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 Post subject: Don McGohan MG-10B Replacement Power Transformer
PostPosted: Aug Wed 07, 2019 3:51 am 
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Joined: Dec Tue 23, 2014 4:12 am
Posts: 51
Location: Belleville, Ar 72824
I picked this up at an auction so I don't know much about it.

I started looking it over and got a copy of the Sam's Photofact (Set 190 folder 8).

Someone in the past has replaced the power transformer. The power transformer currently installed is rated for 550v CT. It is a Triad R-12A. The parts list calls for a 650v CT transformer. The amplifier is designed for 6V6 output tubes.

My question is what will the 100 volt difference make in the output and sound of the amplifier? Should I track down a better power transformer?

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 Post subject: Re: Don McGohan MG-10B Replacement Power Transformer
PostPosted: Aug Wed 07, 2019 2:18 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 5296
Location: Rochester NY USA
Really 50V difference, 275-0-275 vs 325-0-325. If it has a tube rectifier, you can get a good part of that back by replacing it with a pair of diodes (1N4007 will do).

Maximum undistorted output will be less with lower voltage. It's possible that some resistors were changed to make up some of the difference.

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 Post subject: Re: Don McGohan MG-10B Replacement Power Transformer
PostPosted: Aug Mon 12, 2019 1:33 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 10932
Location: Latham NY
If it uses a 5v tube rectifier you can replace it with a 5AR4 and gain some voltage, the wiring may need some changes depending on what the original is.


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 Post subject: Re: Don McGohan MG-10B Replacement Power Transformer
PostPosted: Aug Tue 13, 2019 5:14 pm 
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Joined: Dec Tue 23, 2014 4:12 am
Posts: 51
Location: Belleville, Ar 72824
Thanks for the advice! I have just finished replacing all the electrolytic capacitors. The amp is nice and quiet (minimal hum). I did some voltage checks coming out of the rectifier tube. I substituted a 5R4 (it is all I had). I had about 320 B+ before the first filter capacitor.

Both the 6V6 tubes as well as the 6SC7 are weak. Once I replace those three tubes I will check the sound quality and figure out a plan on voltage.

Thanks again for the advice!

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 Post subject: Re: Don McGohan MG-10B Replacement Power Transformer
PostPosted: Aug Mon 19, 2019 9:07 pm 
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Joined: Nov Fri 10, 2006 12:24 am
Posts: 2414
Location: Thornhill, Ontario, Canada
My guess is that you will be fine with the PT "as is" and new 6V6's. Most listening is at less than 1/2 watt average, but, of course, decent "power headroom" is good to have.
5AR4's are getting pricey, but it would get you a few extra B+ volts, as Lou said. They are "plug and play" with virtually all the common 5 v rectifiers, but not the 5Y4.
Cheers.
Roger

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