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Old 17th Jan 2022, 5:41 pm   #1
Geoff Sherratt
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Default Heathkit MA-12 and UMC-1

I've had this for a long time. Rescued from a jumble sale, well over 20 years ago. Unfortunately I only have the one, presumably it pre-dates the stereo era?
I'm assuming that a pair of the MA-12's might work well together? Any comments appreciated.
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Old 17th Jan 2022, 6:05 pm   #2
G6ONEDave
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Default Re: Heathkit MA-12 and UMC-1

No reason why you can't use two amps to get stereo but you may have to use two pre-amps as well unless the UMC-1 is stereo output. Otherwise you would need a stereo pre-amp. BTW looks like you will need a replacement rectifier as the one in the picture looks like it's gone to air.
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Old 17th Jan 2022, 6:11 pm   #3
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Default Re: Heathkit MA-12 and UMC-1

No, not really, they were well into the ''Stereo Era''. There is a stereo control unit (USC-1?) too to use them as a pair.
Heathkit did stereo valve amps like the S33 and S99 at the same time.

I have a pair I use occasionally (though the pre-amp is a bit like the Leak control units, not as quiet as todays stuff).
I've tried them against a pair of Leak TL12+s and think the Heaths just have the edge, maybe due to their output transformer?

There is plenty on the web about them, another easy project, and they come up regularly on ebay if you want to make it a pair...

Alan

PS, you are not lucky with the GZ34 rectifiers...
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Old 17th Jan 2022, 6:17 pm   #4
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Default Re: Heathkit MA-12 and UMC-1

Quote:
Originally Posted by snowman_al View Post
PS, you are not lucky with the GZ34 rectifiers...
No. 🙄 That's the 3rd one, all on amps that have just been sitting in storage. I sold the other one!
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Old 17th Jan 2022, 6:40 pm   #5
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Default Re: Heathkit MA-12 and UMC-1

Why I wonder
Unless you have a 'stash' of GZ34s, I suggest you use a Russian 5C4M on the MA-12. (http://r-type.org/exhib/aaa1820.htm)
It is plenty man enough for the mono amp and can be found way cheaper than GZ34s. It look right too, that is what is on the pair in the pictures.
(Unfortunately it is just not meaty enough for reliable use on your Stereo 20...)
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Old 17th Jan 2022, 7:08 pm   #6
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Default Re: Heathkit MA-12 and UMC-1

Just extracted the chassis from the chipboard box! It looks to have had a couple of replacement caps at some time in
the past. Also the big 60/250uf is badly bulged and has oozed its contents!
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Old 17th Jan 2022, 7:29 pm   #7
Geoff Sherratt
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Default Re: Heathkit MA-12 and UMC-1

Quote:
Originally Posted by snowman_al View Post
Heathkit did stereo valve amps like the S33 and S99 at the same time.
...
There's a blast from the past, our home stereo system, when I was a child, was powered by an S3-3. Around 1963/4 if I recall correctly.
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Old 18th Jan 2022, 4:54 pm   #8
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Default Re: Heathkit MA-12 and UMC-1

Hi,

I’ve restored two pairs of these.

They are a nice amplifier, however, there is one thing I’ve found and they have a tendency to suffer LF instability/motorboating, which I cured by fitting a CR step network between the phase splitter and EL84’s.

If you search the forum, you should find a few posts about the MA12. The circuitry is not dissimilar to the Leak TL12+

They are quite sensitive and like the Leaks they work with just volume control on the input with modern sources.

As they are Heathkit, kits the documentation is decent, making them slightly more restoration friendly.

Terry
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