McIntosh MC 225 power amplifier appreciation

McIntosh MC 225 power amplifier appreciation

My friend bought a MC 225 recently from 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.

After replaced 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 value to 25K from 22K.

See picture

Every thing ok after replace R51 with 22K correct value.

The sound quality is excellent with "McIntosh style sound"
Personally I perfer MC225 rather than MC240 because it is more detail
and brisk musical feeling with punch at low frequency.

Thank you for looking !