Sure OK.....as always the moment anyone pulls a tube amp board or opens one up the following warning applies... High Voltage is present and it is extremely dangerous so make sure your large caps are drained of voltage and current. You also void your warranty, obviously
I bypassed all the large value caps directly with .033uf 500V NOS K42Y0-2 Russian PIOs, changed the 68 ohm 2W grid resistor on the 2 power tubes to 121 ohm 3W Kiwame (for the 6H30Pi tube mod). I added 4 x Panasonic 1 ohm 3 W resistors to all four tube sockets at the filament V pins (to drop the V to as close to 6.3V as I could get it) as the V to these is unregulated and running higher than spec (my line V is 127V) which means additional heat and premature tube failure.
I bypassed the WIMA 3.3uf 50V MKP output coupling caps with a pair of NOS T-3 Russian 0.1uf 600V Teflon caps that had to be slung underneath the chassis because of their size (big caps). That meant changing the feet on the MK III chassis to large brass cones (Audioselection I think....)
I already use the king of power tubes with it, a costly pair of 6H30Pi-DR's from the 1980's with either Sylvania round getter black plates (6AK5WB mil spec) or Tung Sol 403B drivers. Other great tubes to use are Telefunken CV4010 Diamond bottom, Amperex PQ E95F (rare but outstanding), Mullard CV4015 large shield logo mil spec, Raytheon USN - Jan 6AK5 among others.
Here a few pics of the mods......(sorry about the crappy quality, I had a really old cam at that time). The pics do not show the 1 ohm 3W resistors I added a few weeks later nor all of the .033uf 500V K42Y-2 PIOs (5 in total). The MK III review thread has details of another member pointing out the filament V mod that alerted me to the unregulated V being fed to the heaters. If your mains V is at 115 or 120V then you don't need to add those parts but my line V is always at 127V which put the heater V at around 6.9 or 7.0V which is a little too high. The addition of the 4 x 1 ohm 3W resistors dropped the line V about .5V or so getting it much closer to spec which dropped chassis temps and should make all the tubes last a normal period of time (closer to their MTBF spec).
All of these mods had a positive impact on the SQ of the MK III. Well worth the effort. Some future mods worth doing are subbing the 3.3uf 50V WIMA output coupling caps for a pair of Auricap 3.3uf or if you can find them a pair of PLIO K75-10 NOS Russian caps (2.2uf 400V). Of course you keep the T3 0.1uf 600V using them to bypass the K75-10's.
Changing out the I/O wiring for some silver plated copper wiring or Mundorf silver/gold 23 awg hookup wire. Swap out the cheap brass/gold plated RCA jacks for CMC copper billet/gold plated types and finally change out the pair of 220uf 200V caps for Nichicon FG/KG or if you really want to spend some money Black Gates. The mains filtering caps could also be subbed for some higher quality units (computer rated CDE or Aerovox BHC). The 3 resistors of the PSU section could be subbed for high quality Mills types (5W max but 3W would be ideal). IRC makes terrific resistors for this application (the Mil spec stuff is very good and fairly cheap compared to Mills or Reiken). The generic metal film resistors could be subbed with Vishay/Dale (matched pairs would be a good idea for the signal pathways).
Lots of things to consider.
The MK IV could definitely benefit from the swap of the 2.2uf Solens for some better quality caps like the K75-10 PLIO/T3 Teflon combo or Mundorf Silver/Gold PIOs or any other high quality type. I'd also pull the WIMA box caps and replace those with same value types of your fav brand (size is a limiting factor obviously) but I've never found the WIMA sound to be anything special other than the Black Box WIMAs that cost a fair pile of dough (but would be ideal in this spot). Anyway just a few thoughts on this amp as I don't own one but have sent parts to another MK IV owner to try these suggestions out. (the K75-10 PLIO/T3 Teflon combo and some K42Y-2 .033uf 500V PIOs as bypass caps for the 5 large electrolytic caps ) That member is no longer active at HF so I have no idea how this mod turned out but it should be similar to the MK III in SQ.
These are just suggestions and my own opinions so others results might vary depending on what parts used, soldering skill,execution of the mods, wire selected etc etc...
Peete.
