Comments for page: Eico ST-70 Mods

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Posted by Monte Castleman March 20, 2011 - 07:44 pm
I'm surprised no one's commented so far, but I appreciate you sharing this. I've always like the sound of my ST-70 but the hiss has always bothered me so I'm rebuilding mine with the exception of the AC balance and phono pots. A few comments and questions.

1.) If using a fixed resistor, why parallel R27 / R28 instead of replacing it? I don't use the phono section but decided to rebuild it anyway for completeness. I figured about 56.2 K would be a standard value that would be about right.

2.) Although carbon film resistors might be more "correct", it would seem metal film, especially at the first stage, would lower hiss further. I'm using Vishay/Dale 1% 1/2 watt 500 volts. These are brown so they don't clash visually like the standard blue ones do, if one cares about such things.

3. Rather than having them flying loose, there is (barely) room for a two junction terminal strip for the new grid resistors adjacent to pin 2 of V3/4.

4.) My capacitors C31 and C32 were also low, so it might not be a bad idea to simply recommend replacing them. I used my standard cap, 715P orange drop. For people that like these, the .047 will fit very nicely between the 6SN7s and the 7591s, and, if you get creative, the .1s can fit in the phono amp.

5.) It's not clear from your instructions why C36, C37 were changed to electrolytics, which will go bad in time.

6.) Is there any particular reason why the frequency comp and feedback caps are EPO ceramic instead of silver mica?

7.) If I move the grounding to common but retain feedback from the 16 ohm tap or move it to the 4 ohm tap does the value of the feed back resistor need to be altered? And was the change in the feedback cap from 125 to 120 for any particular reason or just because that value was close and available?

Again, I appreciate you taking the time to write this up.

Posted by Steve L. February 13, 2010 - 08:11 pm
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