McIntosh MC 225 Appreciation with Pictures

mc225amp

My friend bought an MC 225 recently from the local market and listened the good sound quality. Later found that it sounded a bit less punch at low frequency, and all four 7591 output tubes are not hot enough even after few hour playing.

I measured the plate current for each output tube were all about 2 ma but the normal value is about 22 ma.

The B+ supply was only 340V while it should be 400V. The filter capacitors C26 & C27 at voltage doubler dried out and lost most capacitance from 150 uf to 2 uf.

See picture below:

After replacing them with good capacitors and the B+ supply was ok back to 410V. But the output tube plate current only increased to 5 ma, still far away from normal value.

Carefully measured the negative bias of output tubes found -23.5V but it should be -21V.

It was because bias supply dummy load resistor R51 changed the value to 25K from 22K.


See picture below:

Everything is ok after replacing R51 with 22K correct value.

The sound quality is excellent with “McIntosh style sound”. Personally, I prefer MC225 rather than MC240 because it is more detailed and brisk musical feeling with the punch at low frequency.

Thank you for visiting!

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