I have a Yaesu FT101E which has low output because the final 6JS6C valves are old. If i increase the plate voltage on the final tubes from 600 to 800 VDC by changing the mains transformer tapping from 240 V to 220 V will that increase the output power ? What else would that do ?
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2$\begingroup$ Are you sure that a 9% increase in AC mains voltage will produce a 33% increase in plate voltage? $\endgroup$– Brian K1LICommented Apr 27, 2019 at 10:40
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1$\begingroup$ I don't know how tubes age, but as far as I'd guess, it's both air diffusion into the tube's vacuum (which leads to oxidized getter, and plate surfaces, and thus reduced conductivity/higher thermal losses), and things like degradation of the metal surfaces of the heating, the gate and plates due to electron bombardement. While higher acceleration (i.e. higher plate voltage) would counteract that, it sounds like a "dangerous" thing to try in either case, in the sense that I'd guess that it'd significantly increase the speed of degradation. $\endgroup$– Marcus MüllerCommented Apr 27, 2019 at 10:49
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3$\begingroup$ A matched pair of 6JSC6 costs US$75. When you consider the damage that might result from increasing the plate voltage by 33%, stressing other and aged components beyond their specs, this could be a worthwhile investment. $\endgroup$– Brian K1LICommented Apr 27, 2019 at 16:57
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5$\begingroup$ @Andrew if you replace degraded components with good ones, I'd strongly recommend not operating the device out of spec. I mean, you risk breaking your freshly repaired amplifier? And if increasing the output power was so easy, the original designers would certainly have done it to begin with. $\endgroup$– Marcus MüllerCommented Apr 27, 2019 at 23:29
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1$\begingroup$ Continued...increasing the mains power by changing a tap will not only change the plate voltage but every other voltage in the rig. You are changing grid control voltages, maybe pushing currents beyond tube safe zones, and also affecting critical circuits that are dependent on good voltage control (oscillator). Not then way to fix the problem you have. $\endgroup$– K7PEHCommented Apr 28, 2019 at 15:14
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1 Answer
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Should it be confirmed that the old finals are responsible for the low output then there could be only one solution i.e to replace the old finals with new tubes.
Thermionic emission from an old vacuum tube cannot be improved by increasing the plate voltage.
