> Note: Years ago, I had an amplifier which had a burned bandswitch. I
fixed
> the bandswitch and fired it up. Same problem. I then decided it was
> parasitics, so applied all of Rich Measures "cures". I then fired up the
> amplifier and had exactly the same fireworks.
I've found the same thing in testing, but the cause was original components.
WB8BFS reported the same thing here with a TL922, Jack's had a relay
sequencing problem. A K5 had a gas-tube lighting arrestor in the feedline
that was intermittent. Both had installed "the kits" before discovering the
cause.
There are literally dozens of causes of arcs and sparks.
> I do have objections to the approach that almost all amplifier problems
are
> parasitic related. No doubt parasitics can cause some problems. Indeed,
I
> understand some of the tubes replacing the Eimac 3-500Z may have higher
> gain, causing a marginal amplifier to have parasitics.
The interesting thing is the tubes have less gain, not more, in most cases.
Not that grounded-grid gain has much to do with VHF grounding of the grids
and resulting stability.
What seems to have become lost is the fact that slight changes in HV have
more effect on gain than differences fromn tube-to-tube, and that
instability is a complex function of many things the smallest and least
critical of which is tube-to-tube gain variations.
> It is difficult for me to understand how an amplifier that has been
working
> faithfully for 40 years suddenly becomes a poor design because there is a
> failure. And, it is also difficult to understand that no matter what the
> symptoms, the cause and cure is always the same.
Any very simple diagnosis or theory, even if wrong, makes many people
comfortable and satisfies them. This makes it appear that anyone with an
antalytical approach is just "causing problems" except to other who think
the same way. It's a lot like arguing religion.
> Frankly, I enjoy thinking and solving problems. If all there was to
working
> on amplifiers was to install a special parasitic suppressor, I would
> probably lose interest in repairing them.
I like to look at the positive side. I could buy measurement equipment and
textbooks cheap, because few would have any use for them.
Enough of this, why not make some measurements Colin?
73 Tom
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