[AMPS] parasitics
km1h@juno.com
km1h@juno.com
Thu, 23 Apr 1998 10:58:31 EDT
On Wed, 22 Apr 98 22:56:49 -0500 Jon Ogden <jono@webspun.com> writes:
SNIP
>Please, if you have a view explain why and give me some scientific
>reason
>behind your science. I am engaging you, Rich and the whole mail
>reflector on this topic because I am trying to learn what is actually
>happening. So if I have technically gotten something wrong, please
>point
>it out to me. Don't just tell me I am wrong. How will I learn what
>the
>correct answer is?
>
>73,
>
>Jon
>KE9NA
Jon, I guess I didn't make my point clear...sorry but when I am real
short for time I over simplify.
First of all I have no particular disagreement with the use of a somewhat
lossy material in the suppressor. I have commented since the beginning of
this reflector that National Radio went that route in a military tetrode
amp back in the 60's. I use that same strap today in my work.....bought a
roll of it at the final National IRS auction several years ago. The main
difference is that Rich tries to get by with #16 wire whereas National
used 1/2" wide strap around a big 50 Ohm non-inductive resistor.
I do object to using that material in the complete plate to blocking cap
path and prefer the traditional low inductance wide silver plated
strap/tubing strap. If the suppressor is doing its proper job right at
the tube then there is no reason to continue nichrome to the blocking
cap.
Layout is critical and in a sloppy version it may be necessary to use
more nichrome or whatever. But in a good layout I would think that the
lowest possible inductance in order to move resonance above the tubes
ability to oscillate would be the "good engineering" approach.
Secondly, I have harped for a year about VHF resonances in the tank
circuit with nary a bit of comment from anyone. Now I see that Rich has
finally added that possibility to his ever varying repertoire. National
Radio solved that problem very neatly in the NCL-2000 and NCX-1000 by
adding a SERIES cap...not a shunt as has been discussed about here the
past few days. The National engineers added a 10pf capacitor across the
20 and 40M shorting bandswitch contacts which eliminated a particularly
nasty proclivity to arcing.; perhaps you would wish to analyze that
circuit.
I use that idea in stock SB-220's and have had users report no more
switch or Tune cap arcing. I've used it in various Alpha 8874 amps that
had arcing problems...and suppressor R that was as new.
Thirdly, my dis-belief centers around the big bang and that is where I
suggested ( or that was my intent anyway) an article. You are the
engineer, show me how a VHF parasitic has enough energy to bend a 3-500Z
filament.
I say it is a gas/plasma discharge and partially caused by barnacling AND
the lack of proper PS surge protection. It is this area only that I
refered to junk science.
And finally....No, I have not built a ton of amps but there are certainly
plenty of established companies that have. I would prefer to give their
engineers the credit they deserve and cite the megatons of stable amps
that they have shipped to military and other customers. A few persons act
as if they are the sole sources of these revolutionary "new ideas" but in
reality there is absolutely nothing new at all.
The BIGGEST problem in many ham amps IMO is the save a buck attitude of
the manufacturers. They use marginal components, leave out key safety
features and would be laughed out of anything but the ham market.
My own suggestions for an amp would include:
1. HV surge suppressor resistor of a value and wattage that can limit the
instantaneous PS discharge current to a safe non-destructive value.
2. Use of a suitably placed series capacitor in the tank circuit to
provide a low impedence VHF path to the load.
3. Use of a suitably sized parasitic suppressor resistor and phase out
carbon composition. A 5W metal oxide should be the minimum for 3-500Z
size tubes; a pair of them for the 4-1000A.
4. Use of a resistive suppressor L material only if necessary to tame a
known problem tube such as the 3CX1200A7 or to compensate for layout
problems.
73 Carl KM1H
>
>ps: Answers such as: "I've built tons of amps that don't oscillate
>using traditional suppressors" don't cut it. I want facts not
>emperical
>experience.
>
>
>
>-------------------------------------
>Jon Ogden
>KE9NA
>
>http://www.qsl.net/ke9na
>
>
>"A life lived in fear is a life half lived."
>
>
>
>
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