Repair of a McIntosh MC225 power amp

[freddymac406]

Hi all,
I am trying to bring a MC225 back to life for a freind.
At this point I have replaced the film coupling caps in the audio path and have checked the low level tubes and have a new replacement set of 7591s.
When brought up on the variac both channels of the amp start to ocsilate at a freq. between 500 to 2000 Hertz. I can see that it will pass an audio signal on top of this.
When the 7591 output tubes are removed the amp is stable to this point and it passes a small amount of audio from the 12BH7 as it gets its b+ from the outside primary of the output transformer. Also if the input 12ax7 or the 12au7phase splitter are removed it still has this problem.
Never worked on a unity gain transformer output confiquration and dont know what type of problems you can see.
All the voltages look to be correct.
If anyone can guide me in the direction of errorI would be most Greatful.

Thanks Fred

[Eli Duttman]

Fred,

The "old timers" took liberties with 7591 grid leak resistor values. Spec. is 300 KOhms max. They got away with larger values when using the 7591s of the day, but the current production stuff meets spec. and no more (grid leakage current). You may have to change the 7591 grid leak resistor value and the splitter to "final" coupling cap. value.

Jim McShane has addressed this issue on more than 1 occasion.

[kevinkr]

Check for audio on the dc rails.. This could be feedback due to a bad power supply cap. If you find this is the case you can temporarily parallel a suspected bad section with a known good cap of comparable value to help track down the offending cap.

**Don't do this with the power on!**

I don't have a schematic in front of me (buried deep in the garage) but if this amp shares the bootstrapped cathode follower configuration of other mac amps the follower drives the grids of the output tubes directly and the usual grid bias resistor issues shouldn't exist.

[freddymac406]

Thank you for your replies.
Kevin you were correct about the power supply caps.
I always thought it was pretty amazing how balls the 7591 tube has.
Fred

[kevinkr]

The MC225 is widely regarded as one best sounding mac amps, and the 7591 was also used in a lot of good sounding amps by others.