• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Laney Cub 12 tube swap


I have a Laney Cub 12 that I have already moded with a Tayden Retro 55 speaker (much better, louder and later breake up). I also plan to place a switch to take the two opamps on the FX loop out of the circuit when the loop is not in use (see attached schematics).
I see many people saying that one should try to swap tubes in both V1 and V3 (Phase Inverter) to tune the gain and tone of the amp. As I don't want to do this without making some calculation first, I've searched for circuit calculations and formulas but without enough success.
The pre-amp is a high gain stage with gain control between the two triodes of the 1st valve, a second stage that looks like a plate follower and, after the tone stack the phase inverter stage, that is a long-tailed pair with feedback. All pre-amp tubes are 12AX7 (ECC83).
I've understood that the stages gain depend on both the tube amplification factor and internal anode resistance, so changing any 12AX7 with a 12AT7 or 12AY7 will lead to changes in gain not directly proportional to the mu of the tubes.
Can anyone help with this calculations of the impact of tube swap?
I don't have skills to simulate all the pre-amp circuit on Spice.


  • laney_cub-12_2010_service manual.pdf
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