Hello Hans,
> If you short-circuit the grid to the cathode with a number of low
> inductance caps ( chip caps? )in a super cathode tetrode amp - is this a
> good start to get a parasitic-free linear? Without any supressors?
Many amplifiers will be completely stable without suppressors if
the grid connection can be made with a low impedance over a very
wide frequency range.
Such a connection is possible in tubes that use an internal cone-
type connection to the grid with the leads brought out on a flange.
One false assumption is that adding a small capacitor will reduce
the impedance of the grid and improve stability. The problem with
that approach is the impedance is reduced only at one narrow
frequency, and greatly increased at all other frequencies.
That works well with a monoband amplifier, but as a general rule is
a disaster with multiband amplifiers.
The best policy in multiband amplifiers is to ground the grid with as
short and the lowest possible impedance connection. This single
thing does more for stability than any other single change.
> distorsion ( 48 volt final transistors ). I guess the internal structure
> of the tube grid and its leads to the outside world will be resonant at
> some high frequency where the tube gain maybe will be not high enough to
> make the tube oscillate.
Your statement is generally true, with a good layout. If you
measure the transmission loss through the cold tube in the actual
chassis (no tank, no input. Connect analyzer directly to the input
and anode), you will see a sudden loss in isolation at some
frequency or frequencies.
That is the potential trouble spot(s). Contrary to perceptions, anode
resonances where isolation is good don't mean a thing (except for
harmonic suppression issues).
As a matter of fact, resonance can be a very good thing if it makes
the anode have a low impedance. Removing the resonance can
actually deteriorate stability.
Having fancy test equipment is nice, but if you understand the
system an antenna analyzer and a regular receiver or even a diode
detector can tell you everything you need to know. If you don't
understand the system, all the measurements in the world won't
mean a thing.
> I guess you must make this short circuit very broadband? Must be equally
> effective on short wave as on VHF/UHF? A number of 0.01 + 0.001 uF silver
> micas and chip caps may be needed?
Chip caps are excellent. They are the best thing that ever happen
to power amplifiers. A 100nF multilayer chip capacitor can be a
good bypass from a few hundred kilohertz all the way up to VHF
and higher. No more need to parallel combos, although I still prefer
to directly ground the element that is most critical to stability,
either the control grid or the screen grid.
73, Tom W8JI
w8ji@contesting.com
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