I fully agree with Tom points but I like to add that some tube is easy
to be managed while others with high internal parasitic reactances are
ia pain to deal with.
Given a feedthrough capacitance, when the tube already has a relevant
grid leads inductance, for example, its grid is never really grounded.
This problem is expecially evident on higher bands (but not only) where
the oscillations are more easily triggered in a normal operation.
If the external wiring or layout doesn't minimize the reactance of the
added paths, like the grid to ground inductance, then the amplifier
feedback can become too high and the amplifier isn't enough stable and
requires some trick like a suppressor to overcome the problem.
If a suppressor can yes fix instability problems, it's true that it may
have an impact on the amplifier amplification and efficiency (mostly on
the high bands) therefore it's always raccomandable to first deal in
lowering feedback caused by external reasons rather than winding
elaborate coils and burning resistors looking for a "magic" suppressor.
Because of the above, with some tube it's anyway easier to build an
effective and stable amplifier while with others (generally the sweep
type and/or big glass bottles) this is a pain and the final result is
often not satisfactory and proportioned to the effort, not counting the
money spent in tentatives and modifications (better to forget about the
invested time) that make it also an expensive object.
73,
Mauri I4JMY
> Could be many things. How long are the leads from the anode to
> the tuning cap? Is the tuning cap mounted to the same sheetmetal
> as the tube grid is grounded to? Is the loading cap on that same
> sheet metal, or do both the tubes have a very solid very wide and
> very short return path back to the grid area from the tuning and
> loading caps?
>
> What is the suppressor? How did you make it?
>
> What about relays? What type? How much feedthrough around
> relays??
>
> How are the grid pins grounded? How long are the leads?
>
> There are a hundred things that you could do wrong, but the amp
> should be unconditionally stable even if the input and output are not
> terminated when you test it...if you do things right.
>
>
> 73, Tom W8JI
> W8JI@contesting.com
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