From: Bryan (bsmith21@yahoo.com) Date: 10/2/2000 12:44 AM Subject: 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. START NEW THREAD REPLY PREVIOUS LIST Read 324 times From: Rebel420 (giorfida@drifteramps.com) Date: 10/2/2000 5:05 AM Subject: Re: Marshall Mods to JTM 45 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 START NEW THREAD REPLY PREVIOUS LIST Read 313 times From: David Manson (cattgut@hotmail.com) Date: 10/2/2000 6:39 AM Subject: Re: Marshall Mods to JTM 45 what will switch 50 uf to 32 uf filter caps do? that wont change the sound, will it? START NEW THREAD REPLY PREVIOUS LIST Read 300 times From: Trace (voodooamps@aol.com) Date: 10/2/2000 1:31 PM Subject: Re: Marshall Mods to JTM 45 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 START NEW THREAD REPLY PREVIOUS LIST Read 286 times From: John (elgorgon@yahoo.com) Date: 10/2/2000 3:03 PM Subject: Re: Marshall Mods to JTM 45 Ted Breaux, in addition to being an absinthe expert, is quite knowledgeable on this subject. I saved his post from the last time this thread came through (with the hopes of getting one eventually). Here are his suggestions (and I hope you don't mind me posting this Ted): Due to popular demand, I'll post it again. Going from memory, to get a RI JTM45 to sound like an original, do the following in order: (1) Change ALL tubes. Get a Chinese GZ34, Valve Art or GT KT66s, and Ei or Tesla preamp tubes. Rebias the KT66s to around 40ma. (2) Change the filter cap near the GZ34 to a 32X32uF. The other one at the other end of the chassis should be changed to a 16X16uF. These two steps will make a big difference and change the character of the amp. To go further, do the following: (3) Make small parts changes according to the schematic and Ken Bran's handwritten notes which are found on the first schematics page in Doyle's Marshall Book. This involves changing the 470k mixing resistors to 270k, the value of at least one of the small value caps near the Channel I volume pot, and maybe another resistor or cap. You'll need to compare the schematics carefully. I don't have my Marshall Book in front of me, so get your own. This book is a must have for Marshall owners. Needless to say however, the small parts swapping changes the tonal contour further. (4) Change the output tranny to a Mercury Magnetics JTM45 type. Their repro JTM45 tranny sounds spot-on. The amp sounds smoother and has better tonal dimension. FWIW, I believe Obsolete Electronics will have a JTM45 output tranny available soon. (5) If your amp is a repro Bluesbreaker, strongly consider a speaker upgrade. Celestion alnico blues sound like a million $, but the Weber P12B "Blue Dog" is almost as nice, costs less, and handles more power. Unlike Greenbacks (old or new) both of these speakers sound fab in an open back cabinet, and give that mid-60s Brit rock sound. Take your pick. After making these changes, when you play it, you'll find it hard to believe it is the same amp START NEW THREAD REPLY PREVIOUS LIST Read 272 times From: Ian Anderson (colorsound@carrotmail.com) Date: 10/2/2000 5:44 PM Subject: Re: Marshall Mods to JTM 45 What effect will changing the 470k mixing resistors to 270k have? ... Ian START NEW THREAD REPLY PREVIOUS LIST Read 268 times From: Mike Shaw Date: 10/2/2000 5:47 PM Subject: Re: Marshall Mods to JTM 45 And where are the mixing resistors located? START NEW THREAD REPLY PREVIOUS LIST Read 289 times From: David Manson (cattgut@hotmail.com) Date: 10/2/2000 5:56 PM Subject: Re: Marshall Mods to JTM 45 I saw someone selling off Mercury magnetics output trannies on ebay? Are there better ones maybe? where is their website.. i can only find obsolete electronics.. looking for 100 watters.. thanks.. START NEW THREAD REPLY PREVIOUS LIST Read 276 times From: Peter S (Tele52twang@aol.com) Date: 10/2/2000 6:12 PM Subject: Re: Marshall Mods to JTM 45 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 START NEW THREAD REPLY PREVIOUS LIST Read 278 times From: Peter S (Tele52twang@aol.com) Date: 10/2/2000 6:17 PM Subject: Re: Marshall Mods to JTM 45 Ian, Almost no audible effect if you don't have a bypass cap accross the lead channel resistor.......it will give you a little better separtion. If you have a cap accross the resistor, lowering the value to 270k will reduce the apparent effect of the cap and make the bright channel a little less bright......the dull channel a little less dull. peter START NEW THREAD REPLY PREVIOUS LIST Read 256 times From: JimW (nightrider24@hotmail.com) Date: 10/2/2000 6:17 PM Subject: Re: Marshall Mods to JTM 45 Where can you find good caps (coupling)? Are tantalum caps ok for cathode bypass (better/worse than electrolytics? JimW START NEW THREAD REPLY PREVIOUS LIST Read 255 times From: Peter S (Tele52twang@aol.com) Date: 10/2/2000 8:59 PM Subject: Re: Marshall Mods to JTM 45 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 START NEW THREAD REPLY PREVIOUS LIST Read 272 times From: BIG Dave (dave.grover@cox.com) Date: 10/2/2000 10:34 PM Subject: Re: Marshall Mods to JTM 45 Theb URL for Mercury Magnetics is oddly enough... http://www.mercurymagnetics.com/ However if it was me, I'd wait for Obsolete to finish their 100 watt version... START NEW THREAD REPLY PREVIOUS LIST Read 237 times From: Richie{~}==::: (richhall@zoomnet.net) Date: 10/2/2000 10:57 PM Subject: Re: Marshall Mods to JTM 45 Here is a link you can look at to see the mixing resistors. http://www.obsoleteelectronics.com/Marshall_Tech/marshall_tech.htm Richie{~}==::: START NEW THREAD REPLY PREVIOUS LIST Read 235 times From: JimW (nightrider24@hotmail.com) Date: 10/2/2000 11:18 PM Subject: Re: Marshall Mods to JTM 45 Lots of good info- thanks! I'm gonna start keeping notes so I don't ask something again that was posted weeks before. Not meaning to start any flames on caps but I just need a starting place to scrap the caps I have now. These are so small, I wouldn't be surprised if 600v could arc between the leads! They're small 'box' coupling caps (wima mks4 /cecc?) and the cathode/bypass caps are tantalums. How do you weed out the leaky CDEs; with a meter or does it need to be in circuit? Do these companies have web sites? JimW START NEW THREAD REPLY PREVIOUS LIST Read 236 times From: Ian Anderson (colorsound@carrotmail.com) Date: 10/3/2000 1:21 AM Subject: Re: Marshall Mods to JTM 45 Almost no audible effect if you don't have a bypass cap accross the lead channel resistor. So why would anyone want to change the resistor value if there is no apparent benefit or indeed change in tone or anything at all???? ... Ian START NEW THREAD REPLY PREVIOUS LIST Read 222 times From: Peter S (Tele52twang@aol.com) Date: 10/3/2000 2:33 AM Subject: Re: Marshall Mods to JTM 45 Jim, I weed 'em out with a capacitance meter and a DVOM. Yes they have sites. Do a search on New Sensor.....Angela Instruments.....Allen Amplification......Hoffman Amplifiers and you should be able to find the sites. I don't know the URL's I just hit search a look for them. I'm computer challenged. Peter START NEW THREAD REPLY PREVIOUS LIST Read 237 times From: Peter S (Tele52twang@aol.com) Date: 10/3/2000 2:40 AM Subject: Re: Marshall Mods to JTM 45 Ian, It's the same kind of thinking as the people who say a JTM-45 needs 32uf caps to sound good. The original early ones had 270k like a Bassman.....they were earlier, so they must be better. The only audible difference is a little better channel separation, which is kind of moot because all you need to do is rotate the volume control on the channel you're not plugged into to get maximum gain out of that channel. I think Marshall switched to the larger values because they started adding brightness caps accross the resistor on the bright channel. The larger values gives you more of an effect with the cap. I can't realy see any other logical explanation for it. I always use the 470k resistors but it realy doesn't matter in my case because I never bypass the resistor with a cap......it makes the bright channel too bright and the muddy channel too dark. Peter START NEW THREAD REPLY PREVIOUS LIST Read 220 times From: Bryan (bsmith21@yahoo.com) Date: 10/3/2000 3:19 AM Subject: Re: Marshall Mods to JTM 45 Hey guys, thanks a lot for all the information. Sorry if I repeated an oft asked question, but I am new to the site and am primarily an effects building kind of guy. This should keep me busy for awhile. START NEW THREAD REPLY PREVIOUS LIST Read 255 times From: Ted Breaux (tabreaux@neworleans.com) Date: 10/3/2000 4:48 AM Subject: Re: Marshall Mods to JTM 45 Yes, I know absinthe and JTM45s, what a combination. Anyway, my comments can be reposted as you see fit, no problem. My procedure was illustrated as a method of getting a RI JTM45 or BB to sound like the original amps. Many persons seek the fine vintage sound of the originals (which is why they bought a RI in the first place). I realize some of the changes in small parts values seem small, but I find that the sum of small changes make an audible difference. Therefore, consider my proven changes in the intended context. If you want something other than the sound of the original amp, then all of my changes may not apply, although some of them you'll want to make regardless (e.g. tubes and biasing). However, FWIW, I have yet to have a person hear the difference these changes make and be anything but pleasantly surprised. They just don't make 'em like they used to. START NEW THREAD REPLY PREVIOUS LIST Read 208 times From: Dave H. (DHarris@VGGas.com) Date: 10/3/2000 10:33 AM Subject: Re: Marshall Mods to JTM 45 .....it makes the bright channel too bright and the muddy channel too dark. That’s how it sounds to me. Perhaps that’s why people like to bridge the channels to balance the mud with the bright. I like to set it up with no caps across the mixing resistors and no bright cap. I split the cathodes and use different bypass caps to make the channels sound different. If you don’t have caps across the mixing resistors I’d think the only tone difference between 270k and 470k would be the earlier HF roll off with 470k and the Miller capacitance of the next stage but you probably can’t hear it through guitar speakers. Dave START NEW THREAD REPLY PREVIOUS LIST Read 217 times From: Trace (voodooamps@aol.com) Date: 10/3/2000 12:33 PM Subject: Re: Marshall Mods to JTM 45 Perhaps that’s why people like to bridge the channels to balance the mud with the bright. I think most people jump channels to get the extra gain. A great deal of why those amps mush up when the amp is dimed is that the PI is being pounded to death and just goes to mush. It's not a bad idea (for those who dime the amp) to change the .1uF (on the flip side of the PI) to a .022 to help eleviate this problem. Also when the bass channel has 220uF, 250uF or 330uF you're going to get much the more that channel is cranked up. Trace START NEW THREAD REPLY PREVIOUS LIST Read 279 times From: David Manson (cattgut@hotmail.com) Date: 10/3/2000 7:32 PM Subject: Great page - 68 Marshall sound clip... I like some of the clips on this page.. pretty nice... http://www.guitarhacker.com/soundclip.asp?lastMenu=Sound_Clips&lastLink=Listen+to+Clips does this marshall have el34's or kt66's? you think? (under Kris) START NEW THREAD REPLY PREVIOUS LIST Read 184 times From: Ian Anderson (colorsound@carrotmail.com) Date: 10/4/2000 12:08 AM Subject: Re: Marshall Mods to JTM 45 I think most people jump channels to get the extra gain. A great deal of why those amps mush up when the amp is dimed is that the PI is being pounded to death and just goes to mush. It's not a bad idea (for those who dime the amp) to change the .1uF (on the flip side of the PI) to a .022 to help eleviate this problem. Also when the bass channel has 220uF, 250uF or 330uF you're going to get much the more that channel is cranked up. Will this only tame the bass mushyness when the amp is cranked or will it cut the bass at any volume? I tend to run the bright channel on 8 and the normal channel on 7 with the channels bridged. ... Ian START NEW THREAD REPLY PREVIOUS LIST Read 182 times From: Trace (voodooamps@aol.com) Date: 10/4/2000 3:09 AM Subject: Re: Marshall Mods to JTM 45 Will this only tame the bass mushyness when the amp is cranked or will it cut the bass at any volume? It will reduce some bass at any volume. A 220uF or 330uF is a pretty huge cap. most guys find that a 5uF is plenty. The other changes help. When the power tubes start breaking up they also add other harmonics to the tone. I tend to run the bright channel on 8 and the normal channel on 7 with the channels bridged. I think it'll help you. I always left the bass pot off on those amps (stock that is)...same for the Treble. I just turned the mids to 6. Plenty of high-end and plenty of low-end at higher volumes (both channels at 5 or higher) Trace START NEW THREAD REPLY PREVIOUS LIST Read 183 times From: Peter S (Tele52twang@aol.com) Date: 10/4/2000 3:33 AM Subject: Re: Marshall Mods to JTM 45 I like to use a .0022 coupling cap on the shrill channel...No bypass cap across the summimg resistors....split the cathodes using an 820ohm cathode resistor 2.2uf to 25uf(MAX)bypass on the mud channel and an 820ohm cathode resistor with a .68uf bypass cap on the shrill channel. I usually keep the bass pot down to about 2 or 3.....Mids on 10....treble on 4 to 6.....presence control at about 3 or 4......shrill channel volume set on 10 and just add enough of the mud channel to thicken things up a bit, maybe around 5 or 6. Peter START NEW THREAD REPLY PREVIOUS LIST Read 167 times From: Rick Erickson (ampdog@wolfenet.com) Date: 10/4/2000 9:57 AM Subject: Re: Marshall Mods to JTM 45 I think most people jump channels to get the extra gain. My experience on the left coast has been that people couple the channels to ballance the tone. There's plenty of gain from one channel imho, but with a Plexi type Marshall the Bass channel has no highs and the Bright channel has no lows. Put them both together and you get a wonderfully full tone. RE START NEW THREAD REPLY PREVIOUS LIST Read 167 times From: Ian Anderson (colorsound@carrotmail.com) Date: 10/4/2000 12:41 PM Subject: Re: Marshall Mods to JTM 45 I'll have to locate these parts on the PCB of my Bluesbreaker (should be fun). Anyone recommend specific parts - manufacturer, voltage ratings, composition etc... For the record I set my Bluesbreaker as follows... channels bridged normal vol - 6 bright vol - 7 treble - 8 middle - 10 bass - 2/3 presence - 7/8 intensity - 10 speed - 10 I find this gives me a good crunch sound in the AC/DC ball park with my Les Paul which I use all the time (accept no substitute). I use the f/sw to turn the trem off when I don't want it (yes, I do use it) it actually adds a little gain with the trem turned off. I've just added a SS rectifier that Dave H sent me, so I'll see how that works out tomorrow at rehearsals before trying anything else. ... Ian START NEW THREAD REPLY PREVIOUS LIST Read 184 times From: Doug H (dhammond@nospam_gdi.net) Date: 10/4/2000 1:33 PM Subject: Re: Great page - 68 Marshall sound clip... Sounds pretty "Holdsworthy"... Chip's a talented guy. I participated with him in a homebrew tape compilation a few years ago and he submitted an acoustic track that was stunning. Doug START NEW THREAD REPLY PREVIOUS LIST Read 142 times From: Liam (liam@plexi.freeserve.co.uk) Date: 10/4/2000 5:42 PM Subject: Re: Marshall Mods to JTM 45 I've just added a SS rectifier that Dave H sent me, so I'll see how that works out tomorrow at rehearsals before trying anything else. Hate to ask Ian, but you did rebias, right? It will need doing (and I'd hate to hear you fritzed any more tubes):( Liam START NEW THREAD REPLY PREVIOUS LIST Read 141 times From: Ian Anderson (colorsound@carrotmail.com) Date: 10/4/2000 6:33 PM Subject: Re: Marshall Mods to JTM 45 Yup, I rebiased (correctly I hope) according to Lord Valve's biasing FAQ sheet I got emailed off someone - it was really quite well written for a beginner like me. I set the bias to the min available grid voltage first, then set them up for 60% max dissipation which ended up at around 31.5mA each with 438v on the plates. I'm using JAN Philips 6L6WGB's. Guess we'll see tomorrow night if it did it correctly (fingers crossed) ... Ian :) START NEW THREAD REPLY PREVIOUS LIST Read 141 times From: Peter S (Tele52twang@aol.com) Date: 10/4/2000 7:57 PM Subject: Re: Marshall Mods to JTM 45 Ian, Without doing the math, I'd say that you're right in the ballpark with around 30-35ma at 440VDC. Good show! Peter S START NEW THREAD REPLY PREVIOUS LIST Read 129 times From: Ian Anderson (colorsound@carrotmail.com) Date: 10/4/2000 11:10 PM Subject: Re: Marshall Mods to JTM 45 Cheers Peter, I'm looking forward to getting my teeth into this DIY amp stuff as soon as funds allow. It's fun to be able to tweak things without paying someone £15/hour :0) What do you think of a replacement PTP turret board for my Bluesbreaker as a toe in the water? ... Ian START NEW THREAD REPLY PREVIOUS LIST Read 119 times From: JimW (nightrider24@hotmail.com) Date: 10/5/2000 12:25 AM Subject: Re: Marshall Mods to JTM 45 What would be a compromise if you used just one channel: 820 ohm and 1uf and .02 coupling?....JimW START NEW THREAD REPLY PREVIOUS LIST Read 131 times From: Peter S (Tele52twang@aol.com) Date: 10/5/2000 1:08 AM Subject: Re: Marshall Mods to JTM 45 Ian, Msot people would probably recommend a champ style amp for your first project. I can't really argue with this reasoning. They are cheap and easy to build, and they make a great recording/practice and low volume gig amp. I don't se why you couldn't build a turret board for your blues breaker though. They are also easy to build. Get a copy of a Fender bassman schematic and layout diagram and study it for while before you start. The layout is vitually identical to the Marshalls. There are numerous picures of the innards of early Marshall on various websites. I say go for it. Peter START NEW THREAD REPLY PREVIOUS LIST Read 112 times From: Jeff Date: 10/5/2000 7:37 AM Subject: Re: Marshall Mods to JTM 45 I must be doing something wrong. On my old Plexi SL, I run into the top left channel. If I take a patch cord from lower left to upper right channel, I get NO added gain what so ever. Please help. I could definetely use some gain. I dim the treble channel, and then add some of the mud channel to taste (tone-wise). Niether channel has a cap on the pot. Trace, Ian--Where are you guys located? I think this amp could sound spectacular in your hands!!!!Thanks,Jeff START NEW THREAD REPLY PREVIOUS LIST Read 109 times From: Ian Anderson (colorsound@carrotmail.com) Date: 10/5/2000 10:52 AM Subject: Re: Marshall Mods to JTM 45 Peter, Do you know of any pictures of PTP boards that include the trem circuit? - I'd like to retain it when I do the board for the Bluesbreaker. I know there are some pictures on the OEI website, but they're non-trem Marshalls. What specific components would you recommend on the turret board? (i.e. manufacturer, voltage rating etc...) and where can I get these from on the web? (unless someone knows a place in Newcastle?) ... Ian START NEW THREAD REPLY PREVIOUS LIST Read 102 times From: Peter S (Tele52twang@aol.com) Date: 10/5/2000 11:51 AM Subject: Re: Marshall Mods to JTM 45 Jeff, The only way you'll get a noticable increase in gain by jumpering the vhannels is if you have both channels turned all the way up or nearly all the way up. IMO, it's not a very good sounding way to get increased gain.....the bottom gets flabby and the mids get muddy. To me, jumpering is much more useful for balancing the tone between the 2 channels. Check to see if you have a bypass cap on the second gain stage. If not.....add a .68uf cap across the second stage cathode resistor. Try different tubes......especially the first stage. On a plexi SL you should be able to get plenty of gain. IMO, these amps offer about as much gain as you could ever possibly need if you still want to be able to hear the character of your guitar and playing style. I'd say try some different tubes......you might be amazed at the results. Peter START NEW THREAD REPLY PREVIOUS LIST Read 117 times From: Peter S (Tele52twang@aol.com) Date: 10/5/2000 12:04 PM Subject: Re: Marshall Mods to JTM 45 Ian, Check out Randall A's site.....he's got some Polypropylene axial caps that look great.....polyprops sound great IMO, Rebel said they sound very good, which I'm sure they do. The Angela Instruments tin foil caps would also be great, they have a website. Mallory 150 caps also work well in Marshalls and are available from AllenAmplification.com. Minimum rating should be 400VDC. As far as resistor types......any kind will work just fine. These amps don't have enough gain to be concerned with noise. CC's will of course get noisier with age humidity and heat, so I would avoid them if I were you. I can't specifically recall of photo of a trem amp, but I'll keep my eyes peeled. If I get one in for repair I'll take a pic and send it to you. Peter START NEW THREAD REPLY PREVIOUS LIST Read 83 times From: Ian Anderson (colorsound@carrotmail.com) Date: 10/5/2000 10:22 PM Subject: Re: Marshall Mods to JTM 45 Cheers Peter, I'll take a look at RA's site and await some pictures from you (soon hopefully - with any luck) ... Ian START NEW THREAD REPLY PREVIOUS LIST Read 81 times From: Ian Anderson (colorsound@carrotmail.com) Date: 10/5/2000 11:56 PM Subject: Re: Marshall Mods to JTM 45 Jeff, I'm in Newcastle (UK) and I'd guess you're the other side of the pond - just my luck, I'd nigh-on kill to get my hands on a nice SLP (no cash to buy one). Please feel free to loan me the amp - it will definately go to a good home (be still my pretty - lol) :) I plug into the bright channel's top input, then run a patch chord from the lower bright channel input into the top normal channel input. I set the bright channel on 7 and the normal channel on 6 and I definately get a good deal more gain by doing this. On a side note, the trem in my amp (RI Bluesbreaker) has stopped working at normal stage volumes, I can only hear it with the volumes *very* low (1 to 3'ish) - anyone got any ideas? BTW, am I correct in saying that the bright caps have no effect if you're cranking the amp? (anywhere above 6) ... Ian START NEW THREAD REPLY PREVIOUS LIST Read 64 times From: Dave H. (DHarris@VGGas.com) Date: 10/6/2000 1:50 PM Subject: Re: Marshall Mods to JTM 45 I’d think 6 was a bit low. 7 is about half resistance on a log pot. You still have an attenuation of 2 at that point so a bright cap across the top half of the pot will double the gain above a certain frequency (~3kHz). That would be audible. You probably have to turn it above 8 or 9 before the bright cap is ineffective. Dave START NEW THREAD REPLY PREVIOUS LIST Read 41 times From: Mike (jmphilpott@prodigy.net) Date: 10/6/2000 6:39 PM Subject: Re: Marshall Mods to JTM 45 Ian, Did you try tweaking the tremolo pot on the circuit board? I fiddled with this while the guitar was plugged into the normal channel and eventually found a sweet spot where the tremolo effect was at its strongest. Be sure to use a screwdriver with an insulated handle or a plastic tweaker. I got a jolt the first time I attempted to adjust this pot because the small screwdriver I was using had a metal pocket clip on it (ouch!). FWIW, I have to have the Intensity knob set fairly hight (about 6 or so) before the effect becomes noticeable. In fact, on mine the useable operating range for both tremolo knobs seems to be past 5. No big deal, I rarely use the tremolo. The effect is also less noticeable if I have the channels jumped and am running the bright channel fairly high. -Mike START NEW THREAD REPLY PREVIOUS LIST Read 30 times From: Ian Anderson (colorsound@carrotmail.com) Date: 10/6/2000 9:04 PM Subject: Re: Marshall Mods to JTM 45 Mike, My problem is that I can hear the trem effect perfectly well at low volumes at home, but when I crank the amp up at gigs/rehearsals the trem disappears and I can't very well crank it up in the house. I have both the trem pots up full for the trem effect I like. I'll try tweaking the trim pot to the strongest it will go without the trem circuit going bonkers (technical term - lol) ... Ian START NEW THREAD REPLY PREVIOUS LIST Read 25 times From: Mike Date: 10/6/2000 11:02 PM Subject: Re: Marshall Mods to JTM 45 Ian, On mine, the trem pot had a very small range where the tremolo effect was actually appearant. Anywhere outside of this range and it was basically off. START NEW THREAD REPLY PREVIOUS LIST Read 10 times From: Ian Anderson (colorsound@carrotmail.com) Date: 10/7/2000 2:05 AM Subject: dodgy trem continued... Mike, I powered up the amp tonight and it appears that the trem works without the footswitch plugged in, but doesn't work at all (at any volume) with it plugged in. The f/sw definately does work (continuity check) and has done perfectly well in the past on many gigs with the BB. Maybe it's something gone in the trem switching circuit as it only refuses to work when the f/sw is plugged in. ... Ian :( START NEW THREAD REPLY PREVIOUS LIST From: Mark Abbott (abbottmark@hotmail.com) Date: 10/7/2000 4:59 AM Subject: Re:470K to 270K mixing resistors. Perhaps I'm stating the obvious, but doesn't the 5F6A series Bassman sound different when you adjust the volume control of the channel not in use. I know this is definately the case with Tweed Deluxe amps, one would think a Marshall JMP 45 should behave similarly. I hope this helps, if I'm wrong, please correct me. Yours Sincerely Mark Abbott.