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[AMPS] Q and parasitics.

To: <amps@contesting.com>
Subject: [AMPS] Q and parasitics.
From: 2@vc.net (2)
Date: Mon, 18 Jun 2001 18:21:18 -0700
>Rich says:
>
>>/\  good point.  813s won't osc. with no parasitic suppressor whatsoever. 
>
>Dunno about that (I've never tried, but I've never had parasitics in an 813
>stage, even without stoppers), but most of these designs were for triodes. 
>The
>odd pentode - HK257 and the like - but mainly things like T55, 100TH, 812  
>(for
>the 'new' stuff) and so on. I think there's one 813 design in there.
>
>
>>/\    ... perhaps on one frequency.
>
>My feeling, too.  Some of the older books have shown suppressors where 
>there's a
>circuit tuned to the parasitic frequency, resistively loaded, and link 
>coupled
>to the plate circuit. That just seems a complicated way of doing it, but 
>perhaps
>if the tube is one where the self neutralisation frequency is so low that a
>proper suppressor adds too much L, it could be useful.
>
>All in all, I suppose  it's just one of those interesting by ways of
>technological history, rather than anything particularly useful these 
>days. One
>of my books (Wireless Valve Transmitters, W. James) doesn't mention 
>parasitics
>at all - I suppose that they hadn't got tubes capable of them when that was
>published in 1923! (Ultra short waves were about 50 metres)
>
>Rich, have you ever tried ferrite beads? 

/\    We supply a ferrite VHF attenuator bead in our suppressor retrofit 
kit for the Henry 
2K-4 and 3K-A.  The bead helps to isolate the HV-RFC's self resonance 
near 95MHz from the parasitic osc. at 95MHz.  

> I've seen tx's with them used on 6146s,
>but could never get them to work satisfactorily myself. In gate 2 of dual gate
>MOSFETs, I've gnerally found changing to a 22 ohm resistor more satisfactory.
>
/\  RR.  In gate 1, a similar R tames things down.  
>
cheers, Peter

-  R. L. Measures, 805.386.3734, www.vcnet.com/measures.  
end


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