JCM 900 4500 50 Watt Rebuild and mod
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- Опубліковано 18 жов 2024
- This video takes a look at the rebuild and modification of a JCM 900 50 watt 4500. It covers the replacement of filter caps, and transformers, along with a filter choke installation, and a few mods. AND just to see who is paying attention....I have hidden a little something awry in there for the Hot Shot Tech authorities.
Been using Marshall amps since 1984. I love all of them...I have owned many different models, all the known great ones except the first Plexis. The 900 4500 is one of my favorites... It's unique. Been using JCM 2000 DSL 100s for at least 10 years, gigging with them. I love gigging with a two channel amp as I can dedicate one channel for soloing. I have also modded them by replacing the transformers and adding a choke. Very nice upgrade. The correct tubes are huge in these amps as well as a proper bias...on the cooler side, cooler then what Marshall recommended. However, I ran across a really clean 1994 900 4500 for a decent price. I changed the bias network to accept EL 34s and have installed Gold Lion KT-77s. I have also filled the preamp slots with vintage American preamp tubes from the 1960s. What a huge change. Man compared to my DSL 100s in which I can emulate a 2204 almost identically, and then do so much more...the 900 is much more focused and tighter! It also has allot of balls...really cuts with this setup. Great amp for drop D tuning... I ordered a 3H choke and and found a Classictone OT (yes they are still around, but hard to find...OUTSTANDING transformers) and will have those installed soon. One thing I need to add. We all know it is important with a 900 to patch the FX loop if not being used to get more of the preamp signal to the power section as it sits in between... but few know how incredible it is to add a BBE Sonic Maximizer to the loop. NIGHT AND DAY difference! The amp with the BBE is a tone monster...huge improvement in NATURAL soundings dynamics and fidelity... (set conservatively)... I also need to add that unlike the DSL who's reverb sucks tone, the 4500's reverb is EXCELLENT and cuts nothing from the amp's cut and sound. Very underrated, reliable amp...very... Honestly, by adding the BBE you really need to do nothing to this amp. I cannot express enough how much of a different it makes. More so then any other amp I have tried a BBE with.
Wow, a decent comment is a rarity on the Marshall vids. Glad to hear you are biasing cooler. The common concept of biasing hot is just wrong on so many levels. The first being tube life, lack of headroom and definition. Bottom line is higher gain amps need to be biased cooler, so they do not squash out when driven. EVH's for example like to be biased around 20 ma per tube. I have also used KT77's. For a while when EL-34s were hard to come by several years back I used them a lot. Yes, old American and European preamp tubes do sound much better than any of the current production stuff. Unfortunately, I am running low on stash and use them sparingly, and only for special jobs. Sadly a lot of what is for sale out there has been picked over for decades now and much of what is left is unuseable. I chuckled when I read the Sonic Maximizer comment.... locally, a few years back no self respecting metal guitar player was without one.
I have both of those amps unfortunately my 2000 DSL 100 went out, that amp is LOUD! I'm a simple Rhythm player and I like palm muting raking as I play pop and skate punk. I run both with some kind of overdrive which helps to tighten up the amp nicely. I really need to get that JCM 2000 fixed because I like the EL34 warmth, my 900 has Sovtek 6L6 (bought it used with those).
I used to sell these when they were new. I only played the MK III, but here, I'm hearing some nice verb! Pleasant surprise.
I have been searching for you for a while . I have the same amp and it's been in the shop several times and the put new tube sand pots and its still cutting out .hopefully your still working on these amps and if not to expensive for my budget I wouldn't mind doing what you did to this amp. I've never been a fan of Marshall and it was kinda forced on my with a guitar purchase that was a package deal. Originally I tried to buy just the guitar but he insisted I take the amp and 4x12 cab. It never worked good but when it did work I love the tone out of It. No wonder Billy Gibbons used it on his Eliminator tour. From research they were ot reliable. After watching your repair video I understand why .
I finally completed the mod. With a Hammond 194g filer choke and I did not change the transformers as they are not available in Europe. Amp sounds really great thanks you for this video was extremely helpful.
Thanks for the comment, and I am glad it helped you.
Oh sorry hadn't gotten to the ending of the video... Will listen to the sound clip... Thanks for this video
The cleans are chimy ... To my ears I can hear some Collective Soul cleans in there
The force is strong with this customer's wallet. Great stuff!
You would be surprised at the reasons some things were done. Thanks for watching!
The 900 50 watt is my favorite amp. I've had one since 95 or 96 and they can sound AWESOME with the right setup. Mine is all stock, but I would love to get my hands on a second to do some mods on.
Very cool! The dual reverb models are among my least favorite Marshalls, but I have to admit my customers have pushed me to squeeze as much out of them as possible. Some of the results are amazing!
I think mine has a bit of an intangible mojo to it. I have yet to meet anyone who doesn't love how it sounds. When I plug my 79 Les Paul loaded with a DiMarzio X2N into it, it's beastly.
Do you have any sounds clips of this amp? I bought the same amp today... Be interested in what you charged for this work... I don't know if I would want to change transformers but the other mods seem like good stuff
I highly recommend looking through the channel and checking out the other 900 videos, I cover a lot if information there. Plus check the comments, a lot of questions are answered there. Also, I have a reason for changing the transformers. A lot of people feel changing iron does not accomplish anything. They also feel you do not "hear" the power transformer.
However, Performance, tone , and response all depend on the quality of the transformers. Everything you hear is based on the PT. I have my other "real" reasons for changing the OT, and I mention that a couple times in the other videos. Thanks for checking out the channel.
I’m digging it too
I’ve watched all your Jcm 900 video’s and I was wondering if you could work on my Jcm 900 -2100 series? It keeps popping fuses even after I bought new tubes and this tube amps are still new to me
e-mail me at gtrcpr.r58@gmail.com We will discuss it. Thanks for watching!
@@guitarc.p.r.9675 hi I Really liking your videos on the JCM900 and was curious if I can send my 4500 series 50w.head…to have it moded and do what you need to do to it and about how much would it cost thanks
Could you also provide a link for the bias feed cap you changed out to handle the 300+ volts passing through that part of the circuit? Thanks!
It is simply a Xicon 630 volt cap.
Thanks... Digikey has the following options for a Xicon 630 volt capacitor... which one is the correct one for the JCM 900 4500: 2200pF, 4700pF, 10000pF, .1uF, .22uF or .47uF? Thanks again!
The schematic says it is a .047
@@guitarc.p.r.9675 yeah... i found that and realized i just didn't think more before asking this question.
Good job. I wonder why that under valued cap was put there? I love Marshall amp repair videos. My favorite Marshalls are the TSL60 and DSL50.
I do not know but they have blown a lot of 900s.
I've read changing the C5 cap to a quality film helps tone. Your thoughts?
Can't hurt.
Hello, Love your videos and thanks for the tips and mods info. I do have a question that may have been answered in the video but I haven't caught it. What is the capacitance of the 250v bias feed cap that you replace with a 630v cap? Thanks for all the vids and info you put out. I really enjoy the vids.
.047 uf. Read all the comments, this and many other things have been gone over. There is a lot of info in the comments as well. Thanks for watching.
Great video. Does anyone know if any component improvements were made in the 4100 reissue?
Sorry I have not seen one come across my bench yet.
Awesome
I like the nuking of the fake tone control circuit. Did you leave the 100k R55 to ground on the non-inverting input of IC4b? No disclaimer needed, I have been repairing tube amps for 38 years and am aware of the nastiness inside. (Pro tip: Don't use your oscilloscope on the tube section then switch to the IC section, the cap in the oscilloscope will charge to high voltage then wipe out the ICs. LOL Don't ever ask me how I know). And why did they only attach one side of the filter cap when the schematic says both? Ha, Marshall seems to have a hard time following their own directions. One other question, was the effect of changing the slope resistor R61 more low end?
Sounds like we have been in the game for about the same time, I may have a few years on you though. As for R55, I replace it with a 220k. I will mention again, that I see a lot of times the 2nd half of a paralled cap pair being left unattached, one of the great mysteries of the world.......or assembly lines/ QC what ev... :) I am just waiting for another hotshot to correct me for it when they don't even realize which supply it is.....yes it happens...yeeeesh. I changed the slope resistor to shelve the tone stack higher and remove some of the low end muddiness. Thanks for checking out the channel.
@@guitarc.p.r.9675 Thank you for the reply. I'm getting a bit worried about tube supply. Who knows maybe this Russian deal will spell the end of my love. I think you do nice work. One thing is for sure, your production work is far better then mine. Have a Happy Easter and a great week.
Thank you and to you as well. I would not sweat the tube thing too much, I have seen this before. I know of two current cases where it situation is just being used to drive up the cost of product. Yes I have sources.
Great video. Would you be able to share the specs of the components you have used? Thanks.
It is all in the video
@@guitarc.p.r.9675 thanks, I managed to find them all but not this big Mercury power transformer. It does not look like a JCM900-100-P.
It is not. The amp in the video is a 50 watt. That is most likely a Fat Stack for that amp. I can go back through last year's receipts and find the exact model#, but that will not mean anything to the 100 watt. Thanks for watching!
If I may ask, why do you move those caps to the tube sockets? And if my Jcm 900 came with 5881's and I want to put EL-34's in it, do I need to change some resistors or can you just re-bias?
Grid resistors, aka "swamp" resistors are in the circuit to limit ocillations and noise, and stabilize the amp. They are meant to be as close to pin 5 as possible to do their job, not several inches away connected by a lead post resistor. In a Fender style amp they are 1.5k resistors and are mounted on the socket between pins 1 and 5. In a Marshall they are traditionally 5.6k and connected directly to pin 5 and stand straight up lifting the lead as far away from the other leads as possible especially the heater leads. Hope this helps, and thanks for watching.
@@guitarc.p.r.9675 Yes thank you! Im just recapping my 900, so just looking for a few extra ideas mostly! I appreciate it.
Yes, if your amp came with 5881s and you want to go to EL-34s you should change the screen resistors, and grid resistors accordingly. You will most likely have to tweak the bias circuit as well. Also in some cases the feedback loop must also be moved to a different speaker tap. I would also consider an OT change, to match 34s plate load. But that is just off the top of my head.
@@guitarc.p.r.9675 Right, ya that makes sense. I know what you mean though. Its hard to make a plan of attack/give advice without seeing the amp. But thanks, I appreciate the feedback. At this rate, Ill probably just keep my 900 with 5881/6L6's. Ive always liked the way mines sounded, but it just needs a recap and new tubes. Im actually recapping most of my amps this year. May do a few other things while im in them, like replacing that underpowered component, move grid resistors, etc.
Email me..... I want to explain something in more detail concerning converting to 34s. Even if you are not going to do it you should know this.
Where are you located?
Can a 50w 900 be retrofit to a 100w ?
What did the labor cost to do the rebuild in this particular video?
I am in West Virginia. Yes 50 watts can be bumped up to 100. If you are replacing transformers. Two tube sockets will need to be added, plus a new set of output tubes. I refer to this particular mod in the video as "New amp for under a grand" The bump-up would be slightly more.
Cool,
I live in MD, I'll send you an email and try to explain why I asked such a question
Please describe in greater detail what your doing with R61 and C37. Could you provide specific details/specs/part number links for your replacement comps?
You are just messin' with me aren't ya? I changed the ceramic capacitor to a silver mica cap, and raised the slope resistor to 56k.
No... I'm just not an electrical guy. Schematics can be confusing to me sometimes. I'm not afraid of soldering... i just get uncertain when i go to component places and see all the options to pick from. I appreciate your help.
Hi!
I realy enjoyed your jcm900 Videos, since I have a Couple of 'em myself.
Unfortunally the Reverb on my 4501 (50w Combo) doesnt Work, I checked the solder joints inside of the reverb Tank and the RCA Connections (on the Reverb Tank & on the PCB), and changed the Tube in V2 without any success.....
Do you have any suggestions for this issue?
Thanks and greetings from Germany.
Hello Germany! Glad to hear you are enjoying the videos. I will try to give you some direction. It is hard to diagnos a problem without having it in front of me but here goes. OK, first measure the transformers inside the tank. You should have a similar resistance on both. If one is reading open you will need a new tank. If all is good ther move on , does the tank make any sound with the controls maxed and when you tap on the tank? You can also check this by simply touching the tip of the RCA plug for the reverb return. Chances are If you get hum here the return circuit is working. If not then the promlem is in the return circuit. This can be the the return IC 7, or the switching Jfets TR2 and TR3. However if you do have sound from the tank and the return circuit is working the problem may be IC 2 or 3 in the drive circuit. Also make sure that V2 has not had a component failure and that it is actually working. If you have a circuit problem a new tube will still not work. Hope that helps, and thanks for watching!
@@guitarc.p.r.9675Thanks for your reply!
Couple days ago I checked the Transformers in the tank, both showed a value on my Meter, but I cant remember if they were similar, but i will check it next time. The Reverb is maxed out but I can barely hear it when I tap on the Tank (maybe i'm wrong but I have the feeling that the barely audible Reverb is only on the Lead Channel, Not the Clean Channel...?)
I will also check your Suggestion with the Return circuit.
@@guitarc.p.r.9675 Hi. the Reverb is still Not working but I have a more serious Problem with the Amp.
The Tubes were Red-plating and the bias Ran away when I bought the Amp.
I fixed this issues by exchanging the Infamous C15 Cap in the bias circuit, the bias was way to cold when maxed out, so I lowered the value of R7.
After that i could bias the pair of new el34s properly, but the max Amp Volume was still way to Low on both Channels, since the Amp was completely Stock I replaced all electrolytic caps on the Power- and Preamp board.
Still the Amp is not loud enough, of course I checked the FX-loop with a Patch cable > Same result.
The Voltage on the Powertubes seems resonable to me:
3rd Pin: 450V; 4th: 450V; 5th: -32V (on both Tubes)
Now i dont know what Else could be the Problem, do you have some Suggestions what to Check next?
Thanks a lot in advance!
The first thing I would do is plug a signal generator into the return and scope and measure the output signal of the power amp and phase inverter. Also a bit odd that the plate and screen voltages are the same the screen should be a few volts lower than the plate. If all is good there then I would move to the preamp. About 90% of the preamp is IC driven, tip here.... a damaged or bad IC will feel hot to the touch sometimes. Make sure your power supply voltages + and - 15 volts are present. There is a lot that can go wrong in these solid state preamps, check the clipping diodes as well. Pay special attention the the Jfet switching transistors. This is where signal tracing each channel one stage at a time is neccessary. It does not surprise me you are having multiple issues in the preamp, I bet you will find a common problem that relates to all of them.
@@guitarc.p.r.9675 Thanks a lot for your reply!
OK its my bad, I rounded the values of the Screen and grid voltages, actually they were: 453, 450 & -39V / 452, 451 & -39V.
I'll try to have a Look on the Preamp section with the ICs, but since my knowledge is very Limited, maybe i'll have to Bring the Amp to a Tech....
Big Thanks to you for your Support, I really appriciate it!
What is the difference between premium transformers ie the ones built in usa canada and europe. I ask because Mercury Magnetics Transformer for my Jcm 800 are $400 more than from Heyboer Transformers for the power and output transformer and a chock. Is there any difference in tome or just a difference in price?
Heyboer Transformers
There is a difference, they don't just charge more because they can. I have used just about every transformer made, and I choose Mercury as long as the customer will allow me. Most of whom are floored by the results, one particular 900 comes to mind. They also have an incredible warranty. I have had them also make a lot of custom/ special order units for my own purposes. Customer dervice is great . Not so much with other US manufactures, such one which is now defunct thank God. My opinion, you are welcome to form your own.
Hi any idea if this mod would work with a 4501 combo with the jmp50c board
I am pretty sure that is the same amp as a combo.
what did all this cost the customer? I'm asking because id like to have all this done to mine
I refer to this as a new amp for under a grand. If you wnt to discuss it in more detail, my contact info is in the about section.
Can you do the same mod to my JCM 900? It is a 50watt, 1 X 12 with el34's.
Most likely so. Thanks for watching! Email me we will discuss it. My contact info is in the about section on the main page of the channel.
R12 part of the tone stack? I removed R7 and R43
Yes it is. Along with many more than R7 and R43
@@guitarc.p.r.9675 can you tell me which others to remove? 🙂
I copied this from a former comment . Sure, but first my disclaimer....Again these are not How to videos. Amplifiers contain lethal high voltages and should only be serviced by trained, experienced, qualified technicians. Now for your request, it is not rocket science, looking at the schematic you can see that I not only disabled the faux tone stack, but eliminated it completely. Also in order of importance the mods are; Bias mod, then output grid correction and then the preamp mod. So the Guitar CPR Lead channel mod is as follows....
Remove resistors R7, R12,R40, R43, and R55
Remove capacitors C1, C10, and C27
Replace C27 with a jumper wire
Replace R55 with 220k resistor
In the main tone stack..
Replace C37 with a 500 or 470 pf Silver mica capacitor.
Replace R61 (slope resistor) with a 56k resistor
@@guitarc.p.r.9675 i completely understand, yes they can be dangerous. I’ve been doing small mods like this a long time, so i have some knowledge to be safe 🥴 🤣.
Thanks so much man! I’m gonna give this a shot.
So far i removed R7 and R43. Places jumpers over C27 and D10. There is more organic sound now and better low end.
It concerned me that you did not know what resistors were in the tone stack. BTW, do not put jumpers over parts, remove them and replace with jumpers.
Why cant you be in my area?!
I hear that a lot! :) Why are you not in mine... People ship to me all the time.
Sorry but to me it sounds terrible. Maybe better in person and just bad recorded sound. Mine is stock and sounds great for my musical tastes. Non metal person. Upgraded the one cap to higher value and that's it. Blues/rock/light jass
Exactly why I do not do audio clips. The amp sounded great. The audio was accidentally captured after showing him a clip on the camera. I hit the record button while laying it on the glass counter. So I decided just to throw it in. Another reason is cell phone just do not convey well and that is what most people are watching the videos on.
Do you have an email or private contact that I can get in touch with you about some mods?
There is an email address in the about section as a business contact.
@@guitarc.p.r.9675 Okay, awesome. Thank you! ;)