tubes...help!

[youandme]

do 12ax7's need to be a matched pair?

[stratzrus]

It depends, but on the whole, no.

[chervokas]

Usually not. Matched pairs are desireable in the output sections of push-pull amps because the more closely matched the more common noise, like hum, will be cancelled, among other benefits. With preamp circuits you rarely have a circumstance where there's any significant benefit to balancing between tubes. However, because 12ax7s are twin triodes--that is, effectively two triode tubes in a single bottle--there can be some benefit to making sure the triodes of a given 12ax7 are balanced, especially when the tube is being used in the phase inverter slot.

[Blue Strat]

Originally Posted by youandme

do 12ax7's need to be a matched pair?

Only in a stereo situation where you would want both channels to have equal volume at equal settings. Even in this situation the benefits are doubtful unless you have a VERY mismatched pair of tubes.

[cap47]

By using different tubes in different positions, you are able to shape your tone! Most times it is better to try different tubes! Long plate tubes tend to be best for the PI!