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.