Mutual Conductance Valve Analyser

[Sparks]

I have just spent a week building this:
http://www.r-type.org/articles/art-118.htm
and it doesn't work!      Very disappointing.     My supplies to the test valve were all adjustable and "spot on."    
It was all wired up correctly, and there were no wiring errors.     There was a minimum signal adjustment of sorts, but quite inconclusive!     I wonder if anyone else has built it?
I am rapidly coming to the conclusion that valve testing is not all that it is cracked up to be, as I am finding that valves showing very poor on a real valve tester actually work fine, whilst others showing very good, do not work very well, if at all!
Further investigations in progress.
Bob

[Mike Watterson]

It's quite basic. Too simple really. A typical magazine article.
Valve testing is useful, but simple testers were really only of value when brand new valves existed.
As I wrote before, Tektronix had an article as to why they could be misleading.
That's why today something more advanced like an AVO VCM163 or especially an electronic curve tracer, such as Dekker's uTracer is more useful.

A known good radio or amp may be more useful to you.

[Sparks]

Yes, I would agree with that. I am now working on another "Homebrew" tester that will come straight out of my brain, and I dare say it will probably work better than this one!
Bob

[pwdrive]

Whatever you build if it's a DC based tester the anode voltage has to be held constant to obtain gm.

Lawrence.

[Sparks]

I am not all that much bothered about the gm, more in whether the valve works or not. Not difficult to hold the anode voltage at any set value, but what would I then have to do then to ascertain the gm?
Bob

[pwdrive]

Mutual conductance (gm) is obtained by measuring the change in anode current for a change in grid voltage whilst the anode voltage is held constant ie: change in anode current in milliamps divided by the change in grid voltage in volts, multiply the result by 1,000 to give mA/V (milliamps per volt)

Basically the anode supply needs to come from a low impedance source.

It might be worth reading the first parts of RDH4 Chapter 2, it explains the relationship between the different valve parameters eg: mutual conductance, amplification factor and anode resistance and the method of obtaining them:

http://www.paleoelectronics.com/RDH4/CHAPTR02.PDF

Lawrence.

[Sparks]

Thanks,
That sounds like a very simple thing to do!     I did read a bit of the link, but all those complicated formulas "turned me off!"      I did understand it once, but it is no longer something that interests me, but the ascertaining of the gm is clear enough.
Bob

[pwdrive]

Yes, RDH4 was more for designers, it's still a useful reference book though and worth bookmarking.

I posted this up the other day, you might find the reading a bit easier, Inside The Vacuum Tube by John F. Rider, link below in case you missed it:

http://www.tubebooks.org/Books/rider_inside.pdf

Lawrence.

[Sparks]

Thanks, that is a lot better. I have downloaded it and saved it. It brings to mind my first lesson at radio college that began with:
"Matter is anything that has mass, and occupies space, it exists in three forms, solids, liquids and gasseus.......etc" Lots of the other things ring a bell as well, and I believe our lecturers must have had this book when they set up the college a year before I went. The firm, but very basic foundation of my knowledge gained by such simple diagrams has served me well over the years.
Bob

[Sparks]

I have just been looking in my Radio Valve and Transistor Sata Ninth Edition. and find them puzzling.
Taking two valves that I am familiar with, the pentode EF39 and the triode 6C4, this details under gm in mA/V show:
EF39 = 2.2 mA/V (All very clear)
6C4 = 7,700 mA/V (Not very clear at all)
What is the explanation for this?
Most of the triodes are shown up in the thousands. All I can think of it that the commas should be decimal points, and the gm of the 6C4 is, in fact 7.7mA/V
Output valves do not have gm listed at all!
Bob