The Heathkit Amplifier
Article was published in Hi Fi World July 2012 edition
Haden Boardman uncovers a surprisingly common but
forgotten about homemade valve amplifier from
the 1950s. Soldering iron at the ready...
At Hi-Fi World we
have always valued
the 'home constructor'
and experimenter.
Back in the
J 950s, unless you
were seriously rich, a homemade
amplifier was simply
expected. My Grandfather was
one of these home constructors
and, at the time, there
were two options to building
your own amplifier: fabricating
the whole lot from blueprints,
or purchasing a complete 'ready to
go' bolt-together-kit. The latter was
a more expensive option of course,
but had step by step instructions and
would give guaranteed results.
One of the most popular
complete kit makers was Heathkit.
An American Company sensible
enough to set up UK operations
in Gloucester, more than a little
distance from Niles, Michigan in
the U.S.A. The American Heath
Company was founded in the
1930s by Edward Heath, manufacturing
airplane kits. Mr Heath died
performing a test flight back in 193 I.
The Heath Company was declared
bankrupt by 1935, and was purchased
by fellow American Howard Anthony.
Post W W I I Mr Antony launched
the first'Heathkit' product, an
oscilloscope, the OI, built from War
surplus stock.
By the mid 1950s the Americans
had the high fidelity choice of a real
classy Williamson style chassis (with
the option of home-made 'Voice of
the Theatre' loudspeakers), to a tiny
tot of a 6V6 based amplifier. Back
in England, by 1959, Daystrom Ltd
had been set up marketing Heathkit
audio, radio and high fidelity products.
The MA 12 shown here was the
top of the British 1960s Heathkit line.
Clearly based on the Philips/Mullard
classic '5-10' (five valves for ten watts
output), the matching USC-1 preamp
was also clearly Philips/Mullard circuit
based, although fitted with printed
circuit boards.
The Heathkit MA 12 looks rather
a sweet little power amplifier. A
compact twelve by six inch chassis
holds a GZ34 rectifier valve, a pair
of EL84 output pentodes, an EF86
pentode voltage amplifier, and an
ECC83 double triode 'Schmitt' type
direct-coupled phase splitter.
Where Philips/Mullard direct
coupled the EF86 pentode to the
phase splitter, Daystrom chose not
to; maybe this avoided paying any
royalties on the established circuit.
Regardless, what is substantially
better than most contemporary
products is the chassis layout. Real
thought and knowledge has gone in
to this.The GZ34, about the only
component on the entire chassis
not susceptible to transformer
hum field is positioned exactly at
the transformer axis. The output
transformer is located as far away
as possible and rotated to avoid any
chance of hum pick up. The output
transformer is grain orientated steel.
Several manufacturers contributed
this component over the lifetime of
the MAI 2, including Hinchley and
Radford. I have neither measured nor
heard any difference between the
different transformer makes; pointing
to tight quality control at Daystrom.
There's no need to get giddy at the
mention of Radford.
A high value of smoothing block
capacitors are fitted to the power
supply, 60|JF plus 250PF - high
values by vintage standards. An input
volume control is fitted directly to
the EF86 input valve.
Audio input is via little RF
sockets (I do actually prefer these
to phono plugs!) and are easy to
replace with a phono socket if need
be. Circuit layout is neat and tidy,
and very logical. The circuit also
differs from the classic '510 in having
90k/100k anode load resistors on the
phase splitter ECC83 valve.
A 'hum bucker' is fitted
adjust for minimum hum in your
loudspeakers) on the 'low tension'
heater supply, and the usual Octal
socket can feed power supply to the
matching preamplifiers. One word of
caution here, the pre amp umbilical
cable has two plugs rather than the
usual plug and socket arrangement;
this makes a loose cable lethal at the
preamp end! Throw it in the bin now.
It is an easy enough job to by-pass
the mains on the back of the octal
socket, but see our pictures; I used
the existing mains cable hole, and
fitted a proper power switch to the
chassis.
Speaker connections are the
usual 1960s screw terminals and it
is easy enough to fit 4mm banana
sockets. Output impedances are
ideally 2-4 ohm, 8-1 I ohm, and 14-
16 ohm. On testing the output
transformer, 4 ohms reflected 8,000
ohm load to the output valves. EL84s
can deliver 10 watts across 5,000
- 8,000 ohms load, so this little
amp is quite load tolerant. I would
suggest most 'modern' speakers are
connected to the 2-4 ohm tap. To
use the 8-1 I ohm tap requires a little
work on the inside of the amplifier;
The 'ground' wire inside the amplifier,
on the loudspeaker terminal, needs
to be moved from the " C " common
terminal to the normal "3" ohm
output terminal.The loudspeaker
output negative connection then
becomes the 3 ohm terminal, and
positive connection to the 15 ohm
terminal. Frankly, it is easier to stick
with the 3 or 15 ohm set up!
Most of these amplifiers, if not all,
were sold as kits. The instructions
and documentation are stunning.
The layout picture-grams, the text,
the point by point, wire by wire
instructions would enable anyone
who can read, to put the amp
together. The manual is entertaining
and interesting, and of very high
quality. No electronics knowledge
needed, but with enough technical
savvy to satisfy those who are that
little bit more 'technical'. Instructions
on how to solder, and lay things out,
making your bench space; etc. All
very practical stuff.
Quoted specifications are
pretty modest.Ten Watts rms, with
a maximum of twelve Watts output
between '30 c/s and 10,000 c/s'
For full output 120 mV is required
on the coax socket (for 10 watts).
Distortion is quoted as 0.1% at
1,000c/s, 0.2 at 5,000c/s, and 0.3 I at
40 c/s. Damping factor 30, hum and
noise -85dB, and overall negative
feedback 26dB. No better or worse
than any other vintage amp from
the era, but hardly setting the world
alight.
The condition of this pair of
MA 12s was pretty good. Kit made,
budget priced - a lot of variables do
not guarantee stability! They were
powered up on a 'variac' transformer,
given 70v for 24 hours, then slowly
cranked up to 100V (sounds began
to emerge at half power). At I20v
and once running all capacitors were
checked for leakage. When OK, 170V
was applied and the amps double
checked before given the full 240V.
Only the output valve cathode bypass
capacitors needed changing. To
be sporting, I replaced the anode
resistors on the EF86 and ECC83
valves for modern high tolerance
types, as the original 1960s types are
rubbish. On the workbench 15 watts
output was available at the clipping
point.
The fitted input controls made
set up a doddle. A pair of Celestion
Ditton 15 loudspeakers proved a
stunning match, and various digital
sources were connected to the
input terminals direct. From the off,
it was clear these little amplifiers
had tremendous weight and solidity
to the sound. I am not a fan of
the Mullard 5-10 circuit, and even
less so of the Leak TLI 2+ circuit.
Too much gain in the phase splitter
results in much sonic mush; although
most people (including me at one
point) blamed the EF86 pentode,
it is the phase splitter that is in my
humble opinion the weak link.The
sonic differences the Heakthit circuit
has over the 5-10 are nothing but
marginal. It is the layout, and frankly,
the jewel of an output transformer
which marks the Heath out from
competitors, and by a margin.
My favourite Jimmy Smith The
Cat' album really swung.The marked
bass lines, powerful orchestration,
and stunning Hammond punched
through with such gravity, and frankly
clarity, it was quite a shock from a
1950s amp design pretty much 'as is'.
In Prog Rock mode (very rare around
these parts) some 1970s Genesis was
again delivered with clean bass lines
and open vocals; normally you think
400 watt per channel with this kind
of material.The ECM disc 'Officium'
from Jan Garbarek / The Hilliard
Ensemble proved the amplifiers real
clarity...
So impressed was I, I chanced the
Heaths in the main system: driving
l06dB/W horns would show up any
problems. As expected, there was a
bit of hiss and noise, but it was no
disgrace.
Another set of MA 12s turned up
and the original set have now been
tweaked. ECC83 replaced with an
ECC82, 100k on each anode, 22k
common cathode resistor, and the
'second' grid connected to earth via
a 0.1 capacitor and a I M resistor; a
mod I have been doing on the Leak
STEREO 20 and TLI2+ for the past
twenty years (the STEREO 50/60
and TL25+/TL50+ are beyond
serious audio salvage - only
for the deluded).This simple
mod restores treble and lower
bass, reduces gain and noise,
and makes these little amplifiers
very serious. Sadly, I had run out
of decent EF86 valves so these
were ultimately replaced by half
of an ECC83 on the tweaked and |
modified set.
In stock form these little
amps are better than Leak's
TL12+. Rebuilt and tweaked, both I
amplifiers can deliver similar
sonic delights, but I still feel the
output transformer fitted to the
Heath, and the chassis layout are
much superior. Leaks have a name I
and a justified strong reputation.
But the price of TLI 2+ amplifiers
has got to the point of poor
value. The 'homemade' Heaths
are selling for less than half what j
even a 'tatty' set of Leaks sell for.
Don't be put off by the 'home
made' nature, after 40+ years
any of the teething problems will
have gone, and frankly, rebuilt and
modified, these amps are awesome.