[Amps] GS-35B tube issue

[Ron Youvan]

   Paul KW7Y Baldock wrote:

> 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 bias 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.

   I do not know this particular tube, but in some tubes the particles
of the cathode's coating pop off and adhere to the grid wires changing
the  transfer characteristics, usually compensating for this by
changing the bias is all that is needed to compensate for these changes.
-- 
   Ron  KA4INM - Youvan's corollary:
                 Every action results in unwanted side effects.