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Hello!
Watch out for that filament inrush current. HAS DESTROYED MANY A 8877. You
limit it with a resistor in series with the primary of the filament
transformer and short it out after some seconds.
73s de Hans / SM5KI
----------
Fr=E5n: "Jack Brabham" <w4asm@mindspring.com>
Till: <amps@contesting.com>
=C4mne: [AMPS] PA-77 question
Datum: l=F6r 11 sep 1999 04.15
I have been fixing/restoring an Alpha PA-77 for the last few months. It
had apparently been out of service for some time and has had a string of
failures. Fix one thing, something else breaks. It was also covered insid=
e
with tobacco residue, for which I found a great cleaning technique.
The next last event was failure of the 8877 filament. I picked up a yu209
from a reputable hamfest source. After baking the filament for 24 hours =
I
tried to load it up and all the magic smoke escaped from power transformer.=
This was the first time the amp had been "on" for more than a hour or so a=
t
a time.
So now I have a untested tube with a largely new power supply (new p.w.dahl
transformer, new K2AW bridge, new 34 uf @ 5500vdc filter cap, lots of
re-wiring).
All of the voltages look good, HV, bias, and filament. The bias switching
works normally.
I get excessive grid current that I can't adjust around. Can't take more
than 20 watts of drive without exceeding 150 mils on the grid no matter how
I load it. Otherwise the relationship between drive power, plate current,
and output looks normal (all proportionately low). All testing is into a
1KW Bird Termaline.
Now I'm having a hard time figuring out if there is still something wrong
with the amp/PS or if this a bad/weird tube ( I do not have a spare). I
was told that a yu209 was just a "select" 8877 and was a drop in
replacement.
One other oddity, during testing I inadvertently applied too much drive
which resulted in the grid current meter audibly hitting the stops.
However the grid overcurrent circuit failed to trip. This circuit had bee=
n
working before.
I would appreciate hearing any suggestions, speculations, or ideas on this
problem.
73 de Jack - W4ASM
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<HTML>
<HEAD>
<TITLE>SV: [AMPS] PA-77 question</TITLE>
</HEAD>
<BODY BGCOLOR=3D"#FFFFFF">
Hello! <BR>
Watch out for that filament inrush current. HAS DESTROYED MANY A 8877. You =
limit it with a resistor in series with the primary of the filament transfor=
mer and short it out after some seconds.<BR>
<BR>
73s de Hans / SM5KI<BR>
----------<BR>
Fr=E5n: "Jack Brabham" <w4asm@mindspring.com><BR>
Till: <amps@contesting.com><BR>
=C4mne: [AMPS] PA-77 question<BR>
Datum: l=F6r 11 sep 1999 04.15<BR>
<BR>
<BR>
<BLOCKQUOTE><FONT SIZE=3D"2">I have been fixing/restoring an Alpha PA-77 for =
the last few months. It had apparently been out of service for s=
ome time and has had a string of failures. Fix one thing, something el=
se breaks. It was also covered inside with tobacco residue, for which =
I found a great cleaning technique.<BR>
</FONT> <BR>
<FONT SIZE=3D"2">The next last event was failure of the 8877 filament. =
I picked up a yu209 from a reputable hamfest source. After=
baking the filament for 24 hours I tried to load it up and all the ma=
gic smoke escaped from power transformer. This was the first tim=
e the amp had been "on" for more than a hour or so at a time.<BR>
</FONT> <BR>
<FONT SIZE=3D"2">So now I have a untested tube with a largely new power suppl=
y (new p.w.dahl transformer, new K2AW bridge, new 34 uf @ 5500vdc filter cap=
, lots of re-wiring).<BR>
</FONT> <BR>
<FONT SIZE=3D"2">All of the voltages look good, HV, bias, and filament.  =
;The bias switching works normally.<BR>
</FONT> <BR>
<FONT SIZE=3D"2">I get excessive grid current that I can't adjust around. &nb=
sp; Can't take more than 20 watts of drive without exceeding 150 =
mils on the grid no matter how I load it. Otherwise the relation=
ship between drive power, plate current, and output looks normal (all propor=
tionately low). All testing is into a 1KW Bird Termaline.<BR>
</FONT> <BR>
<FONT SIZE=3D"2">Now I'm having a hard time figuring out if there is still so=
mething wrong with the amp/PS or if this a bad/weird tube ( I do not have a =
spare). I was told that a yu209 was just a "select&qu=
ot; 8877 and was a drop in replacement.<BR>
</FONT> <BR>
<FONT SIZE=3D"2">One other oddity, during testing I inadvertently applied too=
much drive which resulted in the grid current meter audibly hitting the sto=
ps. However the grid overcurrent circuit failed to trip. &=
nbsp;This circuit had been working before.<BR>
</FONT> <BR>
<FONT SIZE=3D"2">I would appreciate hearing any suggestions, speculations, or=
ideas on this problem.<BR>
</FONT> <BR>
<FONT SIZE=3D"2">73 de Jack - W4ASM<BR>
</FONT> <BR>
<BR>
<BR>
</BLOCKQUOTE>
</BODY>
</HTML>
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