[AMPS] Big Bang Theories?

[N3XBU@aol.com]

Hi Guys. Any ideas on Mark's problem? Email direct: nm5k@wt.net

Subject: Parasitic suppression?
From: Mark Keith  nm5k@wt.net


Want to pick your brains... I'm sure most have heard the stories about
the vhf parasitic  oscillations in hf power amps. In the past I have
kept a close eye on all this but hadn't made any changes as I think
usually if it isn't broke, why fix it. In other words, I had never
noticed any unusual problems. One small arc/pop about 4 years ago that I
thought might had been a bug that wandered into the hv??? The amp kept
working fine so I didn't think much of it. Normally this amp seems very
docile and stable. It's a Henry 2K classic running a pair of 3-500z's.
Plate about 3200v+ -. The filter caps are getting a bit old and I have
been planning on getting a new set, or replace with a big oil filled.
Now tonight comes along, and I'm talking to a friend of mine using the
amp. All seemed normal as always, and I yakked along for a good while. I
had unkeyed for a minute or two and all of a sudden, BANG!!!!! I mean
loud and sharp like a gun, no arcing sound involved. Actually the sound
was almost the same as a firecracker. It actually rang in my ear for a
second. So my first thought was, well there goes one of my filter
caps.... But, the plate voltage still read normal on the meter. In fact,
I think the amp still worked ok at that point, but I got paranoid that I
had blew a cap and turned it off.So I signed off and pull the covers on
the amp to check out. No visable filter cap damage. In fact so far no
damage at all anywhere I can find yet. I have pulled the RF deck off the
pedastal to check underneath also. No damage that I can find anywhere in
the input circuit or tuning caps etc. No signs of flaking on the tuning
cap either. I have never had any intermitant arcing with this amp that
I've ever noticed. I do notice one cap that has the end plastic a little
crinkled , but otherwise looks ok. This 2k classic an old model that
used 8 computer grade caps. The later 2k classics had a single oil
filled. Now, seems to me if I had blown a cap, I would have a mess,
considering the sound. But all caps are in one piece, and also remember
, the amp was still working when I shut it down. So, seems to me,
barring any other problems, I have had one of these"vhf parasitic
pulses". Big time...But, I so far, I see no damage in band switches,
etc. Of course ,kinda hard to check what it might have done to the tube,
but I'm willing to bet the amp still works fine with normal output when
I fire it back up. So I'm scratching my head here. I have read all the
discussion in the past couple of years between AG6K and all involved.
Some say it's a logical explaination for this, and others say the
problem doesn't exist. I know one thing, it sure as heck went BANG!!!
here, and it wasn't any normal parasitic I have ever seen. I have had
amps go into oscillation before from weird tuning etc., but all they
ever did was peg the plate current until I moved the tuning off the bad
spot. Never have I had an amp in oscillation go BANG!!!. So what do you
all think at this time? Would welcome comments from others who have had
this problem. I'm giving serious thought to buying a set of retrofit vhf
suppressors unless someone can think of a better idea. At least if I do
that, and it does it again, I'll know it wasn't the suppressors fault.
These retrofit ones are the "nichrome wire versions" to get a lower Q of
the resonant vhf circuit between the tube and tune cap. I checked mine
with a grid dip meter, and seemd to have a pretty good dip at about 130
or so mhz. So seemed fairly hi Q I guess. While I have the amp apart, I
think I will change out the filter caps and decide what to do on the
BANG!! operation. I don't like that at all. Just thinking about what
that means for my tubes, meters, etc. gives me the willies. Going bang!
is not an option I will except. Might as well try a set of the
retro-suppressors I guess...What do you all think?MK
-- 
http://web.wt.net/~nm5k


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