On Nov 11, 2004, at 7:24 AM, Bill Fuqua wrote:
At 07:06 AM 11/11/2004 -0800, R. Measures wrote:
On Nov 11, 2004, at 6:13 AM, Bill Fuqua wrote:
When the amplifier is in idle state what impedances does it
see on the input and output. Unloaded amplifiers are prone to
oscillate at times at resonance not necessarily at VHF frequencies.
Bill --- At HF, there is unlikely to be enough be enough feedback
through 0.3pF (20k-ohms at 28MHz) of anode-cathode C to support
regeneration.
You are assuming that the cathode plate capacitance is the only path
for feedback.
I don't know anything about this amplifier but often there is only one
2C contact relay and serves
to switch both the input and outputs. And there is lots of opportunity
for feedback in that area.
I have seen amplifiers become unstable at HF operating frequencies due
something as simple
as lacing the wrong wires together. In one case the HV wire to the
final of a transmitter was laced
nicely in a bundle of wires that feed other lower level buffers etc.
Simply removing the HV wire from the
bundle and moving it some distance away solved the problem. I realize
that this does not necessarily
apply to this situation but it is an example.
Also, the .3pf is not much if the cathode is loaded but unloaded is
another question.
You typically have a tuned circuit there that unloaded may present a
high impedance at resonance.
Remember TGTP oscillators.
Yes.
It can happen in grounded grid amplifiers.
And it does.
In my experiences, VHF parasitic suppressors reduce the chances of
having one, but they do not eliminate them.
I can't understand how a VHF parasitic suppressor could reduce chances
of HF oscillation
They don't. "Having one" refers to having a VHF oscillation. HF
oscillations are avoided by the grid being grounded. VHF suppressors
are only for reducing VHF gain hopefully avoiding a VHF parasitic
oscillation.
if they are practically lossless at operating frequency. At one time I
may have agreed with you but after
giving it some thought over the years I must disagree.
73
Bill wa4lav
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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