My advice is read Dave Gillespie's posts on restoring McIntosh tube amps. And talk to him directly about your MC75s. Definitely my favorite Mac amps.
Refurbishing two MC75 Monoblock Amplifiers
- Thread starter Rodeodave
- Start date
Bill Ferris
Lunatic Member
This forum`s corrupted software for some crazy reason duplicated my post from below here, so I deleted it !
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Bill Ferris
Lunatic Member
FWIW, in my case, although the Mc. tube amps in question were/are very early model Mc. 30s (A-116 labeled) bought used in 1974, and were off set by a few yrs. from each other (I`m guessing from the original owner transitioning from mono to stereo) from the mid~ late fifties, so I chose to rebuild both some 10 + or so yrs. back and chose to update the older amp`s circuit`s minor differences configuration to the later to help assure mirrored electronic circuitry voltages and SQ performance between the 2.
And I also changed over the factory default power transformer`s primary input tap to the 125/6 volt one to account for higher modern AC mains ~120 v compared to the lower 110~117 mains voltage era at that time of the amp`s manufacturing time frame, and would encourage this be checked and done if not already converted, as my 2 different MC-75`s schematics show that input voltage conversion available, though the schematics indicate the higher input voltage tap is factory default connected.
And my (2) A-116 (MC-30) sound as identical as my 70 yrs. old ears can tell, and both exceeded all factory specs in amplification gain, THD+N and IMD as measured on my benches test equipment before placing in service.
And I also changed over the factory default power transformer`s primary input tap to the 125/6 volt one to account for higher modern AC mains ~120 v compared to the lower 110~117 mains voltage era at that time of the amp`s manufacturing time frame, and would encourage this be checked and done if not already converted, as my 2 different MC-75`s schematics show that input voltage conversion available, though the schematics indicate the higher input voltage tap is factory default connected.
And my (2) A-116 (MC-30) sound as identical as my 70 yrs. old ears can tell, and both exceeded all factory specs in amplification gain, THD+N and IMD as measured on my benches test equipment before placing in service.
They should have the same P/N transformers, and you will likely match the tubes and components when you rebuild,. I’d rebuild both to the later amp’s schematic for a stereo pair.
Bill — Do your A -116 amplifiers use the split load phase inverters that predate the long tailed pair inverter that Mac ultimately adopted?
Based on the schematic voltages I’ve seen for this version of the MC-30, the output stage runs more closely to Class AB1. Have you measured the output stage quiescent current draw to know if this is true?
Dave
Based on the schematic voltages I’ve seen for this version of the MC-30, the output stage runs more closely to Class AB1. Have you measured the output stage quiescent current draw to know if this is true?
Dave
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Thanks for your replies. Sorry for the late response; I was doing battle with Windows 11 on three new computers (including, and especially, my wife's who wanted the result to be exactly like her old Win 10...).
Mr. Gillespie has a great many fascinating posts; I have been enjoying them.
I will be rebuilding the older Mac according to the newer circuit, as recommended, although I might try the original first just to see how differently they sound.
The other delay, I rebuilt the newer Mac using all Vishay-Dale metal film resistors, Orange Drops for coupling, etc. Sprague electrolytics, CE/Athenticaps in PSU. Now, after very thorough checking, bringing up on a Variac, adjusting bias (replaced selenium with 1n4007 + resistor) an annoying hum was noted on my test speaker (a venerable Wharfdale W60). Music passed through and sounded otherwise not too bad. Additionally my scope revealed what I took to be a parasitic oscillation at 1.25 MEGAHertz! This was driving me nuts. As I was carefully mapping the strength of this signal, I eventually found that the relatively new Electro-Harmonix 12BH7 was the source. Replacement with two others resulted in silence on no parasites. Has anyone else had a similar problem?
Well, on to the older Mac!
Mr. Gillespie has a great many fascinating posts; I have been enjoying them.
I will be rebuilding the older Mac according to the newer circuit, as recommended, although I might try the original first just to see how differently they sound.
The other delay, I rebuilt the newer Mac using all Vishay-Dale metal film resistors, Orange Drops for coupling, etc. Sprague electrolytics, CE/Athenticaps in PSU. Now, after very thorough checking, bringing up on a Variac, adjusting bias (replaced selenium with 1n4007 + resistor) an annoying hum was noted on my test speaker (a venerable Wharfdale W60). Music passed through and sounded otherwise not too bad. Additionally my scope revealed what I took to be a parasitic oscillation at 1.25 MEGAHertz! This was driving me nuts. As I was carefully mapping the strength of this signal, I eventually found that the relatively new Electro-Harmonix 12BH7 was the source. Replacement with two others resulted in silence on no parasites. Has anyone else had a similar problem?
Well, on to the older Mac!