[Amps] GS-35B tube issue

[Paul Baldock]

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