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Sunday, August 27, 2023

Tube of the Month : The 6AF11


Here is the next example of an interesting Compactron tube as part of the mini series. Meet the 6AF11:

The 6AF11 contains 3 different systems, 2 triodes with different parameters and a sharp-cutoff pentode. Like most Compactron tubes it was intended to be used mainly in TV sets but of course is well suitable for other applications too.

Triode section 1 is of the high-mu variety with an amplification factor of 68. This comes with a plate resistance of 12.4kOhms. The transconductance is 5.5mS. The second triode is classified as medium-mu. But it still has a rather high amplification factor of 41. Plate resistance is a bit under 10kOhm and transconductance 4.4mS. Two triodes with rather interesting parameters for audio. The pentode section is a sharp-cutoff type as mentioned before. It has a maximum plate dissipation rating of 5W. Not much but could maybe serve as the output stage of a headphone amp. The 6.3V heater needs 1.05A of current which seems moderate considering the 3 elements to be heated. For a complete set of technical parameters loom up the General Electric data sheet. This tube could probably be used for a phono stage with the two triodes doing the amplification and the pentode wired as triode at the output. That would be just a single tube per channel. Maybe there is another Compactron phono preamplifier on the horizon with this tube. Let's check the plate curves, starting with the data sheet curves of triode section 1:

And the results from the curve tracer:

Triode section 2:

And lastly the pentode section:

The pentode wired as triode:

With the small signal amplitudes in a phono stage the not too optimal linearity would not matter that much. This tube deserves to be used in a phono stage!

Let's have a look at the 6AF11 from all angles:

And some close ups:

Some more pics:

The base:

Just amazing how they packed all this functionality into this little bulb.

An now the dissection of a 6AF11:

The two bars across the bottom connect the heaters.

One triode section removed, the silver part is a screen between the triodes.

Plate removed:

Grid and cathode:

The shield:

The other triode which is located between the shield and pentode:

Removing the plate:

Grid and cathode:

The pentode section:

The pentode plate:

Beam forming plate, grids and cathode:

The two grids and cathode:

The beam forming plate:

What a marvellous piece of engineering.

I hope you enjoyed the presentation of the 6AF11. Stay tuned for more Compactron tubes to be shown.

Best regards