[Amps] How do I determine Class
R.Measures
r at somis.org
Wed Sep 1 21:18:07 EDT 2004
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
More information about the Amps
mailing list