22-02-2019, 10:57 AM
(21-02-2019, 07:20 PM)LyntonP Wrote: Ended up with a couple of resistors in series to achieve a LT of 1.39 volts.
I'd aim for lower than that. Did you read the thread by Michael Watterson that I gave you a link to?
Quote:Conclusion
Above 1.2V performance is fine. Above 1.35 life is short. Life is x2 at 1.25 and life is 1/2 at 1.43V
Do whatever Philips/Mullard say!
Remember that these valves were designed to be used with zinc-chloride cells (alkaline weren't around in those days!) Although sold as 1.5V cells, nothing could be further from the truth. A fresh cell can easily be much higher than this but, on load, the voltage drops fairly rapidly until it reaches 1.4V, at which it remains for a while before starting off on the inevitable long slope to extinction.
Now, we don't know anything about the condition of the valves you have, other than they work fine, so might I suggest the following experiment? Monitor the filament voltage whilst slowly turning the Variac down until you reach the point where the performance starts to deteriorate, or stops altogether. (I am aware that the HT will also drop while you are doing this but it shouldn't be sufficient to cause any problems.)
Having determined what this low point is, endeavour to achieve a filament voltage approximately half way between this level and 1.35V. This should ensure the maximum valve life as well as good performance.
By the way, did you find the missing 25mA?
