Some help understanding tube testing measurements?

[loz_the_guru]

Is anyone familiar with tube testing values? I'm looking at a few sites which list IA and GM values. These are usually presented twice, e.g. one tube has:

IA: 67 GM: 4.3
IA: 65 GM: 4.0

Another has:

IA: 82 GM: 4.7
IA: 94 GM: 5.8

Both of these are advertised as NOS. I've had a quick google but can't quite work out what they mean, or what sort of numbers/range I should be looking for.

Can anyone help me get my head around it?

 

[Vinny]

Preamp valves?

Forget the numbers - they mean nothing, not least because the running conditions in a preamp are nowhere near testing conditions used by tube testers. Also forget any need for balanced valves.

If you are talking dual triodes (typically preamp valves - ECC 82, 83, 88 etc,), which seems likely, there are two valves in one bulb, so there are two sets of results, one set for each triode.

Valves other than preamp' ones - pass.

No doubt Jez will elucidate in due course.

 

[loz_the_guru]

Sorry I should've given a bit more info - they're a mix of 6AS7, 6080 and 5998 power/output tubes/valves. From reading the descriptions I believe these are dual triode valves which would explain the doubled readings, thanks!

 

[loz_the_guru]

Any of the Friday night crew able to shed any light on this?

 

[Vinny]

Give Jez - Arkless - a prod - he is better than well up on this sort of thing.

I know far too little, but the numbers do not look anything like a match to me, which is important for power amp' valves, or at least some of them...………………. He may well ask what amp' you are looking to fit them in (educated guess on my part).

 

[Arkless Electronics]

6080 is a high power dual triode, usually used in power supplies as a series pass valve.

Ia is anode current in mA and Gm is transconductance, probably mA/V in this instance. The figures will be for a specific set of conditions ie anode Volts and grid Volts, that happen to be presented to the valve by the valve tester in use.

6080 is designed to handle large currents relative to fairly low anode Volts.... by valve standards!

 

[slawekt]

in case of 6AS7 and 6998 is difficult to say if you're buying new or used tube as factory specification shows range of readings for these tubes :
6AS7 - new values:
Ia could vary between :100-150 mA
Gm : 5.8-8.2 mA/V

6998 new values could be in the range :
Ia= 50-125 mA
Gm= 11-17 mA/V

6080 datasheet is more precise:
6080 - new values:
Ia=100mA
Gm= 6.5 mA/V

 

[Arkless Electronics]

in case of 6AS7 and 6998 is difficult to say if you're buying new or used tube as factory specification shows range of readings for these tubes :
6AS7 - new values:
Ia could vary between :100-150 mA
Gm : 5.8-8.2 mA/V

6998 new values could be in the range :
Ia= 50-125 mA
Gm= 11-17 mA/V

6080 datasheet is more precise:
6080 - new values:
Ia=100mA
Gm= 6.5 mA/V

Those specs are only true for a specific set of conditions used in the datasheets.... especially the anode current spec, and may well be very different under the conditions presented by a specific valve tester.

 

[slawekt]

Those specs are only true for a specific set of conditions used in the datasheets.... especially the anode current spec, and may well be very different under the conditions presented by a specific valve tester.

In many cases valve testers match datasheet conditions but you're right , always ask under what conditions measurements were taken ie: plate voltage , grid voltage , heater voltage

 

[loz_the_guru]

Thanks guys, some really useful info here and I had no idea datasheets even existed. I will ask RE test conditions. All are advertised as NOS so interesting they should gijve different values.

Out of interest at what point does a tube fail/is the sound affected? Let's say for a 6080 as the data sheet suggests less of a range there - IA50 and GM3. 0? 20/1.5?

 

[Arkless Electronics]

Thanks guys, some really useful info here and I had no idea datasheets even existed. I will ask RE test conditions. All are advertised as NOS so interesting they should gijve different values.

Out of interest at what point does a tube fail/is the sound affected? Let's say for a 6080 as the data sheet suggests less of a range there - IA50 and GM3. 0? 20/1.5?

That's a definite "how long is a piece of string" question! It's really not possible to give a sensible answer as it depends on so many different factors including how it's used, the circuitry, whether or not negative feedback is used and how much and of course on how degraded the sound has to be for you to find it a problem... it's very gradual and for many people it's a case of they replace their output valves because they have had say 5 years of use but still sounded fine to the user.... and then on fitting the new ones it can sometimes be "wow! what an improvement!"... or sometimes not much improvement...

 

[loz_the_guru]

Thanks, that's really useful advice. I'm switching more to try out some different tubes than anything else, and also as mentioned above to solve a microphonic issue i'm having with my current favourite 6080 tube.