I discovered that my RCA tube book says that a 6SN7 needs 11 seconds of heater warm up time. Not that this is some big discovery, it just got me thinking. I've often read about the dreaded cathode striping, but never ran across any material that explains what actually happens. I've had three people explain it to me three different ways. I like this one......don't know if it is the truth.....seems to make sense. As the heater brings the cathode up to temp, the electrons in the cathode material on the inside (closest to the heater) are up to temp before the exposed cathode material is up to temp. So if there is enough plate voltage present the internal (hot) electrons will blow holes through the rest of the (cold) cathode material to get to the plate, thus striping away the cathode. I've been told that it is based on plate voltage and that 150V is not enough to cause a problem, but 300V is. I've also been told that it is current related and small tubes with little current won't strip, but big power tubes will. Anyway, if anybody knows any better, please post it on the BBS. All I did was use the subject is an excuse to have more fun with my M-60s

The idea is to hold off the +300V and -300V from the 6SN7s for 11 seconds from power up, so just in case there could be cathode striping of the 6SN7s, it won't be happening to mine. The obstacle is, the transformer that supplies heater current to the 6SN7s is also the same transformer that supplies the 300Vs to the 6SN7s. At first I thought that I may need to buy and install an additional transformer to separate the two functions (heater & B+). The solution I implemented was not so massive.

The double pole double throw relay contacts keep the AC voltage from getting to the bridge rectifiers until activated.

The diode, cap, and resistor, which are all wired in series will get the AC voltage from power up.

The relay coil is across the cap.

Eleven seconds from power up the cap is charged up to 55 volts DC, which is where the relay pulls in, thus giving AC to the bridge rectifiers for the +300V & -300V supplies.

The install was easy. All the parts solder to lugs on the relay and the relay has a mounting screw. The assembly is mounted to the existing transformer bolt with the aluminum plate. Electrically the assembly goes between the transformer and the existing bridge rectifier. Only the two AC wires are affected. There is no need to touch the center tap (ground) of the transformer output.