Heathkit A-8A Restoration

[AlexHempel]

Decided to take a break from my balanced 6L6GC amp build to work on this little Heathkit A-8A integrated amp I picked up on CL. First pic shows the tubes it came with, from left to right: 6SJ7, 6SJ7, 6SN7, and two 6L6s. Obviously the 6L6s are mismatched -- the seller gave me two random UOS tubes from his parts bin. Thankful to get any power tubes considering what I paid. Rectifier in the back is a 5U4G.





The PT is Chicago and the OPT is Peerless. Really curious how this thing will perform.





Some of the best info I can find on this amp comes from this AK thread: https://audiokarma.org/forums/index.php?threads/wanted-schematic-for-heathkit-a8a-amplifier.103928/

Apparently the "A" version has an added preamp for mic and mag phono inputs. This is provided by the first 6SJ7 voltage amp, which isn't in the global feedback loop. Afterwards, there's another 6SJ7 voltage amp and a 1/2 6SN7 voltage amp followed by a 1/2 6SN7 cathodyne driver for the push-pull 6L6s.

Not planning to do any modifications on this unit, as they're pretty rare. It will get a full recap, except for the can cap, which I'm going to leave if possible. Measurements to come (using good matched tubes from my stash).

 

[Tom Bavis]

The one mod I'd add is a line fuse - perhaps there's an existing chassis hole to add a fuse clip, or a pigtail fuse could be wired in. The can cap in my '60s HP audio generator was fine for more than 40 years - then it shorted.

 

[AlexHempel]

Restoration is done! It sounds great. Recapped, reverted an incorrect previous repair/mod, and cleaned and re-lubricated the pots and input selector. @Tom Bavis Thankfully it already has a line fuse, even though one isn't specified in the schematic or the Heathkit parts list.

It's good for about 15 watts, which I expected given the size of the iron and the output operating point. THD @ 1 watt into 8 ohms is about -57 db with the random un-matched used tubes I grabbed from my parts bin. 2nd harmonic dominates, so I might throw in matched tubes and see if that improves things. But not bad, especially considering it has tons of gain.

Unfortunately, I haven't had good results with the mic or mag phono inputs, which have even more gain thanks to a preamp 6SJ7. The amp doesn't have a bottom cover and it's picking up a 50 khz signal. I know the preamp is working because it also amplifies whatever signal I feed it, with good performance aside from the HF noise. I'm guessing adding a bottom cover, using a shorter cable run, and removing external noise sources would address the issue.

Pics:

 

[TriodeLuvr]

Nice job! Nothing like that ever shows up on CL near me, unless they want 5 times what I could build it for.

Jack

 

[AlexHempel]

Here are some measurements. There's a more detailed writeup with additional measurements on my website here: http://messingwithelectronics.com/heathkit-a-8a/. Any feedback appreciated -- I love working my way through old, rare tube amps and sharing as much information as I can gather. I would modify the amplifier if not for its rarity. All measurements with 8 ohm load resistor.

Frequency response @ 1 watt (ignore the Y axis units):



Spectrum @ 1 watt output:



@15 watts (nearly max power):



Square wave 1 khz:



Square wave @ 10 khz, load shunted by .1 uf capacitor: