Detail from the Vox "Precision in Sound" newspaper-format catalogue, September 1964. The Top Boost unit kit. Note the "pseudo stereo effects", a slight stepping back from the "stereophonic" of 1961 and 1962.

This page is really the logical extension of two earlier ones: the first on JMI publicity for the Top Boost unit on this page , and an overview of the circuit itself - available here. Both pages are still growing, as this one will.

Below, details of surviving units (assemblies) and some brief notes on numbers and types of amps to which they were fitted.

The assembly

The mounting bracket is L-shaped and made of steel. One limb of the "L" has a slot for the screw that holds the main chassis at centre to the slider board. Bolted to the lower limb, there is a smaller bracket for the valve holder, and opposite, a short section of tagboard.

A unit from 1964. Photos: Stuart Castledine.

A detail from AC30 serial number 6948. The lower limb of Top Boost unit extends under the chassis on the slider board. The centre screw for the chassis holds both unit and chassis in place.

One of the main pots in serial number 6948 has the date code "AK" = January 1963; the pots in the Top Boost add-on - "KK" = November 1963. It is likely that the amp was later returned to factory for the fitting.

Factory practice was to fix the vertical limb of the assembly *behind* the lip of the preamp. The white front-facing traffolyte panel was made such that it fitted perfectly under the lip.

AC30 Super Twin serial number 5726. Further details on this amp below.

The panel

Early panels appear to have been of solid white plastic, the legends engraved on the surface:

AC30 Twin serial number 6948, detail.

One also finds three-ply white traffolyte of the type used for Vox guitar pickguards: a thick black inner layer and thin white outers:

AC30 Twin serial number 4995, detail.

Since Top Boost units were fitted dead centre on the back panel - in other words in the position normally occupied by the serial number plate - plates were generally re-positioned to left or right of the cut-out. Serial number plates of Super Twins were sometimes moved to the edge of the back panel.

Controls

On original early units the BASS control is always at the top, TREBLE underneath. The knobs, which were also used on certain Vox guitars, were white, concial in form, and were moulded with heavily ribbed sides. Some have marker dots, others not.

AC30 Twin serial number 6948

Adoption

Surprisingly few early AC30 Twins with factory-fitted Top Boost units on their back panels survive. Below, a short list, repeated from this page:

4995 - Twin, the Top Boost bracket has two fixing points at top (normally there is only one), no component date codes visible though the electronics are evidently old (and factory installed).

5311 - Super Twin, perhaps a replacement trafolyte facing panel and control knobs. The electronics are old (and probably factory installed).

5329 - Super Twin, Hunts cap. visible but no component date codes. Perhaps sold with Top Boost already fitted.

5648 - Super Twin, the white trafolyte facing panel is now missing, and the foot of the bracket looks to have been shortened. The ECC83 has the date code "JL" = November 1962. The Hunts cap. has "YIW" = 3rd week of 1961.

5726 - the Hunts cap. has the date code "HIH" = 23rd week of 1962. Perhaps sold with Top Boost already fitted.

It is perhaps worth adding that a chassis from late 1962, sold recently on its own (no cabinet, no speakers), seems to have gone back to the factory to have Top Boost added in 1965 (pots with January '65 date codes) - either that, or a repairman installed it from a JMI kit.

1963

6898 - Twin. Serial number plate moved to the right of the opening for the unit.

6929 - Twin. Serial number plate moved to the left.

6948 - Twin. Serial number plate moved to the right.

7167 - Super Twin. Serial number plate moved to the left.

7253 - Twin. Serial number plate moved to the far right hand side of the back panel.

7403 - Twin. Serial number plate moved to the left.

7443 - Twin. Serial number plate moved to the left.

7776 - Twin. Serial number plate moved to the far right hand side of the back panel.

8648 - Twin. Serial number plate moved to the far right hand side of the back panel.

8803 - Twin. Serial number plate moved to the left.

8854 - Super Twin. Serial number plate moved to the left.

Bands

AC30 Twins issued to bands in 1963 all tend to have back-panel Top Boost - those given to The Beatles, Shadows, Rolling Stones, and so on.

At present, the earliest securely dated instance of a Twin with factory Top Boost on its back panel is George Harrison's first AC30, Cavern Club, Liverpool, 22nd August, 1962.

The serial number of the amp is likely to have been in the 5000s. One of the knobs is already missing.

PAUL MCCARTNEY'S AC30 SUPER TWIN: first used in public in Liverpool on 2nd August (with its accompanying "tray" stand). On the 28th July at Great Yarmouth he was still using the T60 amplifier.

A detail of the back of Paul McCartney's AC30 Super Twin amplifier section, Wimbledon Palais, 14th December, 1963. The settings are: Brilliant channel (high gain input); volume at 10 o'clock (from another picture); Tone cut at 5 o'clock; Top Boost: bass all the way up, treble at 9 o'clock.

Below, Brian Jones adjusting the Top Boost controls of his AC30, British tour, unknown venue, last third of 1963. Note that Bill Wyman's AC30 Super Twin amplifier section at right, also has Top Boost, its serial number plate moved to the right to make way for it.

Brian Jones adjusting Top Boost controls, serial number plate far left on the back panel.

Back to the page on the Vox AC30 Top Boost circuit.