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Old 3rd October 2010, 07:10 PM   #11
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Default Here's A Schematic For A Heathkit 12w Amp Released In The USA

Quote:
Originally Posted by SGregory View Post
Good morning Simon. Good start to the day even though our weather here is muck like yours. Good day for DIY.
I was trying to find this amp at several Heathkit sites. Non listed it.
Hi again Scott

Your right these amps are almost impossible to track down on the net.
I have been trying for three days now and have almost got nowhere.

This has to be the hardest schematic I've ever tried to find.
Even totaly obscure recording gear is easier to find than these amps.

I have found this though.

With the 110 AC input it must be a Heathkit monoblock released in the USA.
It's not called the MA-12, but instead, the UA-1, it appears to be the same 12 watt amp as mine.

The pre and phase splitter tubes are labeled with what I think are American style tube numbers, and two EL84's there as well.

I imagine the circuit must be exactly the same, mine have feedback coming from the output transformer secondary (but 2 secondary taps, not 3).
Mine don't have a two way damping switch, but do have an octal power output for preamps just like this USA version does.

So, a 5-10 is a MA-12 is a UA-1 (just thought, does UA mean United states of America-1).

I haven't looked, but yes, I believe the Williamson had a very similar front end as the EF86 was the break-through low noise tube of the era.

The tube microphone I am making at the moment also has an EF86 in it but wired in triode mode not pentode.
This is what the German mic makers like, I think off the top of my head, Neumann did to gain even more performance from the tube (I think the classic U47 tube mic has this scheme).

The Mullard datasheet for the EF86 audio preamp tube, points out numerous advantages it has over previous offerings.

Get a few more thing to put up as well.

Simon...
Attached Images
File Type: gif Heathkit 117AC UA-1 12w Amp.gif (34.2 KB, 431 views)
File Type: gif usa.gif (12.5 KB, 470 views)
File Type: jpg The Heathkit Factory Benton Harbour USA 1950s.jpg (32.2 KB, 420 views)
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Old 3rd October 2010, 07:21 PM   #12
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Classic Mullard did have a boat load of GNFB.

Simon, The last circuit only has four tubes. THe input and splitter share the same V1. It is also a different splitter than the typical mullard.
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Last edited by SGregory; 3rd October 2010 at 07:25 PM.
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Old 3rd October 2010, 07:42 PM   #13
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Default The MA-12 Amp Has 40mv Input Level.

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Originally Posted by mashaffer View Post
What I never understood is why then went with a pentode front end (just cause they had a new one???). Since the PI has some gain and the EL84 doesn't need much drive voltage they must have either used a boat load of NFB or came up with a really sensitive amp it seems.
Hi mashaffer

Yes your right the Mullard and the Heathkit both have an input level of 40mv.

There was a phono preamp available for the Heathkit MA-12, which could be powered from the octal power supply outlet socket on the back of the monoblock.

Heathkit also made a matching radio tuner that had it's own AC power supply.
It could be switched through the preamp (or two preamps, with obviously seperate selector switch and volume controls), there's a thought you could play an old 78 on the left speaker while listening to the news on the right.

I think though, the main reason for the very high input sensitivity, was to allow the direct connection (in either mono or stereo) of a crystal type phono pickup to the system without the need for the preamp/s.
The monoblocks actualy have as standard, a volume dial built into the top of the chassis.

Some people didn't want or need the tuner and preamp, and even as kits, my guess is at the time they were released, they weren't exactly cheap, but they were affordable and 'upgradeable'.

All in all a very modular system, and perhaps even a bit ahead of there time.

Simon...

The pictures below show a generic Mullard 5-10, with and without volume and tone controls.
Attached Images
File Type: jpg 5-10-e.jpg (133.2 KB, 392 views)
File Type: jpg 5-10-l.jpg (108.7 KB, 380 views)
File Type: jpg 5-10-k.jpg (165.6 KB, 376 views)
File Type: jpg 5-10-a.jpg (158.5 KB, 88 views)
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Old 3rd October 2010, 07:45 PM   #14
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Quote:
Originally Posted by The Space Egg Corp View Post
I seem to remember when I switched them on a few years ago the bass sounded pretty chunky, so if I refurbish them and don't use them much, I could always fit an i-pod dock and give 'em to the kids for rap music !!!
I wouldn't, given the rarity of those amps.

Quote:
Originally Posted by SGregory View Post
Classic Mullard did have a boat load of GNFB.

The last circuit only has four tubes. THe input and splitter share the same V1. It is also a different splitter than the typical mullard.
It's a completely different circuit, but I bet someone in NA built a Mullard clone.

Jeff

Last edited by vinylkid58; 3rd October 2010 at 07:47 PM.
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Old 3rd October 2010, 07:45 PM   #15
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Quote:
Originally Posted by SGregory View Post
The last circuit only has four tubes.
The fifth tube is the rectifier......."5 Tubes"
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Old 3rd October 2010, 09:47 PM   #16
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I had the regulator included when I counted four. In the last schema the V1 is a triode/pentode combo tube which serves both as the input and a phase splitter duties
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Old 3rd October 2010, 10:43 PM   #17
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Default # of Tubes ?

Quote:
Originally Posted by SGregory View Post
I had the regulator included when I counted four. In the last schema the V1 is a triode/pentode combo tube which serves both as the input and a phase splitter duties
Hi Scott

I still make that 5 ?

Simon...
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Old 3rd October 2010, 11:11 PM   #18
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Default Finaly Found A Guy Using A Pair Of Refurbished Heathkit MA-12's !

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Originally Posted by SGregory View Post
Good morning Simon. Good day for DIY.
I was trying to find this amp at several Heathkit sites. Non listed it.
Finaly found a guy using a pair of refurbished Heathkit MA-12's !

I think that has to be the longest net search I've ever done, and I still haven't located a copy of the original Heathkit schematic.

There's zillions of Heathkit schematics out there on line but not this one so it seems.
I think I will send this guy an e-mail and see if he has the original schematic.

He seems to be adopting a very small kit, very large speakers approach.
The refurbished monoblocks have been 'stereoed' and are running into big 4 driver, full-range horn speakers with large panel speakers at each side.

Seems to have a few other pieces of hyper-minimal valve kit on his web site as well.

Simon...
Attached Images
File Type: jpg Heathkit MA-12 Modernised 1 (Large).jpg (79.2 KB, 148 views)
File Type: jpg Heathkit MA-12 Modernised 3 (Large).jpg (104.9 KB, 150 views)
File Type: jpg Heathkit MA-12 Modernised 2 (Large).jpg (98.8 KB, 196 views)
File Type: jpg Heathkit MA-12 Modernised 4(Large).jpg (88.8 KB, 208 views)
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Old 3rd October 2010, 11:30 PM   #19
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Here are a pair of Heathkit AA-61's. Same circuit as the UA-1, four tubes each.

jeff
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File Type: jpg P1070139.jpg (96.0 KB, 202 views)
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Old 4th October 2010, 12:10 AM   #20
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Default Cheers For That Vinylkid58

Quote:
Originally Posted by vinylkid58 View Post
Here are a pair of Heathkit AA-61's. Same circuit as the UA-1, four tubes each.

jeff
Hi Vinylkid

Thanks for the photo of the AA-61's.

I have come to the same conclusion as you I think.

There are loads of 'AA' series schematics on the web but I had ignored them as they obviously wern't an MA-12. But...

I think now the amps coded 'AA' something were all sold outside the UK, mostly in the USA, and maybe around the rest of the world.
There's the odd Brit comment about them in one or two forums here, but still no schematic.

Your picture shows an original chassis with no 'missing tubes'.
What I have learned seems to be that when people refurbish and maybe re-chassis the MA-12's, they either modify the EF86's wiring, as all that gain isn't needed these days, or they just get rid of it, leaving 3 tubes and a rectifier per channel.

Am just going to check on the code number of another 'USA ?' issue amp and see if the schematic for this looks right.

Thanks for your help VK

Simon...

Here's a picture of the Heathkit logo used in England.

Here's their factory, in England they were known as Heathkit Daystrom Ltd.
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File Type: png Heathkit Daystrom Logo.png (23.5 KB, 170 views)
File Type: jpg The Heathkit Factory Gloucester England 1968-69.jpg (53.2 KB, 77 views)
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