Re: Mark III to Mark II mod
Post by jrb32 » Tue Jan 29, 2019 9:50 am
3rd post from bottom
http://forum.grailtone.com/viewtopic.php?f=3&t=83605&start=15
Any mistakes you come across feel free to edit and let me know. Easy to compare the two and go from there but the rules are:
A) Send it to Mike B if you're in the USA.
B) Do not attempt unless you have a lot of electronics or soldering experience and cannot send it to Mike B due to geographical location. Lead solder. Iron set to 410 degrees C. The traces can be easily damaged and might need to be repaired. Having a III+ mod not done by Mike B may harm its resale value.
C) The 22K cathode resistors can be left as 22K. Close enough to 15K but you can change them to 15K if you want.
D) Orange drop type (225P vs 715P) resistor type and tantalum cap type vary between IIC+s, especially the orange drops in the power section it seems.
E) For a ++ switchable mod I like to add a 100K resistor off the treble pot with R2 to use C214 only with a 15uF cap instead of a .47uF. You might be able to do something more clever though with the relay.
F) Don't add in the extra D+ voltage rail. It's messy and not worth it.
G) DO add in the B voltage rail for the Reverb like on the C+. It makes a big difference separating this from the other 12AX7 power rails and don't get any interference that the III 120K 2W factory reverb mod tried to address. You have to run a wire though.
Cheers,
Jon
