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Okay, what does this have to do with that old Gibson amp? Dave Anderson, a bass player, tube-amp head, and doctoral student in electrical engineering at the University of Rochester translated calculus and differential equations into English for me and PG readers. Here it is: “6SJ7 tubes are pentodes, which produce a very linear signal with very little distortion. They create a warm sound because they don’t do much to the original signal. They leave it clean, but warm and full. And 6SJ7 tubes do that whether they are in a Gibson Les Paul Junior or a tweed Fender Champ FC-1. The 12AX7 tubes used in Gibson GA-6s and later Fender Champs like the 5E1 and the 5F1, distort a bit more, but keep those distortions in the lower-order harmonics. Amps using those tubes don’t sound like solid-state amps either. The overtone series of harmonics is what gives instruments their unique timbres. That is not the same thing as distortion.”
I asked Anderson if the later amps—models equipped with tone controls—are better. He doesn’t think so: “The advantage of the early amps is that they have no tone control. People say that tone controls eat up signal, but that’s not actually the big difference, because normal circuitry can eat up signal too. The important distinction is that tone controls can interact poorly with other electronics in the 1 kHz to 2 kHz region, just where the desirable effects of tubes are most noticeable. The absence of a tone control leads to a nice overall warm sound due to the flat frequency response.”
As Dave Hunter writes on the first page of the first chapter in The Guitar Amp Handbook, “good tube amp tone really is a very, very, simple thing.” Of course, then he goes on for another 230 fascinating pages to explore the details of this simplicity. But that doesn’t change the truth of his statement. A tube amp can be very simple and still produce a warm, rich sound—the sound so many artists have used to great advantage in the studio.
Maintaining and Upgrading an Older Gibson Amp
Jim Walton recommends that any ’50s or ’60s amp you buy receive preventative maintenance. Here is his list:
- (1) Upgrade to a grounded line cord.
- (2) Replace the chemically impregnated paper capacitors with modern capacitors.
- (3) Check chassis grounding.
- (4) Test signal capacitors for high-voltage leakage.
- (5) Check all tubes and tube-socket connections.
- (6) Clean everything.
Walton cautions that all of this should be done by an experienced amp tech because of the risk of high-voltage electrical shock.
Any tube amp will produce a sound rich in natural harmonics, according to Anderson. “Let’s say that a guitar’s A string, which vibrates at 110 Hz, produces naturally audible harmonics at 220, 330, and 440 Hz. These would be second, third, and fourth harmonics. A 6SJ7 would leave the relative volume of each of these harmonics nearly untouched—what we would call clean and fairly warm. A 12AX7 would likely accentuate the volume of 220, 330, and 440 Hz, and maybe add 550 and 660 Hz, but nothing more. Since the distortion happens in the low-order harmonics and is fairly subtle, it adds to the warmth, but has its own ‘color’ that’s different from other tube amps. Fender, but not Gibson, moved on to the 12AX7 in later versions of their 5-watt amps, adding just a little more warmth.An overdrive pedal will add significantly to these harmonics, as well as adding hundreds more, but the sound will no longer be warm.”
“The initial tone control circuits were badly designed,” adds Anderson, “and interacted with other controls—such as gain and volume—in unpredictable ways. The problem is that when there are too many parameters all wildly interacting with each other, the tone is unpredictable. A signal chain with tone and volume controls on the guitar, tone and volume controls on effects pedals, and tone and volume controls on an amp is a nightmare. The simplicity of an amp with no tone controls can lead to a much more natural, flat sound that is really unachievable with a cascade of complicated controls.”
If Anderson’s description leaves you hungry for even more technical information, there’s plenty available. Check out other articles in this issue and in PG’s previous August “amp issues,” the regular “Ask Amp Man” column, and two books I’ve found very useful in my research: Hunter’s previously mentioned The Guitar Amplifier Handbook and Wallace Marx’s Gibson Amplifiers. And for those who want to jump into this subject in all its geeky glory, the original RCA and GE tube manuals are available online, as are Western Electric schematics ... and even the Radiotron Designer’s Handbook.
All this technical stuff is interesting, but this story’s headline promised it would be about undervalued gear. Are the Gibson GA-5 and its descendants undervalued? Should you buy one? It depends on what you’re looking for. A Les Paul Junior isn’t powerful enough to fill a big room with great sound—after all, it’s only a 5-watt combo. But it is powerful enough to deliver warm, rich sound in your living room or for miking through a PA. For the collector or musician who trades equipment a lot—or for anyone concerned about resale value—an old Gibson amp may not be such a great idea, even if it seems like a bargain when compared to a Fender amp at two or three times the price. For a player on a budget, however, it might be just the right thing.
Nate Westgor and Jim Walton get the last words here.
“As we baby boomers age,” says Westgor, “most of our playing is done at home, so smaller amps have a lot of appeal. In retrospect, the older Gibson amps were overlooked. In the ’50s and ’60s they made some really excellent amps.”
Walton agrees: “The GA-5 is a bargain because it lacks the public association of the tweed Champ with Clapton and ‘Layla.’ In fact, all the amps of that era were copies with modifications of what had been successful in radio. They were all using the same Western Electric circuits.”“If price is no object,” concludes Westgor, “buy the Fender because it will have better resale value. Gibson makes a good product, but a Gibson amp will be harder to sell. It’s like buying a Ferrari—you’ll always be able to sell it. But if you don’t have $1,200 to spend on a tweed Fender Champ, the Gibson is a great little amp.”