I have a home brew GS-35B amp built around 5 years ago. I run about
4KV on the plate. I purchased 2 NOS tubes at the same time. Initially
when I built the amp I needed around 40V of bas voltage for idle
current of about 120mA. Over the next few years, and with little use
the idle current gradually rose and I gradually increased the bias
voltage to compensate and about a year ago at maximum available bias
voltage of 54V the idle current was about 200mA. At this point I
replace the first tube with the second tube, set the bias voltage at
40V and had an idle current of 110mA. Now a year later the idle
current has crept up to 200mA (at the same 40V bias).
Is this typical or did I buy two bad tubes? What causes the idle
current to rise over time.
Paul KW7Y
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