Vox AC15TB and AC15TBX Amplifier
"A Look Under the Hood"

© 1998 - 2017 The Vox Showroom, all rights reserved. No use on online auctions, eBay or Reverb.
Introduced in 1996, the Vox AC15TB and AC15TBX 
were designed by Marshall engineer Steve Grindsrod. Along with the AC30TB and AC30TBX, the AC15TB and AC15TBX were manufactured for Vox by Marshall Amplification.

1996 Vox AC15TB/TBX vs a 1961 JMI AC-15 Amplifier
Aside from the 15 watt power amp stage, the circuitry of the Korg era Vox AC-15 had little in common with a JMI era AC-15.

The JMI Vox AC-15 was a hand wired amplifier. The preamp section of a JMI era AC-15 had four inputs, two channels, Vibravox (a feature combining vibrato and tremolo) and a single tone control. It used one of Dick Denney's favorite preamp tubes, the EF86, in the input position of the normal channel. The preamp also included one 12AU7 and three 12AX7 tubes.

The JMI AC-15 amplifier stage used two EL84 tubes in a cathode biased, no negative feedback power amp circuit. The power supply included a 10 henry choke for improved AC filtering.

Unlike the hand wired JMI AC-15, the Korg/Vox AC15TB and AC15TBX were constructed on three printed circuit boards. The preamp section of the AC15TB and TBX amplifiers used five 12AX7 tubes. It also incorporated the legendary "Top Boost" circuit which replaced the original JMI AC-15 single tone control with individual bass and treble controls.

The AC15TB and TBX added reverb but exchanged the complex JMI AC-15 Vibravox circuitry for a simpler tremolo design. A two button foot switch remotely controlled reverb and tremolo.

Like the JMI AC-15, the AC15TB and TBX used two EL84 output tubes in a cathode biased, no negative feedback circuit. The AC15TB/TBX power supply replaced the 10 henry choke from the original JMI AC-15 design with a 270 ohm, 5 watt resistor.

AC15TB and AC15TBX Preamp Circuit Board

What Preamp Tube Does What?
There were five preamp tubes in the AC15TB and AC15TBX circuit. All of the preamp and power tubes were shielded and secured by the removable steel cage shown at left.

The following explanation of the function of each of the five preamp tubes may help to isolate the cause of a malfunctioning amp. A tube location diagram is located near the top of the page.

V5 provided the first two gain stages for the input signal. The signal coming from V5 was split between V2 and V3. Both triodes of V3 powered the reverb drive (send) circuit. One triode of V2 amplified the reverb return signal, the second triode was the modulator for the tremolo circuit. One half of V4 powered the tone controls, the second triode was used for gain recovery. V1 was the phase inverter (splitter) for the push-pull output stage.

Five fuses were mounted to the lower or "PSU" circuit board of the AC15TB and AC15TBX. Fuse locations are shown at right.

F1 and F3 were T4A fuses on the 6.3 filament heater line. F2 was the main fuse. When operated on 120 volts, a T1A fuse is required. 240 volt operation called for a T500ma fuse. The high voltage, or B+ circuit was protected by F4, a T250ma fuse. Fuse F5 was in line with the 5 volt tap for the heater filaments of the GZ34 rectifier tube.

Power Transformer Failures in the AC15TB and AC15TBX
The Vox AC15TB and AC15TBX have been plagued with frequent power transformer failures. When the power transformer dies, the amplifier will blow the main power fuse (F2) when powering up.

While factory replacement parts are no longer available, Hammond and Mercury Magnetics offer improved replacement power transformers for the AC15TB and AC15TBX. The amplifier shown at left has a replacement Hammond power transformer. The tech that performed the work on this chassis recalled using a Hammond P-T 290PX power transformer. I was unable to locate a part number on the replacement power transformer to verify this information, but it appears to be correct.

Reverb Pan
The full length, two spring, medium delay Accutonics reverb pan used in the AC15TB and AC15TBX carried the part number 4DB2C1D. The input transducer was 250 ohms, the output transducer was 2250 ohms.



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