Hi!
Can I Use some NOS 12at7 in the PI positions on my LXII ? I love it’s tone, just wanting to engage those power tubes sooner with the AT7, using SYN2 into it, would you also recommend running the SYN2 hotter into the LX2 or just keep SYN2 level at noon ?
Thanks!
Welcome to the forum Leo, and thanks for your question!
Changing the 12AX7s to ATs will not drive the tubes harder. The opposite will occur in fact.
You may be assuming that because the AT can drive more current, the tube grids will be driven harder. Unfiortunately there is no free lunch. Tubes don’t create voltage out of nowhere. They convert their applied DC voltage to an appropriate AC voltage for a given application.
While the AT can drive a lower impedance if provided sufficient supply voltage to do so, the AX still has more gain. Since the power tube grid circuit is a high impedance, relatively speaking, the AX can easily drive the power tubes into clipping. The AT, however has lower gain so it will hit it’s own clipping point before the power tubes do. The net result will be that you’ll overdrive the phase inverter while the power tubes will be starved for clean drive voltage. The result is that you will in fact “engage” them less.
I would keep the Syn 2 at noon and turn up the LXII volume.
Cheers,
Dave
Thanks a lot for the detailed explanation, Dave. I really appreciate you breaking it down. I honestly thought it would behave the opposite way… that using a 12AT7, with its lower gain, would have let the power tubes be more engaged. Your clarification makes sense now, and I’ll stick with the 12AX7s as designed.
Is there anything else worth exploring if my goal is to get the power section warmer and more engaged at more reasonable levels, so I’m not just relying on preamp overdrive?