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[AMPS] PA-77 question

To: <amps@contesting.com>
Subject: [AMPS] PA-77 question
From: w4asm@mindspring.com (Jack Brabham)
Date: Fri, 10 Sep 1999 22:15:58 -0400
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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.&nbsp;&nbsp; It had apparently been out of service for =
some time=20
and has had a string of failures.&nbsp; Fix one thing, something else=20
breaks.&nbsp; It was also covered inside with tobacco residue, for which =
I found=20
a great cleaning technique.</FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>The next last event was failure of the =
8877=20
filament.&nbsp;&nbsp; I picked up a yu209 from a reputable hamfest=20
source.&nbsp;&nbsp; After&nbsp; baking the filament for 24 hours&nbsp;I =
tried to=20
load it up and all the magic smoke escaped from power =
transformer.&nbsp;&nbsp;=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>&nbsp;</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>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>All of the voltages look good, HV, =
bias, and=20
filament.&nbsp; The bias switching works normally.</FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>I get excessive grid current that I =
can't adjust=20
around.&nbsp;&nbsp;&nbsp; Can't take more than 20 watts of drive without =

exceeding 150 mils on the grid no matter how I load it.&nbsp;&nbsp; =
Otherwise=20
the relationship between drive power, plate current, and output looks =
normal=20
(all proportionately low).&nbsp; All testing is into a 1KW Bird=20
Termaline.</FONT></DIV>
<DIV>&nbsp;</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).&nbsp;&nbsp;&nbsp; I was told that a yu209 was just a =
"select"=20
8877 and was a drop in replacement.</FONT></DIV>
<DIV>&nbsp;</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.&nbsp;&nbsp; However the grid overcurrent circuit failed to=20
trip.&nbsp;&nbsp; This circuit had been working before.</FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>I would appreciate hearing any =
suggestions,=20
speculations, or ideas on this problem.</FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV><FONT face=3DArial size=3D2>73 de Jack - W4ASM</FONT></DIV>
<DIV>&nbsp;</DIV>
<DIV>&nbsp;</DIV></BODY></HTML>

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