V1182 Westminster Bass Amplifier
A Look "Under the Hood"




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A Major Horsepower Boost
Vox introduced their "top of the line" V118 Westminster bass amp in 1966. The V118 made way for the second generation V1181 Westminster in 1967. The V118 and V1181 both combined a two channel, 60 watt RMS solid state amp head with a 1x18" speaker enclosure. These "made in the US" Westminster amps replaced the JMI Vox Foundation Bass that Thomas Organ had previously imported from England.

The V1181 was as well dropped in the summer of 1968 when Thomas Organ introduced the third generation Westminster, the V1182. The V1182 received a major horsepower boost. Thomas Organ installed the

120 watt RMS power amp module from the V1143 Beatle amplifier in place of the 60 watt RMS power amp used in previous versions of the Westminster. This new, more powerful Westminster was finally capable of providing a bass guitarist the stage level needed to compete with the higher powered guitar amps made by Vox. A picture of this 120 watt amp module installed into a Westminster head cabinet can be seen above.

A cardboard protective panel covered the open bottom of the Westminster power amp chassis. It bolted to the back side of the amp chassis and to the bottom of the cabinet (see photo above). This cardboard panel guarded against the possibility that the power cord or AC plug might slip under the power amp module, potentially causing damage to electrical components on the bottom side of the chassis. Thomas Vox guitar amps such as the Buckingham, Royal Guardsman and Beatle did not require this protective panel as the reverb pan blocked access to the bottom of the power amp module.

Westminster Speakers
The V1182 Westminster head was originally coupled with the V4182 Westminster Bass enclosure. This enclosure featured a roller trolley and one 18" two ohm Vega speaker rated at 100 watts .

Thomas Organ dropped the V4182 Westminster enclosure out of the 1969 Vox line up. The V1182 head was then coupled with two 4x12 "stack" enclosures, each rated at 4 ohms, yielding a two ohm total load. The new amplifier was named the V1261 Westminster Power Stak.

No FET Transistors for the Westminster
Thomas Organ also introduced their third generation Viscount, Buckingham, Royal Guardsman and Beatle guitar amps in the summer of 1968. The updated versions of these amps included a new modular preamp circuit board (Thomas part number 25-5373-2) that featured low noise FET (field-effect) transistors. Thomas Organ did not incorporate either this new preamp circuit board or FET transistors into the V1182 Westminster.

Modular Construction
Modular construction allowed Thomas to trim costs by sharing certain subassemblies over a number of amp models. Such sharing would trim the required number of inventory items and subassemblies that needed to be stocked to produce a line of amplifiers. It also offered the opportunity to reduce part costs through what is known as the "economy of scale." Part prices often decrease as consumption (and order size) increases.

A good example was the steel preamp chassis found in the V1182 Westminster amp. This chassis was not unique to the Westminster. It was also used in the Viscount, Buckingham, Royal Guardsman, Sovereign, Super Beatle and Beatle amplifiers. While the same metal chassis was utilized in each of these amps, the number of features in the amps varied by model, as did the number of controls, switches and jacks. The standardized chassis included all of the component mounting holes required to build any of these amps. A model specific control panel covered any unused chassis holes. The photo of the V1182 chassis at the top of this page reveals a number of such unused control and switch holes.

Another example of modular construction involves the preamp circuitry. Thomas Organ designed a universal modular preamp circuit board (Thomas p/n 25-5222-2) for their first and second generation Viscount, Buckingham, Royal Guardsman and Super Beatle guitar amplifiers. This single circuit board contained the preamp circuitry for the Normal, Brilliant and Bass channels plus the tremolo and "Watchdog" limiter circuitry.

Thomas Organ also incorporated the 25-5222-2 preamp

circuit board into the V1182 Westminster head. As the the two channel Westminster head did not include either tremolo or a Brilliant channel, the sectors of the circuit board supporting these features were not populated with components (see photo above at
right). Aside from tremolo and reverb, the circuitry for the Normal and Bass channels in the V1182 Westminster was identical to the Normal and Bass channels in the Buckingham, Royal Guardsman or Beatle.

The circuit board was hinged, allowing top and bottom access for ease in service (see photo at left).

A color coded illustration of the sectors of the 25-5222-2 circuit board as populated for the V1182 Westminster may be viewed at the top of this page.


The preamp circuitry of all three generations of the Westminster amps was nearly identical. The most major difference was the addition of the "G-Tuner" circuit to the V1181 and V1182 models.

The preamp circuitry of the the V1182 Westminster had only two minor changes from the prior V1181 model. The V1182 eliminated resistor R7 in the audio return of the G-Tuner circuit. The V1182 also relocated inductor L1, a component in the Tone-X circuit, from the Bass channel area of the PC board to an unused area of the board near the Normal channel circuitry (see circuit diagram above). A pair of wires connected inductor L1 back to the same copper circuit traces from where it had been originally located.

Watchdog
The Westminster, Essex Bass, Viscount, Buckingham, Sovereign, Royal Guardsman, and Super Beatle amplifiers all included the "Watchdog" audio limiter circuit. The 1967 Vox catalog proudly proclaimed that "Watchdog lets you to blast clearly without overloading. Play at maximum power with optimum tonal quality. When compared with other amplifiers, Vox amps with Watchdog give more usable power, watt for watt, than others of even higher power rating."

The Vox "Watchdog" limiter circuit was designed to prevent audio levels from increasing beyond a preadjusted point. As the amplifier reached the level where distortion would occur, the Watchdog circuit would restrict the output level of the preamp to keep the audio signal clean. The Watchdog circuit didn't actually increase the total output of the amplifier, it simply allowed the amplifier to work closer to the maximum undistorted audio output.

A chassis mounted threshold control adjusted the point where the Watchdog circuitry capped the maximum output from the preamp (see image at top of page). Click here to learn about the calibration procedure for the Watchdog limiter circuit.

Tone X and the Two Button Foot Switch
A two button foot switch was included with the V1182 Westminster amp head. This foot switch tied into the "Tone X" tone control and pedal amplifier circuitry located in the Bass channel. The Tone X tone control and circuitry made it possible to make a dramatic shift from a low pass filter to a high pass filter with the twist of a control knob.

When the Tone-X control was rotated fully counter-clockwise, the tone was very deep. When rotated clockwise, the tone was brilliant and thin.

The pedal amplifier circuitry, located on the preamp circuit board, worked in conjunction with either button on the foot switch to electronically simulate the effect of turning the Tone X control to the fully clockwise, or treble position. One of the foot switch buttons was a momentary switch (hold the button down to operate), the second was a latching switch (click "on," click "off").


G-Tuner
Thomas Organ added an "G-Tuner" reference pitch generator to the V1182 Westminster amp. It provided an electronic "pitch pipe" tone to assist in tuning a bass. The Vox Sovereign service manual stated that the G-Tuner circuit was "factory adjusted to a frequency of 195.998 cycles, the true pitch of the G string on a bass guitar."

A slide switch, located on the rear control panel, turned on the G-Tuner tone. The external portion of the circuit was mounted to the upper left inside corner of the head cabinet and was connected to the preamp by a cable with a plug (see photo at left). The Normal channel volume control adjusted the level.

If a serviceman neglects to reconnect the G-Tuner to the preamp after service, the Normal channel will be dead.

Westminster Rotary Power Switch
The Westminster amp used a rotary power switch (Thomas p/n 69-5202-2) that was custom manufacured for Thomas Organ by Clarostat. This switch has caused a few service issues, so let's take a brief look at how it works.

The switch had three positions: "Off," "Standby" and "Operate." The switch also had three individual circuits. The main power switch circuit was located in the back of the switch assembly and supplied 120 VAC to the amp. A second circuit supplied 28 VDC to the red and green pilot lamps. A third circuit completed the circuit between the power amp and the speakers.

When the switch was rotated from the "Off" position to "Standby," the main power switch turned on the AC power to the amplifier. The pilot lamp circuit in the power switch illuminated the green Standby pilot lamp. While in the Standby mode, the speaker switch did not complete the circuit from the power amp to the speakers, allowing the amp to be powered up without hearing a "thump" from the speakers.


When the power switch was rotated from "Standby" to "Operate," the main power switch continued to supply AC power to the amp. The Standby pilot lamp was extinguished and the red Power pilot lamp illuminated. Lastly, the connection between power amp and speakers was completed, enabling the amp for performance.

It is not unusual for the phenolic wafer in the rotary power switch that completed the connection from the amp head to the speaker cabinet to snap off and break. Such damage to the power switch was ironically often the result of prior amp service. The damage can happen if the two conductor plug that joined the wires coming from the phenolic speaker wafer on the power switch to the XLR speaker output jack is not disconnected when removing the back. If one neglects to disconnect this plug, a tug on the removable back panel can exert enough pressure on the speaker wafer of the power switch to cause it to fracture.

At left is a photo of the three position power switch that has lost the phenolic wafer that controls the speaker standby function.

As replacements for this custom made rotary switch are unavailable, the easiest solution for this problem is to remove the
two speaker wires (brown wires with white stripes) from the broken portion of the rotary switch and connect them together. Unfortunately, this eliminates the speaker standby circuit but it restores function to the amp while retaining factory stock appearance.




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