Just report the story of my continuing mod for MK IV and MK III...
Edit: Not mean to hijack the tube rolling thread, I moved my MK IV mod to MK IV SE review thread
here, since the mod is not necessarily related to the tube rolling requirement and more MK IV specific.
And I finally take time to open my MK III to do the similar heater resistor mod. The MK III case is a lot tighter and I have to unsolder the RCA connector wires in the back panel to pull the board half way out, and then unsolder all wires on the board to pull it out.
Oh my god, now I know why my first set of 6H30Pi-EH burned out on MK III, look at those nasty burn marks on the resistors of power RC filter section, the two resistors are completely discolored due to overheat. On the back, one of the 150ohm plate resistor for 6N6P/6H30Pi is also burning out. Can you even read the band as 150ohm? All bands are showing black now... This really sucks, I just wish I fully pull out the board and inspect it before replace the 68ohm cathode resistor to 120ohm for 6H30Pi (BTW, MK IV uses 150ohm instead of 120ohm). I thought the heater voltage is the only problem causing the death of my 6H30Pi, but now I suspect lots of power resistors are underrated and overheat ultimately caused the problem. I'm
very disappointed by little dot, they owe me two sets of 6H30Pi. Well, at least I feel lucky I found this before putting 6H30-DR on my MK IV.
On the side note, MK III does not use regulated voltage for 6AK5 like it does on MK IV. So the heater resistors are needed for both 6N6P and 6AK5.
Be careful MK III owners, do not leave your MK III on for more than a few hours, MK III definitely is not designed for 24x7 burn-in.