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Wednesday, April 17, 2013

Tube of the Month : The 6SF5


Last months tube was a medium mu octal triode. This month I am presenting a high mu triode with octal base. The 6SF5 and it's glass equivalent 6SF5GT.

The 6SF5 is even less common among tube audio designers today than the 6J5. Again a surprise since apparently it is a tube very well suited for audio use.

The octal base pinout is shown on the left. The tube contains a single triode system. Being a high mu triode it has a very high amplification factor of 100. This comes with a high plate resistance of 65 kOhm. Which is remarkably close to the ubiquitous ECC83/12AX7. Even more surprising that it is not used more often. Understandable for commercial manufacturers since contrary to the ECC83 the 6SF5 is not made any more. But there are still plenty NOS tubes available for small scale manufacturers or DIYers. A closer look to the datasheet does not reveal anything which would make the tube unusable for audio purposes. The plate curves are as linear as one could hope for:

Measuring a sample on the curve tracer confirms the linearity:

So why is practically nobody using this tube? It seems perfect for small signal amplification purposes. The metal shell of the standard 6SF5 cannot be the reason since it is also available as glass type. Those metal shells can be of significant advantages in a phono stage since the tube has shielding built in.

As mentioned in the article about the 6J5, I am working on a stand alone Octal Phono Preamplifier which will use the 6SF5 as the input tube. A protoytpe is already working and sounds very promising:

This Phono Preamp will be covered in an upcoming series of posts.

Now let's have a look at some 6SF5s from my tube stash. We will start with glass 6SF5GTs. Here two beautiful National Union tubes:

The next one is branded International Service Master. They did not manufacture tubes themselves. They rebranded tubes from the major suppliers.



The electrodes:

 A close up:

The top:

The base, some of the unused pins are not equipped:

And now some metal shell 6SF5s, a RCA in military packaging:


Two different Raytheons:

A Tung-Sol:



Ok, time to open one and see how it is constructed inside. We will dissect this Sylvania tube:

Metal shell removed:

The triode system removed from the base:

Grid, heater and cathode removed from the plate:

The plate:

A close up of the heater, spiral wound in this case:

The grid:

closer up:

Showing the scale of the grid:

I hope you enjoyed this months tube presentation. Stay tuned for an article about a very different tube next month.

Best regards



  1. Fantastic. Exactly what I was looking for. Thank you!

  2. If you were setting these up with cathode bias what restistor and bypass cap values would you recommend?

    1. Hi!

      This depends on your application.
      As an example see here:


  3. Olá meu amplificador, circuito Mullard, tem uma 12AX7 na entrada, um triodo por canal. Pretendo colocar no lugar, duas 6SF5, uma para cada canal. Elas equivalem a meia 12AX7?

    1. They are quite similar in characteristics. You should be able to adapt your amp to use 6SF5

  4. "Even more surprising that it is not used more often".

    Why should this surprise you? When designing for normal, line level (1.0Vrms) input, I don't need that much gain. About the only practical application would be for the LTP, and there are already loads of dual triodes for that: 12AX7, 6SL7, 6SC7, etc. Saves a hole.

    1. There are still many phono preamps out there and new ones being built which need high gain in the first stage


  5. Don Garber of Fi used these in his famous 2A3 amp. One I’d like to resurrect!��