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RE: Which version of the M60 do you have? It makes a difference.

I don't own an M60, but just from the 2 photos and from what Jeff said, I would guess that the middle rear tube of 3 is a CCS (Constant Current Source), solid state in a 3-tube 3.3 and tube-based in the 4-tube 3.1 version. The CCS does not have much effect on sonics per se, but the implementation can have an effect, which is to say that there is no need to use a boutique 6SN7 as a CCS. Two of the remaining three tubes, in a 4-tube input/driver stage, must therefore be in the dual-differential cascode topology. That's what does the work of voltage gain that drives the whole amplifier. Essentially, that's what you are hearing; the output tubes add little to no gain; they serve mainly to convert voltage to current. The remaining tube is a cathode follower, one half of each twin triode drives one half of the total of 8 output tubes. The CF takes the output from the cascode and reduces the net output impedance of the signal from the cascode so as to drive the output tubes. Both the dual-differential cascode AND the Cathode follower benefit from a CCS, so I don't know which one has a solid state CCS or if both do in the 3.3 version.

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