SB-220 parasitic suppressors.

[Multi-Volti Devices]

I don't know whether the parasitic suppressors had an inductance value
specified in the part description, but they consisted of tinned solid copper
wire air wound around 2 or 3 carbon composition resistors. Someone else on
the list can tell us what the resistors values were (47 ohm maybe?)

The Rich Measures mod used a 3W Panasonic metal oxide resistor and nichrome
wire, for lower Q.

I don't know what the Harbach ones look like.

I strongly recommended the AG6K/Rich Measures/QST parasitic suppressor to
customers who called or wrote to Heath Techical Consulting when I was there,
especially for the SB-1000. I personally would leave them there if they are
the metal oxide/nichrome suppressors, because I believe they are more
effective.

1) Generally, the higher the operating voltage, the higher the mu, for a
given tube.

2) According to Measures, the Penta Labs tubes were (when the article was
written) Chinese, and was modeled on the Amperex 3-500Z tube which had a mu
of 200. Since that statement was made, Eimac has left the 3-500Z market, and
sold operations to someone else, (Triton?). Word on the street (Internet)
has it they are making 3-500Z's "to original specs".

I asked the question, "What is original spec?". According to Rich Measures'
research, Eimac reduced mu many years ago from 200 to 160.

If I remember accurately, he stated this was in response to amplifier
engineers' inquiries about tendencies toward parasitic oscillation. Just
because an amp has the grounded grid topology, it's no guarantee of
stability.

When I worked at Heath, we saw an incredible number of SB-1000's vaporize
numerous components for no apparent reason (bandswitch contacts, bias zener
diodes, meter movements). According to Rich Measures, he met with great
resistance from one other amplifier company about his statement that the
parasitic suppressors 'traditionally used' were ineffective. This particular
company insisted the failures were from hot switching or operation
(basically that customers were stupid). Rich's collection of published
photos revealed there is definitely some common factor causing a problem.
Not every ham with a 'linear' is an idiot (although some people listening to
75 or 20 phone might disagree! :o)  ).

We saw much less of a problem with the SB-220. Note that it runs somewhat
lower plate voltage, and the input and output circuitry are much better
shielded than in the SB-1000, where everything is open, on one chassis
shell, with no top side/bottom side wiring. Maybe it's a coincidence that
the few SB-220's (I personally saw VERY very few) in for such problems
didn't come in until alternate sources for 3-500's appeared on the market. I
find Rich Measures' claim about the different mu among different tube mfr's
a useful observation. He told me personally there was nothing wrong with the
Chinese tubes, if they arrived working, anyway. They just had higher gain.
He did point out that there were three mfg. plants (I can't remember who in
China was making them, but a name like Sino keeps popping into my head).
Some of the tubes could be seen visually to be defective, with badly
misaligned anode structures, etc.

I also saw SB-1000's that repeatedly would go into full bore oscillation
while a tech was working on them, and one in particular that fried at least
twice in a row at power up, with no drive. That technician definitely wrote
some new verbal schematic symbols ($#@&%^%).

I got permission to try a Measure's parasitic suppressor kit in service and
begin suggesting them to customers. At that point in history, there was
negligible work being done in engineering on ham products, and the normal
ECN process and chain of command were not available to fully evaluate the
problem and solutions, but we found the Measures' parasitic suppressor in a
non-statistical (due to circumstances) way produced observable improvement.

I don't think the risk of damaging costly, or nearly unavailable, parts is
worth the desire to have an original component that appears to be implicated
in the problem.

The availability of alternate tubes and the alleged change of mu by Eimac
seem to explain why the problem wasn't common for many years.

Thanks for the BW, as people say (I usually don't, I just ramble).

Murray

--- --- --- --- --- --- --- --- --- --- --- --- --- --
To subscribe: listserv at listserv.tempe.gov
and in body: subscribe HEATH yourfirstname yourlastname
To unsubscribe:  listserv at listserv.tempe.gov
and in body: signoff HEATH
Archives for HEATH: http://www.tempe.gov/archives
--- --- --- --- --- --- --- --- --- --- --- --- --- --