I notice with concern, the MP-1 now uses only two 6SN7's per side instead of five. Hopefully we're not losing any of the good things valves bring to the table. Besides lower noise, what's your thinking Ralph? Cheaper to re-tube? Just kidding! Hey, what about tube rolling? Have all my NOS jewels suddenly become worthless?Ha! Actually the MP-1 uses three 6SN7s per channel. In the Mk3.3 version, prior to its release we had spent two years working on a Constant Current Source (CCS) circuit that could outperform the one in the earlier preamps (and amps). In the amps the old CCS drifted about 17 parts per million over an AC input voltage range from 100V to 126V. This in turn assured that the voltage amplifier performance stayed the same over the same range. In the preamp the same circuit was used but the preamp has severely regulated supplies so it was a bit more immune. At any rate, the new CCS is performing at about an order of magnitude greater performance so we were able to improve the differential effect in the voltage amplifiers of the amps and preamps; in the case of the MP-1 this meant that two 6SN7s got eliminated in each channel as they were CCS circuits. But the actual signal path tubes are the same.
To remove the bulb on one of our amps: unscrew the lens (lenses of other colors and texture are available). The bulb is an old-school bayonet mount and so you push on it and twist it 90 degrees counterclockwise and it pops out of the holder. To reinstall is the reverse process. The stock bulb is a type 47 which is a 6V lamp (we run it at 4.5V for greatly extended life and to dim it a bit); a 12V lamp can be installed.