[AMPS] IT WORKS!!!
Jon Ogden
jono@enteract.com
Fri, 12 Mar 1999 23:48:57 -0600
Hey guys,
FINALLY, I got the 4-1000A PA up and working! Yes, I've been on the air
with it all night tonight and it performs GREAT.
I am getting an EASY 1.5KW from about 80 W of drive on 20M and on 10M I
need maybe 90 W to get that (plate V under load is about 5.5KV).
All in all I am very pleased and very happy. I did some efficiency
calculations on 20, 15 and 10M and I am getting about 50-55% on 20, 45%
or so on 15 and 49% on 10. From what I've discussed with others, this is
about what to expect from a GG amp.
The sheet metal is probably not as well fitted and matched up as Carl
would like and the paint on one of the panels doesn't match the others,
but hey, who cares!
Had a minor scare today as I heard some arcing and then no signal out.
Turns out my bandpass filter fried. Don't know why. It's a Bencher and
is rated at 1.5KW and 5KW peak. I'll just have to drill out the rivits
and fix it. But with that out, it works good. I am running an external
antenna tuner so I think that perhaps the filter is not all that badly
needed.
Anyhow, here is the interesting part that is going to make some people go
nuts:
A couple of weeks ago, there was a discussion of adding a coil between
the blocking cap and the Tune Cap. The purpose of this was to step the
impedance down enough so that one can use a reasonable value of Tune C
and still have reasonable Q. Well, I added about a 1.5 uH coil made from
1/4" copper tubing. It's about 9 turns all in all. I ended up having to
replace the 10M coil on the B&W 850 with just a piece of looped copper
strap as I now actually needed LESS inductance on 10M. I tried using a
couple of different "L" coils at the blocking cap and going back to the
original B&W 10M coil, but my efficiency suffered. So I stayed with the
original design.
With this design, I have gotten out almost 1.8KW on 10M with just over
100W drive. Now, here's the good part: The amp is completely stable for
all positions of Tune C. I have keyed the amp up with no input signal an
no load and moved Tune C through its entire range. No increase in grid
or plate current anywhere. If some of you remember I had a problem with
oscillation when I put the lid on. No more! Gone.
What am I using for a suppressor in the anode? Sorry, Rich, it isn't
nichrome wire. They just got too hot and your resistors burned up - not
from parasites but from thermal heat. Heat from the anode is conducted
along the anode output line. It is also radiated from the tube. All
resistors derate significantly as the get up in temperature. They will
reach a point where they won't handle any power. Then they fail. I
believe this is a big source of parasitic resistor failures in general.
After hours and years of being thermally heated, they lose their power
handling capability. After taking out AG6K's suppressors, I got a couple
more hundred Watts of output power. What did I use? Well, I used a
single turn of #10 magnet wire that is about 1.5" in diameter and NO, yes
NO, resistor. Another gentleman on the reflector had pointed out to me
that he used a similar type suppressor with his 4-1000's as his resistors
kept burning up. He said he has had no problems with parasitics.
Neither have I. IMHO, my theory on the "L" coil adding to the stability
was correct. It completely looks like a very large reactance to any VHF
energy. And when checking the anode circuit with a dip meter, I could
not find ANY noticeable dip from 50 to 150 MHz. With all my previous
anode/tank circuit designs, I had a nice dip about 80 MHz. The resonant
capability of the circuit at VHF is gone.
I must say though that I did keep AG6K's suppressor suggestions on the
input of the tube. His 10 Ohm resistor-33pf Cap suppressors on the
cathode pins made all the difference in the world. Without them, the amp
was not stable even with the above discussed configuration. Adding the
little bit of feedback that those do helped tremendously. So thank you,
Rich. I also left his suppressor suggestions for the control and screen
grids installed as well.
Some of you may think I am crazy with this configuration but it works.
I've tried the stability test with three different tubes. Some hotter
than others. All were stable. And as others have pointed out, many
commercially built ham amps might not stand that test.
I've been taking pictures of all this and I'll be posting them on my web
site within the next few weeks.
I want to take a moment to thank everyone who has provided assitance,
support, advice and encouragement. Many people who never go public on
the reflector wrote to me offering their suggestions. Many helped quite
a bit and I owe you guys a big one! In particular, I want to thank Jim
Thomson, VE6NQ. Jim has been a great deal of help throughout this whole
thing.
Anyhow, thanks again all and I want to get back on the air so I'll cul.
73,
Jon
KE9NA
-------------------------------------
Jon Ogden
KE9NA
http://www.qsl.net/ke9na <--- CHECK IT OUT! It's been updated!!!!!
"A life lived in fear is a life half lived."
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