Tubes Primer, Part 2

By Bob Capretto

Last time around I briefly went over some preamp tube basics regarding configurations and what the numbers on a tube mean. Just to refresh your memory, on a consumer numbered tube (basically what you would have in most amplifiers), the first numbers represent the heater or filament voltage. The next two letters represent the model of the tube (this is really where you find the gain factor of a tube). The number following the letters represents the number of elements inside the tube (meaning how many prongs are sticking out of the tube base). So a 12AX7 is a tube using a 12V heater circuit with an AX gain factor and 7 prongs sticking out of the base. Simple, right?

This month I thought that you might like to have a bit of information that is definitely worth clipping out and keeping close by. This chart is just a small part of the cheat sheet chart I keep for myself. You will also need to have it to follow the gain substitution chart planned for part three of this tubes primer.

You can save yourself a lot of money if you are knowledgeable enough to know the European and military equivalents of your preamp tubes. The numbers in this chart are exact replacements for the consumer numbered tubes in your amplifier.

This table is worth my weight in gold for anybody that may be running low on finances and can’t afford a set of new consumer numbered tubes or even those that want to have a second set of tester tubes around (BTW, a brilliant idea). You may be able to find these equivalent tubes in old electronics laying around your house, or check online for the European and military equivalents. You just might find them a whole lot cheaper.

If you have any questions, you can e-mail me at cs-rocinstruments@sbcglobal.net