[Amps] Failed 8877

[Steve Thompson]

----- Original Message -----
From: Steve Thompson <g8gsq@qsl.net>
To: <amps@contesting.com>
Sent: 07 October 2002 07:57
Subject: Re: [Amps] Failed 8877


>
> ----- 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