RA117 serial number N0489 (/cont)

I decided at this point, to concentrate on the under-chassis compartments. In the past, I have left them to last and proceeded straight to the front-end. As previously stated, the RA117 was initially manufactured in response to a US DoD requirement … and like the RA17C12, the RA117 was designed around (or incorporated) North American valves (called tubes!). It looks like the RA117 design benefited from lessons learned from the RA17C12 in that many of the valves are different. For instance, in the UK version of the RA17L, VFO1 is an EF91. In the RA17C12 it is a 6AU6 or EF94. Now in the RA117, where a 6AU6 was fitted, it is now a 6BA6 or EF93. In almost all cases, there is a direct UK replacement valve for the North American ‘tube’ … that is with the exception of the 6AS6. This is the Harmonic Mixer immediately following the Harmonic amplifier. RA17s and RA17Ls manufactured for the UK employed a 6F33 (or CV329 or CV2209). However, there is a subtle difference. Whereas the 6AS6 is electronically identical to the 6F33, it is mechanically incompatible since g2 and g3 are transposed. With this in mind, I decided to lift the can off V4, the 6AS6. Imagine then my surprise when I found not a 6AS6, or a 6F33, as one might expect, but the valve on the right. The lettering is a tad worn, but I’d say it’s a CV2009 (or EF93, CV454, 6BA6W). What gets me is how on earth this ‘rogue’ valve was actually surviving since with respect to the 6AS6, the cathode and g3 are actually transposed. I promptly replaced it with a ‘genuine’ Russian’ 6AS6 and the RA117 was instantly transformed!

Spot the difference!

RA17L 1MHz Oscillator on the left with RA117 circuit on the right … they are quite different. The 17L uses EF91s while the 117 uses 6AK5s

Harmonic Mixer etc. before

Harmonic Mixer etc. after