McIntosh Tube Pre-amp help

[med_designer]

Hello all -

I have a new issue with my C70 that I would appreciate some input on. This is my 2nd unit. The 1st C70 had a MC/MM preamp section that was DOA and did not work at all. It was completely replaced. I have been running the 2nd unit for several months with no issues until the other day. All of a sudden the MC stage has random noise that appears only in the right channel. It sounds like typical tube distortion of the hissy overdriven dirty variety. Sometimes it is very loud and other times it is clearly present but not overly loud.

My next steps:

1. Move both MM tubes to MC stage to see if it clears up problem. If not the problem could somewhere else but not guaranteed.
2. Move original left MC tube (now in left MM) with input driver tube and input driver tube back to left MC tube. If problem still exists then consider swapping output tube.

While I like tubes - they clearly do not like me. Have had nothing but trouble with them. The C70 sits on its shelf to do its job and the issue just starts. This would be the 2nd failure and another in a long line of issues with McIntosh equipment including a D1100 and MC152 (both issues new out of the box). While I like the gear it is becoming more trouble than it is worth and considering another route at this point. Not sure what that would be.

For those of you who are long time owners how unusual is this and how would you proceed. Thanks for the help.

[W9TR]

Usually if a tube fails in one of my units it happens in the first 50 - 100 hours then trouble free for 1000’s of hours after that.
Hope it’s just a tube.
Tom

[Beowulf62]

This sounds (pardon the pun) like tube noise and likely has nothing to do with the first issue with your C70. It is simpler to swap the left and right MC tubes. If the noise moves to the left channel, you know it is that MC tube. If the noise remains in the right channel, then swap the left and right MM tubes. If the noise moves to the left channel, you know it is that MM tube


Tubes can become noisy/microphonic with age. What brand are use using? I would recommend Gold Lion if you are using the original McIntosh set.