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Tuesday, November 24, 2020

Tube of the Month : The 6FY7


Sometimes when I am looking for a certain tube in my stash I stumble upon a type which I had forgotten about but which is interesting to show in the Tube of the Month series. Last time I rummaged through some tube drawers I came across the 6FY7.

The 6FY7 is a Compactron double triode with two dissimilar tube sections. Like many such dissimilar duo triodes it was developed for deflection and oscillator service in TV sets. But also for amplifier applications.

It has a 12 pin Compactron base which is shown on the left. The Compactron series of tubes was introduced by General Electric in the 1960ies as an attempt to integrate more functions into a single envelope. Hence the many base pins. Since the 6FY7 only contains 2 triodes not all of the pins are used which can be seen in the pinout diagram on the left. One of the unused pins, nr 8, is marked IC which stands for internal connection. This means nothing should be connected to that pin at the socket. Section 1 of the tube is a high mu triode with an amplification factor of 65 which comes with a plate resistance of a little over 40 kOhm. Section 2 is a low mu triode with an amplification factor of 6 and a low plate resistance of 920 Ohm. These parameters are

very similar to the 6FM7 which I presented already as tube of the month. So it would be useable in similar applications as the 6FM7. As the prefix 6 of the tube designation indicates the 6FY7 has a 6.3V heater. It needs 1.05A of heater current. Like most Compactron tubes the 6FY7 is largely ignored nowadays although it is still easy to find at low cost. Also Compactron sockets can still be found and are still made in China. A big drawback of the Compactron tubes is the difficulty to insert them into their sockets without bending the pins. Considerable force and care is needed. This is probably a reason for these tubes  not being very popular. Let's have a look at the plate curves of the tube, first the high mu section:

The curve set shows decent linearity especially useable for low level applications. Here the actual curves taken with the tracer:

The low mu section:

I only have 6FY7 tubes made by two different manufacturers. Let's start with one made by General Electric:

The typical all glass Compactron construction, in this case with a top getter.

Some more views:

The larger system on the right in the photo above is the low mu section.

Also note the cooling fin mounted to the grid of the low mu section.

Close Ups:

The base:

And two more photos of this tube:

Next we have another General Electric 6FY7 but this time with the getter on the side and a clear top:

But otherwise similar to the first one.

The only other 6FY7 tubes I have are made by RCA:

Let's see how it looks inside by dissecting this one:

Removing the glass bulb:

Removing the base. The heaters of the two triodes are connected in parallel with the two metal rods:

The heater wires can be easily pulled out of the cathode sleeves:

The high mu section:

Removing the mica on the top reveals the grid and cathode:

Cathode and grid:

The low mu section with the larger plate:

The cathode came right out:

The grid:

And lastly some photos of the 6FY7 in operation:

That's all for the 6FY7.

Best regards