The valve and the switch are working, so any lack of added valve harmonics us down to the low signal level passing through it. It is mimicking the effect of a valve power amp stage, so it needs lots of volume to become noticeable.
No point in changing the valve. The 12AX7 spec is fairly tight so another manufacturer’s valve won’t perform significantly differently in terms of the breakup point. You’d need to change the surrounding circuit to do that. Another valve may have lower or higher noise, or sound slightly different once it starts to distort, but that point will always require about the same input signal level as the gain of the circuit is fixed. The other valve types that are often changed with/are pin compatible with the 12AX7, the 12AT7, the 12AU7 and the 12AY7 have less gain so will need to be driven even harder to distort.
Your amp’s power output is above the range of the attenuators I’m aware if, which normally go up to 100W, so if you want some light drive to your bass sound at low volumes, it’s easiest to get a cheap pedal that does subtle overdrive sounds.
Last edited by Simon Barden; 20-11-2020 at 10:33 PM.
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