[Amps] a common topic here tspa

[R.Measures]

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