Vacuum tube emission (anode current, transconductance) percentage that is considered GOOD

Russian vacuum tube datasheets almost always specifies anode current like Ia = 100 mA +/-20mA at some conditions. The same with transconductance. But western tube datasheets specifies only one central value - for example Ia = 100 mA at some conditions. What is widely accepted tolerance in percentage to consider tube is good?

• What do you mean by "good'??? There's such a huge variance in tube characteristics, even among the same tube in the same manufacturing lot, that you will "never" find a tube that runs exactly at 100mA. That's gonna be a "nominal" value (i.e. typical, normal, usual, etc...) Link to the datasheets and point out what you're seeing, can better explain it then.... The circuits are designed to accomodate these variances.... This can be done with negative feedback or with an adjustable bias supply (that you set when you replace the tubes). Some amps even 'self bias' tubes. – Kyle B Jan 29 at 19:27
• For example - B&K 707 tube tester - has "Repalce", "?", "Good" on its ampermeter. Like percentage - till about 70% tube considered good - that's about minus 25% of its nominal. – Ernestas Gruodis Jan 29 at 19:45
• One thing about "Tube Testers" is that NONE of them have the same "range". In other words, when you measure voltage with a voltmeter, and you read "10 volts", when you pickup another voltmeter and measure it again, you'll again get "10 volts". Tube testers all use their own scales, and what is "good" is somewhat arbitrary. It depends quite alot on what you're putting the tubes into... i.e. is it a power amp with 500V B+?? Or a preamp with only 200V on the plates? Basically, it's a GUIDELINE, not an exact measure. – Kyle B Jan 29 at 21:29
• Basically, if you put the tube in your amp, and it sounds good to your ears, the tube is probably "good". If it sounds weak, loses fidelity, etc... it's "bad". But the point where a tube is no good depends so very much on the circuit it's in, and frankly, how sensitive your ears are. (Assuming this is for an audio application...) – Kyle B Jan 29 at 21:31
• ...what is "good" is somewhat arbitrary... Yes, and that is the biggest question :) In case of russian tubes - while testing - I can easily discard them when some parameter is out of range. With western tubes - I have to think, usually I would use the value of maximum 30%. Tubes are tested for selling later - so it's unclear where they be used.. – Ernestas Gruodis Jan 29 at 21:49