I talked a little about measurements of valve “quality” in a previous post, but as the result of a few discussions about valve testing this week I think its worth a further post.
There certainly are a lot of numbers thrown around by vendors of vacuum tubes when they advertise valve testing results, and the challenge is to sort out what is a meaningful figure and what is not. It’s not always straightforward because there is no guarantee that the starting point is the same and different standards often seem to be applied. Not only that, but different valve testing equipment can and often does give significantly different results.
As an example, significant differences in measures of transconductance can arise due to many reasons, if a measurement is actually given at all.
- Firstly the valve tester has to have been calibrated at some point in the not too distant past.
- The tester has to be operated to given standards and in a consistent manner.
- The original factory ratings and specified operating conditions (voltages, current, bias) for the tube should provide the benchmark for the results. These ratings are not hard to find.
For some folk a “tested good” valve is simply one that “goes” in the amp or radio, there is no actual measurement of anything against factory specifications here!
Alternatively, if a valve is simply quoted as x% good then what is that a measure of and against what standard? For example –
Some testers use a red and green scale for bad/good or weak/strong. What is the exact value at the point of transition from good to bad and where did that number come from? Chances are it’s an arbitrary value determined by the manufacturer of the tube tester, based on around 60% or so of the tube manufacturers rating for the factory new tube. So what is a weak tube and what is a strong one and compared to what? Wouldn’t it be better to have an actual measurement against the manufacturers original rating? eBay is a great example where you can see valves offered measured at 105% or 120% etc but we are often not told what that is measured against. More often than not, it is not a comparison with factory new specification. If you think it’s confusing and at worst misleading then I have to agree!
In summary, I believe that you should know how these measures are determined and against what standard, before handing over your hard earned $! Remember the only stupid questions are those that are not asked. So, if in doubt, ask the vendor and they should be happy to explain further.