There are some things in ARC designs that my tech-guru friends find strange. But then just as many say that ARC knew what they were doing. I don’t know. So far I have to deal with this one, and I think perhaps the only way to get rid of that prolonged instability is to send it to somebody in the know or to ARC. Which May be cost-prohibitive…
If it was Me, I would completely evacuate all the components in the main supply, design an auxiliary board to mount slightly above the main motherboard. I would do away with the 12BH7 all togther, and use possibly an EL84 for a pass tube, implement the 12AX7 in a cascode configuration to drive the 84, and on the front end do a simple zener follower pre-regulator, and drop the input voltage 100V or so, and use a couple of capacitance amplifiers as buffers on the out, one for the line stage and one for the phono. A preamp with that kind of price tag, needs reliability and longevity, and will probably give me better performance.
What’s the point of restoring an SP-8 then? You can most likely easily design your own preamp. An SP-8, modified like you suggest, isn’t worth much. And then, if it were you, an electronic engineer probably. I’m not. If I can’t get this thing to stop doing what it does, - put it goes, for sale as is.
I also want to point out that at some point, before my ownership for sure, this unit wasn’t doing what it does now. I also used to have an SP-10 that I have restored from a basket case (stupidly sold it), which also behaved normally. And it was used by me extensively. I’m sure there’s a relatively simple fix for mine.
Ok. So problem solved. I got in touch with a friend of mine who designs his own tube and solid state gear, and now is going commercial. He looked at the schematic and was really surprised. He said that it has many problems. But not to go crazy. HV and LV power supply in my unit are steady as a rock. He saw R29 (301k ohms, used to be R67, 475k in previous version), and said it’s too high. Parallel 10k with it - violent swings of up to 30v dc on output are gone; they don’t go above 1v, and preamp calmes down to 4 mv in about a minute. Amazing. I have to measure resistance into which phono caps discharge, and see how low R29 can go.
One could do away with the bleeder resistors, attach a 1M ohm resistor across the output, giving a better coupling to the input of the next stage, and shorten the time of the mute delay.
I wonder if this was someone else’s design at the company, I don’t think Mr. Johnson would have designed something like this.
I wonder if this was someone else’s design at the company, I don’t think Mr. Johnson would have designed something like this.
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