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Friday, September 21, 2018

Tube of the Month : The 6CK4


This month I'd like to present a small octal base triode, the 6CK4:

The 6CK4 is a low mu triode designed for the vertical deflection system in TVs. It consists of a single triode in the bottle.

As such it is a small power triode with 12W maximum plate dissipation. The amplification factor is a low 6.6 which comes with a plate resistance of 1200 Ohms. These values are not very far off the 6AH4. The latter has a slightly higher amplification factor. Also the pinout of these two tubes is almost the same. The 6CK4 has an additional grid connection which is brought out to pin 1 and 3 in parallel while the 6AH4 grid is only connected to pin 1. Thus the 6CK4 could be used as a near substitute to the 6AH4 if the circuit can handle the higher heater current of 1.25A. This would result in a slightly lower gain. The 6CK4 could be used as output tube and would deliver about 2.5W in single ended mode. But I would see it as a more interesting tube for simple line stages. The low amplification factor and rather low output impedance are ideal for that.

Above the plate curves taken from one of my samples. The curves from the General Electric data sheet below:

Another interesting sleeper tube which waits to be brought back to some good use. Let's have a look at some 6CK4 tubes from my stash.

Here we have a Zenith.

Some close ups:

The base omits the unused pins.

The top getter:

A 6CK4 made by Sylvania:

The construction looks identical to the Zenith above.

DuMont 6CK4:

Next we have two different RCA 6CK4:

Now let's examine one in detail, here we have another Zenith a bit different than the one above:

This one has a larger plate construction with distinctive holes on the side.

The top showing a cooling fin for the grid and the getter ring.

Side view:

The base removed:

The heater wire:

After removing the upper mica, the gold plated grid is visible:

Grid and cathode:

Close up:

The cathode:


And finally some photos of the 6CK4 lit up.

The heater glows through the holes on the side of the plate:

That's all for the 6CK4. I hope you enjoyed it.

Best regards



  1. Thomas, I think you need to utilize your amazing skill to design two projects for 6ck4. One of them being low-power amp able to power very efficient speakers and/or headphones. Other a line level preamp. The very low output impedance and low mu should be great for these uses. BTW...Love your blog. Read it regularly but seldom comment. Great job and thanks for all you do! Randy

    1. Thanks Randy! So many tubes... so little time :-)