Re: [Amps] How do I determine Class

[R.Measures <r@somis.org>]

On Sep 1, 2004, at 2:19 PM, Bill Fuqua wrote:

> At 09:59 AM 9/1/2004 -0700, R.Measures wrote:
>
> > On Sep 1, 2004, at 8:22 AM, Jim Isbell wrote:
> >
> > >  This is EXACTLY the answer I needed.  The amp uses a pair of 
> > > 4-125As and the screens are at "350V - 400V"   Plates at "2000V - 
> > > 3000V" (but no terminating resistor) so I guess that makes it a 
> > > class AB1 linear.
> >
> > Jim --  Class AB1 amplifiers always have a grid-terminating R because 
> > the grid draws zero current.  .
>
> Not always. You don't  have to have a grid-terminating resistor for 
> AB1 there is always a parallel resistance due to the loss in the grid 
> tank circuit.  Q= R(parallel)/ X. The tank circuit is loaded by the 
> drive impedance also, so it is not totally unloaded Q.

Bill -- What SWR would the transceiver see if the grid-coil R was the 
only load?   My guess is somewhat less than wonderful.
> Many of the old designs were for maximum gain so that very low power 
> exciters could drive them. Here you can vary the drive quite a  bit by 
> changing the link coupling.

For a Bruene bridge neutralized tetrode or pentode, there is no link 
coupling.  The grid is driven either directly (50-ohm termination) or 
through a Z-step up broad band xfmr. (200 or 450 ohm termination) , and 
the grid XC is tuned out by a parallel roller L.
> > > The screen grids have several bypass caps to ground but no resistor.
> >
> > A shunt resistor is needed to sink reverse screen-I.  Typically, 
> > about 25% of normal screen-I is shunted by this resistor.
>
> The 4- series does not have problems with the negative screen current 
> like the 4CX series. I just checked the data sheets.

Interesting observation.  Any theories on why this is the case?

>
Richard L. Measures, AG6K, 805.386.3734.  www.somis.org

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