flipper wrote:
On Sun, 04 Nov 2007 15:42:09 -0800, Robert Baer
wrote:


flipper wrote:


On Sun, 04 Nov 2007 03:10:04 -0800, Robert Baer
wrote:



Some 12Au7 tubes have "unbalanced" triodes when viewed this way,


If you mean the two triodes in one bottle are not 'identical' that's
typical. They're not solid state devices made on a common substrate
with the same process parameters, they're individual mechanical
structures that happen to be placed inside the same envelope but
'triode a' in tube 1 could just have easily ended up in tube 2, or
tube 202.



and
some triodes are rather "peaky" like this one.
Shouldn't there be no peak if the tube was perfectly OK?
What would be the cause?
(BTW, data taken using $3.49 Cen-Tech DVOMs from Harbor Freight, one
used for each grid, and one for the plate via a switch)


I'd be skeptical of that data because direct grid measurements are
notorious for altering grid bias.

A normal tube will have a small grid leak until Vgk approaches 0 and
then it turns positive, like a diode (in theory right at 0 but it
depends on the actual field around the grid and that varies depending
on the structure) )

What 'problem' are you trying to solve?


OK, you found me out.


I apologize if that was supposed to be a secret.


What i have is a GR1862B Megohmmeter which uses a 12AU7 as a
differential detector in a semi-bridge configuration (don't ask why - go
to GR on that Q).
I am getting tired of having tubes slowly degrading so that i cannot
adjust the infinity at max setting (pot setting slowly crawls from near
center to always one end).
I was hoping that i could characterize the 2 triodes and use the data
as a guide - maybe even selecting a tube that would start on one end so
as tholast longer in this application.


Well, tubes do age and I'm not familiar with a GR1862B so I can't help
much on that score but I'd suggest you test your measuring setup by
checking anode current with and without the grid probe attached to see
if it remains constant.
I have been looking at the new graph via Hexcel and am not happy with
it; it appears that it lies.
The meters are hard-wired in series with the grids; if the current is
small then the grid voltage is not much different than the setting
(100mV per 100nA).
Using those el-cheapo DVOMs in this app is ideal in that
"recommended" grid resistance is 1meg and the DCV input is 1meg,
allowing a 200nA FS reading.