When is a tube/valve bad

[MichaelWhy]

Hi All

I have a bunch of old EF80’s that a got for use in robrobs micro designs. I have a little sencore tc109 tester but that just gives a bad to good scale. The EF80’s are all in the good range some only just but had one that didn’t work in an amp, so I just marked it and put it to one side.

I decided to build a tester that would give something more that degrees of good/bad. it supplies the plate and grid with voltages from the tube data sheet and gives the plate current as a ma measurement

I have tested a bunch of preamp and power tubes and am getting readings close to manufactures specs so presume it’s working as intended.

so from looking at the data sheet for the EF80 the plate current should be 10ma and the 10 or so I tested range from 6 to 11ma. the one that didn’t work measured at 3.5ma

what is the plate current telling me , if it’s supposed to be 10 ma but if these are used in the power section would the one that measured 11ma give 10% more power and same for 9 ma would that be 10% less power similarly if it was a preamp tube would that be +/- 10% gain so a good 12ay7 could have the same gain as a weak 12ax7 Or does it not work that way.

1.At what point is a tube considered bad, is there a +/- tolerance
2.what happens if I use one that measures 8ma so the amp works but it’s way off spec
3.are there any other measurements I need to determine how good or bad a tube is
4. is tube matching using a pair or quad of tubes all with similar plate current measurements or a different measurement
5. what +/- is considered a match , I.e. is 9 and 8 ma a match or 8.3 and 8.4

sorry for all the questions I have not been able to find a good/easy to understand write up of what tube parameters are important

 

[schmee]

1.At what point is a tube considered bad, is there a +/- tolerance?
Most that are "bad" are shorted. Does your tester test for shorts?
Others can be weak but they have to be really weak to not at least work.
2.what happens if I use one that measures 8ma so the amp works but it’s way off spec
I like to see them within +/- 10% of each other ma for matching. SO a bias at say 25 ma, I'd want 22 to 27 max spread between two tubes. Preferably better. They can be further off and work fine though.
3.are there any other measurements I need to determine how good or bad a tube is?
In non power tubes many are microphonic and not easily tested for that. Those are often two sided tubes also and one side can be bad. They can be noisy in other ways also like hissing or crackling.
Most testers have a scale that is green-yellow-red. I think most have a failing tube at 60% or so.

 

[dan40]

what is the plate current telling me , if it’s supposed to be 10 ma but if these are used in the power section would the one that measured 11ma give 10% more power and same for 9 ma would that be 10% less power similarly if it was a preamp tube would that be +/- 10% gain so a good 12ay7 could have the same gain as a weak 12ax7 Or does it not work that way.

Trying to correlate testing numbers to actual performance in an amplifier circuit is very difficult. Tubes are very dynamic and will perform differently in every circuit in which they are used. Testing devices will give you a snapshot of a tube's performance at a given operating point but when placed in an actual circuit their performance can vary greatly. Even the tube tester manufacturers from back in the day warned against placing to much emphasis on the testing numbers. I use to have some great links to old tube testing articles but I can't seem to find them anymore.

 

[MuddyWolf]

Put the tube in the amp. If it doesn't work its bad.