on the possibility of using the reverb “send and return” loop as an effects loop. It seems easy: An adapter cable changes the RCA plugs to 1/4". The cables go to your effects pedals and return, instead of the reverb can. The cool part is the reverb control would now mix the wet and dry signals. Will this work? Can you use “Y” cables and a switch to include the reverb as well? Are the impedances so far out to lunch that it’ll never work? Is that why I’ve never heard of anyone doing this?

Wysong Perabula

Question #2 asks about the possibility of using the amp’s reverb circuitry as an effects loop. In essence, it already is an effects loop, but it’s optimized for use with the reverb tank, which is far different from an effects pedal. We can, however, get it to work as a pedal loop.

Fig. 2: This schematic shows how a reverb circuit can be turned into an effects loop. Unfortunately, it’s at the price of the reverb—but there are pedals for that.

The biggest difference between a normal effects loop and a reverb circuit is basically in the “send” department. A typical Fender-style reverb tank has very low input impedance, and this requires a substantial level to drive it. This level is far too hot to feed into any effects pedal, so we must first tame the beast. Looking at Fig. 2, R1 serves as a load on the reverb drive transformer. Next R2 and R3 form a voltage divider to reduce the signal to an acceptable level for an effects device. Since the reverb drive circuit is actually a small amplifier output stage using its own little output transformer, removing the inductive load from the reverb tank and replacing it with the 100-ohm resistive load is similar to removing the speaker load from an amplifier and replacing it with a resistive load. Things get a little “spiky.” To compensate for this, we add the .001 µF capacitor—identified as C1—to smooth out the sound.

Now we have a signal that will be a better match for a pedal. Regarding the effects return, the effects can be connected directly to the reverb return (out) of the amplifier, either by 1/4" to RCA cable or, if you’d like to contain this all in a project box, simply connect the loop return jack directly to the reverb return jack, as most pedals should be able to comfortably drive its 220k input impedance. The loop can now be used for your time-based effects (delay, reverb, chorus, flange, phase, etc.). The reverb knob will mix in the amount of effect, so set your effects for 100 percent wet, if possible, to minimize any phase cancellation problems. The footswitch will now turn the effects on and off as well. As far as including the reverb tank back into the circuit, the output impedance is too low and would more than likely attenuate the output of the effects devices.

Well, there you have it. Enjoy the experimentation!

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Photo 1 — When a blonde Fender Showman head with a blackface control panel came into Jeff Bober’s shop, its tube array provided the Amp Man with a potentially unanswerable riddle.

This oddball 1963 Fender head demanded serious examination.

Hello Ask Amp Man fans! Once again, just when I think I’ve seen about every iteration of vintage Fender amp, another interesting one crosses my bench. This month’s beauty is a blonde 1963 Showman head (Photo 1) that just came into the shop.

You ask, “What’s so special about that?” Granted, we’ve all seen blonde Fender Showman amps before—at least those of us who are fortunate enough to see lots of great vintage gear have—but this one is transitional. Not just because it’s a blonde cabinet with a blackface control panel, but because it has some very rare transitional output tubes as well.

My theory is Fender may have wanted a more hi-fi sound from the amp—something with more fidelity—so they opted to use the
7355 output tube.

If there’s one thing vintage Fender amps are known for, it’s their use of 6L6 (5881) and 6V6 output tubes—as opposed to their EL34 and EL84 counterparts utilized in amps produced across the pond. This was pretty much ubiquitous across the historic Fender product line and was responsible, in large part, for giving the company’s amps their signature “American” sound.

Hi-fi vibe. So why change for this amp? That question may never be answered, but my baseless possible theory is Fender may have wanted a more hi-fi sound from the amp—something with more fidelity—so they opted to use the 7355 output tube. Around the same time, Ampeg was producing amps utilizing the 7591 output tube, which is also a hi-fi-style tube. The Ampegs were great-sounding amps, so maybe Fender took a cue from the guys on the East Coast. I’ve also read there may have been availability issues in obtaining 6L6s at this time. Who knows?

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Photo 1 — With a name taken from the missiles developed to launch astronauts into space, the Gibson Titan’s face looked a bit like the control panel of a rocket ship from the now-classic science fiction films of its day.

A rare find from 1964 lands on the workbench and is up and running after a little ground control.

Hello Ask Amp Man followers. Greetings from Amp World!

An amp crossed my path that I think is interesting enough to warrant an installment of the column, so I’m once again going to forgo a reader question and focus on this somewhat rare brown box on my bench called a Gibson Titan. A friend who recently acquired the head and cabinet pair brought them to me. He said that he’d wanted a Titan set since seeing the trapezoidal head sitting atop its matching extension cabinet in a music store back in 1965, so this was a very welcome blast from his past.

First, a little history on these amps and then we’ll get into the servicing. According to the information I’ve found, Gibson manufactured Titans for five years. In the first two years, 1963 and 1964, they were produced with a brown vinyl covering, and in the last three years, 1965 through 1967, they were covered in black vinyl. Titans were also offered with multiple speaker configurations. The cabinet in the Titan I set came loaded with two 12” speakers. The Titan III had one 15” and two 10” speakers, and the Titan V had two 15” speakers.

The interesting thing about this configuration is that Gibson installed a crossover in the cabinet so the lows and highs would be split between the 15” and 10”’s respectively.

All of these configurations used the same head containing 11 tubes, utilizing a quad of 6L6 output tubes, and it was rated at 65-watts output. The cabinet associated with this head says Titan III on its decorative metal panel, so it’s the one 15” and two 10” version. The interesting thing about this configuration is Gibson installed a crossover in the cabinet so the lows and highs would be split between the 15” and 10” speakers, respectively. That’s not something seen often in guitar world. As for the head and cab, they’re covered in the earlier brown vinyl and the date codes on the parts are split between late 1963 and early 1964, so that should firmly date this Titan as a 1964 model. Nice find indeed!

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At first glance, this SVT looked like a typical ’80s/’90s Ampeg head, but there were plenty of surprises under the hood.

A rare “Skunk Works” production amp is brought out of the woodwork and up to speed.

Hello Ask Amp Man fans. While I’ve been answering your questions for many years now, the last few columns have been about interesting amps I’ve come across, and this column follows that path. I still very much enjoy servicing amps when possible—and occasionally a really cool and unusual piece will cross my bench. Over the years, I’ve seen plenty of vintage Marshall, Fender, Ampeg, Vox, Hiwatt, and Gibson amps. They’re all cool, of course, but sometimes there’s a genuinely unusual one. This month’s featured SVT was one of those. The customer hadn’t played the amp for years and wanted it brought up to spec.

At first glance, it looked like a typical Saint Louis Music-era SVT (Photo 1). It was dressed in black, maybe late-’80s or early-’90s, and possibly an offshore-manufactured unit. But then I saw a commemorative plaque (Photo 2) attached to the rear grate. What could this be? Time to do some digging.

Photo 2

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