I've gotten along nicely with my Heath emissions tester. (Well actually, I don't even use it too much - depending more upon whether a tube works in a radio.)

As a tube ages and wears, the cathode emission often becomes lower. That will eventually cause a tube's performance to drop.

It seems to me that transconductance or Gm, assuming the emission is good, is primarily a matter of the tube's geometry - spacing, size, and position of the various internal elements. Unless something drastic happens, I wouldn't expect a tube's internal geometry to change much over time.

I can see why testing transconductance would be important for a new tube or a new tube design, but is it really important for checking a used tube? Beyond checking cathode emission?

:I've gotten along nicely with my Heath emissions tester. (Well actually, I don't even use it too much - depending more upon whether a tube works in a radio.)
:
:As a tube ages and wears, the cathode emission often becomes lower. That will eventually cause a tube's performance to drop.
:
:It seems to me that transconductance or Gm, assuming the emission is good, is primarily a matter of the tube's geometry - spacing, size, and position of the various internal elements. Unless something drastic happens, I wouldn't expect a tube's internal geometry to change much over time.
:
:I can see why testing transconductance would be important for a new tube or a new tube design, but is it really important for checking a used tube? Beyond checking cathode emission?

Doug - In the book "Getting the Most out of Vacuum Tubes", the author, Robert Tomer, agrees with you for about 90% of typical tube testing. You can download a .pdf copy of the book from Pete Millett's web site at this link, located about the middle of the page:
http://www.pmillett.com/technical_books_online.htm
See pages 119 and 120 of the book.

Robert H.

Hi Doug

I agree with you. Transconductance is controlled by design. As long as grid wires aren't blown or someting else damaged emission will show the tube condition.

Norm

::I've gotten along nicely with my Heath emissions tester. (Well actually, I don't even use it too much - depending more upon whether a tube works in a radio.)
::
::As a tube ages and wears, the cathode emission often becomes lower. That will eventually cause a tube's performance to drop.
::
::It seems to me that transconductance or Gm, assuming the emission is good, is primarily a matter of the tube's geometry - spacing, size, and position of the various internal elements. Unless something drastic happens, I wouldn't expect a tube's internal geometry to change much over time.
::
::I can see why testing transconductance would be important for a new tube or a new tube design, but is it really important for checking a used tube? Beyond checking cathode emission?
:
:
:
:
:Doug - In the book "Getting the Most out of Vacuum Tubes", the author, Robert Tomer, agrees with you for about 90% of typical tube testing. You can download a .pdf copy of the book from Pete Millett's web site at this link, located about the middle of the page:
:http://www.pmillett.com/technical_books_online.htm
:See pages 119 and 120 of the book.
:
:Robert H.

The U.S. military used Hickok-type transconductance testers, right?

I suppose that, for example in a naval ship environment, with exposure to physical shock and vibration, a tube's internals could somehow get physically distorted? And the problem wouldn't show up on emissions.

I'm pretty sure that transconductance was 100% checked by inspectors at the tube factory before delivery to the government. But the navy probably wanted to be able to check transconductance one more time before installing a replacement tube in a piece of shipboard equipment - to verify that something bad didn't happen to the tube during shipment.

Thus, Hickok.

I frequently use my Heath emissions tester to check the condition of '01As. If weak, I use the emissions tester, with a homebrew adapter, to rejuvenate their thoriated filaments.
Doug

:Hi Doug
:
: I agree with you. Transconductance is controlled by design. As long as grid wires aren't blown or someting else damaged emission will show the tube condition.
:
:Norm
:

Someone said a long time ago that in power situations a cathode hot spot can throw things off. With a small portion of the cathode emitting a lot of electrons, a small portion of the grid will be doing most of the work.

T.