My amp came stock with 3 12Ax7 preamps. Is it ok to mix preamp types? For example, would it be safe to put like a 12AY7 or a 12AU7 and then leave the other two as 12AX7's? Also, would I have to bias it if I did this? I was told you generally only have to bias when changing power tubes.

Thanks for any help!
"Good and evil lay side by side as electric love penetrates the sky"
You can mix preamp types. Be sure you know what you're doing with regards to what type does what, though. Preamp tubes don't need rebiasing. It's only with fixed-bias amps that you have to do that.
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You need to get ahold of a tube substitution chart. Some tubes can be substituted for others. Preamp tubes, as has already been mentioned, do not need to be rebiased after changing. Power tubes will need to be biased. Fixed bias amps do not require any adjustment after changing the power tubes. FWIW, most folks experiment with different brands of the same type tube. For example, swapping Tung-Sol 12AX7 with Sovtek 12AX7 tubes.
Ok thanks guys. I looked up a tube substitution chart, looks like these tubes are in the same "family". I guess it should have been obvious from their similar names.
"Good and evil lay side by side as electric love penetrates the sky"