I believe your repair shop guy was 100% correct. The regulator power transistors were an older version, that was discontinued for a much stronger one, which they never upgraded to. They don't even sell those old transistors anymore. Also the zener diode was a very low wattage so couldn't handle a problem if transistors failed. Then we have metal film resistors that should never been in a PSU. They should have had power resistors or wire wound.
The issue is that China is a very, very competitive market and they must cut every corner they can to be cost competitive. They did have some better resistors in older units but stopped that. Then we have issues of the anode resistors to start slowly burning with age because they are not high enough wattage. Today's market you can place physically smaller resistors that can handle double 5w no problem.
Bottom line is that the design was genius, but some of the critical parts used, was poor. Once you fix the PSU, there is basically nothing else that would fail.
My guess is that the super cheap and brittle stranded wire, that is used for the regulator transistors broke, causing failure. Also, if you have the MK6, that is more prone for the transistor issues.
I never had the transistor issues with my MK8se (less output power) and still have the old transistors, but I already bought the backup transistors which are a set. You can't buy one, you must buy both.
Parts for this was cheap. You can order them on "Mouser" website, so I'm not understanding why it would cost $300, so I'm assuming that it is labor.
Also I would get higher wattage diode for replacement.
In the Mod thread, we basically revamp most of the power supply anyways, replacing all that.
Anyways, remember to tell the repair shop about replacing those brittle transistor regulator wires to the board.
If this amp was done properly, with highest grade components, it would easily reach playing field with end-game amps.
As is, the stock performance is nicely mid-tier.
But this can be also said with 100% of ALL tube amps in the market.
I have seen all the top units. I have worked on them. They could all be improved. None uses the best parts.
That's on the owner to do.
The
ONLY tube amps, that are almost 100%
NOT dependant on parts quality, is the designs by member
@SonicTrance
Yes the MK6 can be susceptible to PSU strain with power tube rolling, or defective tube pops, or the dam internal wiring of the two regulator power transistors, sadly.
Recommendation is to not use stock tubes and use good tubes. And do not move unit around. Eventuality with age the anode resistors increase in value and any power surges are less, so the PSU doesn't fail except for heat and wire corrode. That would take some years.
Once you fix the issues it is a very reliable unit.
Up to you to fix. Then the next owner will enjoy.
