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Bringing old units back with a Variac


I purchased a Tena 10 amp variac off of Ebay a few months ago. I bought this to gently apply AC voltage to some of my resto projects. Since buying the unit I purchased a Mc275 and a 10B which had been unused for a decade of more. First up was the 10B which I started at 20 volts and increased by 20 volts a day for six days. This worked well and the 10B is coming along. It had a lot of defective tubes and some other issues but the power supply is A-OK.
I started the same drill with the MC275. It was a virgin unit and I was the first one to crack the screws off the bottom cover. Everything inside was factory stock. After three days and making it to 60 volts I checked the output tube voltages and found the neg. bias voltage to be very high on one side. Upon further investigation I changed the 4- 12 mfd electrolytic and both elec. caps in the bias network. This cleaned up the voltage problem and the amp was stable up to 120 volts. I them checked all of the tubes and installed the original compliment of NOS value Telefunkens and 2 new RCA 12bh7as. For outputs I used a set of 6550C Svetlanas. The amp sounded OK but I felt it was not as good at a current reissue or other old Macs which have all new caps. I have ran the amp over the lest week on my tuner set and the 275's sound has improved. Since it did not blow up I put 4 as new Gold Lion KT88s in it. The 275 responded strongly to the GEC tubes, more so than the reissue 275. The point of all this is that this almost 40 year old amp is running at the correct voltage with no buzz or hiss. Bringing the old sets up on a variac seems to be a way to let the old caps reform and perform at spec.
Compared to a new 275 or an old one that is recapped this old stocker has slightly less deep bass and and a looser sound but does image and much to my surprise has a smooth yet detailed high end.
After working on these two pieces I am a believer in the use of variacs when bringing the antiques back to life. My 5 piece Marantz Model One stereo pre amp is now going through the drill. I am hoping for my third success.
Ron-C


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Topic - Bringing old units back with a Variac - Ron-C 21:38:12 11/26/02 (23)


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