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Tuesday, February 26, 2019

Tube of the Month : The 12J5


This month's tube is a 12.6V heater variant of an indirectly heated triode: The 12J5

The 12J5 is electrically identical to the 6J5 except for the heater voltage. The 6J5 was covered in a tube of the month post already.

Like the 6J5 the 12J5 was made with metal shell and glass envelope, the latter dubbed 12J5GT. It comes with the same Octal base, pinout shown on the left. The heater voltage is twice that of the 6J5, 12,6V and consumes just 150mA. Since the technical details have been discussed in the 6J5 post already I won't repeat them here. The complete set of technical parameters can be looked up in the General Electric data sheet. Odd heater variants are still not getting much love from tube amplifier builders so are a good alternative if a certain tube type becomes scarce. No sonic disadvantages are to be expected from 12.6V tubes vs 6.3V tubes. The 12J5 is as linear as it's 6.3V counterpart. Here a set of plate curves taken from a tube:

Let's start with the 12J5GT made by National Union.

These come in nice boxes which allow testing without taking the tube out.

The insert can be pushed down to fully expose the pins.

To remove the tube from the box, the little seal has to be broken.

A nice measure against the sale of used tubes as new.

The glass is a bit slanted in the base which was quite common with many tubes. This is only a cosmetic issue and does not impact functionality.

Tung-Sol 12J5GT:

Different style Tung-Sol:

Wards Super Airline:

General Electric:

Next we have a metal shell 12J5:

Branded International Servicemaster, made by one of the large manufacturers for them.

Next a brand I have not seen before, Fordom:

Again just rebranded tubes, the name is an abbreviation for Foreign Domestic



This one was made for the military and also carries the military tube designation VT-135.

Again boxes which allow in box testing:

CBS / Hytron 12J5:


And lastly some 12J5GT made by RCA:

Now let's open this one:

Removing the glass:

The base removed and pulling out the heater:

The plate:

Grid and cathode:

And as always at the end some photos of the tube in operation:

Odd heater variants are a great alternative and deserve more attention than they are getting!

Best regards