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The burn-in process for power tube testing and matching

As we began to venture into the realm of matching new production power tubes, we wanted to know the answers to two questions:
  1. How much does burning in affect power tubes’ readings for plate current (mA) and transconductance (gm)?
  2. How long should power tubes be burned in to get a good match?
After performing thorough tests before and after burn in (see details below), we found that the change in readings after burn-in was significant enough to require a 48 hour burn-in period for accurate matching.
 
Here’s how we came to this conclusion.

Pre-burn-in testing

One of best current production brands of the popular 6L6GC power tubes is the SED “Winged C.” So we ordered 50 tubes of this type fresh from the factory and used an AT1000 to test them for plate current, Gm, and gas. The AT1000 is a laboratory grade tester accurate to within 1.5%.

We get the best results on the AT1000 using the manual mode. After an extended warm-up period, we perform test cycles until we achieve duplicate results.

The burn-in process

We used a Maxi-Burn unit to burn-in tubes with both the filament voltage and plate voltage. Folks from Maxi-Test recommended a 48 hour burn-in period to me when I spoke with them on the phone. But the paperwork that came with the unit suggested that it takes tubes 8-24 hours to burn in and recommended a 24 hour burn-in period. So we decided to test that question as well.

This Maxi-Burn unit accommodates 30 tubes at a time, so we filled it up and ran the first batch of tubes for 48 hours. The remaining 29 tubes (1 had a factory defect so it was discarded) were burned in for 24 hours. But because we needed the quantities to land in a factor of 5 for the Maxi-Burn unit, we had to rerun one of the tubes from the 48 hour batch. So, in the end, we had 29 tubes burned in for 48 hours, 19 tubes burned in for 24 hours, and one tube burned in for 72 hours.

Post burn-in test results

The results were as follows:

For tubes burned-in for 48 hours, the mA reading changed by an average of 5.26% and the gm reading changed by and average of 3.02%.

For tubes burned-in for 24 hours, the mA reading changed an average of 4.56% and the gm reading changed by an average of 2.92%.

There were some outliers that changed much more than this. One tube’s mA changed by 17% and gm changed by 16%! And of course, there were some that changed very little.

Incidentally, the one tube that was burned-in for 72 hours increased by 3% on both scales.

Another noteworthy finding had to do with gas readings. On the initial testing, one tube had as much as 20 mA of current passing as a result of gas! A handful of others hovered between 5-10 and the rest were 3 or less, which is considered an acceptable level by the AT1000 test interpretation comments. Happily, after burn in, all of the gas readings came down to 1 mA or less regardless of whether they were in the 24 or 48 hour batch.

Conclusions

So what does this all mean?

If you are purchasing new production tubes and you need matched quantities for your application, it is worth your while to verify that the tubes have been properly burned in prior to matching. This process stabilizes the tubes. If they are not yet stabilized then they cannot be matched correctly.

Additionally, it is important to verify that tubes are matched BOTH for emissions and for gm. We have seen many tubes show large differences in value between these two ways of measuring the strength and life of a tube. Many tube distributors simply use a current draw method of matching tubes (using a “Maxi-Matcher” typically). But this doesn’t mean you’ll get a good match since it’s not based on a measurement of the tube’s actual gain.

If you are going to pay the money for matched pairs, you may as well get the best match you can. But as the description of our process shows, getting accurate results requires significantly more time and attention. We think it’s worth it to get the most out of your tubes.

All new production power tubes we offer are burned in for 48 hours and then tested to get the most precise matches we can for both plate current (mA) and transconductance (gm). We also screen for leakage, gas, and noise.

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TC Tubes specializes in precision testing and matching of vintage and new production vacuum tubes for guitar & bass players, audiophiles, and DIY tube audio enthusiasts. Our website features secure payment options and fast, convenient shipping around the world.

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