![]() ![]() ![]() Diodes generate no heat and have a much longer service life than a EZ81. However, replacing the EZ81 rectifier tube with diodes offered a number of benefits to the AC-15. ![]() As diodes are highly efficient devices, they would not create the audio compression caused by the voltage "sag" of the EZ81. The EZ81 tube for the B+ power supply was replaced with a bridge of four IN4007 silicon diodes. These would be replaced with less costly components that would not affect performance. Vox determined that through modern design the EZ81 rectifier tube and the choke could be eliminated from the AC15C1 power supply. Such pricing would make the amp available to many who could not previously afford an AC-15. Vox hoped to design the AC15C1 so that the street price of the amp might be less than $650. Many feel that the audio compression created by an overdriven AC-15 is an essential component of Vox tone. These drops in voltage compress the audio output of the amp. The EZ81 rectifier tube had the tendency to exhibit a momentary "sag" or dip in power supply voltage when an AC-15 was pushed toward the limit. The choke worked in conjunction with two 16 uf 450 volt capacitors to eliminate the ripple in the B+ supply.Ī bit of Vox mojo was inadvertently created by the EZ81 rectifier tube. The EZ81 tube converted AC voltage from the B+ winding on the power transformer to rippled DC. The original design for the JMI AC-15 B+ power supply included a EZ81 full wave rectifier tube and a choke. In additional to the B+ (high voltage) and a filament heater winding, the AC15C1 power transformer included an additional secondary winding to supply the solid state reverb and tremolo circuits. While the primary winding of the AC15C1 power transformer varied to accommodate regional mains voltages, all four of the transformers listed above had identical secondary windings. The following chart details the part numbers of those power transformers. Vox produced four versions of the AC15C1 chassis, each equipped with a power transformer that accommodated regional differences in mains AC voltage. The Tube PCB was mounted to the five tube sockets and included most of the power supply components.įour additional small printed circuit boards mounted the input jacks, output jacks, power switch and power indicator LED. The "Tube PCB" was located on the inside bottom of the chassis. The Main PCB was mounted vertically against the back of the chassis. The larger of these two boards was the "Main PCB." The Main PCB included the controls plus the Top Boost, reverb and tremolo circuitry. Most of the AC15C1 circuitry was designed onto one of two dual sided printed circuit boards (PCB). This allowed the chassis to act as a heat sink for the tube sockets. While an earlier version of the AC-15 had tube sockets that were mounted directly to the main PC board, the tube sockets for the AC15C1 were fastened to the chassis. The chassis bolted to the amplifier cabinet with four large machine screws. The control panel nomenclature was silk screened directly to the top of the chassis. The top, bottom and inside of the chassis were painted in semi-gloss black paint. The AC15C1 steel chassis had a "C" shaped cross section. It included spring reverb, tremolo and output jacks for extension speakers. The AC15C1 was powered by three 12AX7 and two EL84 tubes. ![]()
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