On 1/28/2012 5:14 PM, Ron Youvan wrote:
> I stand by my statement, "in a perfectly built tube you should see
> none" and "is not strictly normal"
> You say: "Most external anode tetrodes I'm familiar from the 4X150A to
> 4CX1500B can exhibit negative
> screen current."
I think you will find that most Tetrodes in this range *will* have a
range of reverse screen current. To me that means normal
> I agree, "Most" and "can exhibit" but not in 100% of them, in a
> beam power tetrode or pentode
The Pentode is a different animal. They were designed specifically to
address the secondary emission problem in Tetrodes. You should not see
reverse screen current in a Pentode. At least I've never heard of it.
> screen grid is supposed to be in the shadow of the control grid and
> when it is not completely out of
> the stream the electron beam strafes the screen grid a glancing blow,
> the is prime angle to create
> secondary emission, which is the reverse screen grid current. (emission)
> I have worked with many RCA (now Burle) (like 4CX5000R) and Thomson
> beam power tetrodes (like
> TH-331) that did not exhibit any noticeable screen emission.
These tubes are considerably larger and more rugged than the 4CX1500B,
FU728F, or 4CX1000A let alone the smaller ones that operate in the
typical ham environment.
> I am sure lining up the screen "cage" with the grid "cage"
> properly, is the hardest part if
> constructing this kind of tube. (to make as many as possible that
> work well)
I'd use the word, "precisely" rather than "Properly" as properly means
within spec and within spec allows more than enough leeway in alignment
for reverse screen current. Normally I can run anything from a bit over
half power on up without reverse screen current or reports of being
heard up and down the band which means it's working well for me.
OTOH I have the capability of running 200 watts barefoot, so I see no
sense in running under a KW out with the amp.
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