Re: [Amps] Ten-Tec Centurion

[Bill Fuqua <wlfuqu00@uky.edu>]

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. It can happen in grounded grid amplifiers.



> 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
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



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