> >
> > ----- Original Message -----
> > From: Paul Christensen <w9ac@arrl.net>
> > To: <amps@contesting.com>
> > Sent: 06 October 2002 19:47
> > Subject: [Amps] Failed 8877
> > > I narrowed the root cause to a broken 8877 filament....although it was
> > > difficult to find beacuse it is intermittent. At the onset of
filament
> > > current, it opens and after powering down the amp, and checking with a
> > DVM,
> > > the filamant resistance reads fine. I finally got it to read an open
> > > resistance right at the pins with the transformer Molex connectors
> > > unplugged. I can repeat the sequence and by putting the DVM in series
> > with
> > > the filament, I can see the DVM read AC current for a brief instant,
> then
> > > it goes to zero. When this occurs, I can then see the filament read
> open
> > > resistance right at the pins. Once I pull the tube and measure the
> > filament
> > > resistance, 75% of the time, I can then see good resistance. The
other
> > 25%
> > > of the time, it will read open. I do not have a spare 8877 at the
> moment.
> > I have a pair of 8877s that do just the same - they come from the
'quality
> > problem' period and came from industrial generator pulls, but I know
> nothing
> > else of their history. In these, turning them upside down would cause an
> > open, and back the right way would remake the contact, sometimes with an
> > extra tap needed. I experimented with one and found that winding the
> heater
> > up very slowly avoided it going open circuit at switch on, and after a
> 24hr
> > heater burn it seems to have a reliable contact again and runs the
correct
> > current, although I haven't tried allowing inrush current and it hasn't
> been
> > tried at high power either.
> Something I should add here, just remembered. I tested both tubes in a 6m
PA
> built by a friend - one tube ran up just like his, stacks of output
without
> any bother. The other gave nothing at all and that's when I started to
> figure out why and found the o/c heater and the upside down effect. Tried
> the good tube for the same effect, and there it was, just the same.
>
> Steve
>
Steve,
Aren't these the tubes that when you invert them you hear a 'plink' type
noise from inside as they disconnect?
Mike
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