Heathkit also used the Sylvania 6C9 double tetrode in some of its tuners, for example in the AJ-12 FM-only model. This was essentially a 6CY5 pair in a single envelope with a Decar 10-pin base, intended for use as a combined RF amplifier and autodyne mixer in a single-valve front end, presumably with some performance advantages over a double-triode single-valve front end. This was effectively an FM valve, less suitable for use in VHF TV tuners where autodyne mixers did not really fit. As an aside, when FM multiplex stereo was on the horizon in the USA, it spurred the development of new FM receiver-oriented valves, both for improved front end performance and for multiplex decoding functions. GE Compactrons and Sylvania decar-based types featured amongst these new FM valve releases.
Returning to VHF TV RF amplifiers, as well as tetrodes and specialized triodes, another approach was the “shadow-grid” type, apparently introduced by GE in 1960 as the 6FG5. This was basically a beam tetrode with an additional grid between the control grid and the screen grid. This was the shadow-grid, which was aligned with the screen grid and connected to the cathode. It directed the electron flow into sheet beams that largely avoided contact with the screen grid, with resultant low current for the latter. The mechanism is shown here:
The 6FG5 anode curves showed some kinking, but so did those for the 6CY5 tetrode:
The later 6FS5, with screen current down to less than 2% of anode current, had only marginal kinking:
The 6FS5 was used at least by Tarzian in its “Silver-Sealed” switch-type TV tuner.
Cheers,