Gas test - AVO VCM mk 1

[Bushbaby]

Hi All,
In all the time I have owned and used my valve tester I dont beleive that I have performed a gas test on a valve. Does anyone know how to do this and at what point the gas button can pressed during the testing cycle?

Thanks

Rob

[pwdrive]

Hi Rob, I don't have one of those but in the cycle I would say after the shorts test, I know nothing of the Avo circuit either but in most testers a dropper resistor is introduced into the grid circuit, any grid current dropping across this will oppose the bias, the meter I guess would display the Ia difference.

Lawrence.

[Robert Darwent]

In all the time I have owned and used my valve tester I dont beleive that I have performed a gas test on a valve. Does anyone know how to do this and at what point the gas button can pressed during the testing cycle?

I've an AVO VCM Mk.I too and I don't think I've ever pressed the 'Gas' test button on an actual valve I was testing either!

However, according to the Mk. I operating manual the instructions for carrying out the 'Gas' test are given on page 20, specifically paragraph 7 which states;

"7. Where a valve is suspected of passing too much grid current, a measure of the magnitude of grid current at the desired conditions of applied electrode voltage may be made after having measured the mutual conductance of the valve in question. After having set the valve up and backed off the anode current to zero as for mA/V test, the button maked 'Gas' should be pressed. Any grid current flowing will set up a DC grid voltage across the 100,000 ohm resistance introduced into circuit."

If you haven't got a copy of the op manual I can post a pdf copy to this thread if you like?

Regards

[Joe]

The manual would be worth having for The Archive in any case, Robert.

- Joe

[Bushbaby]

Hi Robert,
That would be most helpful

Many thanks

Rob

[Robert Darwent]

OK no problem, here you go. Hope it proves useful.

Regards

[pwdrive]

Hello all, Interesting circuit, some similarities to the set up in the Hikock testers, I have a passing interest as to the patents that the valve tester manufacturers took out in order to protect their interests, is this something you or anyone would have any info on?
The PDF below is of a Hikock designed tester, see what you think, the best description etc is further down in the manual.

Lawrence.

[Bushbaby]

Hi All,
Many thanks for all your help and I've sorted out the gas test. The only thing that I cant appear to get my head around and to actually perform is the replace/good conductance test. I have no problem in finding Ia and mv/A values but the conductance test is a mystery. I've always been very cautious in undertaking any testing that could damage the meter if performed incorrectly.

Regards

Rob

[Radio Fixer]

I had reason to test some valves today Rob and refreshed myself with what I believe you refering to here. I dont normally bother with the Set mA/V control and the Good / Bad scale that this is used with.

Like you I am happy to just measure Ia and Gm but probably the control and Good / Bad was included to be like testers who give this supposedly simple indication of a valves goodness.

As it says in the Instructions that you now have, back off Ia and switch to the Ma/V position on the Meter Selector switch and set the expected Gm on the mA/V control. Press the mA/V pushbutton and the indication will be either in the Good or the Replace on the meter. I cant see how you can damage the meter ...

But why bother?

Also, it seems to me that the best test of a valves goodness is the emission. How can Gm not be OK if this is so? Gm must relate to the valves geometry and unless this has gone off (unlikely) or was not correct at the time of build (possible) then its unlikely to be low ... higher than spec seems more common.

cheers Gary

[Bushbaby]

Hi Gary,
I tested some more valves too and tried the emission test as you have indicated. I agree, having tested the Ia and Ma/V there still seems little point the in emission test as it doesn't inform you you anything that you haven't already determined.

Regards

Rob