ECC88 etc

[Mike Watterson]

Makes you wonder why the PCC88 was needed at all, if they could make an ECC88 or equivalent with 300mA heater.

Aren't there some series heater types that are not the same as same number of E series?

A reason for 7V PCC88 vs 6.3V ECC88 is likely Heater - Cathode leakage/breakdown voltage. Perhaps a higher isolation voltage needed more heater power for the same cathode temperature? Certainly if the 6.3V is at 6.3V or above within tolerance, but not at the lower limit, the PCC88 will be fine. At 10% the 6.3V range is 5.7V to 6.9V. For 7V @10% the range is 6.3 to 7.7V.

Note life of a direct filament is doubled if at 10% below nominal and halved if 10% above nominal. Hence don't put fresh Alkaline or Zinc cells into a Battery Valve set each time you go to use it. A 6V or 12V regulated DC, just slightly below the nominal voltage will greatly increase filament or heater life. Emission life is very much more complex.

The 6.3V was originally picked as 3 x 2V Lead Acid cells, which had peak of 6.9V (historically 2 x 6.9 = 13.8V for 12V car batteries). The VW Beetle in late 1950s was one of the last 6V battery system cars. Modern Lead Acid 12V batteries that don't gas on a suitable charger will rise sharply to 14.2 V after 13V is reached but drop back when charging is stopped and battery under load. Older direct filament 2V, 4V and 6V valves, certainly before 1935 would not have been intended to be used while a battery was charging; the cell or battery was taken to the cycle shop to be recharged, and if you could afford it, you had two batteries/cells.

I know there are mains series radio sets where specific valves have to be at the "earthy" end of the chain for isolation, though some Italian models ignored it. Most series mains/battery valve sets have the series arrangement for reasons of hum on mains or stability. Usually the output valve(s) are at full LT+ and the detector with audio preamp is at 0V end, with the mixer-osc next.