![]() ![]() T he Vox AC-30/4 was also equipped with a tone control named "Bass" to roll off treble response for both channels simultaneously. The simple circuit rolled off treble response above 5000 hz. The circuit for the Brilliant control had only two components, a two position rotary switch and a 250 pf capacitor. The Brilliant control affected only the Normal channel. The AC-30/4 featured two tone controls, "Brilliant" and "Bass". The capacitor values and part numbers in the table come from the factory schematic OA/032. Conversely, decreasing the value of the capacitor allows less bass frequencies to pass. The only differences between the AC-30/4 Normal and AC-30/4 Bass amplifiers were the values of the coupling and bypass capacitors shown in the table above.Īll one needs to know to understand the table above is that as the value of the capacitor increases, more bass frequencies are allowed to pass. The black and gold AC-30/4 control panel covered the two additional input jack holes. This allowed JMI to use the same chassis to build both the AC-30/4 and AC-30/6. It is interesting to note that the control panel area of the AC-30/4 chassis was punched to accommodate six input jacks (see photo above). This served to further electronically shield the preamp from the power supply. Aluminum is even less likely than steel to pick up hum and oscillations from the power supply and output amp section of the amp. The vertical aluminum section of the chassis enclosed the preamp circuitry and secured the control panel. "L" brackets mounted to the top of the power and output transformers strengthened and supported the vertical aluminum portion of the chassis. ![]() T he Haddon power transformer and output transformers were located at opposite ends of the chassis base to provide proper balance. The use of steel in the chassis base not only provided strength but also electronically isolated the high voltage (and hum producing) section of the power supply and output amp from rest of the amplifier. The steel base of the chassis supported the power supply and output amp sections of the AC-30/4. This chassis metal work was adapted from the second generation AC-15. The ingenious chassis design of the JMI Vox AC-30/4 combined a pressed steel horizontal base with a verti cal assembly made of aluminum. While it was "AC-30/4" in print, it was "AC thirty four" when spoken. The model number was listed correctly in all other JMI printed materials. This was probably a misprint was caused by confusion between the written and spoken version of the amp's name. The title box for schematic OA/032 offers the name "AC-34" for the AC-30/4. Schematics that were supplied to outside service organizations started with the letters "OS." The schematic indicated that the drawing was for "works use only," or as an factory manufacturing assembly reference. ![]() The circuit diagram for the AC-30/4 is found on JMI schematic OA/032, dated April 29, 1960. The AC-30/6 had a new and unique circuit design that was prefered by Vox lead engineer Dick Denney. Finally, the unique design of the AC-30/6 was not merely an adaptation from the AC-15. This eliminated the problems with microphonics common to the AC-30/4. Secondly, the AC-30/6 circuitry did not include the troublesome EF-86 tube. Having three channels with individual volume controls, the AC-30/6 offered the ability for three instrumentalists to play through the amp simultaneously. First, it was not unusual for an English band to pool their money to buy one good guitar amplifier and share it. The AC-30/4 made no appearances in Vox catalogs and price lists after 1961.Ī number of opinions have been offered why JMI would decide to retire the AC-30/4 in favor of the AC-30/6. The AC-30/4 continued to be offered in the 1961 price list, probably to reduce existing inventory. By the time the 1961 Vox catalog was released in mid 1961, the AC-30/6 was included but the AC-30/4 was not. Thoroughout the first part of 1961, AC-30/4 and AC-30/6 amps were sold concurrently by JMI. ![]() This facilitated the addition of a third channel in the AC-30/6. The AC-30/6 replaced the single element EF86 preamp tube used in the AC-30/4 with a dual element 12AX7. When JMI introduced the four input AC-30/4 in mid 1960 it was their "top of the line" amplifier but it was joined by the six input AC-30/6 before the end of the year. As the AC-30/4 and AC-15 shared the same preamp circuitry, the AC-30/4 also included an EF86 tube in the preamp stages of the Normal channel. Vox head engineer Dick Denney so liked the higher gain and rich harmonics of the EF86 pentode tube that he designed it into the preamp circuits of Vox AC-4, AC-10 and AC-15 amplifiers. ![]()
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