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Tuesday, August 27, 2019

Tube of the Month : The 6BX7


In last months tube presentation of the 6BL7 I briefly mentioned this tube. Have a look at the 6BX7.

The 6BX7 belongs to a family of 3 medium mu double triodes along with the 6SN7 and 6BL7. They differ in amplification factor, transconductance and plate resistance.

All 3 of them share the same octal base. The 6BX7 has the lowest amplification factor of them with a mu of 10. But this comes with a low plate resistance of 1300 Ohm. Wire both triodes in the bottle in parallel and this halves to 650 which is in the territory of a 2A3 output tube. This is one hefty triode for such a small bottle. Its characteristics would also make this an excellent driver tube with quite large voltage swing capability. Besides a flea power tube amp it would also serve well in a headphone application or line preamplifier. Another tube which is rather forgotten but deserves more attention. I did not have any of these in stock, but after covering the 6BL7 last month I thought it would be a good idea to get some for completing the series of 3 cousin medium mu double triodes. To my surprise they were easy to find at modest prices. As always we will start with a look at the plate curves both from the datasheet and from a real life sample.

I only got some 6BX7GT from 2 different manufacturers. Here some made by General Electric:

From RCA I have samples with three different construction styles.

The first one has a clear top and the getter on the side.

Those little boxes on the top are cooling fins for the grids.

Next we have a coin base version:

And lastly a version with the getter applied in the top dome.

And now the tube in operation:

What a neat device. So many tubes, so little time to try them all.

Best regards



  1. Thomas, Never heard of this tube until today that is.... Any chance of a project in the works using this tube soon? Sounds like a very good candidate for a headphone amp! Randy/U.S.A.

    1. Hi Randy, I have too many other things going on so will probably not get around to work with this tube for a long time.

  2. Thomas, So many tubes.... so little time! Gotcha. Randy/U.S.A.

  3. Hello Thomas - I am the happy owner of a little collection of these tubes. Over on the headphone forum Head-Fi, there is a gentleman who builds a custom output transformerless headphone amplifier using these tubes. It has an SRPP output stage using three 6BX7 in parallel per channel, total of six tubes. I have to say, the results are excellent. As such I have collected several makes of 6BX7: General Electric, RCA, Tung-Sol, Fivre, and Toshiba. The Japanese Toshiba's are my favorite of the bunch, followed by the Italian Fivre.

    Thanks for writing an article on these great tubes.