Odd gm readings when testing valves. Maybe 42 isn't the answer to this one?

[thevalveset]

Whilst testing my latest acquisition of 'auction site' vintage valves, this chestnut of a question has popped back into my mind.
So I pose the question to all the experts out there to hopefully end my consternation:-
Over the 40 (or more) years I have been tinkering with wireless I have noticed when testing thermionic valves, that some seem to have higher mA/V test results than they are supposed to have.
The valve can test perfectly regarding Heater continuity, H/K & inter-electrode resistance, (both Hot and Cold).
However, when coming to the final 'goodness' emission test, the valve has an excess of output, sometimes two, three or more times the mA/V value stated in the valve tester book.
Clearly this doesn't mean I have a 'super' valve, something must have changed in the valve but what?

Is it a distortion of the spacing of the electrodes within the structure?
Has the filament has worn thinner and so is getting hotter , increasing the electrons available?

I have puzzled over this for years but never seen an explanation.
The valves still respond to varying negative grid volt variations but of course the characteristic curves will not be what the wireless design is expecting?
Has anyone studied this and found the definitive answer.
Please share...
Thanks in anticipation...
Dave

[Herald1360]

First question I would ask is:

"Is the valve tester doing the gm test at the electrode voltages specified in the datasheet?"

[thevalveset]

Hi thanks for your swift reply.
Yes..... presently I am testing early balloon triodes 210HL etc and some test exactly to the book, some are completely exhausted and some read extremely high output. Its these 'super' valves that I can't understand.
However....
I remember throughout the years, (for instance) when I was at work servicing Tektronix 500 series 'scopes I and my colleagues all remarked about this excessive emissions situation with some valves.
It has intrigued me down the ages...lol!
Dave

[Robert Gribnau]

The data in a datasheet for a specific valve type is the data of an average valve of that type. There is always some spread in the data between individual valves of the same type. So individual valves can have somewhat higher (or lower) gm than this average.

Some types of SQ valves have smaller spread (for applications that need more precision if it comes to gm and/or other parameters).

[Radio Wrangler]

Slight gas can distort Gm measurements by a disproportionate amount.

David

[thevalveset]

Interesting thoughts thanks...keep them coming.
So.... in amongst the 2V early triodes (I am testing at the moment)
I find that a 210HL should have an anode current of around 1.6mA with Va of 150V and grid voltage of -3V.
The Avo VCM data book requires these same settings and a 'good' 210HL falls pretty well into this output performance. However a second 210HL (without changing the VCM settings) delivers an anode current of nearly 11mA which to me is well beyond any expected distribution 'spread' !
The 'super' valve still responds to varying the grid voltage so this isn't some internal 'short' and can be completely 'shut off'. I haven't actually plotted the Vg vs Ia curves though it may be interesting to compare them with the attached original datasheet.
I was hoping someone else had noticed this phenomenon and discovered the source of the mystery?
Dave

[Skywave]

Just out of interest, what is the valve tester you are using?
An Avo VCM, for example?

Al. / June 4th.

[thevalveset]

HI,
Yes an AVO VCM Mk3 which we had from brand new in the University's electrical engineering laboratory where I worked since 1967. I completely restored the machine in the 1990's including stripping the rotors and gold plating all the contacts .
So, I have been guardian of this machine for well over 50 years and it has never let me down, yet always bemused me when I was confronted with the odd 'super valve' result.
Dave

[crackle]

could the valve be oscillating?

[paulsherwin]

I have also noticed this phenomenon when testing a minority of valves. The valves in question seem to be perfectly functional in all other respects. This was using a borrowed VCM.

[thevalveset]

Hi Crackle,
No the 'erroneous' readings are too repeatable and stable to be attributed to oscillation. I don't think I have ever seen oscillation using the AVO VCM?
And.....
'Hurrah' .... someone else who has experienced this 'super' valve phenomenon....so it wasn't all in my imagination after all!
Thanks Paul for your input, I really hope someone can supply a definitive explanation to this effect, I will have it inscribed on my headstone
As you say, the valves in question exhibit normal 'valve action', and yet have an exceptionally high anode current for a given negative grid voltage.
I suppose it would be very interesting to lash up a single valve audio amplifier and compare the results between a 'normal' testing triode and a 'super' triode. It may show some nonlinearities that aren't apparent just doing 'static' gm tests?
Keep your thoughts coming everyone, they are much appreciated.
Thanks,
Dave

[TowerRadio]

I have noticed this,it would seem to be more usual in older,pre-war valves.I have a HSVT and the valves test fine in every other respect except on emission when some give an indication very high on the green zone.Only one EL84 does this and some time soon I may be able to put that in a radio to check it.Does this indicate the older valves have a vacuum less than perfect? Les9h

[thevalveset]

Hmmm interesting theory.
Harping back to the 20th century and my rudimentary grasp of schoolboy physics, I would imagine that if the vacuum was poor, then the emission would be reduced not enhanced?
I think that air molecules would hinder the flow of electrons within the 'vacuum' and as such, reduce the number of electrons arriving at the anode due to collisions.
Please correct me physics gurus?
Dave

[snowman_al]

Current flows more easily through 'air' than a vacuum.
Interesting thread, I think anyone who has tested valves has seen one or more of these super valves?
Is there a 'Gas test' button on the tester?

[John_BS]

I'm far far far from an expert on valves, but my instinct veers towards gas molecules.

1) Have you tried observing these valves in pitch darkness, and
2) if there was grid current, could it raise the actual grid-cathode voltage above that set by the VCM potentiometer?

John

[Herald1360]

If the source resistance of the VCM grid voltage supply is high (as in similar to typical grid leak resistors) then grid current could easily upset the grid-cathode voltage (cf UL41s in DAC90As).

Have any of these "super" valves been tried in a dc circuit with low impedance electrode supplies?

[thevalveset]

Hi John,
Thanks for your suggestions,
No I haven't tried viewing the valves in the dark, though most of these early 'balloon' triodes are pretty well silvered all over the insides of the envelope.
Others are covered on the outside in gold or grey coloured 'metallisation' so it is almost impossible to look for the 'gassy' blue glow.
If my VCM is working on the 'gas' setting, then I am not observing any grid current but of course the magnitude of the current is still only milliamps.
I will await darkness.....
Dave

[PJL]

This is how I understand it: Electrons hit the air molecules knocking off electrons which head to the anode leaving positively charged air molecules. These are attracted to the -ve grid causing a grid current to flow. Assuming the grid supply has some source resistance, this raises the grid voltage and consequently the anode current increases.

Testing power valves on the HSVT which operates the valve in normal DC conditions, I often see the anode current shoot up for a while and then slowly settle as the valve heats up and the air is absorbed by the gettering. I have also noticed that radios that are newly restored with valves that have been dormant for years often improve after a period of use.

The similar UL41 grid voltage symptoms are caused by leakage between pins caused by a thin film of impurities (carbon?) that occurs on the inside of the valve between the pins.

[Bazz4CQJ]

I don't think I've ever seen a "super valve" on my VCM163. I have seen NOS new valves light up and show above-spec results that diminished quite a bit over the next 24 hours!

Are you getting super results on valves of all eras including the more modern B7G & B9A? Curiously, we have another thread discussing gas and test results.

B

[thevalveset]

So..
I have just been testing some Mazda 'octal' (a little more modern) valves and I had two very similar (curve wise) Triode/Hexode valves.
One a TH233 and the second TP25.

Firstly the TP25
Triode section should give an Ia of 3.7mA @ -1V grid, it gave Ia=4mA
Hexode section should give an Ia of 1mA @ -1.5V grid, it gave Ia = 1.1mA

Now to the second valve, the TH233
Triode section should give an Ia of 4mA @ 0V grid, it gave Ia=20mA
Hexode section should give an Ia of 2.6mA @ -2V grid, it gave Ia = 8.5mA

Both sections needed around -5.5V grid voltage to bring the readings to the book values. The valve still responded to varying the grid bias and could be completely 'shut off'.

Both valves have a grey metallisation coating and so I can't see if there is any 'soft' blue illuminations in daylight.
I left the TH233 'cooking' in the VCM for 20 minutes and there was no change in the readings over that period.
I will leave it plugged in and wait until dark, as I can peek in the top where the metallisation avoids the top-cap.
So, I am seeing the 'super valve' effect in a range of valve ages and I am sure I remember the 'odd' B7G/B9A in my dim and distant servicing days.
The investigation continues
Thanks everyone for your input.
Dave