Re: [Amps] 4-1000A in Passive grid-driven use?

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

On Apr 30, 2004, at 6:32 AM, Bill L. Fuqua wrote:

> I don't know if anyone has actually made a cathode driven biased 
> 4-1000A AB1  amplifier but it should be very low in distortion 
> compared to either zero bias grounded grid or common cathode 
> operation. That is because of the  fact that there would be very 
> little grid current

ï  AB1 means there is zero grid-current.  AB2 means there is some 
grid-current.

> and the plate RF current would be the same as the drive RF current and 
> the power gain would come from the ratio of cathode drive impedance 
> and plate impedance. If the plate impedance was 5000 ohms and drive 
> impedance 200 ohms then the power gain would be 25 or so.

ï  Since there is zero signal potential applied between the grid and 
the cathode, the power gain of a Class AB1 / "super cathode 
driven"stage  is roughly the grid-screen amplification factor -- which 
is typically 4 to 6 in most tetrodes.

> It would be interesting to try. I expect that the  cahtode impedance 
> would be somewhere between 100 and 200 ohms. It should be somewhat 
> greater that gg due to the greately reduced grid current.
>     And just about all of your drive power would pass thru to the 
> output.

ï  Agreed, Bill.  cheerz
> 73
> Bill wa4lav
>
> 73
> Bill wa4lav
> -----Original Message-----
> From: "R.Measures" <r@somis.org>
> To: n7ioh <n7ioh@cybertrails.com>
> Date: Wed, 28 Apr 2004 20:39:43 -0700
> Subject: Re: [Amps] 4-1000A in Passive grid-driven use?
>
>
> On Apr 28, 2004, at 12:38 PM, n7ioh wrote:
>
> >    Are there any problems in using the 4-1000A in a passive
> > grid-driven mode on 160 - 10 meter or on 6 meters?
>
> ï Above 30m, it's a good idea to tune out the grid's XC.  See:
> http://www.somis.org/D_a_05.GIF
> The XL of L1 tunes out the grid's XC for whatever frequency you choose.
>   note -- this is the first step in tuning up such a machine.  .
>
> > If not, what kind of output can I expect to get?
>
> ï  Depends on anode V and screen V.  With 7kV and 1kV, you should be
> able to get 2.6 out.
>
> > Second, I have seen amps that use a roller inductor and not a band
> > switch, any thoughts?
>
> ï  They provide the best match over a wide frequency range.
>
> Richard L. Measures, AG6K, 805.386.3734.  www.somis.org
>
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org

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