The 12ax7 is one of the most commonly used vacuum tubes in guitar amps and shows no sign of losing its popularity.
The 12ax7 is nearly 70 years old, having first been produced in 1947. In all that time it's never been out of production. Why? Because its design makes it hugely popular with guitar amp manufacturers, meaning its a key component in amps like 65Amps' Whiskey.
At the risk of getting way too technical, the 12ax7 is a miniature dual triode vacuum tube with high voltage gain. It was first developed in 1946 by RCA in New Jersey.
Designed for audio applications – at the time vacuum tubes were used in everything from television sets to record players – the 12ax7's design makes it the perfect component for a pre-amp.
Stick with us, we're going to get seriously technical now, with the help of Wikipedia. Here's what it says about the 12ax7. 'Typically a 12ax7 triode is configured with a high-value plate resistor, 100k ohms in most guitar amps and 220k ohms or more in high-fidelity equipment. Grid bias is most often provided by a cathode resistor. If the cathode resistor is unbypassed, negative feedback is introduced and each half of a 12ax7 provides a typical voltage gain of about 30; the amplification factor is basically twice the maximum stage gain, as the plate impedance must be matched. Thus half the voltage is across the tube at rest, half across the load resistor. The cathode resistor can be bypassed to reduce or eliminate AC negative feedback and thereby increase gain; maximum gain is about 60 times.'
Got it? Good. No? Don't worry, all you need to know is that if you need a replacement for a high-gain valve for a pre-amp, the 12ax7 is the one you need.
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