6L6G emission too high

[pilovis]

I tested some 6L6 tubes I have, some of them are new NOS (4 Russians and 1 JAN RCA), two others are used but full functional tubes.

All new tubes tested have 100% emission, the two old "Fivre" 6L6G (used) tubes have about 140%-150% emission.

I can't understand it, can someone explain this to me?

 

[Osvaldo de Banfield]

If too high anode current may indicate a very good tube or a gassy one (become gas or air inside), and then faulty.

Can you re-test them with cold heater?

 

[pilovis]

If too high anode current may indicate a very good tube or a gassy one (become gas or air inside), and then faulty.

They do not seem to be gassy, the getter is perfect, also the two tubes sound good in my 6L6 S.E. amplifier.

 

[Osvaldo de Banfield]

Thne enjoy them.

Salutti per tutti!

 

[pilovis]

I noticed the others 6L6 sound better than these two Fivre tubes.
I suspect Fivre did some trick.
Anyway, they are not in spec., 50% more anode current is not acceptable.

 

[DF96]

Spec? What is the current spec for an ordinary 6L6? Only 'special quality' valves have an upper current spec, and even then it can be 20% up on nominal. 50% up may be a bit high but it can't be out of spec if the spec does not mention it.

 

[Doz]

Please place all the out of spec tubes in a box and ship them to me, where I will wear them out a little for you and return them in 3 or 4 years time

 

[pilovis]

Please place all the out of spec tubes in a box and ship them to me, where I will wear them out a little for you and return them in 3 or 4 years time

The two tubes are 50 years old and too used to have such high emission.

I still do not understand...

 

[pilovis]

Maybe screen grid current too high?

 

[DF96]

How did you test them? What value of grid resistor was used? You do realise that the getter can look normal for a gassy valve? You do realise that a gassy valve could, in some circuits, sound fine until it drives itself into thermal runaway?

 

[pilovis]

As far as I know the tester verify emission connecting the tube as a rectifier diode with grids connected to plate.

 

[DF96]

That sort of cheap tester does a rough and ready check of whether the valve is alive or dead. It tells you nothing else. Your valves are alive.

 

[kevinkr]

Emission testers are almost useless as an indication of tube performance, what you need is a mutual conductance (transconductance) tester.