At 10:15 PM 09/10/1999 -0400, you wrote:
>>>>
<excerpt><smaller>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.
</smaller>
<smaller>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.
</smaller>
<smaller>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.
</smaller>
<smaller>I would appreciate hearing any suggestions, speculations, or
ideas on this problem.
</smaller>
<smaller>73 de Jack - W4ASM
</smaller>
</excerpt>
Hi Jack,
I have been following your adventures with the PA77 with keen interest.
I have a PA77 that a friend of mine wanted converted to a "two holer". I
have completed the work and I too noticed the grid current pinning the
needle in order to get good power out. I will look at the circuit and see
if I can change the resistor you mentioned.
BTW, I had a heck of a time tuning to get power out on 15 and 10 meters.
I did manage to get 1500 watts out on 10 meters but the tuning was
extremely touchy. I wondered why that was, so I started measuring the
Pi-L Network values with my HP4815A Vector Impedance Meter. The problem
is the minimum tuning capacitance is a little under 100 pF!
Here's a breakdown of the measurements:
1) 8877 mounted in socket: 26 pF, two tubes: 52 pF
2) Minimum vacuum variable capacitance: 42 pF (This requires an
explanation. Even though the minimum cap of the vac. variable is around
10 to 12 pF it is mounted so close to the power supply wall that it picks
up an extra 30 pF to ground there!)
3) RF Choke: 3.5 pF @ 15 mtrs and 2.5 pF @ 10 mtrs(The choke is no longer
inductive at those high frequencies)
So 52 + 42 + 3.5 = 97.5 pF on 15 and 96.5 pF on 10 mtrs
I ran the Pi-L computer program that comes with the latest ARRL Handbooks
and it said the Qo of the network on 10 meters would be almost 50! The
design goal most people use is a Qo of 12 which yields a tuning
capacitance on 10 mtrs of around 30 pF. You can get power out up there
but it is extremely touchy.
Good luck with your PA77 and keep posting your progress.
73 Hardy N7RT
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