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.
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 RA117s.
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 (1N916) semiconductor diode with
minor circuit modifications. This valve was not used in the RA17
Mk.2.
Andy Cowley, M1EBV