AC30HW2, AC30HW2X & AC30HWH "Point to Point" Hand Wired Amps
A Look "Under the Hood"


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Introduced in 2010, the AC30HW, AC30HW2 and AC30HWH were the first amps built by Vox since the mid seventies that featured true point-to-point, hand wired construction.

AC30HW Features and Design
The AC30HW was a two channel, point-to-point hand wired amplifier powered by three 12AX7 preamp tubes, four EL84 output tubes and a GZ34 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 15 or 30 watts RMS. Neither reverb or tremolo were included.

Chassis
The steel AC30HW 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 on a steel platform that fastened to the outside bottom of the chassis. The chassis was mounted to a wooden slider board and the entire assembly was retained in the amplifier cabinet by four large machine screws.

B+ Power Supply
The original design for the sixties Vox AC-30 B+ (high voltage) DC power supply included a GZ-34 full wave rectifier tube and a 10H choke. The GZ34 and choke worked in conjunction with two 16 uf 450 volt smoothing capacitors to eliminate any residual AC ripple in the B+ supply.

A bit of Vox mojo was inadvertently created by the GZ-34 rectifier tube. The GZ-34 rectifier tube had the tendency to exhibit a momentary "sag" or dip in power supply voltage when an AC-30 was pushed toward the limit. These drops in voltage compress the audio output of the amp. Many feel that the audio compression created by an overdriven AC-30 is an essential component of Vox tone.

The design of the Vox AC30HW B+ power supply was very similar to the original JMI AC-30. The AC30HW power supply included a GZ34 rectifier, a slightly larger 15h choke and a number of 450 volt filter caps. The larger, 15H choke aided in the reduction of 60 cycle hum and increased bass response

All Tube Signal Path
The AC30HW 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. Four EL84 output tubes (V4-V7) created the classic 30 watt Class A, NFB Vox AC-30 output stage.

Top Boost Tone Control Circuit and "Hot/Cool" Switch
The second channel of the AC30HW 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 AC30HW 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.

Fuses
The AC30HW has four fuses. Fuse F4 is the main fuse and is located inside the AC recepticle socket. The 120 VAC version of the AC30HW requires a T2.5AL/250V, the 240 volt version requires a T1.25AL/250V.

The heater filament circuits are protected by fiuses F1 and F3. Fuse F1, a T5AL/250v, protects the 5 volt filament heater circuit. Fuse F3, a T6.3AL fuse, protects the 6.3 volt filament heater circuit. The B+ circuit is protected by F2, a T500mAL.

Fuses F1, F2 and F3 are mounted in fuse holders that are accessible from the bottom of the chassis.


Figure 1 - Original placement of 3300 ohm control grid resistors on tag strip


Figure 2 - Revised tag strip without 3300 ohm control grid resistors


Figure 3 - 3300 ohm control grid resistors relocated to EL84 tube sockets

"Grid Stopper" Resistors
The AC-30HW had 3300 ohm control grid or "grid stopper" resistors installed on each of the four EL84 power tubes in the output amplifier stage. The primary purpose of these resistors was to block parastic oscillations (squealing) and radio frequency interference in the output stage of the amplifier.

For maximum effectiveness, grid stopper resistors were often installed directly to the tube sockets. Oscillation was minimized by keeping the lead length from the tube socket to the resistor as short as possible.

The original design of the AC30HW called for the grid stopper resistors to be mounted to the tag strip, as shown in Figure 1 at left. A wire lead extended from the tag strip of the four grid stopper resistors to pin two of each EL84 tube. No problem with oscillation or radio frequency interference was detected at the time of product testing.

However, when some customers replaced the factory installed Ruby brand EL84 tubes, their amps developed an oscillation problem.

Vox addressed this issue by relocating the grid stopper resistors from the tag strip (see Figure 2) to pin two of the output tube sockets (see Figure 3). Should you own an amp that has developed an oscillation problem after a change of output tubes, it would be wise to check the location of the grid stopper resistors.

It only takes a qualified electronics technician a few minutes to complete the resistor update.



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Photos and editorial content courtesy Gary Hahlbeck, North Coast Music


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