I enjoyed my SP-6b for 14 years. The power light stopped blinking after about 3 minutes. I normally let it warm up for an hour before listening. I suggest checking the bias voltage of V2 and V5 (The two middle tubes in each series of three). Unless your preamp has been modified, the bias voltage must be adjusted manually. If it is too far off, the unit will go into automatic mute mode (blinking light)at times.
If you are not handy with an electronics screwdriver and a voltmeter, have someone do it for you. It has been a few years, but as I recall, I set V2 at 155 Volts and V5 at 150 Volts. There are adjustment potentiometers next to each of those tubes. Check voltage from the capactitor next to each of those tubes to ground. It is shown in the schematics. Be careful. Don't go poking around in there too much. Voltages are very high. One time I put my voltmeter lead on a transistor case and measureed 550 Volts! I still have an old analog voltmeter that I used for calibrating bias voltage. A digital DMM should be fine as well. Just that I prefer analog :)
