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Increasing The Lifetime Of V18

Intrigued by the worn-out appearance of V18 in my SP-600-JX I decided to exploit the design margins of this 1950 vintage beauty.
Warning! Do not simply assume the following as suitable without adaption for your beloved SP-600. Please do your own measurements, calculations and draw your conclusions. Tolerances of component values, esp. 0A2 voltage ratings can be considerable!

Varying various controls (Rf Gain, CW/MOD, Send/Rec) show that the regulated section draws 13.6-14.8mA at 147.3-150.5V. The B+ voltage varied from 230V to 256V, and the current through R85 (2.56kOhm measured) 32.3-41.2mA accordingly. This results in a current draw for the 0A2 of 18.7-26.4mA which seems unnecessaryly high for today's standards of mains supply (urban areas at least).

Assuming a minimum B+ voltage of 215V due to low mains voltage, minimum current for 0A2 of 5mA (according to datasheet), and maximum (to stay on the safe side) regulated voltage of 150V, the maximum value for R85 would be

(215-150)V / (15+5)mA = 3.25kOhm.

Adding a 2W/470 Ohm carbon resistor in series to R85 reduces the current through V18 up to 4.5mA. This may not seem much, but will reduce the dissipated power by approx. 17%. Considering the exponential behavior in speed with increasing temperature of many physical processes (Arrhenius' Hypothesis), a doubled lifetime is quite probable...
As an unexpected plus I found an improvement in voltage regulation: 146.6-149.4V soon after power on, and further decreasing to 146.6-148.6V two hours later. Enjoy!

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Last update: 2000-03-18, ira@oe1ira.at
http://www.oe1ira.at/hu/0A2.html