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RE: Confusion at MX110 12AX7 tube results

Posted by Michael Samra on December 29, 2016 at 23:17:17:
I rebuild a lot of Mac amps and preamps.Whenever I get an MX110z that was modded by Mr Modaferri,I always put it back to the original later MX110z design.He raises the resistance values of the plate and cathode resistors and it moves it from the original operating points that produced low distortion and perfect linearity in the critical interim stages.The mod on the MR78 tuner he did was fine which Mac had updated in a service bulletin.
It sounds as tho you are lacking body to the music that the preamp had at one time and that usually indicates filter caps and coupling caps are getting tired.Your preamp may have the stock value components but take a look and see if any new caps were changed such as the can filter caps.If it is a CE on the filter cap with a little sticker,it was changed which it should be ok.
When the filter caps get old,the develop leakage and high ESR and that raises havoc. If you could pop the bottom off,you can see if they used decent coupling caps or some cheap metallized polyesters as some places do.Maybe shoot us a picture because I don't think tubes are going to correct the issue you are having to your satisfaction.