master volume mods

[keith moon]

 Greetings! I own and love my sceptre. I use the thing for bass guitar and it sounds great! For guitar, it has the best clean sound that I have ever heard! Very warm and tubie. I may be getting a Sunn 350L. I don't know much about that amp and can't really find anything about it on the web. I read on this forum that it is either a 100s,or a 1500s with a 350L sticker pasted over the original model logo. Any additional info on this amp would be very helpful and would help me decide whether to purchase it or not. The sceptre has a tube rectifier I believe,does the 350L? Well my main question is this: I would like to be able to get a bit of overdrive from the sceptre without turning it up past the pain threshold.
I have heard that a master volume mod would be the thing to do. I just want to be able to get a little chug ala Chris Squire /Geddy Lee out of the thing. Nothing too major,just a little overdrive. I love the amp and won't be hacking it up. I would probably mount the extra vol control on the back of the amp. How hard is this to do and how the hell do I do it? I can't find any info on how to do the master volume mod on a Sunn amp. Info on fender and marshall's is plentiful. So, what I need to know is:
1) how hard is it to do this mod?  I believe it should be somewhat simple.
2)how is it done? I have the schematic for the amp.
3) what type of pot should be used?
Also how hard is it and how can I change that dreaded 2 prong power cable to the safer 3 pronged grounded type?
I am a drummer of 24 years who also loves playing bass. Please excuse my lack of knowledge of electronics. Oh,I have had the old sceptre since 1996.  
All my Very best,
                         Todd aka keith moon

[JoeArthur]

I've never seen a 1200s schematic.  If it is close to the 1000s and/or 2000s, then installing a master volume is pretty much a piece of cake.

Look at your schematic and find the treble control.  The center control wiper should lead directly into the power amp inverter/driver tube... pin 8 of the 6AN8 tube.  If there is a 1meg resistor to ground at this point like the 2000s has, remove it and replace it with a 1meg pot - the wiper of which goes to the 6AN8 tube, the treble control wiper now connecting at the top end of the pot, and the other end goes to ground.

If there is no 1meg resistor, no problem... just connect the pot up the same way - and 1 meg is a good value to use.  I would suggest that you use shielded cable to connect between the treble wiper and the top of the volume control... and from the wiper center terminal of the master volume pot back to pin 8 of the 6AN8.  Only ground the shield at the volume control - only one end needs to be grounded and this is the easiest way to do it.