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Tuesday, February 18, 2020

Tube of the Month : The 12AY7


This month's tube is a miniature (noval) twin triode for low level applications, the 12AY7.

I have stashed away some 12AY7 many years ago and used it in one of my very early phono stage builds. But later stuck with my other favourite tubes and these collected dust in the drawer. But since it is an interesting tube it deserves it's own post.

As mentioned above the 12AY7 was specifically developed for low noise performance in applications working at low levels, which makes it an interesting candidate for the first stage in phono or microphone preamplifiers. The General Electric data sheet lists an operating point specifically for such low level applications. The tube has a 9 pin noval base. As the prefix '12' indicates the heater voltage is 12.6V and only needs 150mA. The heater has a center tap which is connected to pin 9. This enables parallel connection of the heater halves which results in a 6.3V/300mA heater. The 12AY7 has an amplification factor of 44 which comes with a plate resistance of 25kOhm. Here the plate curve diagram from the data sheet:

And actual curves taken from a sample:

Let's start with a 12AY7 made by RCA:

Some views from different angles:

Next we have a Tung-Sol:

And lastly Sylvania:

The RCA 12AY7 shows a nice glow in operation.

Best regards