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Marshall Mods to JTM 45


 
10/2/2000 12:44 AM
Bryan
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Marshall Mods to JTM 45
Hello,  
I've recently purchased a JTM 45 reissue and was curious about some minor mods to sweeten it up a bit. Also, will a NOS rectifier knock out the rattle? Thanks for any help.
 
10/2/2000 5:05 AM
Rebel420
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The mods to perform in order  
1) good output tranny  
2) set the amp up to use real KT66 tubes, and not the 5881s they come with.  
3) good speakers  
4) change the filter caps from 50uf's to 32uf's  
5) replace any of the shitty 1/4 watt resistors, and cheap box caps to better types  
6) (only if you plan on messing with the circuti, and modding etc), go toa turret board from the PCB... this will NO(T change the tone! I repeat will NOT change the tone, but will make changes MUCH easier to do
 
10/2/2000 6:39 AM
David Manson
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what will switch 50 uf to 32 uf filter caps  
do? that wont change the sound, will it?
 
10/2/2000 1:31 PM
Trace
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quote:
"what will switch 50 uf to 32 uf filter caps  
do? that wont change the sound, will it? "
 
 
Yes is does change the sound but not drastically. 32uF's allow for less filtering and hence the amp will sound as "uptight" (to quote what many call it). The original plexis' and a few others did have 32uF filter caps.  
 
I like to use just enough fitlering to do the job and let it go at that. Adding more filtering will "stiffen" the amp up and add more low-end but it's a fine line we dance and if it's a gain-amp then it can get in the way. It all depends on what someone is going for. Les filtering allows for a more "open" sounding tone and the highs are more "natural" sounding (for lack of a better word).  
 
Trace
 
10/2/2000 6:12 PM
Peter S
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David,  
I agree with what Trace said, but to sum it up a little differently, 32uf caps will give your amp a mushier sound at high volumes.....50uf caps will tighten up the sound a little and deepen the bass response. The effect on high end is pretty much negligable. If you like to dime your amp, IMHO the 50uf caps sound better. Keep in mind that these changes are extremely subtle, and the tube rectifier will sort negate most of the advantages of having the 50uf caps in the amp. There are other factors at play as well. I've done ALOT of AB'ing Kimerik K-50's with 32uf caps(tube and ss recto) against K-50's with 50uf caps and the difference is very slight. I've also AB'd the K-50's against plexis and other "boutique" amps and the resul has been the same. Bottom line is, if your a blues player and a stickler for that bluesy sound, AND you DON't crank your amp all the way up to get your sound, then you'll like the 32uf caps. If you play rock and like to wail at full tilt, keep the 50uf caps. Don't use Chinese GZ-34's if you avoid it. The actual cost of a Mullard if you consider it's life expectancy is about the same and the Chinese ones suck. If you can find some Sovteks...they are good.  
 
Peter
 
10/2/2000 6:17 PM
JimW
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Where can you find good caps (coupling)?  
Are tantalum caps ok for cathode bypass (better/worse than electrolytics? JimW
 
10/2/2000 8:59 PM
Peter S
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Jim,  
Tantalum caps are dry electrolytic types....I highly recommend NOT using them. Use conventional electrolytics.....you can bypass an electrolytic with a mylar cap to improve high frequency response if you need to. In most guitar amps the difference is negligable and not worth the bother IMHO. Good coupling caps.....this is an intersting question....opinions differ quite a but on this. Sprague 715/716 caps are available just about everywhere.....New Sensor sell them. They are good caps, and I love them in amps that need a little sparkle added to the top end. My personal preference for new production caps are the Tin foil caps that Angela sells....they have a smoother high end IMO. Mallory 150 series caps make excellent Tweed Bassman and Marshall replacements, because they are metalized film and again are not quite as efficient in the high frequecy department, which is great for Bassmans/Marshalls because these amps already have a tendency to be overly bright. I also love the old CDE Black and Green Cats(Somewhat hard to get, but Angela may have some).....BUT there is a major caveat.....they have terrible tolerances and tend to be leakier than an old faucet. You have to spring for a bunch of them and cull the garbage out. What you'll have left IMO are some of the best sounding coupling caps available for guitar amps anywhere, especially if you want to overdrive the whole amp and are looking for a smooth distortion sound. I've been told by someone, whose opinion I respect highly, that all of the above observations are psycho-acoustic in nature, but I respectfully disagree, since I've conducted many blind tests, using good players, non-players and I've had another tech install the caps in some of my K-50 amps and conduct the blind tests on me too. The results are consistent, although not scientific by any means, and the sample is rather small, so there is a large margin for error, but there must be something going on here to get such consistent results.  
 
Peter
 
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