A while ago, I embarked on an ambitious project -- to rebuild & mod the ugly old Eico ST-70 integrated amplifier into a real performer . . . both in specs and appearance.
I stripped an old chassis and had it professionally cleaned and refinished. I bought an old Eico from another AKer and pulled off the iron and some other parts.
The basis of my work was to follow Steve Lafferty's excellent article: http://www.tronola.com/ST-70A_mods_package.pdf
In addition, I wanted DC for the filaments preamp and first amp gain stages; I also used K&K Audio's constant current sources for the 6SN7 splitter / drivers. I used a much simpler selector switch and an Alps pot (which could be a Chinese copy).
I also removed the tone control section, and by doing so, made a critical mistake. Steve's mods and measurements depend upon the presence of the (modified) tone control circuitry.
Without the "gestalt" of the entire circuit, the result was lackluster. The output power was reduced, and it was missing a certain "something."
Somewhat from desperation, I reached out to fellow AK'er Dave Gillespie, who lives about an hour up the road. (Dave was as kind and generous in person as he is on the forum).
He spent a bunch of time analyzing the circuit and making recommendations, which included adding parts and measuring the results. The main things he discovered included:
1. Feedback is too high without the tone control circuit;
2. The way I had arranged the volume and balance controls had severe frequency roll-off due to the Miller effect;
3. My grounding needed improvement;
4. The circuit would benefit from a regulated screen supply to the 7591s;
5. A pair of my tubes had been "matched" in a tester, but they were not a match under full load, increasing the distortion.
As you can imagine, it was a huge investment of his time and trouble. Not many folks will go the extra mile, and I was damn grateful.
So I set to work to fix the problems. The feedback loop was changed; I added a cathode follower to deal with the Miller Effect.
The results were really stunning. Great clarity; no hum; plenty of power. Unlike any Eico ST-70 I've heard or seen. I've attached a schematic that includes Dave's circuit suggestions, in case anyone wants to remove the tone section from an Eico.
Thank you, Dave (and Steve) !
I stripped an old chassis and had it professionally cleaned and refinished. I bought an old Eico from another AKer and pulled off the iron and some other parts.
The basis of my work was to follow Steve Lafferty's excellent article: http://www.tronola.com/ST-70A_mods_package.pdf
In addition, I wanted DC for the filaments preamp and first amp gain stages; I also used K&K Audio's constant current sources for the 6SN7 splitter / drivers. I used a much simpler selector switch and an Alps pot (which could be a Chinese copy).
I also removed the tone control section, and by doing so, made a critical mistake. Steve's mods and measurements depend upon the presence of the (modified) tone control circuitry.
Without the "gestalt" of the entire circuit, the result was lackluster. The output power was reduced, and it was missing a certain "something."
Somewhat from desperation, I reached out to fellow AK'er Dave Gillespie, who lives about an hour up the road. (Dave was as kind and generous in person as he is on the forum).
He spent a bunch of time analyzing the circuit and making recommendations, which included adding parts and measuring the results. The main things he discovered included:
1. Feedback is too high without the tone control circuit;
2. The way I had arranged the volume and balance controls had severe frequency roll-off due to the Miller effect;
3. My grounding needed improvement;
4. The circuit would benefit from a regulated screen supply to the 7591s;
5. A pair of my tubes had been "matched" in a tester, but they were not a match under full load, increasing the distortion.
As you can imagine, it was a huge investment of his time and trouble. Not many folks will go the extra mile, and I was damn grateful.
So I set to work to fix the problems. The feedback loop was changed; I added a cathode follower to deal with the Miller Effect.
The results were really stunning. Great clarity; no hum; plenty of power. Unlike any Eico ST-70 I've heard or seen. I've attached a schematic that includes Dave's circuit suggestions, in case anyone wants to remove the tone section from an Eico.
Thank you, Dave (and Steve) !
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