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RE: Replace 12ax7 with 12at7

Posted by Jim McShane on March 31, 2012 at 11:49:48:
We've all been through this before - a change to a 12AT7 doesn't change the gain much.

While indeed the 12AT7 has 40% or so lower mu than the 12AX7, it also has much lower plate resistance; at 250 volts the 12AX7 has a plate resistance of about 62.5K while the 12AT7 is about 10.9K at the same voltage. This means that much more of the voltage drop will occur across the plate load resistor instead of the plate. And that means more gain from a common cathode stage. With a fully bypassed cathode resistor and a 100K plate load resistor the gain of a 12AX7 is about 61.5 and the gain of a 12AT7 is about 54. So there is not very much difference, only about 13% less with a 12AT7 - all else being equal.

Add in the gain leveling effect of NFB - either local or loop - and it's going to be difficult to find much gain difference in most cases.

The largest issue you face in a swap is that the operating point of the 12AT7 and the 12AX7 are significantly different. Generally I find the 12AT7 likes to run about 3X the plate current of a 12AX7.

So swapping the two really has little effect on gain, but some effect on tone. Some like the tone after a swap, some don't.

To get any significant gain drop you need to use a 12AY7/6072 which has 1/3 the plate resistance of a 12AX7, but it also has only about 40-44% of the mu. Using the same operating conditions as above, putting a 12AY7 in the circuit would yield a gain of about 34.4, roughly half of a 12AX7.