[Amps] Ten-Tec Centurion

[R.Measures]

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