Ampex 620 manual

Since they will be used a lot, I would replace every capacitor in them except perhaps micas, after testing even those. Then do a full test and make sure that it is at least performing as per original design specs.

You may have to replace some resistors as well but I, personally, am not of the "shotgun" replacement mind. As far as resistors are concerned, I replace only those that need it after testing. Note that, while an excellent speaker system for its time, it is not currently going to be able to reproduce 20Hz - 20KHz The original spec was 65Hz to 10KHz. So if that's the goal here, I'd find some other solution, or you will be facing some rather extensive modifications which are probably a, not worth it and b, unlikely to fully succeed regardless.

Actually the wife is the main opposition to anything like this. So to make these viable within that constraint, I am advocating the addition of a tweeter coaxially if possible. My plan for the amps is to remove the designed-in EQ from the back end and replace it with the same simple linear negative feedback found in later ampex monoblocks of basically same topology. I have a pair and they are quite linear way beyond 10K.

I am assuming the stock speaker in this is the biggest single limiting factor. Complicating all this are the multiple versions produced over time. There are at least three variations of the thing; sams print does not agree at all with the real thing we have on hand here. But another print from elsewhere on the web is much closer. First you could get a coaxial speaker to get the tweeter in there but it doesn't stop there.

The am has freq response issues at both ends of the spectrum as was stated earlier. To fix the low freq response you will need new output transformers designed to go down to 20hz and up to at least 20khz. These will take up more room and that chassis is small. The coupling caps will also have to be changed as well as the negative feedback network.

It would seem unrealistic to re-engineer the thing to get down to 20hz; the speaker might be some sort of holy grail to the cult but it has zero prayer of making anything happen below 50 or 60 so I would not care about that. Owner would use a powered sub anyway. I have some ampex monoblocks that use outputs with a bigger triad OPT than this thing has. The component values surrounding the EF86 and 12au7 drivers are identical on both these amps.

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