Mutual Conductance Valve Analyser

[AlanBeckett]

I think you'll find that 7,700 is the 'Plate Resistance' - American - on the RCA Data Sheet and the 'Transconctance' is quoted in MicroMhos. Mhos are 1 over Ohms so 2200 MicroMhos is 2.2 mA/Volt.

Alan

[pwdrive]

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

You need the valve data sheets, here's one for a 6C4, mutual conductance is clearly stated for two sets of operating conditions:

http://www.r-type.org/pdfs/6c4-1.pdf

Valve data sheets for output valves should also give mutual conductance figures, here's an example for the EL84:

http://www.r-type.org/pdfs/el84.pdf

EDIT: Whoops...post crossed with Alan's...He's right, be careful not to confuse with ra or micromhos

Lawrence.

[Mike Watterson]

Look at shape of Ia vs Va for fixed grid voltage for a triode and a Pentode. You'll see triode has slow start and then shoots up (like a rectifier or forward diode). The Pentode starts quick and then at the "knee" goes nearly flat, the flatter the better (a little like a Zener (reversed diode) or transistor).

gm is only one parameter.

As pwdrive / Lawrence hinted, the shortform catalogues are not very detailed, you need the makers data, which can be 2 to 30 pages.

[Sparks]

The top of the column for the 6C4 says  gm (mA/V)
It is listed as 7,700.       All of the triodes on that page show gm as between about 2,500 and  91,000!
Looking at it again, I have decided that they have put the wrong titles on the columns.   The 7,700 is the ra in Ohms, as the ra is shown as 2.2 for the 6C4, and that could not be!    
I shouldn't believe all I read, should I?
Bob

[pwdrive]

If it's Iliffe books I have an original 8th edition, all's well with the column designations in the one I have.

Lawrence.

[Sparks]

It is startlingly obvious now, I have been "bending my brain" for weeks over it, and should have looked in the ra column when all would have become clear!   - Live & Learn
Bob

[pwdrive]

Sure do.

It might help for a better understanding to look at some typical valve data sheet characteristic curves, most will show at least a set of anode family curves and sometimes a set of grid family curves but remember they are static curves eg: no anode load, as soon as an anode load is introduced everything changes, for that you need to draw a load line and if needs be a dynamic transfer curve, sounds high and mighty but it isn't and is simple enough to do.

A load line drawn on a set of anode family curves will tell you what the anode voltage and current should be for different grid bias voltages, the bias voltages given are normally in increments of 1 volt or at best 0.5 volt, obtaining the dynamic transfer curve from the anode family of curves will show what the anode current should be for any grid voltage from zero to cutoff.

Both RDH4 and Riders books show how to draw a load line and a transfer curve, doing it kinda takes you through the motions as to what's what, worth doing as a simple exercise.

Lawrence.

[Sparks]

My grid bias supply is smooth from 0 to 12 V! My HT and Screen supplies are also smooth from 0 to 300V, so I can easily get whatever voltages I wish, although I am well aware that changing one, often influences the others, so it is a matter of balacing them until I get exactly what I want! I remember doing load lines at college, and not having much problem with them. All this is just to exercise my brain and practical skills, and is of little importance apart from that, but thank you all for the clues, that are all I need to proceed.
Bob

[Craig]

The ultimate in bridge type testers was that General Radio Vacuum Tube Bridges. They did a series over the decades, and typical of GR they made use heavily of specialised transformers and custom differential variable capacitors. The 561-D manual is here http://www.ietlabs.com/pdf/Manuals/GR/56...Bridge.pdf

Again typical of GR you need another bunch of stuff to make it work - signal generator, DC electrode supplies and an AC null detector (or headphones). For examples http://www.vacuumtubes.com/testequipment...ridge.html and http://vintagetubeservices.com/laboratory/

As a result they are rarely used, and difficult to obtain second hand.

[Cleggy]

I have an AVO VCM, but the issue remains as to how do you know it's giving good results.

There are various checks which are described in the Avo Service Manual, but I decided that I wanted to build a really basic DC test unit and get some results (Ia and gm) on one or two valves which I would then use as "Test Standards" for the Avo.

This proved to be a bit more difficult than I expected; for one thing, these damn valves went in to oscillation quite easily, so I had dump the "rat's nest" version and build it properly in a diecast box. I did eventually make the DC rig work and got good agreement on the Avo, but it took more time and effort than I had expected!

C