Matched Pairs of Tubes for BeePre2

[bombay317]

I’ve never seen it explicitly stated (that I recall). Do we need to have matched pairs of the 300B and the 6CM7s? I thought they are separate circuits and matched pairs is not explicitly required.

 

[Paul Joppa]

That is mostly correct. The caveat is that if the gains (mu) of the tubes are different the left/right balance will be off proportionally. This is not usually audible, given the vagaries of room acoustics, but it can be measured.

This is also true of any preamp that does not use feedback.

 

[bombay317]

I assumed that with different gains you would have slightly different levels in the L/R channels, but wanted to make sure it was not impacting functionality.

The question stems from tube rolling. Ive been looking around for certain 6CM7s, but I haven’t been finding a lot of matched pairs around. If I wanted to get some RCA 6CM7s I might could find a pair, or I just try and get some that are close enough in gain that I can’t tell that there is a difference.

 

[Paul Joppa]

The 6CM7 in BeePre is used as a cathode follower (essentially 25dB of feedback to control gain) and shunt regulator (under control of the 431 chip) so there is no need for matching.