• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

The Heathkit MA-12 - A Mullard 5-10 In Disguise

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Hello vintage EL84 tube lovers out there.

Just woke up the spiders and dusted off a pair of Heathkit MA-12 monoblocks that have been resting (and rusting) in my garage for the last 20 years !

They were saved from extinction at the dawn of the digital decades; when a bunch of guys I used to work with who listened to them in their lab, moved out and left them behind (if your reading this guys, too bad, your not getting them back now !).

Having lots of other diyAudio things to do, they had been sadly neglected, until now that is.

I'm sure there are loads of tube-head members out there who know loads more than I do about this stalwart little amp, and it would be great to collect this knowledge under one roof for all the Heathkit MA-12 / Mullard 5-10 listeners out there (and the many other guises this classic original design appeared as).

It seems the design was originaly first published in the UK magazine 'Practical Wireless' of October 1954; and was pened at the Mullard research labs by their white-coat, back-room boys, to promote the wider sale of the company's audio valves.

The incredibly catchy name it was given, 5-10, says it all, 5 valves - 10 watts.
I think they could have called it 'The Mullard Minor' after the Morris car of the same name.
Just as the Morris Minor was intended to bring good, reliable and affordable driving to more folk; the 5-10's success depended on offering good reliable and affordable 'high-fidelity' to more folk, which it succeded in doing very well.

I can imagine one of these little babys hooked up to my grandad's 1930's high efficiency horn speaker, would have had the neighbours complaining about his new high-fidelity system in no time at all !

The downside for manufacturers in producing such a good quality and reliable design, in an era when the word 'up-grade' was just a dream in the marketing boys minds, was that they never needed replacing. I guess Mullard succeded in uping their valve sales quite a bit though.

Never mind, along came 'stereo' so all the happy 5-10 owners already deafening their neighbours with a 30's horn speaker and mono amp; rushed out and got a second one getting twice the power for their new 'stereo high-fidelity' system.

The two Heathkit MA-12's I have, were clearly made some years apart.
Although the chassis are identical, slight differences in components and the AC transformers are in evidence, and the diyer who made them clearly 'up-graded' from mono to stereo.

I also saw a photo on the net of another pair of MA-12's, one chassis chipped and rusty, the other shiny and new which suggests the same.

Here's a picture of a generic 5-10 monoblock.
The front cover of the 1952 Mullard catalogue.
An early Heathkit by Daystrom logo.
& Heathkit MA-12 brochure.

More to come shortly.

Cheers for now. Simon.
 

Attachments

  • A Generic 5-10.jpg
    A Generic 5-10.jpg
    73.1 KB · Views: 1,195
  • Mullard 1951-52 Catalogue.jpg
    Mullard 1951-52 Catalogue.jpg
    11.5 KB · Views: 1,128
  • Heathkit By Daystrom Logo.jpg
    Heathkit By Daystrom Logo.jpg
    14.2 KB · Views: 1,119
  • Heathkit MA-12 Brochure.jpg
    Heathkit MA-12 Brochure.jpg
    109.4 KB · Views: 1,153
Last edited by a moderator:
Mullard Back-Room Boffins Create A Legend In Their Own Lunch-Time.

Hello again EL84 lovers & fans of the tiny tube amp.

Here's the original Mullard 5-10 article which appeared in the UK magazine 'Practical Wireless' October 1954 edition.

The Heathkit MA-12 is based on the classic Mullard 5-10 design.

Simon.
 

Attachments

  • October 1954 Page 1.gif
    October 1954 Page 1.gif
    49.4 KB · Views: 1,127
  • October 1954 Page 2.gif
    October 1954 Page 2.gif
    53.1 KB · Views: 451
  • October 1954 Page 3.gif
    October 1954 Page 3.gif
    61.9 KB · Views: 253
  • October 1954 Page 4.gif
    October 1954 Page 4.gif
    49.6 KB · Views: 240
  • Mullard Pocket Data Book Autumn 1956.jpg
    Mullard Pocket Data Book Autumn 1956.jpg
    11.1 KB · Views: 198
The GZ34 Rectifier Valve Used In The MA-12 Monoblocks.

Heathkit opted to use the GZ34 rectifier valve in the MA-12 monoblocks.

A change from the original Mullard 5-10 design.

Here is the GZ34 rectifier valve, it's pin outs, a power supply schematic and the Philips datasheets from 1954 & 1958.

The Mullard GZ34 rectifier valve with it's 'double halo' getter is thought to sound particularly good.

Simon.
 

Attachments

  • Mullard Wide Base GZ34 Rectifier Valve.jpg
    Mullard Wide Base GZ34 Rectifier Valve.jpg
    60.8 KB · Views: 94
  • GZ34 Rectifier Valve Pin-Outs.gif
    GZ34 Rectifier Valve Pin-Outs.gif
    7.5 KB · Views: 55
  • GZ34 Rectifier Valve Schematic.png
    GZ34 Rectifier Valve Schematic.png
    27.6 KB · Views: 79
  • GZ34 Rectifier Valve Fired-Up.jpg
    GZ34 Rectifier Valve Fired-Up.jpg
    36.1 KB · Views: 98
  • Philips GZ34 Rectifier Valve Datasheets 1954-58.zip
    313.4 KB · Views: 31
How Many Tubes ? The 5-10 Has 5 Tubes & 10 Watts.

Simon,
How many tubes on this thing including the Phono Stage? Does it look something like this?: Heathkit Virtual Museum | A-9C

Hi Scott, How are you today ?

Yes the Mullard 5-10 has 5 tubes & 10 watts, and so does the Heathkit MA-12.

The only difference is the Heathkit MA-12 uses the larger GZ34 rectifier tube.

The name suggests a slightly higher output power of 12 watts also (MA-Music Amplifier-12-watts).

There is no phono stage on them, they are monoblocks.

The total complement of tubes is:- GZ34, EF86, ECC83, 2x EL84.
The EF86 was Mullards new 'super low noise' preamp tube, a replacement for an earlier design.
The ECC83 being used as a phase splitter, before the EL84 output tubes.

Simon.
 
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. There was a couple of comparisons to a williamson style amp, hence the link.

The tube compliment would work for either a Williamson or Mullard. May have to back engineer the phase splitter to determine what type it is.
 
Photograph Of A Heathkit MA-12 Monoblock Pair.

Simon,
How many tubes on this thing? Does it look something like this?

Hi Scott

I should have posted some photos of the monoblocks, but haven't taken any yet.

I shall take some decent photos of them tomorrow, and pop the hoods for an inside look at their guts.

Don't think that'll impress you much though, having seen your neat and tidy construction. Whoever made them got the job done but it 'aint pretty.

In the meantime here's a photo of a pair of MA-12's from the net.
They come in any colour you like as long as it's industrial battleship grey.

See what I mean about slightly different pairs like mine. Ones got a bad case of iron oxide outbreak, and the other looks shiney and new.
When stereo came along I figure folk just made another one to add to their existing mono setup. No built in obsolesence there.

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 !!!:idea:

Cheers Simon
 

Attachments

  • Mullard 1926 Catalogue.jpg
    Mullard 1926 Catalogue.jpg
    41.7 KB · Views: 112
  • Heathkit MA-20 Monoblocks.jpg
    Heathkit MA-20 Monoblocks.jpg
    119.6 KB · Views: 379
  • The Heathkit Logo 59.png
    The Heathkit Logo 59.png
    7.9 KB · Views: 354
Simon,
I should have looked at your first post last picture. It shows that it is a Heathkit. The additional pictures are of great help.

My reason of questioning whether it was a Mullard or Williamson was related to some of my initial digging when I was deciding on what to build. Somewhere, I have in one of my computers a schema for a williamson with ef86 input 12ax7 phase splitter. Hence my line of questioning.
 
Here's A Schematic For A Heathkit 12w Amp Released In The USA

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...:D
 

Attachments

  • Heathkit 117AC UA-1 12w Amp.gif
    Heathkit 117AC UA-1 12w Amp.gif
    34.2 KB · Views: 576
  • usa.gif
    usa.gif
    12.5 KB · Views: 612
  • The Heathkit Factory Benton Harbour USA 1950s.jpg
    The Heathkit Factory Benton Harbour USA 1950s.jpg
    32.2 KB · Views: 560
The MA-12 Amp Has 40mv Input Level.

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.
 

Attachments

  • 5-10-e.jpg
    5-10-e.jpg
    133.2 KB · Views: 523
  • 5-10-l.jpg
    5-10-l.jpg
    108.7 KB · Views: 511
  • 5-10-k.jpg
    5-10-k.jpg
    165.6 KB · Views: 498
  • 5-10-a.jpg
    5-10-a.jpg
    158.5 KB · Views: 112
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 !!!:idea:

I wouldn't, given the rarity of those amps.

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:
Finaly Found A Guy Using A Pair Of Refurbished Heathkit MA-12's !

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...:p
 

Attachments

  • Heathkit MA-12 Modernised 1 (Large).jpg
    Heathkit MA-12 Modernised 1 (Large).jpg
    79.2 KB · Views: 197
  • Heathkit MA-12 Modernised 3 (Large).jpg
    Heathkit MA-12 Modernised 3 (Large).jpg
    104.9 KB · Views: 199
  • Heathkit MA-12 Modernised 2 (Large).jpg
    Heathkit MA-12 Modernised 2 (Large).jpg
    98.8 KB · Views: 257
  • Heathkit MA-12 Modernised 4(Large).jpg
    Heathkit MA-12 Modernised 4(Large).jpg
    88.8 KB · Views: 275
Cheers For That Vinylkid58

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...:D

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

Here's their factory, in England they were known as Heathkit Daystrom Ltd.
 

Attachments

  • Heathkit Daystrom Logo.png
    Heathkit Daystrom Logo.png
    23.5 KB · Views: 212
  • The Heathkit Factory Gloucester England 1968-69.jpg
    The Heathkit Factory Gloucester England 1968-69.jpg
    53.2 KB · Views: 89
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.