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Marshall Super P.A. Blasphemy...or maybe not
Posted: Thu Dec 31, 2015 1:08 pm
by stratonenator
So, I have a late 67' Super P.A. head. It's a fantastic sounding amp, I've modified it a bit to slightly more superlead spec. Split cathode, etc. Now I am faced with a dilemma. The amp doesn't have original the PT or OT transformers so it's not quite a mint example of the amp. I am thinking about redoing the power section though, upgrading it to 68' spec, instead of the dual solid state rectifier with 32uf cap situation it's currently got. What do you guys think, would it be blasphemous to change the power section, or should I keep it stock?
Re: Marshall Super P.A. Blasphemy...or maybe not
Posted: Thu Dec 31, 2015 1:24 pm
by coldengray
Keep it stock, build a new amp.
Re: Marshall Super P.A. Blasphemy...or maybe not
Posted: Thu Dec 31, 2015 5:11 pm
by axeman
+1
Re: Marshall Super P.A. Blasphemy...or maybe not
Posted: Sat Jan 02, 2016 9:31 am
by neikeel
I tend to agree, you could up the value of the filter caps but I would not rip out the rectifiers. What iron did you fit (the PT will have needed a split centre tap to use with the dual rectifiers)?
Re: Marshall Super P.A. Blasphemy...or maybe not
Posted: Tue Mar 22, 2016 11:37 am
by stratonenator
So, finally got back to working on the Super P.A. and there have been some important updates. It appears that the replacement iron was from some time in the mid 80s and was marshall supplied. The OT was a Dagnall C2998, which I replaced with a Metro C1998 and that alone made massive improvements in tone. The power section is still a bit loose for my liking but the RS 32uf caps are still working flawlessly after all these years. It looks like the correct power transformer wasn't available back when the iron was replaced so I think it's just what would have been a stock PT from that era, plastic bobbins and lots of yellow tape. To get around this the tech installed some resistors which look to be bridging the two rectifiers, the fully functionality of which I am not completely sure because the dual rectifier situation is a bit foreign to me. I would be tempted to up the mains filtering a bit, get it more into the 50uf range, but likely just with a parallel capacitor to keep the amp as near stock as possible, does that sound reasonable?
I've also tweaked a little bit of the preamp, basically done a EVH 12000 on channels 3-4 and more of a Hendrix/Eric Johnson Superbass on channels 1-2, its pretty epic having the flexibility that provides.