Using a 12au7 in place of a 12ax7 in a phono pre-amp??

[philloamps]

Hi,

The 12AU7 biases differently and if installed in a normal 12AX7 circuit, will pull considerably more plate current. I have seen guitar players do this swap in Fenders only to have the amp fail because of blown 100K plate feed resistors.

Current flow through a 12AX7 or 12AU7 is dependent on the value of plate and cathode resistors. Since most any voltage amplifier is self biased with just a grid resistor there generally are not any bias issues. So if you have a 12AX7 circuit with a 100K plate resistor and a 2K cathode resistor the amount of current going through the tube is limited by those resistors. Popping a 12AU7 into the circuit should not make much difference, if any, in current flow because you still have the same circuits values. Even if the tube were biased full conduction the most current that could flow with 300 volts B+ and 100K plate resistor would be 3MA, well below the rating of a 12AU7. On the other hand, 3MA through a 100K resistor will dissipate almost 1 watt so a 1/2 watt resistor would probably fail. So I guess one might conclude the 12AU7 will not be damaged but there is a chance of circuit damage. If you have a volt meter you can measure the voltage across the plate resistor(s) with a 12AU7 in the circuit by powering up the amp just long enough to get the voltage reading(s) then shut it down. Now divide that voltage reading by the value of the plate resistor to find the current flow through the resistor E / R = I. Then you square the current and times that by the resistor value to find the watts being dissipated (I X I) X R = watts.