ust want to share some experience with tube rolling in MC275 MKVI. My tube rolling in MC275 was limited to small tubes, mostly the two 12AX7 in V3/5 as phase inverter. The preamp I used is McIntosh MX110Z recently fully restored at Audio Classics. The speakers are Rogers Studio 1A. I first replaced the 2 stock 12AX7 with Amperex orange globe 12AX7/ECC83. The sound became more transparent and neutral (not cold or thin). I also tried Siemens E88CC in V3/5 and it sounded really good (more vivid and grainless). When I played Mendelsohn's violin concerto op.64 thru Spotify (I used dCS Debussy DAC), it was quite obvious to me that Siemens E88CC sounds much better than Amperex 12AX7. I am not sure if I can ever find a pair 12AX7 that would sound as good as Siemens E88CC. Maybe I can try some 5751.
This is the interesting part. I hear some people claim that E88CC(6922) and 12AX7 are not interchangeable simply because of the difference in filament heating config. The suggested voltage for E88CC filament heating is 6.3V between pin4 and 5 (pin 9 is connected to the shield), while for 12AX7, in series, a 12.6V is applied between pin 4 and 5 with pin9 floating, and in parallel 6.3V is applied between pin 4+5 (4 and 5 are shorted) and pin 9. I measured the voltage drop between pin 4 and 5 in MC275 for 12AX7 with multimeter and found it was ~12.5V. The service manual shows that two 12A*** tubes are on 24V DC in series. Pin 9 is floating. So why would E88C even work in MC275? Why didn't 12V filament voltage burn E88CC? Or maybe 12V filament voltage can still make E88CC work, but can significantly reduce its life?
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