[Amps] 4CX1000A

[Roger (K8RI)]

On 1/28/2012 3:23 PM, Ron Youvan wrote:
>     Val LZ1VB wrote:
>
>> Thank you very much for the explanation. I completely agree.
>> The question is how a tube, showing maximum reverse screen current leading
>> to an excessive screen dissipation could be used in an amateur amplifier.
>> Yes, the screen is in a danger only when the amplifier is moderate driven,
>> but the PA is supposed to work at any drive level from 0 to max.
Typically all I've used ran well at reduced power levels even with 
reversed screen current, but I've never seen a need for running an amp 
at less than half to three quarters power.  The idea just never made 
sense to me.
>     I'm not mike, but I think the reverse screen current is not strictly normal, in a perfectly built
> tube you should see none.
All the Tetrodes I've used have an area of reverse screen current with 
low drive  As long as it's within the tubes parameters/ratings and the 
IM doesn't go to pot I don't worry about it.
The "big boys" tell me even the larger tubes will do the same.

BTW some one mentioned minimizing screen current. To tune a Tetrode you 
start out tuning for max output just like you do with a triode. *THEN* 
once the screen current has gone positive  you tune for *Maximum* screen 
current while increasing drive and remaining within the ratings of the 
tube.

73

Roger (K8RI)

> These tube are built with the screen grid components hiding in the electron shadow of the control
> grid elements.  When the alignment isn't perfect the electron beam strafes the side of the G2
> elements which produces emission via secondary emission.
> I suspect most new good tubes of this kind show negotiable emission.  Those that do emit
> significantly may find their way onto the hands of HAMS more easily.