Date: Sun, 25 Dec 2016 19:30:22 -0800
From: Paul Baldock <paul@paulbaldock.com>
To: amps@contesting.com
Subject: [Amps] GS-35B tube issue
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
## I read about that effect a few years back. Dunno if it was on some of
eimacs tech notes,
or was on svetlanas website from years ago. I believe their was also a
reference to it on one or more of
the tube rebuilders sites.. like Econco.
## IF I remember correctly, and I think this was for directly heated metal
tubes like the 3cx-XXX
and 4cx- XXX variety, for broadcast use, at the end of their useful life, the
emission went down, but
the idle current increased. And this was well after the fil voltage had been
increased several times.
Normal deal on the directly heated tubes was the fil was run at rated
vac..measured at the socket,
for 150-200 hrs. During the 1st 150-200 hrs, the emission increases, then
hits a peak. At that point, the
fil V is reduced way down, til PO just starts to drop off..then increased by
.1 vac. Fil V is subsequently
increased as the emission drops off. Once the fil V is raised til the fil V
is now 5% on the high side, the
tube is sent in for a re-build. Just prior to the rebuild, the idle current
will increase for a given amount
of bias Vdc.
## But the GS35B is an indirectly heated tube. Dunno if any of the above
even applies.
Do you measure at least 12.6 vac on the fils ?? Did the gain of the tube
change at all ?
Jim VE7RF
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