The HEATHKIT MA-12 a MULLARD 5-10 In Disguise. - Page 3 - diyAudio
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Old 4th October 2010, 01:57 AM   #21
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Quote:
Originally Posted by The Space Egg Corp View Post
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.
It would be interesting to see if the MA-12 schem. is an exact copy of the 5-10.

Quote:
Originally Posted by The Space Egg Corp View Post
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..
That would make sense. Easy to check your amps to see if the 12AX7's are wired the same as the 5-10.[/QUOTE]

jeff
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Old 4th October 2010, 02:39 AM   #22
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There seems to be some confusion in this thread about what constitutes a Mullard or a Williamson. Another schematic also appeared showing what seems to be a Dynaco circuit. While it doesn't really matter in the grand scheme of things what labels we put on these different topologies,such vagueness can lead to misinterpretation.

To clarify:

*The Mullard approach (pentode IP stage, unbalanced LTP triode splitter/driver and pentode/UL P-P OP tubes) is normally quite stable even using a great amount of GNFB but has excessively high loop gain, especially for easily driven EL84s. It is common practice to modify the 5-10 or 5-20 to improve a number of things, e.g. reducing loop gain, balancing the splitter with a CCS tail, using a lower Rp double triode as splitter/driver, applying fixed bias).

* The Dynaco approach (pentode IP stage, triode concertina splitter and pentode/UL P-P OP tubes), works perfectly well for easily driven EL84s but is a poor solution for driving bigger OP tubes. This is an extremely basic approach and its unsatisfactory performance with bigger OP tubes is the reason for the existence of many after-market improvement kits.

* The Williamson approach (triode IP stage, triode concertina splitter, twin triode differential driver and triode (or pentode UL) P-P OP tubes) overcomes the problem of a poor concertina driver in the Dynaco but icludes an extra stage to the GNFB loop, making stability a challenge for the unwary. I see a Williamson as overkill for EL84 OP tubes.
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Old 4th October 2010, 02:44 AM   #23
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Default Yes !

Quote:
Originally Posted by vinylkid58 View Post
It would be interesting to see if the MA-12 schem. is an exact copy of the 5-10.
That would make sense. Easy to check your amps to see if the 12AX7's are wired the same as the 5-10.
jeff[/QUOTE]

Hi Jeff

Yes the 'Heathkit Daystrom Ltd' MA-12 is quite simply a Mullard 5-10 clone, like a few other similars, such as the Leak 20 ?

I havent checked the wiring yet, but it just has to be.
It even uses the Mullard recomended mains and output transformers made in England by 'Hinchley'.

This is why there is a bit of talk on one or two of the English forums on how to rewire the EF86, or get rid of it.
I think the favoured way from what I can make out is to leave it in and wire it as a triode like the Germans did in microphones.
As far as I can tell no American Heathkits ever had an EF86 in them.

The MA-12 was produced some time between 1954 and 1962, as this is when Daystrom Ltd had an interest in the company, in 1962 Daystrom Ltd got into the oil buisness for some reason.

The guy I thought had refurbished MA-12's a few posts ago, actualy has a re-boxed AA-151 which looks in terms of parts and transformers, just like a MA-12, but is American made and is another variant of the GZ34 / EL84 scheme that ALL the 10/12 watt-ish amps had.

I didn't manage to find a schematic for the AA-151, but I found a couple of photos of what it would have looked like before re-casing, so I'll post those just to confuse everyone even more ! (just kidding Kid).

Cheers Simon...

PS - The MA-12 has the bigger GZ34 rectifier, where the original Mullard 5-10 had a smaller B9 type.
I asume Heathkit Daystrom Ltd just threw this in as their units had to power the optional preamp as well. Pheeww!
I think thats the story.
Attached Images
File Type: gif logo-1956.gif (2.8 KB, 428 views)
File Type: jpg aa-151a.jpg (8.0 KB, 376 views)
File Type: jpg aa-151.jpg (17.3 KB, 373 views)
File Type: gif logo-1957.gif (1.3 KB, 425 views)
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Old 4th October 2010, 03:30 AM   #24
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Default Hi there Ray Moth

Quote:
Originally Posted by ray_moth View Post
There seems to be some confusion in this thread about what constitutes a Mullard or a Williamson. Another schematic also appeared showing what seems to be a Dynaco circuit. While it doesn't really matter in the grand scheme of things what labels we put on these different topologies,such vagueness can lead to misinterpretation.
Hi Ray Moth

Yes, there was no confusion over Williamson and Mullard.

The main point of this investigation into the MA-20's was to try and establish whether the MA-20's had any connection to the American EL84 Heathkits some of which do resemble Dynaco type products.

It has been most surprising to me, not to have fairly easily found documentation on the net for the MA-12, perhaps the reason for this is that the Mullard 5-10 is so well documented there is no need for MA-12 details as they are in fact the same thing.

It is still surprising however that since a vast number were probably sold, that there isn't even documentation for the Heathkit Daystrom preamp which sold along side the MA-12 and which I in fact used to have a pair of as well.

Got there in the end.

Cheers Simon...
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Old 4th October 2010, 06:15 PM   #25
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Quote:
Originally Posted by The Space Egg Corp View Post
I didn't manage to find a schematic for the AA-151, but I found a couple of photos of what it would have looked like before re-casing, so I'll post those just to confuse everyone even more ! (just kidding Kid).
I'll email you a copy of the schem. Be prepared to be disappointed.

jeff
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Old 4th October 2010, 06:30 PM   #26
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There is a site with a good write up on the 5-10 that includes the chassis layout:

Mullard 5-10 Ten Watt Amplifier

There is an interesting Heathkit UA-1 Schematic on the Vintage Radio site:

http://www.vintage-radio.info/heathkit/

It uses only 4 tubes, but it looks like it might originally have had 5 and switched to a combined Pentode/Triode (6AN8) to reduce package count.

Last edited by TheGimp; 4th October 2010 at 06:45 PM. Reason: Ref UA-1
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Old 4th October 2010, 09:00 PM   #27
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Default Heathkit AA-151 Schematic Of Preceding Amp Photos

Quote:
Originally Posted by vinylkid58 View Post
I'll email you a copy of the schem. Be prepared to be disappointed.

jeff
Hi Jeff VK

Thanks for you interest and help with the quest again.

There's no way I will be disappointed to see the AA-151 Schematic.

The photos of the amp rebuild a few post's ago, clearly shows what one music lover has chosen to do with his old Heathkit EL84 amp.
It would be great Jeff if you posted it here with the title 'Heathkit AA-151 Schematic', it looks like it might be one of those slightly hard to find ones and it would be great if search engines picked up on it for others to see.

I'm going to post again soon with the question to members who might be reading this:-

"OK we know what it is now and we've gone through a bit of history along the way. But the question the very first post in the thread hints at is; We've got these amps here that need a bit of TLC, what are we going to do with them ?"

Cheers... Simon...
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Old 4th October 2010, 09:23 PM   #28
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Default Thanks Very Much The Gimp

Quote:
Originally Posted by TheGimp View Post
There is a site with a good write up on the 5-10 that includes the chassis layout:

Mullard 5-10 Ten Watt Amplifier

There is an interesting Heathkit UA-1 Schematic on the Vintage Radio site:

Heathkit Schematic Diagram Archive

It uses only 4 tubes, but it looks like it might originally have had 5 and switched to a combined Pentode/Triode (6AN8) to reduce package count.
Hi TG

Thanks very much for your links and your interest in the thread.

I have a feeling that this could be an amazing journey of history and discovery, deciding what to do with these amps and how to refurbish them.

It's great to be on the site where who knows what gems of knowledge and information members will come up with next.

There must be loads of members out there listening to loads of different EL84 amps with loads of stuff to say about them, not just in technical terms, but also how they sound, and how to get the best from this small but very good sounding Mullard valve.

It looks like the MA-12's I have are essentialy Mullard 5-10's as the thread suggests.

The question now is WHAT to do with them ?

My intention is not neccesarily to 'restore' them to exactly how they were.
But to 'refurbish' them into something that will be of maximum benefit to me today.
ie. if I don't come up with a plan that results in having a pair of amps that I want to and indeed do listen to then the plan will have failed.

I do think faithfull restorations are totaly valid if that is the intention of the builder, but I'm not nostalgic for these amps or running a museum, but would love to end up with some thing I'd be happy to listen with for many years.

Any further EL84 rebuilding info and listening opinions from you or other members would be most welcome.

Thanks again for your interest TG.

Cheers...Simon...

Last edited by The Space Egg Corp; 4th October 2010 at 09:27 PM.
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Old 5th October 2010, 12:56 AM   #29
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Default Hinchley 51/507 Output Transformers.

Hello everyone.

I was hoping there maybe someone out there that has, or can help me find any information about the Hinchley 51/507 output transformer.

These are used on my Heathkit MA-12 (Mullard 5-10) monoblocks after 2x EL84 output tubes.

The secondary taps are for 3 and 15 Ohm's, the primary taps are of unknown value.

The possible power handling of the transformers is also unknown.

Cheers... Simon...
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Old 5th October 2010, 01:32 AM   #30
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Default Heathkit AA-151 Schematic

Quote:
Originally Posted by The Space Egg Corp View Post
It would be great Jeff if you posted it here with the title 'Heathkit AA-151 Schematic', it looks like it might be one of those slightly hard to find ones and it would be great if search engines picked up on it for others to see.
Ok, I hope you guys can read these.

jeff
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File Type: jpg Heathkit aa151-2.jpg (319.9 KB, 353 views)
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