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Saturday, August 28, 2021

Tube of the Month : The 12AX4


Oh no! Not another TV Damper tube. I know that I show a lot of these but they are my favourite rectifier devices. So they deserve being covered. Most of the odd heater variants of the 6AX4 have been shown already, the only one missing is the 12AX4.

All other heater variants of the 6AX4, namely the 17AX4 and 25AX4 have had their appearance as tube of the month. The 12AX4 completes the list.

The 12AX4 of course shares the same pin out as the other variants. While the 6AX4 is specified to be heated by a voltage of 6.3V the 12AX4 heater is designed to operate at a specific current, which is 0.6A. This would result in a voltage of around 12.6V across the heater, hence the 12 prefix in the tube designation. Such heater variants are meant to be heated in series with other tubes which require the same current. But they can also be heated from a 12.6V source. Besides the heater specification all other technical parameters are the same as those of the 6AX4. See the RCA data sheet for all details. Since such heater variants are not very popular, these tubes can be picked up at cheap prices. Maybe the 12AX4 would be the right choice for a project which uses 12.6V heater tubes throughout.

Here we have a Sylvania 12AX4GTB with coin base.

The 'GT' in the suffix stands for glass tube and B indicates a later version with improved specs.

A few detail shots.

On the base unused pins are left out to increase creepage distances for high voltages.


Another Sylvania also with coin base.

Next we have a Westinghouse with full base:

And lastly 3 slightly different looking General Electric 12AX4GTB:

Although internal construction appears to be identical in all 3, they differ in color and placement of the printing.

The next one:

And the third:

Since I already dissected a lot of TV Damper tubes I did not intend to break one of these. But since I accidentally dropped one while taking photos, we might as well have a look at it's internal construction.

The getter ring:

The heater:

The cathode:

The plate:

That's all for the 12AX4.

Best regards


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