Peter Chadwick wrote:
>
>Colin says:
>
>>I believe most of the modern film resistors are made with a spiral design
>>on a surface. Thus, they will have some inductance.
>
>So where's the difference between them and nichrome inductor, except that the
>resistance is higher?
>
Inductance is smaller. If you scrape the paint off and count the turns,
the normal inductance formula gives a pretty good approximation. Because
resistors are rather small in diameter, the inductance is much less than
you'd get with a typical coil of wire.
By using a bundle of several higher-value resistors in parallel, you can
get the inductance down to quite low values. This works as a grid load
for 50MHz and even 144MHz, so it will work for parasitic suppressors
too. (There's also a way to get the inductance down to exactly zero, but
I won't steal someone else's lines.)
>Let's not lose sight of the fact a nichrome inductor will have a Q that
>increases with frequency - at least, until the frequency is reached at which
>the
>self capacity affects the Q.
>
Let's not get too hung up on the Qs of individual components. What
ultimately matters is the damping load that is presented to the tube at
the frequency of the potential VHF parasitic. Calculating this load
requires a lot of parallel<>series impedance transformations, which
involve ALL the other circuit values: tube capacitance, RF choke
inductance and self-capacitance, stray series inductance and even the
setting of C1 in the pi-tank.
All these other circuit values affect VHF stability - not just the
parasitic suppressor - which incidentally explains why a particular
suppressor design may work in one amp but not in another.
Until I got down and crunched the numbers, I hadn't fully realised that
the value of the suppressor Q is literally *useless*. To do the
calculation, you need both the L and the R of the suppressor network -
the two separate numbers. Rolling them together into a single value of Q
creates a "dumbed-down" number that you can't actually use. (In other
words, it's a mathematical proof of something we already knew - that the
arguments about "my suppressor Q is better than yours" are completely
vacuous.)
There's an Excel spreadsheet on my web site that you can use to explore
these effects, and also design pi- and pi-l networks. It uses the
accurate design equations from recent ARRL handbooks, but has additional
facilities to include parasitic suppressor networks and circuit strays.
The "under construction" sign is still up because the two workmen have
moved off to other projects and haven't been back for more than a year.
You can still drive through if you know how to handle Excel with a
manual shift, and keep checking the ARRL handbook for directions.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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