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Old 7th April 2009, 01:51 PM   #11
rknize is online now rknize  United States
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I don't have as much experience as a lot of the other folks here, but I find that tubes that are weak due to "normal" wear (weak emission) can keep going longer in fixed bias amps than in cathode bias amps. The operating region is shifting across the chart as you "put the spurs" to the bias to get the idle current up, so the linearity may be affected. It really depends on the tube in that respect.

Now there are plenty of other problems tubes can have besides weak emissions, like gas, contaminated grids, sagging screens, wobbly cathodes (DHTs) that will make an other wise strong tube sound wrong.

I do like a tube tester for quickly evaluating the health of a set of tubes, but I much prefer having a known-strong tube to compare against. My tester is not the best (Eico 666 "Satan's Choice"), but I've made a few mods that allow accurate gas tests and fairly consistent results.

The ultimate test is always behavior in the amp and putting the amp on a scope will tell the story. However, this is kind of a pain and that is where the tester comes in handy....
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Old 7th April 2009, 08:58 PM   #12
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The reference data for good/bad valve parameters is well-shown the Mullard/Philips Special Quality (SQ) series.

look at the E280F data sheet:

http://www.mif.pg.gda.pl/homepages/f...09/e/E280F.pdf

page 2 shows the data arranged across 3 columns:

I. The nominal value (eg Ia = 20mA)
II. The range of acceptable Ia for the same operating condition, across all samples of NEW E280Fs: eg. Ia = 18.8 to 21.2 mA
III. The end-of-life value, when the valve is considered out-of-specification (eg Ia =17mA.

So or the SQ series, we see the BAD level of Ia is only 15% down on nominal.

On page 4 of the E280F data. See how the valve is guaranteed to have Ia=17mA or more, even when it has been used for 10K hours!

OK, so this is SQ, which is a professional quality item, and can expect better tolerances than commercial equipment (like amplifiers).

But -60% is no good to any person or application! I would say 20% down is the absolute limit for any use at all.

Even then, a valve that has been used so long that it is 20% down will often be microphonic and noisy.
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Old 7th April 2009, 09:33 PM   #13
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According to GOST (I don't remember the number) good power tube means less than 15% of plate current drop and less than 10 times increase in the 1'st grid current. The less is humidity and solar radiation, the longer they will be good.

However, GOST defined conditions of tubes used then in USSR, you criteria may vary...
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