On Sun, 17 Jul 2005 22:58:35 GMT, 800 Whiskeys
wrote in :

On Sun, 17 Jul 2005 13:08:44 -0700, Frank Gilliland
wrote:

Here is what I found so far;

1) The amp is zero bias. Sounds like Darth Vader with added distortion
on sideband.

Ok. I fixed that with a diode clamp. At least mine is in AB1 now.


Check the idle current on the collector -- you might be suprised.



The collector had no connection of any kind back to the B+. It only
had a ferrite bead going to ground. It was entirely up to in the rf
input signal from the radio to turn on the transistor.


Correction in follow up post noted, but you should still check your
idle collector current.


4) Here is the weird one. The SWR without the amp on is 1.1, but
when I turn the amp on the SWR goes to 3.x:1 or higher, and I'm
talking about the SWR displayed on a meter AFTER the amp. Not the SWR
on the input side of the amp.

Hmm. This one stumped me, but some other smart fellows suggested the
amp has parasitic oscillations and the frequency (or frequencies) of
the oscillation(s) are outside the bandwidth of the antenna. I can
take another amp with two 3-500z tubes, and put it on the same antenna
with no substantial increase in SWR from the antenna. The Palomar
Elite 300 is only good for about 100 watts. The two 3-500s are good
for 1300 watts. The antenna is rated at 5000 watts.

I'm still working on the fix for this one. Tuned input and output
circuits would probably fix it, but there is not alot of space inside
the amp to work with.


The reason for this problem is obvious -- the output impedance of the
amp is not even close to 50 ohms. It's that simple.


Just in case you were right, I tried changing the number of windings
on the broad band output transformer, but it still did not get rid of
the SWR after the amp. There were some number windings that the meter
showed more output on than others, but the reflected power back from
the antenna varied proportionally with it.


You are forgetting that impedance = resistance + reactance. Your
transformer may have the right number of turns, but the collectors of
the transistors are very reactive and that reactance is reflected to
the amp's output through your transformer. You need to cancel out that
reactance. That's why SS amps have a large capacitance on the output
end of the transformer (no, those caps aren't there to make a tuned
output tank).


I think the other guys are
right in suggestion the amp is generating frequencies outside of the
bandwidth of the antenna, and those out of band frequencies are
generating the reflected power back from the antenna.


If the amp was oscillating you would have probably smoked your
transistors by now.


An SWR meter hooked up after the amp should be measuring the
reflected power back from the antenna verses the forward RF coming
from the amp. I don't see how the amp could cause reflected power
back from the antenna due to an impedance mismatch inside the amp when
the SWR meter is hooked up between the amp and the antenna. The coax
is 50 ohms even of the amp is not, and the reflected power back from
the antenna is caused by an impedance mismatch between the coax and
the antenna.
If you can logicaly explain how an impedance mismatch in the amp can
cause reflected power back form the antenna I am more than willing to
listen. On the truth with fix this amp, but I really don't see how an
impedance mismatch before the SWR meter can cause reflected power back
from the antenna.


Forget the coax for a moment: If your forward power is highly reactive
(as I suspect) and it's feeding an essentially non-reactive load, the
result is an impedance mismatch, and therefore you will have reflected
power. What you do is hook your amp to the directional coupler and a
non-reactive dummy load (using SHORT cables), then adjust the amp's
output capacitance (as stated earlier) for a 1:1 SWR.









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