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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 =
inside 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 at 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 =
been working before.
I would appreciate hearing any suggestions, speculations, or ideas on =
this problem.
73 de Jack - W4ASM
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<DIV><FONT face=3DArial size=3D2>I have been fixing/restoring an Alpha =
PA-77 for the=20
last few months. It had apparently been out of service for =
some time=20
and has had a string of failures. Fix one thing, something else=20
breaks. It was also covered inside with tobacco residue, for which =
I found=20
a great cleaning technique.</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>The next last event was failure of the =
8877=20
filament. I picked up a yu209 from a reputable hamfest=20
source. After baking the filament for 24 hours I =
tried to=20
load it up and all the magic smoke escaped from power =
transformer. =20
This was the first time the amp had been "on" for more than a hour or so =
at a=20
time.</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>So now I have a untested tube with a =
largely new=20
power supply (new p.w.dahl transformer, new K2AW bridge, new 34 uf @ =
5500vdc=20
filter cap, lots of re-wiring).</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>All of the voltages look good, HV, =
bias, and=20
filament. The bias switching works normally.</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>I get excessive grid current that I =
can't adjust=20
around. Can't take more than 20 watts of drive without =
exceeding 150 mils on the grid no matter how I load it. =
Otherwise=20
the relationship between drive power, plate current, and output looks =
normal=20
(all proportionately low). All testing is into a 1KW Bird=20
Termaline.</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>Now I'm having a hard time figuring out =
if there is=20
still something wrong with the amp/PS or if this a bad/weird tube ( I do =
not=20
have a spare). I was told that a yu209 was just a =
"select"=20
8877 and was a drop in replacement.</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>One other oddity, during testing I =
inadvertently=20
applied too much drive which resulted in the grid current meter audibly =
hitting=20
the stops. However the grid overcurrent circuit failed to=20
trip. This circuit had been working before.</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>I would appreciate hearing any =
suggestions,=20
speculations, or ideas on this problem.</FONT></DIV>
<DIV> </DIV>
<DIV><FONT face=3DArial size=3D2>73 de Jack - W4ASM</FONT></DIV>
<DIV> </DIV>
<DIV> </DIV></BODY></HTML>
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