NAD: 73 Marshall SuperBass

[fusedbrain]

Just picked this up for a VERY reasonable price. Looks pretty unmolested, but will need a little TLC. Sounds pretty good right now but I don't want to run it too much until I deal with the issues. Came out of a collection and I don't think it's been played very much in the last 20 years.



Judging by the dye on the PT and the impedance selector, both tranny's are original.


There's some monkey business with a top of the board diode repair, and one of the 10k droppers looks like it's on the way out. The standby switch is toast, and there are no snubber caps on the amp in the secondary feed to the diodes. Gonna fix all that and do a complete cap job and see where it's at. All the electrolytics are original.
Also, check out the 2 - 250pf caps on the bright channel mixer resistors. Marshall run out of 500pf caps that day??

Added bonus is the .680uf mustard on the presence pot, but the pot is 5k, not 25k. Perfect for a SuperLead conversion, 'cause those .680uf mustards are hard to come by. I can move the .680uf to the empty spot on the board, and I have a spare .1uf mustard for the presence.
Double bonus: the 3 preamp 12ax7's are labeled "Original equipment - West Germany" and they all test strong.

Curious what you guys think about the amp and what I should do with it?? SuperLead?? SuperBass??
I'm not sure what to do for filter cap values either. They are all 50 x 50uf right now, but if I want to do a SuperLead conversion, maybe that should be adjusted a bit.

 

[SpiderWars]

Sweet!

Do you mean you are going to split the cathodes using that .68uF from the Presence? Looks shared right now.

I recently got a '76 SuperLead and all I did was replace the bias supply filter caps (the 2 each 10uF/160v), change some resistors in bias supply (6550-to-EL34 conversion was incomplete/sloppy), moved Presence to back (in 4th speaker jack hole), and put a PPIMV in Presence spot. I used the PPIMV with dual-250k pot and 2M2 resistors (you can see it on the Ceriatone Chupa/Yeti layout pics). Oh yea, I wired the OT straight to the remaining 3 speaker jacks. One jack is the 16ohm, two jacks are 8 ohm, and I taped up the 4 ohm tap. Those old impedance selectors are risky.

Regarding the filter caps; mine still has original Daly's. I already bought F&T 50x50 replacements but I'm not sure I'm even gonna replace the old ones. Some say replace them and some say if they still work and you use the amp regularly it should be fine.

I would leave the Normal channel as-is and make the other channel SuperLead but maybe adjust the bright cap (my amp had it disconnected and I just removed it). IMO, a PPIMV is almost a necessity and allows me to actually enjoy the amp and I ALWAYS regret putting it on the back (the Presence control essentially won't work at low MV settings anyway). Whether you change the PI output caps from .1uF to .022uF is another decision. I'd try it as is at first.

One thing I noticed on mine was that the head for one of the power tube ground bolts had broken off even though the connection was still made. When I tried to check/tighten the other three bolts, the head of one of them snapped right off with almost zero pressure. The other two were fine. Something to look at, they are the bolts closest to the rear edge at each power tube socket.

EDIT: I found a replacement switch on Reverb but it was like $18, not including that nut that goes on front (those are easy to find). Dude said he has more, PM me if you need that contact.

 

[paulyc]

I like leaving the last two coupling caps before the phase inverter at Super Bass values instead of converting to Super Lead ones...sounds fuller, fatter.

 

[fusedbrain]

Thanks for the tip on the switch. I've already received 2 switches and some other stuff from ValveStorm.
I'm gonna do all the filter caps. The 2 on the mains are badly bulging on the bottom.
When I was saying superlead, yeah I meant splitting the cathodes and using the .680 uf for v1b. I've got a .0022uf mustard for the first coupler on the bright channel as well. There isnt a bright cap on the volume control right now, and I probably will leave it off. I'll probably stick with the. 01's on the pi for now as well.
Planning on doing a Lar/Mar master as these amps are stupidly loud.
Looks like I'll have the first round of repairs done in a day or 2 and I'll post the results.
Any one else got any suggestion, keep 'em comming.

 

[SpiderWars]

Planning on doing a Lar/Mar master as these amps are stupidly loud.

That's the same PPIMV I was talking about. I think 'Lar' is Larry who used to post on amp building forums (maybe still does); I think he was from Germany. 'Mar' I think is Mark aka Rockstah. I guess they developed it (or maybe 'popularized' is a better term). I had developed my own MV almost identical to it (my 2M2s went across the whole pot) prior to ever hearing about the LarMar. It's pretty straightforward idea though so not unusual to see several people using it independently. The whole shebang just needs to be close to the stock 220k bias splitter values. Using dual 1M pots and 270k resistors works too but the sweep isn't as nice ime.

 

[fusedbrain]

Yup, the Larry and Mark master volume. First found out about it on the Metro amp forum. I've got one in a 1987x and it rocks. Some good info about Marshall's over there.

 

[sg guy]

-bad ass amp!!!

-check out MICHAEL RT's clips and Tweak's-

 

[fusedbrain]

-bad ass amp!!!

-check out MICHAEL RT's clips and Tweak's-

Thanks!! Yeah Michael RT & his Major are freakin' awesome
I always check out his clips. He's a great player and his amp is off the charts great sounding.
A while back he posted a layout for a version of the "one wire mod" done on a 76 SuperBass. I have it saved for future reference.
I think the SuperBass circuit is a better candidate for this mod than a superlead. The one wire mod can be a little much, and the shared cathode on the bass circuit keeps the gain a little more under control. It's certainly something I'm considering doing. I just want to hear the amp stock with the problems fixed first, before I get to making any changes. Sometimes these amps kinda tell you where they need to go. Sometimes they're perfect as is.

Might have time tomorrow to get the first round of work done.

 

[bbaug14]

Looks awesome! I rock a 100W Super Bass clone and it's a fantastic amp not only for natural breakup, but with pedals as well. Much better with my pedals than any lead amp I ever had. I'd leave it as is. Maybe change a couple values if the bass is too flubby or in your face.

 

[novosibir]

That's the same PPIMV I was talking about. I think 'Lar' is Larry who used to post on amp building forums

Correct.

I had developed my own MV almost identical to it (my 2M2s went across the whole pot) prior to ever hearing about the LarMar.

Bad idea, cos then the 2M2s don't have the safety effect, if the pot's wiper is failing and if it does, your power tubes will die

 

[SpiderWars]

That's the same PPIMV I was talking about. I think 'Lar' is Larry who used to post on amp building forums

Correct.

I had developed my own MV almost identical to it (my 2M2s went across the whole pot) prior to ever hearing about the LarMar.

Bad idea, cos then the 2M2s don't have the safety effect, if the pot's wiper is failing and if it does, your power tubes will die

Thanks! I read that a while back (you probably posted it then too). I don't really know what I'm doing, I know just enough to get myself into trouble.

 

[novosibir]

I don't really know what I'm doing, I know just enough to get myself into trouble.

Learn more, here:

http://forum.metropoulos.net/viewtopic. ... 656#p78656

Larry

 

[SpiderWars]

I don't really know what I'm doing, I know just enough to get myself into trouble.

Learn more, here:

http://forum.metropoulos.net/viewtopic. ... 656#p78656

Larry

Yes! I remember that thread!