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Thursday, July 27, 2023

Tube of the Month : The 6AG11


Regular readers of my blog are aware that I am quite fond of Compactron type tubes. So here is another specimen from my stash, the 6AG11.

The 6AG11 is a duplex diode and twin triode in one bottle. That is 4 elements. It makes use of all of the 12 pins of the Compactron base. The tube was developed for use in FM receivers.

As such it is a great example of the Compactron series which was introduced as an attempt to integrate more elements into one bottle. For some background on the history of Compactron tubes, read the 6GE5 tube of the month post. The 6AG11 contains two triode systems with the same parameters. They have an amplification factor of 66 which makes them high mu triodes. That amplification factor comes at 8500 Ohm plate resistance which translates to a quite healthy transconductance of 7800 Micromhos. This makes it seem quite useable for phono stages. In addition there are 2 diodes. These are rated for very low 5mA of continuous current. They are meant for use in the multiplexer part of the receiver. Don't think bout using them for power supply rectification. Except maybe for extremely low

current supplies, for example a bias voltage. However the data sheet does not give any indication of allowed plate voltage or peak inverse voltage. These are most likely quite low. That still leaves two interesting triode sections in the bottle. As the prefix of the type designation indicates, the heater voltage s 6.3V and the heater consumes 0.75A. Like most of the Compactron tubes this type is largely ignored nowadays and if you find any they are probably very cheap since tube sellers want to get rid of them. Don't have too high expectations with regard to linearity as that was not necessarily of priority for the intended use of the tube. Here are the plate curves:

Note that the plate curves also extend into the grid current region. Here the curves taken form a sample with the tracer:

I would not use it for applications which require large voltage swings. But if an operating point in the linear region is chosen this would be well useable for small signal applications like phono stages.

Let's have a closer look at the tube. I only have some samples that are Westinghouse branded but probably made by General Electric.

Here we see the 6AG11 in all its glory:

The smaller plates with the holes are the triodes.

The heaters of the two triodes and two diodes are wired in series and those then in parallel. The white wires across the top make the series connection.

The base:

A beautiful tube. I like that chubby shape. Let's open one up.

Pulling the heaters out of the cathodes:

One of the diode sections:

The cathode:


The triode sections:

The grid is remarkably close to the cathode and precisely shaped to get the transconductance:

Gold plated grid wire:

The plate of one of the triodes:

A remarkable piece of engineering! Let's see how the 6AG11 lights up:

I hope you enjoyed the presentation of this tube. I plan to do a miniseries of more Compactron types in upcoming Tube of the Month posts. Stay tuned!

Best regards


Friday, July 7, 2023

The New Stereo Single Ended 300B Power Amplifier - Part 1


I am currently working on a new chassis style for the stereo 300B power amp.

So far I built the stereo 300B amp with external power supply. Either in two tower chassis or in the portrait style chassis.

The new version shares the same circuit and the same amount of transformers and chokes. Space is saved by the use of more compact capacitors.

But still without any electrolytics in the high voltage supply.

The first assembly steps are completed.

All signal relevant parts are mounted closely beneath the top plate.

The power supply and filter sections will be on separate modules.

The chassis is 305*305mm and 400mm high (plus feet and tubes)

Stay tuned for updates as assembly progresses.

Best regards