Paul, sounds to me like your filament voltage may be too low, or else
you are cooking the grid by driving the tube too hard (grids are
usually plated with gold or other metal that will evaporate if the
grid is overheated). Either will greatly shorten the life of tubes
with indirectly heated cathodes. Are you getting normal power output?
Sent from my iPhone
> On Mar 8, 2018, at 10:09 AM, Paul Baldock <paul@paulbaldock.com> wrote:
>
> 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
>
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