Re: [Amps] 4CX5000A question

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

On Aug 9, 2004, at 2:04 PM, Stan C. wrote:

> Hello.
>  I am thinking of building a amp using the 4CX5000A tube. I have a few 
> question that I can not seem to come up with the answer to. So I am 
> hoping that some of the knowable folks here on the list can help me. 
> Keep mind I am not a engineer,so please give me your answers in layman 
> terms.
>
> 1) How do you determine the value and rating of the screen bleeder 
> resistor.

Stan --  This subject could be of benefit to other builders of tetrode, 
AB1 amplifiers, so I posted it to the entire group.
-  30mA of screen-shunt bleeder-I seems to be enough to protect an 8170 
or 8171 from (runaway) reverse screen-I condition during tune-up. ///   
note -- reverse screen is entertaining for those who enjoy fireworks.
> 2) How do you regulate the screen supply.

One way is:
http://www.somis.org/D-a-08.GIF
However, the raw DC must be >10V higher than the desired screen-V.

> I was going to just use a transformer that rectifier to the right 
> voltage and call it good. Please let me know if this will work.

This approach will work, but the filter-Cshould be the resonant-choke 
type and the filter-C should be larger than normal to reduce screen-V 
fluctuation during voice modulation.

> Is the concern in the voltage going to high or sagging???

Both.  Screen-V needs to stay put for best linearity during voice 
modulation, however, with a sloppy (high ESR) screen supply, and using 
a 2-tone test with 1kHz or so tone spacing, the IMD test will look 
better than reality because screen-I stays pretty much constant , so 
the screen-V will not vary during the test.
> 3) And last....what is the correct bias voltage for this tube?

Correct bias is whatever it takes to produce virtually zero (<100µA) of 
grid-I)  during maximum driving signal conditions.  However, ideally, 
one should be able to adjust the drive, the screen-V and the grid-bias 
V to achieve this since a ZSAC of 0.5A is needed to produce good 
linearity.  Actual numbers are c. -340Vwith E-screen = 1400V.
/// note -- the rated screen potential of 1250V-max can be safely 
ignored with this tube since the 8170/4cx5000A has the same innards as 
a 4cx10,000D, whose screen is rated at 1500V.
> Thank you for your time. Please email me direct at kf6ikc@sbcglobal.net

-  cheerz, Stan

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

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