ARC SP-6 Warmup time

Yes, as I said earlier and I guess it never made it to this thread; Because the SP-6B/C have long DC time constants, in order for the circuit to stabilize, the timer should not release after one minute or you might hear a thump and watch your woofers move in and out for a few cycles. It would be especially dangerous if you were using a DC coupled solid state amp. Usually the cause if the timer releasing early is the wrong value cap in the circuit. The circuit diagram I believe shows 2mins. 35 secs. with the right value capacitor. Minor changes to that time are strictly a function of the tolerance of the cap.

Just try some new tubes before doing anything else. High quality vintage gear is good stuff and can have many more years of service. The things to watch out for are corrosion and dried out electrolytic capacitors. I think it can be particularly rewarding updating vintage gear with modern high quality components. I bought my ARC SP-6b preamp in 1989. A few months later I had it modified by a buddy who was skilled in audio, a hobbyist and in the business of upgrading gear. Circuits weren't changed, just upgraded internal wiring with Litz, gold plated RCA jacks with teflon dielectrics, Wonder Caps and wire wound resistors in place of the carbon filled ones. My buddy also added better quality trim pots in the bias circuits. These were many of the things ARC incorporated into their later models. I used that preamp for the next 14 years. It sounded great. I only changed models eventually, because I wanted to get into the low output MC cartridges. I discovered NOS tubes too late in life. I'd love to go back and hear my old SP-6b again with NOS tubes.

Capacitor next to u555 timer ic is shot.
Happened to my sp10ii a few years back.