The V1133 Royal Guardsman Amplifier Head
A Look "Under the Hood" of the Modular Preamp and Control Section

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PC Board Layout and Circuit Design of the Vox V1133 Royal Guardsman Amp
Thomas Organ, the US licensee and manufacturer of the Vox product line in the United States, released the third generation of the Royal Guardsman amp head in mid 1967. Known as the V1133 Royal Guardsman, the new version introduced the use of low noise FET transistors. It also streamlined production through the use of a new universal preamp circuit board.

The preamp and control section used in the V1133 Royal Guardsman was identical to that found in the V1123 Buckingham and V1154 Viscount. The only difference between the preamp section of the V1133 Royal Guardsman and V1143 Beatle amp was the inclusion of the "Repeat Percussion" effect in the V1143 Beatle.

The preamp section used in the first (V-13 and V113) and second generation (V1131 and V1132) amps utilized two or three modular circuit boards to support preamp functions, reverb and distortion. The third generation preamp board used in the V1133 Royal Guardsman amp was designed by Stan Cutler of Thomas Organ. It consolidated all of the preamp and effects circuitry onto one compact and universal board given the Thomas part number 25-5373-2.

The 25-5373-2 circuit board allowed Vox to incorporate "modular" construction into their guitar amps in a new and innovative way. The 25-5373-2 circuit board was arranged into sectors (see color coded diagram above). Each sector supported a particular preamp channel or effect. Thomas Organ would install electronic components only into the sectors of the preamp board that would be consistent with the features included in a particular amp model. No electronic components would be installed in circuit board sectors where the channel or effect was not included with the given model.

The list of features included in the V1133 Royal Guardsman head, V1123 Buckingham head and V1154 Viscount combo amp required that electronic components be installed into all but one of the modules in the 25-5373-2 preamp board. The "Repeat Percussion" area of the preamp board was left blank in the V1123, V1133 and V1154 as this feature was reserved for the V1143 Beatle.

A zinc plated steel chassis enclosed the preamp circuitry and also served as the control panel.

The 25-5373-2 preamp circuit board was hinged in the chassis, allowing easy access to the bottom of the board for service.

The photo at left shows the preamp board hinged to the open position, revealing large electrolytic capactors mounted to the back of the chassis that were used in the preamp power supply.

Thomas Organ/Vox engineer Brad Plunkett contributed a new low noise preamp circuit utilizing FET (field effect) signal transistors to the third generation V1133 Royal Guardsman head. Additionally, the V1133 incorporated modern FET switching transistors in the MRB and Distortion Booster circuits to replace the electro-magnetic relays used in prior Royal Guardsman models.

The Westminster, Essex Bass, Viscount, Buckingham, Sovereign, Royal Guardsman, and Super Beatle amplifiers all included the "Watchdog" audio limiter circuit. Thomas Organ proclaimed in their 1967 Vox catalog 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.

V1133 Royal Guardsman Reverb Pan
The full length, medium delay reverb pan for the V1133 Royal Guardsman head was mounted horizontally to the bottom panel of the head cabinet. The Thomas replacement part number for the original two spring reverb pan was 53-5004-2. For those seeking to purchase a replacement pan, the modern Accutronics replacement part number is 4BB2A1B.

Thomas Organ added an "E-Tuner" reference pitch generator to the V1133 Guardsman amp. It provided an electronically produced "pitch pipe" tone to assist in tuning a guitar. The Vox Guardsman service manual stated that the E-Tuner circuit was "factory adjusted to a frequency of 323.628 cycles, the true pitch of the E string on a guitar."
A slide switch, located on the rear control panel, turned on the E-Tuner tone. The actual circuit was mounted to the upper left corner of the head cabinet and was connected to the preamp by a cable with a plug (see photo at left). The Bass channel volume control adjusted the level.

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

Royal Guardsman Rotary Power Switch
The Royal Guardsman 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 switch did not complete the circuit from the power amp to the speakers, allowing the amp to be powered up without hearing a turn-on "thump."

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 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.

North Coast Music manufactures many replacement and restoration

parts for the Royal Guardsman under license from Vox. These parts are
available exclusively at North Coast Music. Some are shown below.


The VOX Showroom!

Photos and editorial content courtesy Gary Hahlbeck, North Coast Music

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