I have two of these amplifiers. One has unfortunately been Bubba'd, but looks like it can be fixed and has the power transistors pictured below. The other is very clean and original except for the electrolytics I just replaced and uses 2N3055s. That amplifier is the one below. I am thinking I will swap the resistors as well as the original filter caps, but are there any other obvious improvements that I should do without fundamentally changing the amplifier? Some thing(s) they just weren't aware of in the mid-60s? These are amazing amplifiers given they're from 1965-1966ish (I think they're excellent compared to any of my other amplifiers). I am aware of the significance of the circuit as well. I posted about these on AK, but it didn't really go anywhere.
Schematic
Schematic
This actually was not a James Bongiorno design, which is a common misconception, it was Dawson Hadley's own (as was the Marantz 500 along with some other Marantz units). Bongiorno would've been in his mid-20s when he joined Hadley Labs. I believe he worked as a tech and possibly worked on some revisions to the amplifier in the time after the release, but he was not the original designer. He likely took what he learned from Hadley and made a career of it, but Hadley was on another level as evidenced by his electronic fuel injection patent. This amplifier is an X-design predecessor. I know that Mr. Pass is well aware of this amplifier and its design.
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Never heard of them but they look beautifully made. Do you have any other information?
Love these classic amps from the days when owning a stereo was an 'aspirational' thing for most folks.
This is the only other piece of documentation I've found:
Thanks. Specs look pretty good for an early solid state amp. The JBL SA-600 was another great amp from 1966 Amplifier History: The JBL SA-600
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Funny you bring up the SA-600 since I'm finishing one up today. Never again will I restore one of these, felt like the capacitors were endless in quantity and it's not easy to work on in any way. The power supply is a mess. Hopefully I don't have to do anything else to it and I'll get to see how it sounds.
I have an ancient Pioneer receiver from 1966 that's under the bench awaiting my attention. The whole thing is solid state except for the FM tuner which uses 2 tubes. It works except for one of the channels - amplifier section is faulty.
You can see it was designed by people used to working with tubes - no way you'd build any piece of electronic gear like that today. Nevertheless, this is a piece of history - a product made just before Japanese hi-fi took off in the West and for that reason worth restoring IMV.
Anyway - good luck with the JBL!
Thank you! Makes me appreciate the Hadley even more. The one I worked on was far easier to access, though still cramped, but it all comes apart at least without removing 30+ screws. Hadley did rivet in the filter caps along with the inputs and some other things, which is annoying. The Hadley is more aesthetically pleasing inside as well, though the JBL is much nicer outside, but I have a hard imagining the SA-600 will sound better than the 622 but will let compare and report.
I got the rougher of the two amplifiers basically back together and one channel works great! That being said, I haven't plugged the other channel into speakers yet because I'm getting a very stable .875vdc at that set of outputs. other channel zero'd out and I was getting ~0 +/- 2mv without much fiddling. I could see no obvious issues with my flir camera - both boards appear nearly identical in terms of heat and both are identical with all new resistors and electrolytic caps. the amplifier in total was consuming about 24 watts at idle. Heat sinks never got above 85 degrees. All power transistors are new matched pairs, but the rest are a mix of various kind and vintage. I'm guessing a transistor but does anyone else have a better idea?
Solved! Since I thought this was a transistor issue I began changing all of them out based on a model I created and then checking for the presence of DC between each swap. Well, I got all the way to the phase splitter transistor (the TO-5 above the white caps) before anything changed. Immediately after I changed that transistor, I noticed that while still present, the amount of DC and how that DC behaved was different, so on a hunch I swapped out the white 8uf film caps above. Immediately the problem was solved, no more DC, they were leaky or they were simply the wrong kind. Now, I went all the way with these transistor swaps. Many many hours of research and data sheet reading. The difference between the two channels was obvious. While they were amazingly still the same volume, the noise was greatly diminished on the side with the swapped out transistors. So much so that I could hear the difference while music was playing. Heatsinks were a little warmer after the swap, but just about 90 degrees F vs. 85 on the side with the original transistors. Flir camera uncovered no heat issues.
Did some looking around.
Hadley was the chief engineer at Marantz at the time, but there were several other well known engineers there also. I honestly do not believe Hadley designed the 500, it is far more advanced than what I see here. Completely different in just about every way.
Marantz products were designed by a team, not just one engineer.
-Chris
Hadley was the chief engineer at Marantz at the time, but there were several other well known engineers there also. I honestly do not believe Hadley designed the 500, it is far more advanced than what I see here. Completely different in just about every way.
Marantz products were designed by a team, not just one engineer.
-Chris