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Bugera amps use the following tubes:

12AX7A is highest gain, not necessarily balanced. one side may be 110% while the other is 90%. Typically used in the first gain stage.

12AX7B is lower gain, not necessarily balanced, 90%/80% would be common. Typically used after tone stacks to bring signal back up.

12AX7C is balanced, could be high or lower gain, 95%/93% or 85%/87% typical.

These are used in the inverter so the positive and negative signals going to the power amp are equal.

Tube recommendations depend entirely on what type of tone you are trying to achieve. a wildly unbalanced 12AX7A in the inverter will cause asymetrical power tube distortion adding many odd harmonics, this is great for playing metal, not so great for everything else.