It is what it is... an upgraded MKIIC. Nothing more... Nothing less. Yes, It is a legit C+ after factory modification. If someone wants a factory original C+ with a serial number between about #12500 thru 14000 with the proper factory stamped pcbs... tell them to go buy one. Simple. And the...
New tubes (pre & power)... new caps (pre & power)... and check for any mods/repairs. Clean/spray the pots/jacks. That should get it back to fighting shape.
Unplug the reverb cables and see if it goes away. Also spray some Deoxit on an instrument cable plug and insert/remove it several times from both the fx loop send & return jacks.
To make a long story short... sell the MK3 and buy a JP2C.
The JP2C is a true 3 channel amp and will do everything you want. No matter what you do to the MK3 it is still essentially a single channel, multi-mode amp. If you were going to keep the MK3 and add the JP2C... that’s a different...
... I think you might have taken some if his info out of context or not in the proper context. I have 4 "++" Marks... and I'm working on #5. So I'm pretty familiar with them.
The + and ++ mod are 2 separate and distinct mods. The + takes either a IIC or MKIII (no stripe, black, red, purple...
Although the 105P puts out a higher PV... the total wattage (75W) and headroom (DualClass) is reduced unless it is a HRG (which is somewhat rare... a HRG with 105PT). I’ve found the standard MKIII transformer in the Red-Green to be actually tighter sounding with addition of a pentode/triode...
The outer pair of tubes are wired in triode in the Blue... all 4 tubes in the Green are in pentode = more power. And you can always add a pentode/triode switch. Subtle differences.
I have 2 of these and it does not involve some of the changes mentioned in this thread. Mine came from Mike B at MB. They are switchable via the mid pot pull out to switch the mod on. Not going to divulge the specific changes, but much of what’s been mentioned here so far is off base. The + mod...