Notes on the valves used in the RA17

V4 - The Harmonic Mixer Valve

  • British : CV2209, CV4064, S6F33

  • North American: 6AS6W, 6AS6
    • Although described by RACAL as a pentode, this is a rather strange beast. It is variously described as a 'short supressor base pentode' or a 'dual control pentode'. It is designed to operate as a mixer, with one signal fed to the control grid (G1) and another to the supressor grid (G3). The supressor grid is very closely wound, with a spacing comparable to that of a normal control grid. This allows very small changes in the G3 voltage to have significant effects on the anode current. It also has a diode connected to it. The diode prevents the grid from going positive with respect to the cathode. It is believed that the reduction in the number of grids, when compared with a conventional heptode mixer, is a means of reducing the noise figure.

    V20 - The Power Rectifier

    The original valve fitted by Racal to the RA17L was the CV1377/GZ34. This is an extremely reliable and long lasting valve, especially in the RA17L, where it is operated well within it's design limits. It should not normally be necessary to replace it. However, this indirectly heated rectifier is now much sought after for hi-fi and guitar amplifiers and New Old Stock (NOS) examples command high prices. Many of the modern production valves marketed as 'GZ34' are inferior to the originals and may fail in normal use. If it is necessary to replace it, the GZ37/CV378 may be used. This replacement draws significantly more heater current and has a slightly higher voltage drop, but the RA17L mains transformer is rated to provide the current needed and the small drop in HT voltage will not cause any problems. NOS GZ37's are currently available for less than a third of the price of NOS GZ34's. The GZ37 is much larger physically than the GZ34, but fits easily into the RA17L. The GZ37 was the standard rectifier fitted to the RA17 Mk. II, which immediately preceeded the RA17L.

    Very late versions of the RA17L, and those designed for export or made in the U.S. or Canada, had a different mains transformer, lacking the 5 volt winding and with a different HT winding, and used solid state rectifiers. This was also the case with all RA117's.

    V24 - The ALC Diode

    V24 is the 'missing valve' in the RA17L. It is a sub-miniature wire ended diode and is neatly concealed under the chassis in the 100Kc/s IF strip compartment, below V16. (see Figure 13, BR 1171). Later models of RA17L and all the North American versions had this replaced with a CV7757 semiconductor diode with minor circuit modifications. This valve was not used in the RA17 Mk.2.
     
    Andy Cowley, M1EBV