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Friday, October 18, 2019

The 6KN6 Mono Amplifiers - Part 2


The 6KN6 Mono Amps are finished. In this post I will share the selection process for the driver tube and show photos of the finished amps.

To reach an output power of about 10W I selected an operating point at the maximum plate dissipation of the 6KN6 with about -100V grid bias. That requires a voltage swing of 200V peak to peak from the driver. As usual I wanted the driver to be interstage transformer coupled. This requires a tube with low enough plate resistance. But that usually comes with rather low gain. Especially since I want my driver to have large headroom which excludes choices like the D3a or 417A. Also just for consistency and style I wanted all tubes to use the 12 pin compactron base.

I first looked at the 6FM7 which has a low mu section with very low plate resistance and a high mu section. However the amplification factor of only 5.5 is very low and would mean a 3 stage circuit is necessary to get enough gain. I wanted the option to stay with 2 stages only. The 6AK10 would fit the bill especially with all triodes wired in parallel. However the 6AK10 is not linear enough for such large voltage swings to my taste.

I finally ended up with the 6FJ7, a compactron equivalent of the 6DN7. The 6FJ7 is a dissimilar double triode. The low plate resistance section has an amplification factor of about 15 at 2kOhm plate resistance. It is very linear and can deliver large voltage swings. But a gain of 15x is still not quite enough to get a usable input sensitivity. There are two possible choices to solve this. Add an input transformer with a step up to reach the desired sensitivity or add another gain stage in front of it. The other triode section in the 6FJ7 has an amplification factor of 22. As such a bit much but RC coupled and with unbypassed cathode resistor, about half of that would remain.

After tossing both variants back and forth in my mind, I decided to use both and a switchable set up. A direct input which drives the lower mu section through a step up transformer, making this a pure power amplifier and the higher mu section RC coupled with volume control potentiometer in front of it to make this an integrated amplifier.

This offers the possibility to start with the amp as integrated and later upgrade it with a preamplifier like my 6AH4 linestage or even a 10Y line stage. This would also give an easy way to compare the sound of such a line preamplifier with transformer volume control against a simple resistive potentiometer and RC coupled stage. Just with the throw of a switch.

With the use of a 1:4 step up input transformer the power amp direct input needs about 1V RMS to reach full output power. The input section measured at about 12x gain, close to the predicted half of the amplification factor. So about 300mV sensitivity for the high gain input which gives enough gain headroom for low volume sources.

Since the amps are built as monos the separate volume controls provide an easy way to adjust balance as well if needed. Since I used stepped attenuators with defined positions it is easy to adjust the left and right volume at the same levels.

So here they are, the finished 6KN6 integrated mono amplifiers easily switchable to power amp configuration. Since I often get requests for integrated amplifiers I decided to build them this way also to have the possibility to demo the sonic benefits of a separate linestage.

To complete the discussion of the tube line up, the choice for the rectifiers was easy since there are plenty 12 pin compactron TV dampers. I decided to use the 6BE3.

In part 1 about these amps I showed the initial construction steps. Here the top plate completely wired:

All chokes and the interstage transformer are mounted on a separate plate:

The completely assembled amplifier:

And with the bottom cover mounted:

Some pictures of the amp in operation:

Some more views:

This was a fun project which shows how many possibilities there are to build an amplifier with uncommon tubes.

I will probably bring these amplifiers to the High End show in Munich next year. Come to the show to listen to these amps or visit me in Lindau for a demo.

Best regards


Monday, October 7, 2019

The 6KN6 Mono Amplifiers - Part 1


As mentioned in the 6KN6 tube of the month post, an amplifier project with this tube is planned.

Here some photos of the initial construction steps.

A pair of mono amps with the 6KN6 wired in triode mode is in progress. Here the top plate of one of the amps with some parts mounted.

Output transformer is a Hashimoto.

Power transformer and OPT will be hidden under transformer covers for a uniform look

Some initial wiring done:

Stay tuned for part 2 which will reveal the choice of driver and rectifier tubes.

Best regards


Thursday, September 26, 2019

Tube of the Month : The 6KN6


Although I am manufacturing my own directly heated triodes since the acquisition of the assets of ELROG, I still enjoy digging up unusual tubes which are well suited for audio purposes. With the 6CB5A and 6GE5 I already showed some sleeper tubes which work nicely triode wired as single ended output tube. Here is another tube of that category, the 6KN6.

The 6KN6 is a 12-pin Compactron tube. It was developed at the end of the era of tubes when that technology was matured and perfected. I have expressed my fondness of Compactrons in other tube of the month posts about such tubes, like the 6GE5, 6U10, 6AK10, 6BE3, 6FM7, 19DE3 and the marvellous 6HS5 and 6HV5A.

The 6KN6 was developed as horizontal deflection amplifier tube for TV sets. It is a beam power pentode. The data sheet and pinout diagram indicate that it has two systems internally wired in parallel. But not all 6KN6 have such a structure. When it was introduced they probably started with two existing pentodes which they could fit into a compactron bottle. But later developed a new version which consists of a single pentode internally. At least most of my stock shows only a single system inside. The 6KN6 needs a hefty 3A of heater current which indicates a healthy emission capability. There is also a series heater version, the 42KN6. The data sheet gives a triode wired amplification factor of 4.5. I measured a very low triode plate impedance even lower than that of a 300B or 2A3. The max. plate dissipation is 30W. I am not afraid to run such TV tubes at their maximum plate dissipation as they proved to be rugged and long lasting. And replacements come rather cheap. So triode wired they can be expected to deliver up to 10W when pushed. Here are the triode plate curves:

This looks very promising and prompted me to start development of a single ended 6KN6 amplifier. More about that in upcoming posts. Let's look at some tubes.

Since General Electric introduced the Compactrons, it is appropriate to start with that manufacturer.

Like most of the higher powered deflection tubes the 6KN6 has the plate wired to a top cap to handle the large voltage spikes.

Some might be put off by the 12-pin base since the sockets can be a bit hard to find and the many pins require some substantial force to plug in the tubes.

On the close up photos some of the delicate engineering details are visible.

The construction is quite advanced and rugged.

Another General Electric 6KN6:

Next we have a RCA:

Here is a tube with a rather unusual feature:

The tube carries the Realistic Lifetime name which was a house brand of Radio Shack.

Gold plated pins on this one, and they had a green ring applied. It is some kind of coating which is applied inside the tube. Probably to cover the ring of black deposit which used to develop in such tubes over some time of use.



Next we have a sample which has two internal systems, it is Zenith branded:

Canadian General Electric:

Next we have two different Raytheon tubes.

This one also has two systems wired in parallel.

The other has a single pentode inside:

Now let's cut this one open to see the construction more in detail:

The massive heater:

The plate:

Quite unusual structure with some additional fins.

Cathode with grids and beam plate:

The screen and control grids are extremely well aligned:

Only when viewed from an angle the gold plated control grid is visible underneath the coated screen grid.

These photos show how far tube manufacturing was advanced in the 1960s.

The beam plate removed:

Revealing cathode and the 2 grids:

Separating the screen grid:

Cathode pulled out:

And now a 6KN6 in operation:

As mentioned above, development of a 6KN6 amplifier is underway. The metal plates are already made.

The top plate, connection plate and support plate which carries the internally mounted parts. Stay tuned!

Best regards