[AMPS] PA-77 question, round two

[Jack Brabham]

Alek and all who replied,

Thanks for the helpful ideas.

I opened the grid return line above the bias switch (Q5) and inserted a VOM.
When the VOM indicates 100 ma. of grid current, the panel meter reads 250
ma.   If I tune using the VOM indication for grid current the amp seems as I
would expect.   Makes about a KW on low power with about 50 watts of drive
and 100 ma of grid current

So I feel fairly comfortable that the panel meter is off by about 250%.
The meter is a multi-meter and works normally for the other readings, so I
think it must be basically ok.  It reads about 32 ohms resistance, but I
don't know what it should read..

All the resistors in the circuit are within reason for 28 yr old carbon Rs,
the 3.3 ohm reads 4.0 ohms, the 680 ohm reads 711.    The circuit is... grid
current is returned to ground thru the 4 ohm. The 680 comes off of the high
side of the 4 ohm and goes to the 0 to 1 ma meter, reading voltage drop
across the 4 ohm.

Oddly enough, when I run all these values thru good old Ohms law, the thing
seems to working as designed

Look like to me that the 680 should have been more like 1600 ohms, maybe I'm
missing something.

Problem with this is it implies that the amp was designed wrong and has been
used that way for 28 years.   Pretty far stretch, too far for me.

I'm not familiar with 8877s, this is my first one.   I'm wondering if the
"ideal" grid current is 80 to 150 as indicated in the manual or is it the 40
to 60 you actually get when tune up per the book (considering the meter
error).

So I'm still working on it.   Ideas?

73 de Jack - W4ASM



.
  ----- Original Message -----
  From: Alek Petkovic
  To: Jack Brabham
  Sent: Saturday, September 11, 1999 1:28 AM
  Subject: Re: [AMPS] PA-77 question


  Jack, (Any relation to the famous Australian formula 1 driver?)
  Sounds to me like your grid current metering circuit is playing up. I
think your grid current meter is telling you lies. Check your meter shunt
etc.

  73, Alek.
  VK6APK




  At 10:15 PM 09/10/1999 -0400, you wrote:

    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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