[Phlogiston]

Or any 2 - 2.5 volt common cathode power tube.

Have you ever put a 45 or a 2A3 into your 533, 600A, TV7, checked for shorts (ALWAYS), pushed the test button, only to watch the needle go up and then start to drop?

This can be a bad sign on small signal tubs on a tester like a I-177. It MAY be putting too much signal (,5Volts AC) onto the grid, BUT that is not what I am talking about here.

I have heard several people say that they had discarded tubes like the 45 (245, 345) for such poor test results. Then wished that they had not.

First lets start with the tester cal. When you are setting the filament voltage on a lot of testers, you have to aim at 6.3V and up to 12V as accurately as possible since these are the most common tubes that you will test. However, many testers will have a significant error towards the upper and lower ends of the filament voltage spectrum. This is usually ok on the top end since the error is less significant in heating the filament to a valid test point and those 50V tubes tend to draw a lot less current.

On the low end, 1V, 1.5V, 2V, 2.5V the tester may be a bit low BUT the relatively high current consumption is going to cause the filament voltage to sag even more. On my 533, testing at the pins of a 45 gives me about 2.2V. AND when you push the test button the filament voltage sags even further -  this is assuming that you 0ed the line to start with. SO that 2.5V tube sees the filament voltage drop further to around 2.1V.

Now, I have heard, and it appears to be somewhat true, that tube testers were built to accommodate this issue and I see that even with a reduced plate voltage the bias is lower than would be expected in circuit.

If you test a brand-new 45 tube under these conditions there is no problem. If you test a well used tube there are issues. The used tube may have more problem with the reduced plate voltage in the tester and specifically with the reduced filament voltage.

How can I easily validate this? Well, a tester like the 533 has a "life test" that duplicates these conditions. Problem is that you are doing the life test without engaging that function. AND as the filament voltage drops further under test the output of the tube drops further which is observed by the dropping meter.

So what to do? I have marked the line setting in my 533 for 2.5V test value which is about 2 marks higher than 0(ed). This is not alone the solution. Instead what I am doing is the "Life test". A new tube is going to perform well at the reduced filament voltage and there will be little gain (possibly a loss) in the mho reading as the filament is adjusted to 2.5 V (which also raises the other test voltages). A well used tube will instead spring to life with the increase in filament voltage. A used-up tube will stay low with no or little improvement - it is still bad.

So what I am suggesting is creating a valid "Life Test" for these tubes before you decide to throw one away. A tube that tests well at 2.1V probably does not need to be tested at increased filament voltage. But you may want to save an expensive tube by retesting a tube that gives you 1000/1850 at 2.1V. This tube may give you 1850 at 2.5V and will work just fine in an old radio. The "bad" one will still test bad. You may also find that a filament has gone to sleep at a lower filament voltage and after testing at 2.5V the test results may improve at 2.1V. Do not confuse this with thermal inertia - let the tube cool a bit down to the 2.1V temperature before claiming improvement.

Do not test the tubes above 2.5V (filament).