[AMPS] SB1000 parasite question for Tom and Rich+

[Tom Rauch]

> Tom,
> 
> So what you are saying is that a 3-500Z will oscillate at 1 ghz just as
> well as a 2c39?

No, if I wanted to say that I would have said that.

What I said was the fact the tube has useable gain at a high 
frequency has no direct relationship to instability. As a matter of 
fact the most stable tubes (in HF amplifiers) are  those useable to 
UHF frequencies.

That's because the frequency where the tube tends to oscillate 
would be so far removed from the operating frequency, making 
suppression (if required at all) very very easy.   
  
> Seems that whatever tube has a tendency to oscillate at VHF would also
> stand to sustain a parasitic oscillation at VHF when used in the HF
> range...  Or do the laws of physics and common sense not prevail in RF?

The laws of physics always apply, and precisely why all of this 
parasitic paranoia is mostly nonsense. Common sense sometimes 
does not seem to prevail. 

> On a side note, I noticed in quite a few of the "10 meter" amplifiers of
> yesteryear, (with multiple sweep tubes) manufacturers placed ferrite beads
> at the anode clip.  This tamed many a amp from eating tubes (with the
> subsequent "big bang")...  Seems that the ferrite would also act as a
> reatance at VHF, too.
 
Sweep tubes "bang" because the anodes warp and hit the screen, 
or because the anode or grids melt from excessive current and 
voltage. That's because the rated anode dissipation is 30 or 35 
watts, and the grids are much much less.

Bangs are caused by internal tube arcs, not by parasitics. If 
parasitics caused arcs, so would excitation at the operating 
frequency.


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73, Tom W8JI
w8ji@contesting.com

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