Prior to 1962, a bass guitarist wishing to purchase a Vox bass amplifier had two choices, the AC-15 Bass or the AC-30 Bass. Either amp had an open backed speaker cabinet that tended to cancel bass response. Both used electronics that were primarily designed for guitar use. These amps simply did not have the potential for full, rich bass tone. Recognizing this problem, Vox decided to develop several new amps designed specifically for bass. Two models were introduced, the tube powered 50 watt 1x18" Foundation Bass (1963) and the solid state 2x15" T.60 (1962).
Vox had obtained a bit of experience in solid state design during the development of the Continental Organ. One of the members of the Vox Continental engineering team, Les Hills, created a radical new solid state power amp concept that was incorporated into the T.60. Unlike tube amplifiers that required a heavy output transformer, Les Hills design coupled the output transistors directly to the speakers. Eliminating the cost, size and weight of the output transformer would help to create a trimmer amplifier package. Additionally, as solid state amplifiers did not have the high voltage and current demands of a tube amplifier, the power transformer could be modest in size, reducing weight and bulk from the final product.
JMI lead engineer Dick Denney once told me Vox tried to make their amp heads no larger than the size of a "lunch pail" whenever possible. That certainly was true in the case of the small box AC-100, small box AC-50, and T.60 heads.
The completed design for the Vox T.60 amp head used a pair of germanium output transistors that were capable of about 30 to 40 watts RMS and 60 watt peak output power. The amp had two inputs and three controls. A rotary voltage selector allowed the amp to be adjusted for the mains voltage anywhere in the world. This same chassis also powered the "AC-30 Solid State," a 3 x 10" amp offering from this period.
The original version of the T.60 speaker cabinet included two 15" speakers, as shown in the 1962 Vox product catalog and in a 1963 Vox magazine advertisement fetauring Jet Harris of the Shadows.
At just 36" x 19" x 12," the T.60 speaker enclosure was barely large enough to accommodate two 15" speakers. The cabinet side, top and bottom panels were constructed of 3/4" 13 ply baltic birch plywood; the baffle and back panels were 1" thick.
The 1963 Vox catalog offers a revised version of the T.60 speaker cabinet that featured one 15" Celestion bass speaker and one 15 watt 12" Celestion Alnico Blue speaker. A crossover capacitor filtered the lowest bass frequencies from the 12".
Im 1964, Vox brought back the 2x15" version of the T.60 cabinet, renamed as the "Vox AC-100 Bass Cabinet." It featured either two 50 watt 15" Celestion or Fane bass speakers. This was the cabinet that Paul McCartney powered with an Vox AC-100 amp head during the touring years of the Beatles. Many people call this 2x15" cabinet the "T.100," but this was a name Vox never officially used in catalog materiials. However, even Dick Denney refers to this cabinet as the "T.100" enclosure on page 54 of his 1992 book, "The Vox Story."
Paul McCartney originally played a Vox a T.60 amp and cabinet briefly with the Beatles (see advertisement above), but not for long. After the release of the amp, Vox discovered that the germanium output trnasistors powering the 30 watt RMS T.60 power amp stage were highly susceptible to heat related failure.
While germanium transistors were dominant in the design of solid-state devices from the mid 1950's through the mid 1960's, dependability was always an issue. These transistors were especially prone to self destruct if they became hot. Early Motorola auto radios equipped with germanium transistors were known to suffer heat related failures in cars when exposed to nothing more than the summer sun with the windows closed.
Despite several revisions to the T.60 circuit, failures to the germanium output transistors continued to occur. A successful solution to this output transistor failure problem was never implemented.
The T.60 was discontinued in 1966 with the introduction of the next generation of Vox bass amps, the UL430, UL 460 and UL 4120. The UL Series amplifiers had solid state preamps but the output amp stage was tube powered. Due to the problems with the T.60, Vox was not yet ready to introduce another amp with a transistorized power amplifier.