[Amps] Tetrodes in Grounded Grid, vs. Grid Driven?

[Rob Frohne]

On Apr 24, 2004, at 9:31 AM, Joe Subich, K4IK wrote:

>
> Rob,
>
>> I've read some comments here on the list that led me to believe that
>> grounded grid with a tetrode may not be as good for some reason as
>> running grid driven.  Can someone more knowledgeable bring me up to
>> speed on this issues involved?
>>
>> I'm looking at options for a Heathkit SB-230 that I purchased with a
>> smoked 8873.
>
> The problem is the geometry in the tube ... most tetrodes (other than
> the old 4-250/4-440/4-1000) just don't like having their grids tied
> together and being operated as a high-mu triode.
>
> If the filament winding of the SB-230 will support the extra 3 A of
> current, you would be better served by adding a screen supply, air
> plenum and blower to support a pair of 4CX400A tubes.  You could even
> use low cost (surplus) 4CX250s.
>
> Unfortunately, the single 8873 used in the SB-230 had a plate
> dissipation
> rating of only 300 Watts and was good for, at best, 500 Watts PEP 
> output
>
> if operated safely and cleanly.
>
> 73,
Thanks Joe,

I have had great experiences with the 4-400 and 4-1000 in grounded grid
which partly prompted my question.

On the SB-230, I have considered going to a single 8874 which I think 
is the same as the 4CX400A, and I still might do that, but it appears 
that I could keep the conduction cooled feature of the linear if I use 
an 8560AS.  The plate dissipation of the 8873 and the 8560AS are both 
only 200 watts, though it looks like the 8873 is a little beefier in 
other ways.  The difficult thing with the 8874 is that the amplifier 
will require extensive mechanical modifications.  The socket must be 
moved.  The beryllium heat sink must be removed and a new back panel 
must be installed in place of the heatsink.  The 8560AS has a different 
pin out than the 8873, but seems to have the same socket and physical 
dimensions.  It is also quite reasonably priced.

What is it about these tetrodes that don't like having the screen and 
grids tied together.  Is it a power dissipation in these elements, or 
what?

Thanks for the answers!

73,

Rob
--
Rob Frohne, Ph.D., P.E.
E.F. Cross School of Engineering
Walla Walla College
http://www.wwc.edu/~frohro/