It's time for a Compactron tube again! This month I am presenting: The 6AK10.
For some background and history about the Compactron base tubes, please see the 6GE5 tube of the month post.
The 6AK10 is a triple triode. Three independent triode systems in one bottle. This means it utilises all but one pin of the 12 pin base. 3 pins for each of the triodes and 2 pins for the common heater. The pinout is shown on the left. While the pinout looks very untidy with only the second triode system's connections grouped together, there probably was a good reason for that. Like many compaction tubes the 6AK10 was developed for use in color TV sets. More specifically it was intended as color difference amplifier. But of course as with most tubes this does not mean that it cannot be used for other purposes - like audio - as well. The three triodes all have the same parameters. They have an amplification factor of 53 at a transconductance of 7mS (7000umhos). This results in a plate resistance of 7300 Ohms.
The data sheet shows a typical operating point with 10mA of plate current. So a rather healthy triode with parameters which appear quite usable. What to do with so much amplification though? A rather practical phono stage could be built with only 2 of the three triodes yielding a gain of well above 40dB after subtracting the loss in the RIAA network. For a line stage the amplification of the third triode is too much, but not if used as cathode follower or, what I would prefer with a step down line output transformer to get the gain into a sensible region and resulting in a low output impedance. Or, since such compactron tubes usefully are dirt cheap, it is affordable to leave one of the triodes unused and pick a second tube with an amplification factor which fits your needs whichever they are. It is a shame that there are so many compactron tubes collecting dust in warehouses since nobody buys them. Admittedly I have never used the 6AK10 myself. So many tubes, so little time. But maybe this post motivates someone out there to give the 6AK10 a try.
Don't let the plate curves shy you away from using this tube. At the low levels in a phono stage and operated in the middle of the plate curves this will not be much of an issue.
Here a measured set of curves from a tube. I did realise that the individual tube section's transconductance spread as quite big. Here the set of curves from another triode in the same tube:
This was from a tube with better matching. So if used in a circuit which requires close matching, make sure to test a batch of tubes before using them.
I only have 6AK10s made by General Electric in my stash.
So not a big variety to show.
Here we can see the three triode sections arranged in parallel.
Some close ups:
Those compactrons are little marvels of tube engineering.
Checkout the delicate wiring in the bottom.
I cracked one open to have a closer look inside.
The three heaters of the triodes are wired in parallel.
One of the triodes separated from the others:
Grid and cathode:
Gold plated grid wire to eliminate secondary emission:
Tube manufacturing processes reached a high level of maturity, automation and precision at the time the compactrons were introduced. Let's finish off with some photos of a 6AK10 lit up.
The heated cathodes are nicely visible through the holes in the plates.