Rod Pentode TRF

[Mike Watterson]

Thanks for this. Now I am reminded I do think you may have mentioned it before but I had forgotten. It does make me wonder all the more about how the Russians handled such variation.

Design, multiple HT rails and dividers on g2 (which isn't really a variation issue).

Regular valve variation is huge. On indirect heaters the cathode resistor hides it. Some designs and parts of circuits are more tolerant. A good radio may still work well with only 30 % emission on some valves. The UHF tuner valves had the shortest life, though an EF80 elsewhere in a TV might be more worn and it wouldn't matter.

When o/p valves used a grid battery rather than a cathode resistor they had a suffix for grid bias setting or a coloured dot.

Transistors, especially for the first 30 years, had immense spreads of gain and VCEo and VCBo ratings. It wouldn't surprise me if the early 1950s coloured dot ones were select on test as RF, IF or preamp and if later a BC107, BC108 and BC109 were all simply select on test for max operation voltage (BC107) or max gain (BC109). They even had a, b & c gain groupings on some.

Modern VMOS FETS, when miss-applied as CB RF PA and driver need the bias voltage adjusted. There is a big variation in voltage at threshold.

You'll find the 2N3819 j-FET has quite a variation of Id at VGS=0 and cut off voltage.

But the issue is magnified by the rod pentodes because when Ia= max, the Ig2 is nearly zero, uA anyway, but at Ia nearly zero the Ig2 is maybe a 1mA. Thus ONLY a series resistor to 80V HT varies Vg2 too much.