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Monday, March 11, 2019

Tube of the Month : The 12SF5


Last month we had the 12J5, let's continue the 12V heater theme with the 12SF5.

Like the 12J5 being a 12V heater variant otherwise identical to the 6J5, the 12SF5 shares all electrical parameters with the 6SF5 except for the heater voltage and current.

The 12SF5 comes with the same Octal base as the 6SF55. Pinout shown on the left. The heater voltage doubles to 12,6V and the heater current is only 150mA. For all technical aspects of the tube refer to the 6SF5 tube of the month post or look up the General Electric data sheet. The 6SF5 seems to have gained some popularity since more and more people realised that it is a great Octal base alternative to the ubiquitous 12AX7. It offers a tad more gain as one half of a 6SL7. 12V heater variants of popular tubes are still largely ignored, so this is another sleeper tube which can be found cheaply. No disadvantages in terms of sound or linearity are to be expected from the 12V heater. Here a set of plate curves taken from an actual tube:

Like the 6SF5 it was made with metal as well as glass envelopes, the latter dubbed 12SF5GT. My collection mainly consists of the glass variant. Let's start with 12SF5GT made by National Union:

Three different box styles.

The tube in the cardboard insert:

The tube:

The unused pins are omitted from the base:


Tubes testable in box without breaking the seal:

This one has a metal shell around the base.

RCA Victor in 'Nipper box':


Different style Tung-Sol:

Close Ups:

JAN (Joint Army Navy) military tubes made by Tung-Sol:

These are from a large batch I bought years ago:

Sylvania 12SF5 :

4 different boxes and 4 different construction styles.

A variant with metal base:

Older style:

The oldest Sylvania 12SFGT:

With leaf logo on the base:

Metal envelope 12SF5 by Sylvania:


Newer version:




General Electric:

Canadian General Electric:

The tubes in these boxes were made by Tung-Sol:

Another example of the rebranding and cross branding which was very common.


A Hytron with metal clamp on the base:

And lastly a Dumont 12SF5GT:

I found this tube with loose metal base:

A good candidate to slaughter.

The metal base is just a ring clamped around the tube. A small metal strip connects it to pin 1:

The base:

Bottom of the tube, note the extra insulation on the pate wire:

Glass removed:

Pulling out the heater wire:

Grid and cathode:

That's all for the 12SF5. I hope you enjoyed this months tube presentation.

Best regards