[Amps] GS35B Idle current increasing

[Jim Thomson]

Date: Thu, 08 Mar 2018 09:09:00 -0800
From: Paul Baldock <paul at paulbaldock.com>
To: amps at contesting.com
Subject: [Amps] GS35B Idle current increasing

<I built a GS35B amp a few years back to be used with my home brew AM 
<rigs. Initially I set the idle current at around 110mA/4KV requiring 
about -35V bias. Over a few years of very little use the idle current 
drifted up so I increased the bias voltage to get it back down. 
Eventually I was up to -55V with the idle current at around 180mA, 
and so I installed another tube (purchased at the same time as the 
first). Same deal as before, started at -35V, now at -55V.

Is this 2 bad tubes, or could I be doing something wrong? I'm getting 
ready to install a third tube purchased from a different source.

- Paul KW7Y

##   dunno about indirectly heated tubes.... but looking at eimacs tech notes
for directly heated tubes, its normal for  idling current to increase, as the tube
approaches the normal end of its life span.   Your tube is not being overdriven,
thats not the issue here.   I suspect the tubes available these days are all pulls,
or pulls of pulls.    And if you try and increase the  filament  voltage even more,
the idle current will also increase somewhat. 

Jim  VE7RF