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McIntosh MC60 Circuit Differences / Restoration

H

Hi-FiGuy

AK Subscriber
Subscriber
I am recapping a pair of MC60 amps for a friend and the serial numbers are 3XXX and 7XXX there are lots of differences between these two amps. The board layout is completely different and the bias section is different as well. The first amp I replaced the selenium rectifier with silicon and changed the 1.8k resistor to a 5.1k and replaced all the capacitors and the amplifier works perfectly. The second amp (7XXX serial) has a completely different board layout and uses the M-171A Power transformer which has a bias winding, following this winding through the diode and the bias circuit, the resistor divider network in the amplifier is also different. I changed this diode from selenium to silicon and I wanted to see what value of resistor to use to bring this back to factory voltages. I was planning to clip in a pot between the winding and the diode to dial in the value and solder in an appropriate resistor. There is very little information online about this version of the amp. Another thing I noticed is that only half of the first 12AX7 is being used. If these amps are to be used as a pair, are they going to sound the same and perform equally?
 
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In doing some reading online, I learned a bit more about this amp. I changed the 12k 2W resistors as well as the 220 ohm 1W units, they are now matched to 1%. I variac'd up the amp and measured the bias voltage at -53v. I will use a resistor to get it down to -45v. I put in a signal and tested for noise and this amp matches the other channel. So now to listening tests and burn in.
 
Through some trial with a 5k pot, I have found the value of 1.7k is the sweet spot. I am going to use 1W part and a terminal strip to implement this mod.
 
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