A brief history of 12ax7 tubes

A brief history of 12ax7 tubes

The 12ax7 tube is arguably the most popular audio vacuum tube that has ever been created. It’s popularity is driven by use in many Hi-Fi tube amplifiers and especially guitar amplifiers, where it’s pushed really hard and burns out fairly quickly. Introduced in May of 1947, the tube gained almost immediate popularity due to its very high gain – mu: 100, so it amplifies voltage 100x! Although I’ve seen JJ and Sovtek 12ax7 tubes go as high as 115, but that’s not up to spec.

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Tube Matching 101

Tube Matching 101

There is a lot of confusion about matching tubes. It starts with some vendors, especially on eBay, matching tubes by the way they look (visual matching) to matching on multiple electrical parameters at the same time.

There are 4 parameters that could be used to match preamp tubes:

  1. Transconductance – Gm
  2. Voltage Gain – Mu
  3. Plate Current – Ip
  4. Tube Age
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