Vox AC50CP2 Amplifier Combo and AC50CPH Amplifier Head
A Look "Under the Hood" of the Classic Plus Series


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Like their counterpart from the sixties, the JMI Vox AC-50 Mk IV, the 50 watt Vox AC50 "Classic Plus" series amplifiers were powered by four 12AX7 preamp tubes and two EL34
output tubes. Two models were offered in the AC-50 "Classic Plus" series, the AC50CPH head and the AC50CP2 combo amp.

The AC-50 Classic Plus amplifiers were designed under the leadership of former Vox lead engineer Steve Grindrod. Prior to his employment at Vox, Grindrod spent many years as the lead engineer at Marshall Amplification.

All Tube Signal Line
The signal line for the AC50 Classic Plus amplifiers was tube powered, but transistors and integrated circuits were included in the circuitry for the Line In and Line Out jacks, the D.I. (Direct In) jacks and the reverb.

Chassis
The Vox AC50CP2 combo amp and AC50CPH head utilized a rectangular steel chassis pan. To insure even weight distribution, the power transformer and output transformer were placed at opposite ends of the chassis. The output tube sockets were mounted to the chassis to facilitate heat dissipation from the EL34 tubes.

The chassis was suspended from the top of the AC50CP2 but mounted to the bottom of the AC50CPH. On the AC50CP2, channel two was on the left side of the control panel, channel one was on the right. As the chassis was inverted on the AC50CPH, channel one was on the left side of the control panel and channel two was on the right.

Circuitry
The circuitry for the AC50 Classic Plus amplifier was designed onto nine printed circuit boards. The Preamp PC Board included the 12AX7 preamp tubes. The Reverb, D.I. and FX Loop Board was mounted towards the rear of the chassis. The Power PC Board included portions of the Class AB power amp circuit plus the high voltage section of the DC power supply. The Bias Adjustment PC Board included two potentiomenters for adjusting the bias of the output tubes. The controls for each channel were mounted to separate boards. Three additional small circuit boards were used to mount the input jack, master volume control and the speaker output jacks.

The Preamp, Reverb and Power boards were stamped with the model number "VX100 Modern." As these boards were also used in the 100 watt version of this amp (AC100CPH), it suggests that Vox was originally planning different model numbers for the Classic Plus Series.

Preamp PC Board
All four 12AX7 tubes were located on the Preamp Board. The following chart explains the function of each preamp tube.

V1a
12AX7 (low noise)
Initial Gain Stage Channels 1 and 2
V1b
12AX7 (low noise)
Tone Stack Gain Recovery, Channel 1
V2a
12AX7
Tone Stack Gain Recovery, Channel 2
V2b
12AX7
Gain Stage for "Fat" Switch, Channel 2
V3a
12AX7
Final Gain Stage, Channel 1
V3b
12AX7
Final Gain Stage, Channel 2
V4a
V4b
12AX7
Phase Inverter

Channel 1 offered a clean and chimey tone, reminiscent of the AC-30. The additional gain stage in Channel 2 (V3b) allowed this channel to develop overdriven "Plexi" style tones. While neither channel employed the traditional Vox "Top Boost" circuit, each channel included a "Bass," "Middle" and "Treble" control.

Reverb, D.I. and FX Loop PC Board
Solid state devices powered the reverb circuit in the AC-50 Classic Plus amplifier. The reverb drive circuit was powered by three NJR2147D integrated op amps. The reverb return circuit was powered by a pair of NJM2120D integrated op amps. A pair of board mounted RCA jacks connected the drive and return circuits to a three spring reverb pan.

The D.I. and FX Loop circuits were also powered by a series of integrated circuits and transistors.

Power PC Board
The Power PC Board converted the AC output of the power (mains) transformer to useful DC voltages needed by the AC50 Classic Plus amplifier. The power transformer had four secondary windings.

The first of these secondary windings supplied the B+ (~500 VDC plate voltage) for the EL34 output tubes. The AC voltage from this winding was converted to DC by four discrete IN4007 diodes and six 100 uf 400 volt capacitors mounted on the Power PC Board. A chassis mounted 15H choke further smoothed the AC ripple. The B+ supply was protected from overload by a T500mAL fuse mounted to the power board (FS1)

A second winding on the power transformer created the -150 VDC bias supply. The AC voltage from the power transformer was rectified to DC by a KBU bridge rectifier and two 47 uf 250 volt capacitors mounted to the Power PC board. Three 51 volt zener diodes, connected in series, limited the bias voltage to ~150 VDC.

A third winding on the power transformer supplied the 28 VDC and 15 VDC nodes for the solid state circuitry in the AC50 Classic Plus amplifier. Rectification was handled by an integrated diode bridge (W01) and several 2200 uf capacitors.

The fourth winding powered the tube filament heaters. This winding powered the filaments for both the preamp and output tubes. The preamp portion of the filament heater circuit was converted to DC to minimize 60 cycle hum, The filament winding was protected from overload by a pair of T6.3AL fuses mounted to the power board (FS100 and FS101).

The main fuse for the amp (FS3) was mounted inside the AC receptacle. The 100 and 120 volt versions of the AC50 Classic Plus amplifier required a T3, 15AL 250V main fuse. The 220 and 240 volt versions of the amp required a T3, 6AL 250V main fuse.

Bias Adjustment Board
The Bias Adjustment Board was located behind a small removable access panel labeled "Output Bias Adjustnent Underneath" on the rear of the chassis.

Removing the access panel exposes two potentiometers (VR101 and VR102) and a trio of brass test points. The test points are provided as a handy way to measure bias voltage. The potentiometers adjust the bias voltage. The following procedure should only be performed by a qualified electronics technician.

To adjust the bias voltage, attach one lead of a digital volt meter to the center test point and the second lead from the meter to left test point. Adjust the left potentiometer until the voltage is set to 35mV. Next move the lead from the left test point to the right test point. Adjust the right potentiometer to 35mV. The AC-50 Classic Plus output circuit will now be biased to factory specs.

Reverb Tank
The AC-50 Classic Plus Series amplifiers utilized a full sized Ruby 3EB2C1B three spring, medium delay reverb pan. This was a three spring pan with an 800 ohm input impedance and a 2575 ohm output impedance.




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