Ca 1933/34 Gibson prototype amp. At least two were made. One is shown together with a late '20s L-4 in Andre Duchossoir's Gibson Electrics and another in the June 1998 issue of Guitar Player, Encore section. The two are different in respect to the clover-shaped speaker openings. One of them have the clover cutouts on the +-axis, as shown here, and the other on the x-axis. The carved cabinets were made of mahogany and housed two 5" Utah speakers and a five-tube chassis. Twin 43 power tubes and twin 25Z5 rectifiers were used.

What turned me on to Gibson amps was a project to build a guitar tube amp I started a long time ago but never finished. As I was browsing through schematics and other material, trying to come up with something that could serve as a suitable model to my project, I found this article(1) on old Gibson amps. A few schematics were also presented in the article. This intrigued me as I had never heard of or seen any Gibson guitar amps before that. Quite in reverse to the by most everyone well known and much coveted guitars by the same company.

Even so, you may surprisingly often find an old Gibson amp, in the shop, at the auction or in the classifieds section in a magazine. If you make inquiries about a Gibson amp, the information you get is often a bit shady when it comes to what year it was made, how many watts it produces or how rare it's supposed to be - or at least this is how it used to be.

This overview, which I hope will shed some light on the subject, will cover the various styles of almost all Gibson guitar and accordion amplifiers up to the point when amp production was moved from Kalamazoo in 1968.

There are still many faults and oversights left in this text which I hope readers will point out to me.

The source-date codes have been in use since 1949/50 and consists of six or seven digits. The first three signify the company that made the component. Common numbers are 220 (Jensen), 328 (Utah), 285 (Rola), 137 (CTS), and 134 (Centralab). If there are seven digits in total, the fourth and fifth digit represent the year of manufacture. If there are only six digits, the fourth digit represent the last digit of the year. The last two digits represent the week of the year.

In many early Gibson amps you will often find Utah speakers. Rola was used as a supplier of speaker elements beside Jensen in the late '40s and early '50s. Gibson literature mainly specified Jensen speakers throughout the 1950s. In the early '60s Gibson switched to CTS speakers, especially for its student and budget lines of amps. During the '60s, Gibson also offered JBL speakers as an option for some of its amplifier models.

All speakers from this period had voice coils wound on paper formers. The heat transfer was poor which confined the power handling of the speakers. The speaker cone perimeter suspension, aka surround, was also made of paper. This may have become fragile and torn over the years and after heavy use. It is not unlikely that a speaker has been replaced at some time, and one should be wary when dating an amplifier from the EIA codes found on a speaker.

Note: Gibson serial numbers stamped onto amp chassis seems to be quite random and therefore possibly meaningless.