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AC15HW1 Features and Design
||Introduced in 2010, the AC15HW1 and AC15HW1X were the first amps built by Vox since the mid seventies that featured true point-to-point, hand wired construction.
The AC15HW was a two channel, point-to-point hand wired amplifier powered by three 12AX7 preamp tubes, two EL84 output tubes and a EZ81 rectifier. The circuit design incorporated a "Bright" switch in the Normal channel, a "Hot/Cool" switch in the Top Boost channel, a "Master Volume" control and a "Master Volume Bypass" switch. An "Op/Mode" toggle switch allowed the amplifier to operate at either 7.5 or 15 watts RMS. Neither reverb or tremolo were included.
The steel AC15HW chassis had a "C" shaped cross section. The control panel was located at the top of the chassis. The hand wired tag strip was mounted to the inside of the chassis and the transformers and choke were bolted to the outside. The tubes were mounted vertically to the bottom of the chassis. The chassis was mounted to the amplifier cabinet by four large machine screws.
B+ Power Supply
The original design for the sixties Vox AC-15 B+ (high voltage) DC power supply included a EZ81 full wave rectifier tube and a 8H choke. The EZ81 and choke worked in conjunction with two 16 uf 450 volt smoothing capacitors to eliminate any residual AC ripple in the B+ supply.
The design of the Vox AC15HW B+ power supply was very similar to the original JMI AC-15. The AC15HW power supply included a EZ81 rectifier (V1), a 7H choke and a number of 450 volt filter caps.
All Tube Signal Path
The AC15HW featured an all tube signal path. Please see the chassis photo near the top of the page for tube types and locations.
The signal from the "Normal" input jacks was preamplified by one half of V3 (12AX7). The signal from the "Top Boost" inputs was preamplified by the other half of V3. V3 had a spring loaded, can style aluminum tube shield to guard against noise and radio frequency interference.
The classic "Top Boost" circuit (Bass and Treble controls) was powered by V9 (12AX7). A third 12AX7, labeled V2, served as the phase inverter. Two EL84 output tubes (V4 & V6) created the classic 15 watt Class A, NFB Vox AC-15 output stage.
Top Boost Tone Control Circuit and "Hot/Cool" Switch
The second channel of the AC15HW chassis incorporated the classic Vox "Top Boost" tone control circuit. Top Boost was Vox parlance for individual bass and treble controls. Prior to the introduction of the Vox Top Boost circuit in 1960, Vox amps only included a simple "Tone Cut" control that rolled off treble response.
The Top Boost circuit appears to have been lifted, part for part, from the tone control design of a 1950's era Gibson GA-70 amplifier. While extremely simple in design, the tone controls of the Gibson GA-70 were strangely interactive. Advancing the GA-70 bass control also affected the tonality of the mid and high frequencies. The interaction between the tone controls was caused by an unusual ground connection on the bass control. While a more conventional tone circuit design might either leave off the ground or install a resistor between the bass pot and ground, the Gibson GA-70 tone control circuit grounded one of the legs of the bass control potentiometer. As all "Top Boost" Vox amplifiers share the GA-70 tone circuitry (and the unusually grounded bass control), they exhibit the same peculiar tone control interaction.
Here is where the story takes an interesting twist. Vox added a new feature, the "Hot/Cool" switch, to the Top Boost circuit for the AC30HW. When this switch was in the "Cool" position, one of the legs of the bass control was grounded, as in the classic Gibson GA-70 / Vox Top Boost tone control design. Flipping the "Hot/Cool"switch on the AC30HW to the "Hot" position lifted the ground from the bass control, increasing gain and allowing the bass and treble controls to operate conventionally.
Tone Cut Control
The AC15HW included the classic Vox "Tone Cut" tone control. The "Tone Cut" control was linked to both channels and was located in the power amp circuit between the phase inverter and output tubes. Here is how it worked.
The signal from the preamp is directed to the phase inverter. The output from the phase inverter is split into two signals. One of these signals is 180 degrees out of phase with the other. When "out of phase" signals are combined, they cancel each other out. The "Tone Cut" control works by combining the high frequency signals from one side of the phase inverter with the other.
The AC15HW had three fuses. Fuse F4 is the main fuse and is located inside the AC recepticle socket. The 120 VAC version of the AC30HW requires a T1.25AL/250V, the 240 volt version requires a T1.630mAL/250V.
Fuse F3, a T6.3AL fuse, protects the 6.3 volt filament heater circuit. The B+ circuit is protected by F2, a T250mAL. Fuses F2 and F3 are mounted in fuse holders that are accessible from the bottom of the chassis.