[Heath] Concerning the SB-200 and 4CX250Bs ...

[Kenneth G. Gordon]

On 19 May 2014 at 12:00, Wilson wrote:

> Has anyone looked at the 4CX tubes triode connected?

Yes. It has been done, but one has to be pretty careful attempting that with 
certain ones of those particular types of tubes.

> 4-125A and 4-400A work great that way.

Yes, they do, but others in that same series do not.

> Wilson
> W4BOH

Hi, Wilson:

Yes. Some do, and some do not, work well as triode-connected tubes. You 
must review the characteristics of each tube you consider for use with this 
method.

As I remember it, in several of those external-anode types, what happens is 
the screen current runs to several amperes under certain conditions.

This is not (obviously) conducive to long-life for those tubes.

It IS possible to operate tubes like the 4CX250B in "grounded" grid (cathode 
driven) service, but that must be done with big bypass capacitors, 
"grounding" the elements for RF, while still applying the usual necessary DC 
voltages to them.

This, essentially, eliminates the "simplicity" of the usual GG method.

I've settled on the "swamped grid" method as it is somewhat simpler and 
more reliable....and 4CX250Bs work especially well using this method.

One advantage is that this method does not require switchable tuned input 
circuitry. There is just a fairly big 50 ohm non-inductive resistor involved.

Ken W7EKB