[AMPS] 4CX1000A advice??

[Ron Hooper, W4WA]

On 13 Mar 98 at 4:54, Larry Molitor wrote:

> Date:          Fri, 13 Mar 1998 04:54:14 +0000
> To:            amps@contesting.com
> From:          w7iuv@axtek.com (Larry Molitor)
> Subject:       [AMPS] 4CX1000A advice??

>     Over the Last thiry years or so, I have designed and/or built literally
> dozens of power amps, both at home and on the job. These span the range of
> audio thru L-band, from a pair of 1625's (??) to a multi-KW L-band solid
> state job. Of all of these, my 4CX1000A amp has given me the most grief.
> It's been over ten years since the last time I rebuilt it, so I thought it
> might be fun to do it again. (Sure!)
> 
>     I have been going thru the last several months of list archives looking
> for clues, hints, revelations, etc, for passive grid tetrode amps hoping
> that I might find something that I overlooked in past design efforts. I am
> amazed at the amount of knowledge and experience (and EGO) possesed by the
> list members in general. In spite of this, I could not find answers to two
> points that are currently bothering me. 

Hi Larry,
I have been successful building screen grid tubes in many anps. 
Unless you have a low drive requirment, I would scrap the idea.
 
>     First question: Many references have been made to grid damage in the
> 4CX1000A and similar tubes; what are the detailed symptoms of said damage?
> (Assuming the tube still sorta operates)

The 4cx1000 has little tolerence for any grid current. The tube will 
not work when the grid becomes damaged. 

> 
>     #2: I want to include NFB cathode resistance this time around. Looking
> at the Measures' and Daughters' versions, I see no mention of filament
> chokes. With the cathode either connected to the filament at the factory
> (4CX1000A) or by preference (4CX1600), I don't see how this can work. The
> filament is either bypassed with caps or thru the filament transformer
> windings and therefore the cathode resistance is bypassed as well. Am I
> missing something here? If a filament choke is required as I suspect, then
> that is yet another reason why one should never consider a tetrode for a HF
> amp unless one gets one with handles for FREE!

Yes, you dont put the drive on the filiment so no need for a choke, 
it still goes in at the grid. Passive means the grid is swamped down 
with resistors to make it match 50 ohms and require a higher drive 
level. Impedence transformation is usually done by winding a small 
transformer in addition with the resistors. This also serves to 
neutralize the amp. Good construction pratices, to seperate the input 
and output circuits are still employed in the design. 

Another suggestion is to not use the 4cx1000 and use the 4cx1500. It 
has twice the grid rating and runs cooler from the anode design. It 
is bascially the same tube other than that.   

Ron W4WA
--
Ron Hooper, W4WA

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