Vintage Tube Sound With The Heathkit A-7 Amplifier, And For Less Than $300.00

The Heathkit A-7 amplifier was produced over a relatively long span of time from about 1952 until the early 1960's. Four versions are known to exist including the A-7 thru A-7D. The smallish chassis was finished in a gray hammer tone enamel, and had a detachable front plate with the model designation and control markings screened on it. (This may have been to facilitate cabinet mounting. The chassis has no provisions for a bottom plate or cage. Some models have phono amplifiers and some do not. The tube complement is one 12SL7 (phono version only), one 12SQ7 utilized as a tone control amplifier, a 12SN7 as a voltage amplifier, and cathodyne phase inverter, and a pair of cathode biased 12A6/VT134A beam power pentodes in an ultra-linear connection. Output power is estimated at about 7 watts, and although the original output tubes are metal, the glass 12A6GT can be substituted. The transformers are reputedly of Chicago manufacture, but I had problems with the power transformer in the older of the two units.

These amplifiers are not terribly common, and what little information I have is of questionable accuracy.. The two units I have differ only in the presence of a phono amplifier in the newer unit, whilest the clearly older unit (as evidenced by the nature of its parts) lacking the phono stage, appears identical to the only photograph I have seen which identifies a very late model according to the poor information I have available. The transformers in both however have the same codes and part numbers, and the amplifiers are otherwise identical...

As I mentioned earlier the power transformer in the older of the two units was bad. I determined that the failure was a partial short across some portion of the high voltage secondary, and the only cure was to replace the transformer... Obviously new potted transformers for an amplifier this old is out of the question, so I removed the old core from the can by heating in the oven until the tar based potting material softened sufficiently that I was able to remove the core. I measured its dimensions and noted how the leads exited the core, and based on electrical measurements on the good unit I designed a replacement which I had fabricated by a local vendor. The new core fits perfectly within the old housing, and is electrically equivalent to the original.

I replaced all the coupling capacitors as well as any other parts that I deemed necessary with parts that would have been used during the era.. NO modifications of any kind have been performed. Within its limits the amplifiers perform well, they won't win any awards for resolution, but they are pleasant sounding, and dynamic. I am using Radio Shack Optimus 77 loudspeakers which are really pretty crummy for what they cost - I have observed that they sound much better off axis than on, and they seem to have very beamy treble, but on the plus side they achieve reasonable spl with these low powered amplifiers. I plan to modify the drivers and cross-overs at some future date to tame their excessive hardness, beamyness, and mid range honkiness.. These amplifiers would probably work well with some of the smaller NHT, AR, PSB mini-monitor types.

I did not bypass the tone controls or in any way strive to alter the sound of these amplifiers, but I do recommend efforts of this nature to those who are so inclined. Improvements could include film caps in the cathode bypasses, metal film or Kiwame resistors, better pots and connectors, and ultimately bypassing the bass and treble controls.

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This page first published 10/3/96.