On the 7th February 1961, a capacitor was added to the circuits of three amps - the AC15, AC30/4, and AC30/6 - the aim being to suppress any unwanted noise in the HT line of V8, the ECC82 Vib/Trem modulator valve. For the AC15 and AC30/4 its value was to be 0.033 mfd (C42 on the circuit diagrams); for the AC30/6, 0.005 mfd (C41) - see further below.

The AC30/4 - capacitor C42

OA/032 information panel, bottom right hand corner of the sheet.

OA/32, note of the modification, top right of the sheet.

C42 in place - .033 MFD, over the 47K resistor in the HT line for V8.

Detail from OA/031 for the AC15, third circuit. C42, also noted as a modification on 7th Feb. 1961, is again .033 MFD.

Below, a small series of AC30/4 chassis that were never fitted with C42, starting with those in TV Front cabinets, the position of the 47K resistor (yellow, purple, and orange rings in ascending order) indicated with an arrow.

TV Front cabinet, serial number unknown. Hunts capacitor dated 39th week 1960.

Modified TV Front cabinet, serial number unknown. Hunts capacitor dated 39th week 1960; pot dated October 1960.

TV Front cabinet in blue, serial number unknown. Pot dated August 1960.

No cabinet. The picture has been enlarged slightly beyond its natural size for better legibility. One of the cathode bypass caps in the preamp looks to have the date code "TTY" perhaps "TYY" = 44th or 40th week of 1960.

Note that in the pictures above numbers of black WIMA Tropydurs are still in place, untouched by the repairman's iron. Below, a detail of the chassis of an early split-front AC30/6.

AC30/6 - capacitor C41

For the AC30/6 the added capacitor - 0.005uf - was numbered C41 - as marked at the head of the list of changes to circuit diagrams OA26 and OS/065. In practice, a 0.047uf cap was sometimes used.

Detail of OS/065.

Detail of OS/065.

AC30/6, split front cabinet, serial number unknown.

A further split-front AC30/6 (no serial number) may also have lacked C41 originally, but the amp has had a good number of repairs, so one cannot be certain.

Naturally, one of the factors that comes into play is the retrospective fitting of C42 in the AC30/4 and C41 in the AC30/6. In most instances, it is exceedingly difficult to work out what was there (or not) originally. Below, a detail of a late TV Front amp, a red print WIMA in position as C42. Was it added to replace an earlier 0.033 MFD capacitor? Perhaps.

TV Front cabinet, serial number 4x73, Hunts capacitor dated 29th week 1960, one of its pots with the code "CI" = March 1961, C42 a red-print WIMA.

As for the date - 7th February 1961 - that may simply record the point at which the change was added to OA/031 and OA/032, not the date of the change in production itself. It serves, provisionally, as a useful terminus ante quem at any rate.

For the mis-fitting of the capacitor in certain AC30/6s, see the note at the end of Glen Lambert's page here. In AC30/6 circuit diagrams, the capacitor is C41.