> The reason why I say enlightening is because in my field of electrical
> engineering that includes the design of commercial power systems,
> lighting, traffic signal instrumentation etc., the engineering principals
> are cut and dried and there is no interpretation of basic applied
> engineering principals such as voltage drop and system fault analysis. In
> the field of RF Power Engineering it appears, however that their is great
> deal of room for interpretation.
There isn't actually much room for interpretation. Everything can be
confirmed by physics, math and proper measurement.
The problem is getting people to understand the problems,
because they are complex. Most people want a simple "one thing
causes all" answer rather than the truth, that in a complex system
many things interact.
> Both Richard Measures and Tom Rouch are very intelligent, highly skilled
> and respected engineers within the RF Power Engineering community. The
> have both contributed many publications advancing the knowledge base in
> this field and have both contributed to the "design" and "development" of
> products at the commercial level. Having said that however, it is
> disturbing that their various 'egos' interfere with the presentation of
> their information.
I wasn't aware that Rich designed any commercial products, but
that shouldn't matter at all to anyone. It's what a person SAYS and
how well he can explain it that are key to proving his technical
abilities.
> Here is a basic summary of what I should do to improved the performance of
> my SB-220 based on information posted in this group.
>
> 1. Replace the existing bias diodes with diodes having a 3A rating.
You have plenty of diode rating for ICAS, but if you want to improve
the biasing stability use a zener...especially a low power zener
with a transistor wrap-around. A zener will have a sharper
clamping action and make a more stable bias source than a string
of rectifiers.
That may not be a major issue in amateur service, but zeners
around 5 volts also have a nearly zero temperature coefficient. I
can't understand why people use rectifiers for zeners.
> 2. Increase the L in the existing parasitic suppressors.
Increasing the L will help make the parallel R a bigger part of the
anode circuit, if you have a VHF stability problem.
> 3. Add current limiting protection in the form of a ten ohm RCD 175P
Yep, then when the tubes arc the current will be limited by ESR of
the electrolytics, circuit wiring, and the ten ohm resistor (actually
20 ohms would be better, if you can use two resistors in series).
> series HV pulse rated resistor. 3. Replace the existing 3-500Z tubes with
> new tubes (I would assume that you would recommend getting good quality
> Amperex 3-500ZG instead of the generic Chinese variety?).
The later Amprex tubes have a tendency to develop filament to grid
shorts, but that is a non-catastropic failure. It generally shows
within a few months of initial service, so be sure you get a
warranty. There are very few outgassing problems (HV arcs) with
the Amperex tubes, and they hy-pot well over 11 kV as a general
rule.
The Chinese tubes are very poor quality, mostly due to materials
problems like melting glass.
> Although "tube replacement" was a gray area in discussion, it would
> probably be a good idea as they have an unknown history.
Why not hypot the tubes? Or you could add a fuse resistor at the
anode of each tube to check for arcing, if you don't operate above
twenty meters. On ten or fifteen meters you might get too much
circulating current in the path to the tubes...and burn up the
resistors.
A small metal film 2 ohm 1 watt resistor will often blow apart during
a fault, and point you to the bad tube.
It's a pretty sure thing you have bad tubes, from what you
described...but it would be nice to have absolute evidence.
73, Tom W8JI
w8ji@contesting.com
--
FAQ on WWW: http://www.contesting.com/FAQ/amps
Submissions: amps@contesting.com
Administrative requests: amps-REQUEST@contesting.com
Problems: owner-amps@contesting.com
|