On Apr 5, 2005, at 9:53 AM, Vic Rosenthal wrote:
> Phil Clements wrote:
>
>> It is obvious that suppressors on an 8877 amp are optional, depending
>> on the
>> design and use requirements.
>
> The handbooks describe procedures for checking if an amplifier is
> stable at VHF. If not, and this of course depends on the layout and
> components used and the parasitic tank circuits created thereby, one
> way to stabilize it is by adding the usual suppressor to the plate
> circuit. It's desirable to use the minimum inductance needed; if
> you've seen the ones in some Alphas they consist of a carbon resistor
> soldered across a copper strip not much longer than the resistor
> itself! It would be even better, from the point of view of
> efficiency, reliability and cost, if it could be eliminated entirely.
>
> My point is that it is part of engineering an amplifier -- the layout
> and components used (including the tube type) will determine if there
> is need for a suppressor and what form it should take. I presume that
> the designer of the AL1500 checked the amplifier on all bands and
> under various conditions and determined that it was stable without the
> suppressor.
One problem with VHF parasitic suppressors in a HF amp is that the less
L-supp one uses, the higher the VHF parallel-equivalent R on the anode
and the higher the VHF amplification at the anode's VHF-resonance.
Thus, to reduce VHF-gain use more L. However, the trade-off with using
more L-supp is that it increases the dissipation in R-supp - especially
at 29MHz - and if R-supp opens, parallel-equivalent R increases, VHF
amplification increases, and the chance of oscillation is a sound
wager.
-- Murphy said that nothing is as simple as it first appears.
>
> I'm sure that some amateur and professional designers would put one
> in, "just in case". As an amateur, I would! But it would be better
> engineering to perform the appropriate analysis and testing on which
> to base the decision to include the suppressor or not.
The fly in the ointment is that every tube ever made has feedback-C, so
the only game in town is to reduce VHF amplification by reducing the
VHF parallel-equivalent R on the anode.
As I see it, VHF amplifiers are slightly easier to design than HF
amplifiers
>
> --
> 73,
> Vic, K2VCO
> Fresno CA
> http://www.qsl.net/k2vco
>
>
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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