Another possibility is that the filaments don't really both want to run at 100 mA. With 110 V applied to the resistor and filaments string instead of 95 V, the filaments get a little hotter than nominal, but probably not decrease tube life significantly. This is probably due to filaments being rather forgiving in voltage. That only comes out to 95 V instead of the expected 110 V. That doesn't quite add up with the 200 Ω resistor dropping 20 V due to 100 mA thru it. It seems the filaments are 45 and 20 V, for a total of 65 V in series. You then have to find a way to run the two fillaments and the indicator lamp. That takes care of isolation, the rectifier tube wearing out, and the inevitable hum this amp has on the output.īut, it's not quite that simple. However, my preference would be to replace the whole power supply in the lower right corner with a off the shelf modern DC supply, if you can find something that puts out 130 V or can be adjusted to that. ![]() It specifically says that it's designed to work with 50 or 60 Hz power. See the note next to the power plug at lower right.
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