The HEATHKIT MA-12 a MULLARD 5-10 In Disguise.

[The Space Egg Corp]

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.

[The Space Egg Corp]

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.

[SGregory]

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

[The Space Egg Corp]

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.

[The Space Egg Corp]

Originally Posted by SGregory

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.

[SGregory]

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.

[ray_moth]

The tube compliment would work for either a Williamson or Mullard.

What makes you think that?

[The Space Egg Corp]

Originally Posted by SGregory

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 !!!

Cheers Simon

[SGregory]

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.

[mashaffer]

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.