Marshall Jcm900 4100 Schematic
Hoping someone has a bit of insight for me here. I have a friends JCM 900 model 4100 hi gain dual reverb.
Comes to me with a blown 4a mains fuse, and the V2/3 500ma fuse blown. So - I do some reading. Amp says its for 5881 tubes. Okay this has JJ 6L6 GT tubes.
Cornerstone of the Marshall “Sound of Rock”, which we have modified over the years to cope with the changing demands of the evolving guitar player. The JCM 900 models split into two distinct product ranges, the Master Volume SL-X and the Dual Reverb. Though they share common ancestry and have similar rear panel functions their.
- Why did Marshall reissue the JCM900 4100 so soon, especially when one from the 90's could be bought for 1/3 of the price of a reissue? I think Marshall should've waited at least 10-15 more years and reissue around the year 2030 MAYBE.maybe longer, if at all.
- Where is the wiring diagram for the Marshall mega stack? Why do none of the wiring sites show the wiring for this amp?
I read some more and from what I gather this should be okay too, but should be biased accordingly. I imagine that this was done at some point, but who really knows.First I took out all the tubes and cleaned the pins and tube sockets. Replace the obviously blown fuses. Fire it up and of course it's blowing fuses.
So I am going to move tubes around to see if there is a bad tube. 1 and 4 apparently are one pair and 2/3 are the other.Assuming we are (for now) looking at tube issues, what is a good method for setting the bias on this amp without installing 1 ohm resistors to ground, and without buying a bias rite type socket tool?I tried to measure the voltages on the tube sockets and the first pin I touched my probe to blew a fuse right away.So - does anybody have a bit of experience with these beasts? Not tons of room around the tube sockets, and lots of panel/pcb mounted components that I'd rather not move if not needed.Any input is appreciated! My suggestion.with best intentions.for the time being.Step away from the amp.FWIW, V4 and V5 power tubes are on one side of the OT's primary.V6 and V7 are on the other side. There is really no 'pairs' there. IF one wanted to 'pair' them off while running only two power tubes, one could run V4 with either V6 or V7 or one could run V5 with either V6 or V7.so there are 4 'pairings' possible there.Your blowing a fuse while trying to take a voltage measurement might indicate that some study of tube pinouts would be in order.
Google 'Duncan Tube Data' and go to 6L6.draw out the pinout that you find there.that helps put things in the mind, ime.You can find the schematic online and study that, too.you need it to understand what is going on there, ime.Have you done any studying on basic electronics and tube technology.and amplifier schematics and layouts? The failure of that 500ma fuse is indicative of a problem associated with that side of the OT primary.power tube/s, resistor, OT???The inside of a live amplifier is a dangerous place if one doesn't have some basic understanding of what one is doing in there. It is dangerous enough if one does have some idea of what is going on in there. Thank you Wally. I know that it blows no fuses without power tubes installed. Without tubes I also know that I have 535 v on the plate and the grid #2 of all 4 tubes.
I have a -50.4 on grid #1 on all 4. The screen resistors installed are 470 ohm, which is what is called for on this amp when using 5881 or 6l6 tubes. I also know that they are 6L6 GC not GT. The 1500 ohm resistors on grid #1 on v4 and v7 are fine as well. I labeled the tubes a,b,c,d.
Tried them all in v4. 'B' blew the output fuse and the rest had voltages of 503 to 510 on plate and grid 2. The 'bias' reference between -50 and -51. I took 'd' and tried it in v5,6,7 and had similar readings. I have a bad tube.
Now I proved what I thought before starting out. It is a tube thing. IME at least.
So now I'm back to the getting the bias set right thing. Pin 8 with only one tube installed is.015v. Marshall recommended to someone on a different forum that to bias the amp you measure grid#1 and set it between -48 and -50. All this sound right?Also tube data shows I shouldn't have more than 450v on grid 2. I know I only have one power tube in and it's over 500. I did try two tubes for a few seconds and it's still 497v. Is this sounding high?
Marshall Jcm 900 4100 Manual
Ridesglide.you have to have at least one tube on each side of that OT's primary.and have an appropriate load matching situation. That means if you have a 16 ohm cab.you can't do that becasue you would need a 32 ohm tap on the OT. If you have an 8 or 4 ohm cab, you can run two power tubes at a match by selecting the next higher tap on the OT.8 for the 4 ohmload, 16 for the 8 ohm load. Then, you could bias for that situation.
IF you are going to be concerned with voltages and biasing for 4 tubes, you need all 4 power tubes in the amp and working properly.I also don't think you want to see that votlage on pin #2 of a 6L6 tube. That is a heater filament.you would want to see either 6.3VAC or 3.6VAC depending on how you are measuring.
Marshall Jcm900 4100 Schematic Pdf
Are you measuring pin 4 as pin 2? PIn 4 should show nearly if not the same votlage as pin 3-the plate.Pin #1 on a 6L6 is unused.it is not attached to any part of the tube's internals. In this case, it is used as a contact point for the biasing voltage. The bias is applied at pin #5.read that voltage there.after the 1.5K stopper resistor.This is my last post on this one.
I advise stepping away. IF you have some electronic experience and know how to keep from electrocuting yourself, have at it. IF not and if you are just getting lucky in your endeavors, then.again.it is your choice. I cannot continue to give any advice. As always Wally. I appreciate your help. It is grid 1 and grid 2 as I typed.
Not pin Wally. Grid - on the data sheet for the tubes it specifies pin 4 as grid 2 and pin 5 as grid 1. The plate is pin 3 and heaters are 2 and 7.
8 is the cathode.I am fairly certain I have this worked out now. I can get the bias current from a couple places on this amp.
I will pick up a tube one my way home from work.Again thank you for taking the time to look over the post. But just so you are not as worried about my safety please note that I was referring to the grids and not pins. Thus far I have done a fair job of trouble shooting the issue. Well at least the one that I can check without 4 working tubes.My two questions on last post were about the voltages to the screens and grids.
You answered that, and I appreciate that. I found the answer to my other question on a Marshall forum.Mark. MY bad, Mark.nto enough cfafeine yet! I am accustomed to pin numbers. Since you are refrerring to the grid numbers, you are doing great. That is an interesting way to find a bad tube.Since the schematic's voltages are with 4 power tubes, I would think that untiil one has 4 power tubes in the amp and it is bias to some point, one wil not know what to think of the voltages. And.suggested bias voltages on a schematic do not establish one certain current draw figure and therefore that bias votlage does not establish one certain plate dissipation figure.
That is what one wants to know when biasing.not just the bias voltage.not just the current draw.but the plate dissipation.With 497V on the plates, one would want to see 42.4ma of current draw for a 70% of maximum plate dissipation.with a 30 watt 6L6. IF one is running a different 6L6, then that equation changes.IF you run two tubes in that amp, then reread that post I gave about mathcing the load to the OT.coudle the load's value when selecting the OT tap. ONe would prefer a match of the OT and the load, imho. Tehn, check your current draw to see what is going on.Say you see.038 amp.38 ma at 497PV.
Youwoudl have a plate dissipation of 62.9% with a 30 watt tube.good to go. But.if you were drawing 19.5 ma at 497Pv, then you are at about 32% of max plate dissipation.kind of cool. IF you were to adjust the bias voltage to increase the current draw, that plate voltage will come down.Good luck.and agin.my apologies for my lack of understanding. All good Wally.
I learn something from nearly everything you post - and I read nearly everything you post ( if you want to be a good putter, hang out with good putters). Having said that, I am going to wait until I have have four tubes.
And having said THAT, I am thinking a quad matched set best as I don't have a tube tester. I still find it interesting that ( if it's true) that the Marshall tech told someone on a different forum that they just measure grid 1 and set it to -48 to -50 and don't get too hung up on the actual current.
Marshall Jcm 900 4100 Schematic
And I also find it interesting that the plate voltage is so close or equal to the grid 2 voltage. Again I can't thank you enough for the advice on the amp and on the safety.I will report back as things move forward. Ridesglide, imho.better than biasing to one certain bias voltage, I woudl advise using the eyes and ears method.adjust the bias to the hot side unitl you see that redplating just begin.and quickly back it away just enough to stop that redplating. Play.if you like it, that is as hot as the amp shoudl be run.
IF you want, back the bias pot away to decresae t he curretn draw soem more.until you hear what you like. There is a wide range of 'correct biasing'.I like it warm. Some people push past that 70% of MPD.I don't. I find the rewards small and the wear on the tubes high.Grid 2.that voltage has a direct bearing on how the tube acts.more subtle than bias voltage but an effect nonetheless. Most guitar amps have a screen grid votlage that is very close to the plate.
The screen is of interest. Study soem ultralinear outputs some, and you will see that UL OT's have a different effect on the screen than non-UL OT's. Don't tell some of those boutique amp owners, but there are UL OT's in some very much praised boutiques.and they are owned by folks who wouldn't buy a UL Fender for any amount of money!?! Well maybe I'm not so smart after all - Or maybe my meter isn't working - I am using an old Fluke 77 from the 80's but I'm getting no current readings on it. I have the red lead into 300mA, and the dial set to Amps DC -I do have the tubes in, the amp plays (sounds pretty good too) and I have the -49 set on PIN 5 with the plate voltage now is down to 477 with all four tubes -This I read and tried to follow -The output transformer shunt methodThis is the way most pro techs measure plate current. A.good.
quality DMM is required for this measurement. (When it comes to good DMMs, you have three choices.Fluke, Fluke, and Fluke.) This section assumes that you know a bit more about your amp, and how to use your testgear. If any of it is unclear, DON'T TRY THIS.WHAT YOU WILL DOA) Read the current flowing through each leg of the output transformer's primary.B) Read the plate voltage.C) Use the above readings to calculate the static dissipation wattage.D) Adjust the bias to obtain the best tone, while keeping the tubes within specifications.For this particular reading, you'll need to change your test leads to the CURRENT input jacks, and select the 200 mA DC range. The two probes are applied to the center tap and either of the ends of the output transformer's primary.
(On a Fender, for instance, the center-tap is RED, and the two plate wires are BLUE and BROWN. On a Marshall, the center tap is BROWN, and the plate leads are RED and WHITE.)On some amplifiers, the easiest way is to put one probe on pin THREE of either socket (or of either of the two sockets on each side) and the other on the center-tap, which will be located at some distance from the socket. Some amps (like the Marshall JCM 900 series, for instance) have all the wires soldered to terminals on the bottom of the output transformer, conveniently sticking up right where you can reach them.The current that would normally flow through half of the transformer's primary winding is 'shunted' through the meter, and thus measured. A small amount still flows through the part of the winding you are shunting, but the transformer's resistance is much higher than your meter's internal resistance. IF you have 6L6GC tubes in that amp.or any other 30 watt max plate diss. Tube.at 477Pv and 40ma of current draw, then your plate dissipation is(477 x.04)/30 = 63.6% of max plate dissipation.Your first current draw fugre of 44.8ma at 477PV yields a plate dissipation of 71.23%.just a bit on the hot side of 70%.which is that plate dissiaption factor that most of us hold as a 'hot as it needs to be' point.Plesae note that with a change of current draw, one needs to recheck the plate voltage. It will 'swing' a bit with the change in current draw.
IF you backed off of the 44.8ma to the 40ma current draw, you will see that plate voltage go up jsut a bit.maybe no more than a volt.depends on the size of the change in the current draw, right? I kept playing with that Wally. I found it interesting that the statement about making the reference voltage (my take on pin 5) is so close to accurate. Get it either side of -48 to -50 and you are either too low or too high. Low 60%s to just over 70%.
So, having said that I have it up where i thought it was about 65%. There was a red glow on all the tubes, but nothing that looked violent. And, IMO, and since I am not really a 'Marshall Guy', the amp sounds pretty good!
I was able to dial in tone that I could live with if it was my amp. Of course it won't be my amp. I hope he gets a few years out of this set of tubes. He plays a lot and he plays at volumes I have not been familiar with since HD stopped making shovel heads. Did I really play a Twin for 25 years?!?!? Hard to believe now.