McIntosh MC-75 in unknown condition, what do I do?

That is a valuable and highly prized collectable that you rescued!

The value is highly sensitive to cosmetic condition, however. A stereo pair of these amps with near-mint cosmetics and excellent chrome would easily command several thousand dollars, and possibly much more. However, poor chrome reduces the value drastically. And one amp is worth considerably less than one-half the value of a pair, for obvious reasons which you alluded to.

Obviously finding another one, especially at a reasonable price, will be difficult and require patience. All I can suggest is watching the offerings here, at eBay, etc.

You are correct to be cautious about powering it up. Vintage equipment, especially equipment which might not have been previously powered up in a long time, should have its ac power brought up slowly, over a period of several hours or more, by means of a variac or other variable ac power supply. That will allow the electrolytic capacitors to "reform," and will allow overheating or other problems to be detected before more serious secondary damage occurs.

Here is a little bit of info on it. A little Googling will turn up much more.

http://www.roger-russell.com/amplif1.htm#mc75

Best of luck with the great find!

-- Al

Also, I'd suggest not operating it without a speaker load. That might result in higher than normal internal operating voltages, which could overstress capacitors or other internal components. I use a $10 or so Radio Shack speaker driver when I do initial test and troubleshooting of older amplifiers.

Regards,
-- Al

Almarg,

Thanks for the info. Is there a reliable way to check tubes or is this something that really requires a tube checker? The tubes appear to be originals. I suspect the lab bought the amp for some long forgotten experiment then allowed it to sit for decades. The tubes are 3 Telefunkens, 1 McIntosh by RCA and 2 Genelex KT-88s.

The amp has a reasonable amount of surface rust. Nothing that keeps you from reading the labels but far from clean. Any suggestions for cleaning the chrome?

Oddly enough I have a Radio Shack driver from the same pile! It's been modified such that the air displaced by the cone is pumped into a 1" pipe. I'm not at all sure if the amp and the driver were ever used together.

Nikki, with all due respect to Almarg, you don't want to operate it AT ALL! The electrolytic caps are probably shot, and even if they aren't, they have to be brought up to power slowly with a Variac. Don't even worry about the tubes at this point!

The best person to put this little gem back in shape for you is Terry DeWick in Knoxville TN.
http://www.mcintoshaudio.com/dewick_repairs.htm
Terry is a McIntosh wizard and does most of the important McIntosh repair and/or restoration for owners all over the country (including me!)

As for another one, singles often come up here on Agon, on eBay, and you can ask Terry as well. He can give you tips on cleaning the chrome and other info, so don't hesitate to call him. Also, if your amp has a pair of ORIGINAL KT-88's (made in the UK) and they have good getters (still fairly silvery on the top and both sides of the tube) those alone are worth $200 to $500 a pair!

Definitely a keeper ;-)
.