Quote:
Originally Posted by Mike. Watterson
Quote:
Originally Posted by Davewantsone
If the recommended LT supply is 1.35V, would the valves not be overdriven if the radio had been battery powered with a 1.5V LT cell?
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The 1.5V Zinc Carbon battery isn't 1.5V!
It's nearly 1.6V with no load and freshly made. It droops with age and load.
For motors & torches the cell endpoint is taken as 0.9V
Battery valve design was most critical for the Mixer/Osc. The DK91, DK96 etc. The first was an Sylvania octal in 1938 and 1st B7G was the RCA in 1940 that the DK91 is the Philips copy of. There were German metal Y base types that were designed for parallel only and NiCd use. The Sylvania / RCA / Philips all designed for either 50mA series or later 25mA series and nominal 1.4V (1.35V on mains operation). Usually the most critical is the heptode or octode Mixer oscillator and typically designed to work down to 1.0V. As it ages the minimum oscillation voltage might be 1.1V filament. That's actually the endpoint off NiCd and not far from endpoint of Zinc Carbon, maybe about 20% wastage.
The average over time of the Zinc Carbon is around 1.4 to 1.3V depending how fresh from factory the cells were.
A mistake is to put a fresh Alkaline cell in each time you demo. Let the Radio use up the cells! |
A zinc carbon cell when the voltage falls to just under 1.5V under no load is usually exausted. My original question was the heater voltage would be higher than the rated 1.35V for the valves. Could valves fail due to this? The MR2 has some form of shunt to limit the voltage on a.c. A alkaline cell has a slightly less output than a Zinc carbon cell when new but a more constant output voltage until exausted. The best cell would seem to be a Nicad only delivering 1.4 V fully charged dropping to its rated !.2V when nearly flat.