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Marshall Studio 15 - 4001 Question ... Trace, Ray, et.al


 
2/26/2003 8:03 AM
Lee
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Marshall Studio 15 - 4001 Question ... Trace, Ray, et.al
HI:  
I have 3 Studio 15's. I am going to keep 2 dead-on stock, but the other one I wish to mod so I can run 6L6, 5881, 6550, EL34, KT88, etc. Can this be done? I love the tone stack that's aready the pre-amp. I figure all that needs to be done is to put in a PT that will handle the various tube demands, tinker the dual bias adj's so they do their thing per each tube type, etc. If it is possible, can someone either, help me, do it for me, and/or if so, how much and what are we talking about in changes?  
 
All input is appreciated. If it is way too expensive, I'll probably opt for a THD Bi-Valve, but the idea really intrigues me because I love this little amp, but there are some venues I need 40+ watt power to do the gig, and the studio 15 can crap out in some places; although it has served me pretty well.  
 
TX In Advance!  
Lee
 
2/27/2003 2:05 AM
Ray Ivers
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Lee,  
 
I modified mine a long time ago to have pentode/triode switching, single-ended/push-pull, and reduced driver-tube B+. Now I occasionally run it in triode mode, but the other switches always remain in stock-circuit position. The tone stack seems pretty unique, being split between stages as it is. I also put a push-pull switch on the mid control that cuts in a 100K resistor between the mid control and ground, but again, I usually end up using the stock circuit. I always dime everything.  
 
My buddy used one of these live for years. He ran his guitar into the 4001, then from the line out (4001 speaker still connected) to a rack effects unit, and then in "stereo" (wet side/dry side) to two early-70's Marshall 100-watt half-stacks. I never heard him get a better sound than that rig, and he went through a ton of gear.  
 
You could drop in a Hammond 374BX PT and a 3.4K OT like a Hammond 1650K, use KT88 outputs, dump the cathode resistors, and your power would probably triple; these are mods which you could do yourself (bias supply resistor R10 will need some minor tweaking). Complete the package with an Eminence Delta Pro 12, and you will no longer have any problems at gigs. This would probably run a couple hundred bucks in parts alone, though. Have you tried just running the amp through a 4-12 cab? You might be surprised at the results.  
 
Ray
 
2/27/2003 8:15 AM
Lee
email

I'm not into making it become a monster, just a bit more "push" and headroom for the times I work a bit bigger venue. This amp has been great by itself with the 1x12, through a 2x12 or 4x12 cab in small (100-150 people) venues, and while I have never been disappointed, I would think I can squeeze bit more out of it with the proper re-design of the power section. I don't want to do anything to the pre-amp section 'cuz it is everything I want already. I was thinking an OEI transformer switch, if they make one that would work, because I'd be suprised if their's isn't a touch over-engineered for the good. 40 watts was just something to throw out as a possiblilty, but doesn't have to be that. It may be that I just need to tinker the power output section to where it can take the 6L6-5881-EL34-6550 variety tubes without burning up ...  
 
I have not tried running it into another more powerful amp yet, I do have JCM800 50w and 100w heads, but didn't consider this idea ... (DOH!) ;) I have run both JCM's with attenuators and they really are awsome, but lately I'm too lazy to carry the buggars, plus my back ain't what it use to be ... I just want to carry one good amp to the gig. (I'm gettin' old and lazy ... but mostly old!) :D  
 
I like your idea. If you don't mind, could you email me a schematic and instructions of how to 'hook up' your idea?  
 
Keep The Ideas Coming!  
TX!  
Lee
 
2/27/2003 12:42 PM
Ray Ivers
email

Lee,  
 
At 50-60 watts output after the mods, the amp really won't be a 'monster', just noticeably louder and more dynamic. IMO you really need to boost the power this much to get a real, "I'm glad I went to all that trouble" kind of improvement.  
 
I chose the 374BX and 1650K (the OEI 3.4K OT should also work well) to drop right in; you will have to drill new mounting holes, of course, but you will not have to modify the power supply at all (the caps are rated for 500V, and this PS will push about 470 volts). Most probably you will have to change R10 to get about -70 volts at the top of C9 (check the voltage rating of this cap), but you may get lucky with the stock R10 value. The OT will also drop right in, but you will lose the balanced line out. Replace the cathode resistors with jumpers, and you're done. It's really just tranny replacement coupled with a few component changes.  
 
Ray
 
2/28/2003 8:27 AM
Lee
email

Thanks Ray!  
I am pretty much a novice when it comes to mods, etc. I can do 'em when I have 'em mapped or explained, but I'm no E.I.:(  
 
I thought that it may be as simple as you explained because I figure I'm not touching the tone control/preamp circuit; just the power output & tubes, etc.  
 
If it's at all possible, could you make me a simple drawing and I can follow it pretty well from there. If you don't mind, I will also touch base with you on what exactly, and where to get the parts to get this done. I can't do it right away, but was thinking maybe around May or June ... (schedule permitting) ;)  
 
Thanks again for all your help ... BTW, you are the reason I've come to love these lil' 4001's so much. I talked with you about them about a year ago, and I got G.A.S. for them really bad! :D  
 
Keep Rockin'  
Lee
 
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