Tube of the Month : The 6S4A

Hi!

This month we have a look at a 9 pin miniature medium mu triode, the 6S4A.

This tube, like so many, was developed for use in the deflection system of TVs but can serve in other applications as well including AF amplifiers.

It is a single triode on a 9 pin base. The pinout is shown on the left. It has an amplification factor of 16.5 which comes with 3700 Ohm plate resistance. The plate can dissipate up to 8.5W which is not bad for such a small tube. As the 6 in the designation indicates it has a 6.3V heater which takes a moderate 0.6A of current. Like so many TV tubes you will  hardly see this one used nowadays although probably still easy to find. The low amplification factor and the fact that absolute linearity was not a priority when this tube was designed limits the usability somewhat. But I would see it working nicely in a linestage or as DAC or phono output stage. Check out the data sheet and decide for yourself if it is worth to give this tube a try. As always we compare the data sheet plate curves with those of an actual tube:

And here a few samples, starting with RCA:

Some close up shots:

Higrade branded 6S4A:

Westinghouse in military box:

General Electric:

Canadian Westinghouse:

And lastly a few photos of the tube in operation:

Best regards

Thomas