Carl wrote:
>
>On Sat, 9 May 1998 00:31:47 +0100 "Ian White, G3SEK"
><G3SEK@ifwtech.demon.co.uk> writes:
>
>Lots of SNIPs
>
>
>>increases faster as the frequency is reduced into the HF region.
>
>
>Ian, I have a problem understanding your last statement. If the L was
>properly selected for a particular tube AND circuit why would the Q
>always detrimentally increase at a lower frequency? I look at the
>suppressor network as a series trap and would expect the response to be
>somewhat uniform on either side of its optimum frequency. The ideal
>suppressor would then be only frequency dependent over the "parasitic"
>range of the particular amp.
As the words "series" and "trap" are normally used, that would imply
that the suppressor is resonant - which it ain't. It's a resistor
paralleled by an inductor, and the network analyser measurements show
that the conventional LR suppressor behaves very close to theoretical.
As you increase the frequency, the network behaves progressively more
like a resistor; as you decrease the frequency, it behaves progressively
more like an inductor.
Truly resonant suppressors do exist, particularly for mode killing in
cavity amplifiers, but here we're talking exclusively about "lowpass"
suppressors which are intended to be lossy at VHF but transparent at HF.
>I know of no modern tube that requires any excessive R dissapation at
>30MHz. Indeed, the parasitic frequencies are well above 50MHz and if the
>suppressor was designed for the parasitic it should then run cold at
>28MHz.
>
Sure - no problem with that. The higher the VHF resonance is that you
need to suppress, the more chance there is of designing a suppressor
that looks lossy at VHF but also runs cool at 30MHz.
>One very good argument for a lossy L is that it works equally well...or
>poor over a much wider frequency delta. The "classic" suppressor may very
>well be far superior at some particular narrow range of frequencies...but
>what is its characteristics outside of that area?
Check the graph of measured Q from N7WS. On a log-log plot, the classic
LR suppressor gives an almost straight sloping line (just as it should)
with absolutely no sign of resonance effects below 200MHz where the
measurements stopped.
The nichrome suppressor has significant losses in the inductor as well
as in the resistor. That's the only difference. The question is whether
this is an advantage or not...
>The issue, as I see it, is a "monkey see, monkey do" attitude. Someone
>published a suppressor design back in ancient days and it has been copied
>ever since as a cure all for one particular tube in one particular layout
How true!
>. Ian, you have a similar problem trying to educate people about tetrode
>screen supply design.
Yes - but please, Please, not in this thread!
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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