[Johnson] Bang! There go the finals.

[Sherrill Watkins]

I really do appreciate everyone's kind efforts to enlighten me about
amplifiers and parasitics. I have found a copy of "Power Grid Tubes"
published by Eimac and plan to read sections of that book.  Almost every rig
I owned, even those with neutralization circuits, on 10 meters the plate of
the tube would quickly turn red when transmitting. The most unstable rig I
ever owned was a National NCX-5 transceiver. The receiver worked well but the
transmitter was prone to instantly flashing the bandswitch contacts of the
final tank/output tubes. I discovered this happened if the final lost
excitation. Years ago, one day when I attempted to transmit, suddenly a white
hot arc occurred in the final bandswitch and instantly distroyed the ceramic
switch contacts. It sounded like an electric welder; zap! I discovered what
had happened was one of the sideband carrier oscillator crystals had failed.
This removed drive to the final 6JG6 sweep tubes; POW! I got the impression
that upon loss of drive, that the final went into a wild and uncontrolled
oscillation. I have no idea if it was a high or low frequency oscillation but
it was white hot! The rig had been neutralized and had paracitic suppressors
on the plate leads. I ordered a new sideband oscillator crystal and a
replacement bandswitch from National and was able be repair the rig. However,
afterwards, I frightened to key the transmitter for fear of another
explosion. (I sold the rig and bought a Drake TR-4C which I still have in
storage. However, the Drake will overheat quickly on 10 meters when tuning up
and I know it is neutralized.) Also, a friend who is a ham for 50+ years, had
a similar experience with the National NCL-1000 amplifier. He stated that
when transmitting one or both tubes suddenly would fail with a loud bang. It
sounded like a shotgun! He attributed that to parasitic oscillation. The
NCL-1000 used grid driven, high gain tetrodes. He said the noise was so loud
he sold the rig from fear of a repetition. Also, something else has been
brought to my attention. I just realized that I an not fully sure I
understand the difference between a grid dip meter and a absorption
wavemeter. I think they are similar but a dip meter is an active oscillator
and a absorption wavemeter is a passive divice that takes energy from an
operating circuit? To be frank, it has occured me that very few hams fully
understand what is going on in their transmitters! (This is especially true
for transmission lines and antennas.) Also, I have also wondered why final
amplifier tubes do not have tube shields??? Would this reduce the effect of
parasitic oscillation by shielding the tube from outside effects? Thank you
for your interest and help in this matter. Finally, I was told by an
electronic engineer at National about 30 years ago that in the NCX-5 or any
transmitter using sweep tubes, if the output tubes were not electronically
matched, the transmitter would not neutralize. I have found this to be the
case with other equipment using sweep tubes. What I fail to grasp is what has
electronically matched tubes got to do with neutralization? I really
appreciate everyone's thoughts and comments.-73 - Sherrill W. k4own. 

-----Original Message-----
From: Richard Peterson [mailto:zapp11 at hotmail.com] 
Sent: Tuesday, October 05, 2004 11:21 AM
To: Sherrill Watkins
Subject: Gee whiz


Gee whiz. I am sorry my comments on Richard Measures caused such a 
firestorm. But trust me, he is not an especially good guide. Nope. But it 
hurts me to see so much anger from people who do not want to make a study of 
the circuits they use. I hope they study things a little more closely than 
that before they vote next month!

Now, as to burning out final tubes, I am curious about what rig you are 
talking about - and what tubes? Or, is this a home-brew amplifier? I am 
surprised they burned out - we need to discuss this.

But, yes, as I might have mentioned before, you can have parasitic 
oscillations even with a parasitic choke installed. Every rig I have heard 
of that had parasitics had parasitic chokes. The problem is that these 
chokes need to be tuned to the correct frequency. This requires alignment - 
such as compressing or expanding the windings on the coil. Too often the 
"one size fits all" approach, as suggested by Mr. Measures, is doomed to 
failure. I wish I were wrong about this, because life would be easier. But 
I'm not wrong, despite all the tributes and accolades to Measures.

A grid dip oscillator is not the right way to find the problem. I regret to 
say that an absorption wave meter is the way to go. If you don't have access 
to such a beast, you are at a disadvantage.

The coil around the resistor does not convert the oscillation to heat. It 
provides L (and C), and actually lowers the frequency of the parasitic. That 
is the purpose of the coil.

To answer your question: The tank circuit for the parasitic oscillation is 
outside the tube. Of course, the oscillator requires an active device, in 
this case, a tube. Transistors can do it too. They're a pain to deal with. 
Beem there, done that. The little suckers can get hot as heck with no drive. 
Not what you want! I had a 40 meter solid state final I built that had more 
output without drive than it did with drive. Now that was a parasitic!

Now, tell me, what did your rig do that made you believe it had parasitics? 
I just wanted more detail. I would hate for you to be chasing parastic or 
neutralization problems when that might not be the problem! Thanks.

Richard, WB5NEN

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