Final update on this question: I got the problem fixed. When all is
well, the resting plate current should be zero or nearly so with no
excitation. Whether in QSK or PTT/Vox mode, standby or operate, the
current should be zero. If you key the amp via PTT or QSK with no
excitation, the plate current should be around 100 ma. Before, the
outside cabinet temperature where the tube exhaust air leaves the
cabinet was 198 degrees F. After the fix, the cabinet temperature at
that point was 98 degrees F.
In my case, Q2 on the Filament-AC board was shorted as was one of the
output transistors on the QSK board (either Q4 or Q5). I also replaced
the ICs on the QSK board with sockets and new ICs. I know that the
previous owner (SK) had experienced a tube short and that apparently
caused the problems.
If you have resting plate current above zero when the amp is not being
keyed, you likely have a failure in the bias circuit on the Filament-AC
board, or the QSK board which controls the bias circuit on the
Filament-AC board. Or, you may have an early Centurion with the
original bias circuit. If that's the case, you can easily add the
updated circuit to the original Filament-AC and QSK boards. See the
Centurion manual currently on the Tentec website for up to date
schematics of those boards.
My query via email to Tentec regarding parts for the update (previously
offered by Tentec) went unanswered.
73, Floyd - K8AC
Date: Thu, 23 Jun 2016 16:41:50 -0400
From: Floyd Sense <k8ac@k8ac.net>
To: tentec@contesting.com
Subject: [TenTec] Fwd: Tentec Centurion bias question
Message-ID: <245abbbe-16ab-8563-99d9-1114f0ed13ce@k8ac.net>
Content-Type: text/plain; charset=utf-8; format=flowed
I recently acquired an early Centurion (serial 10A10221) from an SK's
estate. The resting plate current, PTT not keyed, is around 100 ma and
the 3-500ZGs have a nice orange plate glow. I had read in the archives
that later changes resulted in a much lower resting plate current and
set out to see what the current state of circuits is in this amp. ( Yes,
I know that level of plate current is OK, but it generates considerable
heat on the desk that I don't need.)
The amp has obviously been updated at some point as the Filament-AC
board has the new bias circuitry and the QSK board is revision C. The
Filament-AC board circuit as shown in the 10/94 rev C diagram agrees
with the schematic. So, I'm guessing that I might have all the latest
changes in regards to bias. The question is: Is the observed 100 ma
resting plate current what others are seeing in the Centurion with the
later bias changes? If that level is normal, I was thinking about maybe
using the Ameritron ESB-1H board which would reduce the resting current
to just about nothing with no excitation. Any thoughts? Tentec has not
responded to an email on the subject.
73, Floyd - K8AC
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