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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Laney Cub 12 tube swap

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Hi,


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.
 

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  • laney_cub-12_2010_service manual.pdf
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