>
>
>> I have gone back thru some old e-mails and re-read some concerning
>> "Parasitics" and "parasites" and ETC.
>> what is most interesting to me is not so much how to control and?or
>> eliminate them , but the fact that they were recognized "Way back" in the
>> twenties and such, and yet there are those folks today who seem to believe
>> that such things don't exist.!!!! I don't have the books from those days ,
>> so i'm having to assume that the info does in fact exist., but believe me,
>> I'm following the thread all the time.
>> It's very interesting !!!!!
>
>Any "doubting Thomases" can do an easy experiment. Just breadboard up
>a circuit with a glass tube sans suppressors, and observe the results. I
>don't
>think anyone could possibly deny the existence of VHF parasitics. We have
>been living with them since at least the 1920's. The argument is how to
>identify them and prevent further oscillations. As ceramic tubes became
>popular, with their shorter leads and compact designs, it was found some
>could be tamed sans suppressor,
However, the tradeoff with ceramic tubes is that they are rated for
"Amplifier and Oscillator Service" to higher frequencies, so there could
still be surprises lurking about,. For example, the 3-500Z is rated at
110MHz and it has 0.15pF of feedback C. Since the 8877 has only 0.1pF of
feedback C, it would seem to be a more stable device. The sticky-wicket
is that the 8877 is rated at several times the amp/osc frequency of the
3-500Z, so the 8877 actually has fewer reactive ohms of feedback at its
F-max than does the 3-500Z. However, although this may be bad news for
amplifier builders, for oscillator builders, it's good news.
> or with only an inductor (3CX3000A7, etc.)
>or with any old suppressor laying in the junque box. In other words, with the
>advances in tube technology, suppressor design had room for error.
>
>There now seem to be four camps out there.
>1. Those who believe ALL tubes and layouts will have VHF parasitics.
>2. Those who believe that a suppressor of a certain makeup and design
> is better than anyone else's design.
>3. Those who start out from scratch on a new amp design, and tackle each
> problem as it comes up, installing only the preventative devices necessary
> (if any) then quitting when the beast is tamed.
>4. Those who will bet with you either way that the Sun will rise in the East
>tomorrow morning.
5) My position: Trying to squeeze out the last watt at 29MHz is
probably not going to do much for reducing VHF gain.
History tells those who will listen that betting against Mr. Murphy is
neither likely to be good engineering practice or likely to be profitable.
> ...
have phun, Phil.
- R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K,
www.vcnet.com/measures.
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