[ARC5] Smart People - OT Why pentods fail (was Triodes &Pentodes)

[Geoff]

On Nov 14, 2013, at 1:16 AM, Leslie Smith <vk2bcu at operamail.com> wrote:

> "The screen failure will not be due to voltage over max but excess
> dissipation."
> Aw- Ken!
> The above is nonsense, and I'm onto you.   :-;    It's not dissipation
> (I-squared R heat) that causes the damage.
>
> The screen sags from the weight of accumulating electrons; ...
> As for the the plate - it doesn't sag even a little.  Being thicker and
> more robust, it can take the weight.

But, but !

I have an 811 (the non-A version, apologies to the R-390 mailing list) that 
has a hole in the plate.
The plate does seem to have sagged quite a bit around the hole.
No doubt this is due to some phenomenon I’m not too familiar with, possibly 
"11-meter overdrive".

> We get the word, impedance from the latin, "impedire", meaning "to
> shackle the feet"

Maybe the earlier 811 (non-A) has thinner plate construction, not being made 
with the much acclaimed RCA multi-layer plate material.  (Or was it Tung-Sol 
and their also much acclaimed 6550’s that had that plate material?)

Boatanchor historical reference:  The Collins 30L-1 may have been equipped 
early on with the 811 (non-A), and there was the early Central Electronics 
200V SSB no-tune transmitter that used 6550’s as the finals.

Roy


Roy Morgan
RoyMorgan at alum.mit.edu
K1LKY Since 1958

The difference between the 811 and 811A is the small "fins" on the anode 
that brought the dissipation from 40 to 65W in 1949. The 812 and 812A are 
similar. Hams were destroying themall along and the Heath, Gonset, Dentron, 
and others with 4 of them preceeded CBers!

These days it is the various Ameritron AL-811's using Chinese crap. The last 
one I got in for repair for a senile citizen that took forever to tune had 
holes in all 4 tubes. I installed 572B's and he can dawdle and drool all he 
wants and wont hurt them.

Carl