18Watt.com

I've been wondering what the handwritten numbers you see on (for example) EL84's mean? Obviously, they were written by someone who was testing or matching them.

For example, I have a matched pair of EL84's marked: 21/4.9 I also have a single EL84 marked 21/5.5

What do these numbers represent. More importantly, what does changing them DO in a circuit? (in other words, hypothetically, going from 4.5 to 5.5 makes for more gain and affects the plate voltage by... Or whatever the answer is.)

Andre

You need to ask the person who wrote the numbers.

Well that's certainly ONE possibility, but since I have tubes from a few sources, all with an apparently consistent numbering system (granted they're all JJ's), I figured it might actually mean something slightly more universal to those in-the-know.

has to do with the bias matching of power tubes. I am curious what the exact numbers mean also...

from thetubestore.com

Many amplifiers are designed to require power tubes that have similar cathode current. Even tubes of the same type made at the same time will have variations in their cathode current, and this is why testing and matching is required.

Power tubes are paired up based on their current draw, or "plate current". Measuring plate current draw under operating conditions is generally considered to be the most accurate method for rating a power tube, and it is the method we use.