Mazda 30F27 RF tetrode for TV tuners.

[Synchrodyne]

Yes, I think it could have been that the EF8 arrived at a time when conventional pentode performance (at low VHF) was still improving, and with the technology of the time, this offered an easier way to the desired performance.

Then as you say, the shadow-grid valve was, by the late 1950s, a relatively easy option in manufacturing terms, and provided a means to go beyond the performance plateau provided by the best consumer-type pentodes.

One of the reasons for departure from the cascode for VHF TV RF amplifiers was a desire for simplification, with its concomitant cost reduction. Another, perhaps more cogent reason was to reduce the HT voltage requirement to a level easily provided by AC-DC type power supply circuitry, say around 125 volts. The series cascode circuits based upon the 6BK7 et al typically required 250 volts. So, a single-valve RF amplifier was highly desirable.

The en masse arrival of AC-DC TV receivers in the USA came circa 1955, following the valve industry’s introduction in 1954 of a full line valves for of 600 mA series-string service. That said, the technique did not become universal. The Standard Coil Neutrode tuner, with a 6BN4 or 2BN4 triode RF amplifier requiring only 125 volts HT, was available by the start of 1956.

I’d say that not everyone saw the triode as the ideal solution to the VHF TV tuner RF amplifier problem, hence the subsequent development of special tetrodes, guided grid triodes and shadow-grid valves in an effort to do better, or at least enable a different trade-off amongst noise, gain and the need for neutralization. Clearly the Nuvistor should be included here, although in this case RCA had a much broader range of applications in mind, industrial as well as consumer.

On paper at least, the 1960s incarnation of the shadow-grid valve looked quite good. For the 6GU5, RCA quoted a noise figure of 5.9 or 5.7 dB at 200 MHz, according to anode voltage. This compares with the numbers quoted by Fisher, as shown in post #8 upthread, of 5.5 dB for the PCC84 and 7.5 dB for the 6AK5. The latter was one of the best VHF pentodes, developed during WWII, but generally viewed as too costly for most consumer applications. Anode-to-grid capacitance (Cag) of the 6GU5 was 0.018 pF, lower than the 0.03 pF of the 6AK5, although a lot higher than the 0.002 pF of the best HF pentodes. To the extent that the shadow-grid form of construction made it difficult to get very low Cag numbers (as suggested in some of the EF8 commentary), it was better suited for wider-band VHF applications than for narrow band HF.

I don’t have a real sense as to in what proportions 1960s US VHF TV RF amplifier applications were distributed between cascodes, triodes (of several types), tetrodes and shadow-grids. Possibly significant is that in its 1966 paper comparing valves and solid-state devices for TV RF and IF applications, RCA used triodes to represent both conventional-grid and frame-grid valve performance, a 6CW4 (Nuvistor) in the former case and a 6GK5 in the latter. One would have expected it to include a Nuvistor anyway, so perhaps it saw the frame-grid triode as the best proxy for “all the rest” in the valve group, suggesting that by the mid-1960s it was the dominant type. And as if to reinforce that, in 1969, Zenith used the 6HA5 frame-grid triode as the reference standard when developing a high-performance solid-state VHF TV tuner.


Cheers,