On Nov 3, 2004, at 10:58 AM, Will Matney wrote:
Steve,
That's the way I always looked at it too. But, and here's a big but, I
read a few webpages that says VHF oscillations are a bunch of bologna.
These very authors was or still might be high up in the ARRL, I'm not
sure, and did work for, or still do, the amp manufacturers. They say
that it's impossible and that it couldn't cause arcing in components.
Now to me, if the lead(s) length is correct, and the gain high enough,
the internal capacitance of the tube along with the inductance of the
lead(s) could make up a VHF oscillator. Another thing is that most
tank circuits in production amps are hand wired. The lead lengths
aren't strictly controlled so they cant tell from one to another if
they hit the sweet spot or not!
The sticky wicket is that, in a HF amplifier, there is always a VHF
resonance formed by the anode C, the anode lead, and the Tune C. This
would not be a problem if the (grounded) grid resonance was higher in
frequency than the anode resonance. Unfortunately, in a 3-500Z, the
grid resonance is c. 88MHz and the anode resonance is c. 20MHz higher.
Even though the grid can do a fair job of shielding the cathode/input
from the anode/output below its resonance, it can not do so above its
resonance.
It would look to me that over this many years of using parasitic
suppressors, and having found direct formulas for them from a lot of
experimentation, anything that says VHF oscillation don't exist would
have been caught. As far as I know, the ones saying it doesn't happen
has only been in the past few years. I would think that 60+ years is a
loooong time to go before catching it? From this, I'd like to hear a
good discussion and or debate about it.
A month or so after "Parasitics Revisited" appeared in *QST*, I
received a telephone call from Charles Thomas Rauch, Jr., W8JI --
a.k.a., Tom Rauch. He wanted to discuss this article. In regards to
the SB-220, he stated that he had repaired about 400 SB-220s and that
many of them exhibited parasitic-oscillation damage. When I mentioned
that the AL-80 used a parasitic oscillation suppressor design that was
virtually identical to that used in the SB-220, he began speaking into
the telephone so loudly that my wife Susan, N6FEY could understand what
he was saying about 6-feet away from the telephone.
Best & 73's
Will Matney
Steve Thompson wrote:
On Wednesday 03 November 2004 18:23, Will Matney wrote:
To all,
A quick question here, and may open up a big can of worms, if there
is
no such things as parasitics in amplifiers, why do they all come with
parasitic suppressors on the anode leads?
There certainly are parasitics (unwanted vhf oscillation) -
especially with tubes with long grid leads - but not every amplifier
will have the gain/feedback/load to allow oscillation if suppressors
aren't fitted.
It's likely that few amplifier builders have the equipment to
properly assess whether their amplifier needs a supressor, and/or
whether the suppressor they fit actually does the job of reducing the
gain by the right amount at the right frequency.
Steve
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Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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