Since this has been one of the most viewed videos and has generated the most comments, I feel the need to add something overlooked. First of all I want to mention that is was never meant to be a "How to Video". If you feel the need to ask questions about what was done, you may not have the ability to do it correctly. Now, that being said, this ONLY WORKS on the PCO112 JMP50 Board. If you choose to attempt it, be sure it is that board only. Similar boards have the same resistor numbers in the same places on the board but are entirely different. In some cases R26 and R27 are 150k bias feed resistors, not grid resistors as in the video. Just a heads up and thanks for watching!
Great video. It's kind of hard to see what you did with the R24, R25, R26 and R27. It looks like you removed them all and added jumpers across R24 and R27. What kind of bridge work was done underneath? What value resistor did you put on the tube socket?
i got a pretty old jcm 900 100 watt head that has been sitting in a smokey basement for years…original problem is that it kept blowing fuses, just really never had funding to get it properly fixed. Sadly, i think i need to sell it to pay for other life stuff…. anyone have any idea of what i should attempt to sell it for? …truly wish i could use it again, but i no longer play with live bands
I've got one on the bench right now and spent a whole day trying to figure out and studying all their schematics (all versions, 5881 and EL34) on the grid resistor issue. Makes absolutely no sense whatsoever. I know this is old news for you, but thanks for the collaboration. Replacing the "X' class bias cap with the 600+ volt is the right move also, in my opinion. That's what I do.
I'm having a problem with a JCM 900 4100 with the same version circuit boards. The two outer power tubes are drawing idle current but the two inside tubes are not. I have good bias voltage and plate voltage on all tubes. Any suggestions?
Check screen voltages. The screen resistors have a bad habit of the solder joints failing. If the amp has failed previously and new tubes were installed, the screen resistors could also have failed. The more likely scenario is that the 500ma fuse on the cathodes has failed. If so the tube failure led should be lit up. There is one fuse and led for each pair of tubes, center and outside pairs. Just guessing though.
I gather the grid resistor cock-up is peculiar to the 100 watt. In learning the "language" of the amp so I could work on it after coming from a black face Fender perspective I found my 50 watt bias system is as it should be and it's a pretty early example. First thing though, I replaced the volume and sensitivity pots which were shot. I then eliminated the resistors on the board the way you do and put 5.6k 1/2 wat on the tube sockets as well as 1 ohm 5 watt biasing resistors. Actual "mod" mods will at a later date.
Yes the main problem with the grid resistors being in series is only on the 100 watters. However, the grid resistors should not be on the board and that is common on both the 50 and 100 watt models.
GUITAR CPR> Is the Chokes storing current like a reservoir like the filter capacitors or is the Choke doing something else to the AC ripple, not sure if the Choke is regulating the current? Most Power tubes 6L6,EL34 use a B+450vdc on the plates, if you increase the B+ to +600vdc would this increase the AC ripple but why would it increase the AC ripple? i would think it would decrease the AC ripple because more B+ voltage mean less current draw from the power tubes. When you're biasing the power tubes in an amplifier circuit how do you bias the compression of the power tubes the 1dB compression point of when the power tubes start to compression? because its say to bias tubes at a 30mA bias current per tube to be safe but how do you know when the tube is compressing?
I do not see the correlation to the video but I will try to answer some of the questions. First up, a choke does not store current or voltage. A choke is used in series with the power supply usually between the Main B+ and the screen supply to smooth and filter out 60 and 120 Hz hum and ripple. Think of it a a large wire wound resistor with an expanding and collapsing magnetic field. As the tubes are driven the screen current goes up. The choke's IR drop during power demands reduces voltage going to the screens and creates a natural compression factor on the output tubes stabilizing their operation. The input choke on tweed style amps is a typical PI filter. This type of filter drops the voltage of the entire amp under current demand resulting in compression commonly known as "sag" Second, raising B+ voltages is not a solution to ripple, and for some reason the industry has been leaning to stupid high voltages. Third, The -1db compression point is a point on the linear operation curve of an amplifier. This is the point that the amp no longer responds linearly to input signal saturates, and gain drops off. It has nothing to do with biasing. As for biasing, The dumbed down catch all of 30 ma. is a overly repeated falsehood. Bias involves Plate Current ratings, plate load, Power supply and Screen supply voltages, and other variables. An amp with a plate load of 3.5 k will draw more current than with a 6.6k load with all things being equal, therefore it will need more negative bias voltage. Likewise a low PC# tube will run cooler than a higher PC# at the same voltage. Hope this helps. Thanks for watching!
@@guitarc.p.r.9675 Thanks for the help. When you Decrease or increase the B+ voltage it changes the screen voltage and plate voltage which is changing the bias operations of the preamp and power tubes which is voltage biasing? 1.) How much current does one power tube draw at its maximum? because each filter cap in the power supply can only holds & stores about 1 amp of current because the formula is Capacitance X rectified voltage = current stored in filter capacitor 4700uf X 450vdc B+ voltage = 1 amp of current stored in filter cap 2.) When using very low capacitance guitar cable when the amps input Z impedance is 1Meg it will "roll off high frequencies" but when you change the amps input Z impedance to 3 meg or 5 meg it will not roll off the high frequencies, any reasons why? 3.) When using a Variac like eddie van halen & Randy Rhodes did by setting it to 90VAC this lowers the B+ voltage on the amplifier. Does the ripple voltage increase or decrease when you use a Variac set to 90VAC? Because most amplifiers in general used a B+ voltage at +450vdc so lowers the 120VAC to 90VAC the B+ voltage is I'm guessing +250vdc to +300vdc. Biasing is balancing the voltage & current through the power tube. Because when you lower the plate voltage B+ voltage to the power tubes this is also biasing the power tubes? 4.) If an amplifier is designed for 6550 and you install KT88 or KT66 tubes. The power amplifier output impedance is designed for 6550 or EL34. What sets the power amplifiers output impedance is it the tubes plate DC or AC impedance? Because you would have to change the output transformer because the output transformers primary impedance was designed for 6550 or EL34 tubes and not KT66 or KT88?
Great video! I just recently picked one of these really super cheap, but it has some problems. I have one of the power tubes getting extermly hot. Any ideas what this could be? Wished you was in NC I would let you work on it... Not many people work on amps around here. I do little work on them, but not much
If it is a 100w I would venture to say you have 1 bad tube. But you should use a "good" matched set. If it is a 50w the same still applies except you may have a bad Phase Inverter coupling cap. I have even seen PI tubes cause a current problem. Check the Blues Jr. video. You could always ship it to me, a lot of people do.
OK Guys...for those of you who keep questioning the replacement of the .047 250 volt bias feed capacitor....I replace it with a 630 volt cap because it is connected directly to one side of a 375 volt AC winding. Wonder why the originals fail?
@@robert2948 OK, first it is a polypropylene film capacitor. There is also a similar version known as a "safety cap." Mouser Electronics has high voltage ones. Now, forget about these caps and get a good quality .047 uf 600v-630v poly cap, ie; orange dropetc., I use the Xicon here because it fits the circuit board holes easily. Thanks for watching.
That .047 cap is a class x cap. Designed to short if they fail blowing a fuse and not electrocuting anyone. Should be replaced with class x. Your thoughts?
Hello, I've just bought a Mk3 2502 50W combo. Checking the power tube sockets on mine, R24 and R27 are connected through green and orange wires only to pin 5. Pin 6 doesn't have any connection whatsoever. I see in the video that the resistors are jumping pin 5 and pin 6, so I assume if I'm removing R24 and R27 from the board to install them in the tube socket, I would have to jump pin 5 and pin 6 too. Or is the 50W different than this 100W from the video, and I can leave pin6 alone and only add the resistor to pin5 (in series with the wire)?
To add to my comment, doing a bit of research, EL34 datasheet doesn't even mention pin6, and also checking my EL34EH tubes, they don't even have a physical pin for pin6, so I guess it is safe to jump pin5 and pin6 with the resistor. I was measuring my bias current, and it was 38mA in the outer tube (the one next to the filter caps) and 28mA on the inside tube. These are supposed to be matched tubes, but I didn't check with another matched pair (I'll do that later), but would you account this 10mA difference to the resistor placement?
You are correct Pin 6 does not have a connection in the tube. Connect 5.6k grid resistors directly to pin 5 alone. There is no need to connect them to pin 6. Stand them straight up. AND change that bias cap! As far as the current on the tubes, They should be within 5 ma of each other to be matched. I like to measure plate current only so if you have a screen problem it does not reflect on the plate current. You may have a screen resistor problem on the low side. I also highly recommend looking through this channel for the other 900 videos, watch them, and read the comments as well. There are a lot of questions answered there. Thanks for watching, I hope this has helped.
@@guitarc.p.r.9675 Thank you for the quick answer. By bias cap you mean that one that's 250V but should be 600V, right? Mine currently has a 47nF 250V. I don't have a replacement currently, so I'll order one online. The unmatched tubes, I will test tomorrow with a pair that I know for a fact is matched because when I biased my plexi-1987 last week they were only 2mA apart. If those also give a higher difference in the 900, then it might be a problem indeed. Cheers!
my jcm900 head has been out of commission years before u posted this video, ( and im just stumbling across it!) ...my head was never modified in any way, it just kept blowing fuses left n right...and RadioShack was never any count as far as fuses go...ive replaced the tubes ONCE because someone 'professional' advised it...no dice!. the last shop i took it to wanted to charge $300 just to look at it!...so here it still sits, almost 15 years since last used...really wish i had someone with your expertise nearby, because of all the gear ive let go of, THIS one is one i just dont wanna part with
@@guitarc.p.r.9675 , while i very much appreciate the offer, i probably couldnt even afford shipping charges, let alone your cost to fix it and send it back! LOL...im top-notch broke at the moment, and my wife would definitely have objections. LOL
Hello sir, I could use your help - it appears C15 has given up the ghost - oddly this happened when I had the chassis out to bias new tubes - everything else looks to be ok. In order to replace this component the entire board has to come out right? I was hoping to not have to go to that length but I don't see any other way to get at that cap. So not only is it prone to faiure, it's a pain to get to also ;)
Excellent work. For the love of God, why would anyone change an MkIII over 6L6's? They aren't bad in a Dual Reverb or SL-X, but it just seems like heresy to convert in MkIII.
@@guitarc.p.r.9675 You verbally and visually pointed out how a previous tech installed 6L6 output tubes when you turned the chassis towards the camera to show us. That is the relation to my comment.
Ok, It has just been quite a while since I did that video, and I was not sure exactly which 900 video it was. Actually, I sometimes use the JJ 6L6s in the 5881 models for the really heavy guys. They do not "mush out" as much as the 5881, the bottom is much tighter and defined. They handle gain better, and they "chug real good". In this particular amp he did not like them as he said they were just too much in the bottom, and wanted to put 5881s in plus take care of some age issues. Turned out his bass issue was actually a WAY off spec bass pot. Thanks for clearing that up, and thanks again for watching.
Understood. I too have 6L6’s in my Dual Reverb. The amp in this video is an MkIII and all of those were manufactured with EL34’s. So someone (previous owner and previous tech) went out of their way to convert it. At any rate your videos are great and a wonderful learning tool. Thanks for making them.
I must apologize, I had completely forgotten about that video, and was thinking of another. I am a repair guy not a cinematographer. I have a few 900 videos. I had forgotten how messed up that amp was. I may have to refresh my memory on this one ....hummmm 2017. Old one. Thanks again for watching, and I am glad you are finding them at least a little helpful.
That JCM900 model 2100 is using a JMP50B PCB.....it looks like you are looking at a JMP50C PCB version which is why it doesn't make sense. The JMP50B version should have 2k2 resistors at the sockets in series for all 1k5 resistors to all tubes but I'm not seeing them here in the 'before' images so perhaps they were removed prior by somebody(?). So they should all be the same. Check here: drtube.com/schematics/marshall/cd0192-iss7.pdf . I could be wrong, I'm not an expert. Marshall can be confusing sometimes by using different types of PCB's for the same model depending on the year. That mod for the resistor at R28 for the bias pot range makes a huge difference though. I've done this before and I no longer need to special order tubes
Thanks for the comment. And you are correct with the schematic you are showing, but that was not the case with the amp in the video, and it was also dead stock when it came into the shop. Yes, it was an early one with a wrinkle to be worked out. The series resistors you mentioned are still a bad idea, in this case adding up to 3.7k. Also a reason I mention changing these to 5.6k and moving them to the sockets for stability.
Man, I thoroughly enjoyed the video my friend... I haven't had the time to play in a couple of years due to chasing my 3 yr old and 2 yr old twins around being daddy....hehehe but I plan in the future to teach my boys how to play guitar... I started when I was 15 around 1980 after being so lucky to have the once in a lifetime opportunity to strum Paul Stanley's original Ibanez Iceman while it was on loan for display at Strings and Things in Memphis... Paul was using his new broken mirror Ibanez during the Dynasty Tour at that time and he was buddies with the guitar shop owner and the guitar was still in its fligh case when he showed it to me. I was blown away... so, a couple weeks later I bought my first guitar due to Ace's favorite axe the Gibson Les Paul Custom... mine was brand new 1979 GLPC wine/gold with chainsaw case... I wish I still had it bit like a dummy I took to college with me back in the 80's and some corksucker sticky fingered it... took me until 2012ish to decide to get another 2008 GLPC Oxblood... nice but nowhere near the 79... Anyways, if I ever need any work done on my dual reverb 100w 900 amp I will ship it to you... I tried to get someone to install some jj 5881's in it but it immediately made sizzling sounds... so I told him to put some new jj el34 back in it like it came originally... sad to say the only amp man near me passed away a few yrs ago.... I think there's one at Backsatge Music in Starkville, MS about 70 miles away... but I doubt he can even come close to your knowledge on Marshall amps. Enjoyed and subscribed and rang the bell my friend... peace
I am so glad you liked the video. Funny thing the 900 videos ( I believe there are 3 of them ) are the most viewed videos, and they have had mixed reviews. Regardless I stand by thier info. Thanks again for the comments.
have you done many mods on 900's? What are some mods you would reccomend? Ive heard of people bypassing a so called(crappy) tone stack to essentially make the amp into a jcm 800?
The 250 volt cap it too low and it fails, a lot, taking out the bias and the output tubes. Yes, I have also bypassed the faux tone stack the other channel and it does help immensely.
I have a 1993 JCM 900 SL-X and I noticed that r24 and r25 are mounted on the pc board. should I put them on the tube socket as you did here and remove the resistors in r24 and r27? what do I do with the resistors from r24 r27? Thank you.
Looking at the schematic, looks like the very same situation. Replace R24 and R25 with a jumper, and move these resistors to the tube sockets. I would also use 5.6k resistors instead of the 1.5ks at the tube sockets on pin 5. IF YOU HAVE ANT DOUBT have a competent pro do it for you. Be cautious and thanks for the comment.
@@rustyh62 OK, with the 50w version you do not really need to move the resistors, although the rightful place IS on the tube socket. 5.6k is the typical Marshall value and it will also limit the slap to the el34s as well.
Much of the actual pre-amp and gain circuitry of these amplifiers is done with ICs in an attempt to get larger amounts of gain with less noise and micro-phonics from tubes. The diodes in the pre-amp are also for clipping the signal.
I’ve got a jcm 2000 tsl 122 on the bench and I suspect that class x safety cap to be the reason my bias is starting at 90mV per side after changing out the motherboard to the new revision 20 one. I see it is also 250v, should I replace it with a 600v? Kinda strange the two caps by the HT fuse are 22n 630v which are on the same board as this 47n 250v? Why would those two be 630v with this 250v on the same board?????? Why wouldn’t they all be 630v?
So, I take it the main board was changed due to thermal runoff? Regardless yes, change the cap to a 600v. I use the Xicon 630v it fits the board well. Take a good look at the bias board as well. As for the why the 250v cap, that is one of the great mysteries....boggles the mind.
@@guitarc.p.r.9675 yep, already changed both bias trim pots and hit all of the solder connections. Thank you so much for you reply, you’re the only one so far who did out of four videos I commented on 👍🏽.
I have a 50 watt jcm 900 too, and the EL34 power tube next to the transformer visibly is redder then the one next to it. thanks for posting the video, my amp has the same resistor configuration. it looks like you jumper-ed R24 and soldered the resistor directly on the tube socket. is that correct?
That is probably the same model I found the problem in originally quite some yrs ago. Get back to me w/ amp model #. I'm pretty sure your problem is an uneven PI drive. .....And please explain "redder".
Thanks for responding to my post. It looks like I may have more issues then I thought, I realize now that my tubes are red plating, and one is red plating more than the other one. The model is a jcm900 50watt 4500 that was made in 6/26/90, I couldn't find a serial number, but the board revision number is a JMP50B. I can only find the schematic for JMP50A, and JMP50C. In my amp V5 has two resistors in series connected to pin 5 of the el84. The values are (r27) 1.5k ohm and a 2.2k ohm resistor that looks like someone before me soldered directly to pin 5. V4 has 1.5kohm resistor connected to pin 5. I measured the plate voltage (pin 3), and it stabilized to 294 volts. This voltage seems pretty low when compared to what values that I've seen from other peoples youtube videos. The tubes that I have are tesla el34 with a 37 code, and I have no idea how long that they were installed.
Definitely check the bias cap I mentioned. Your voltage is low due to the outputs pulling too much current . I will look into this more for you. Your tubes are already toast, pull them to check the amp, you will need a decent set of matched tubes. Look for my facebook...Guitar C.P.R. PM me.
So 3 weeks ago I took my Marshall jcm 900 4100 to an amp repair guy. I had bought it on reverb and it was shipped from Italy. When it arrived i plugged it in and it had really low volume, crackling when i turn the knobs, and reverb did not work in both channels. After that there was no sound coming from the amp. I called the repair guy last week and he said they had to order a transformer for the amp. I hope i get to play the amp soon.
Good question. Given it is a used amp sold with problems it is impossible to make an educated guess of what the problems are, and I said problems because it sounds like there are several different ones. The crackling in the pots is usually dirty pots or broken solder connections on the board or even a damaged pot. The transformer in question may be the reverb return transformer, though I have never had to change one, it could fail That would explain not having reverb on either channel, or it could be a problem with the IC circuitry and there is pleanty of it in these amps. The no sound thing cold be as simple as a dirty effects loop jack or as ugly as an output transformer. However I tell people all the time, until it is in front of me on a bench we are just guessing and actual trouble shooting is the only thing that will give solid answers.
nice video...is it possible to put varicac to supply the final valves(5881) with less voltage for changing headroom and dreaming have a better sound without high gain-volume (similar like van halen)? or put variac between output of jcm900 to cabinet 1960a is it dangerous? i'd like to have sound like when i left maximum value of gain and volume is amazing and reach of armonic frequency, but of course i can't do it cause neighbor..how can i have that sound with low volume?
Thanks for the nice comment! OK, first thing! Do NOT put a variac between the amp and cabinet. A variac is to vary AC voltage, I firmly believe the only place one should be is on a tech's bench and sometimes not even in that case. Yes you could use a variac to drop the power supply voltages, but keep in mind that it is also dropping the 12 and 6 volt heater voltages which may cause a few issues, but most importantly the 4500 preamp is mostly IC driven which runs on a +/- 15 volt supply. I have seen a LOT of amps damaged by such things. A variac varies all the voltages in an amp. It also lowers the plate voltages for the preamp tubes which also reduces gain and browns out the tone some. Possibly more than the output tubes. Lower preamp voltages compress and grit up quicker too. This is why power scaling is becoming more popular. The thing with the VH sound is you only get it loud. Even variaced to 90v it is still a non-master Marshall dimed.
my idea was put the variac between plate and ground without touch the chatode .but i fear about condensator and transformer what would have been happened?is the plate supplies in dc or ac ? jcm 900 have a lot operational amplifier and other components supply in dc like you had showed me.(+-15 volts).may be it had been better
if i do with old model amplifier like 1959 plexy or up with only valves tube circuits it would be better to supply the plate only adjust 70 to 90 volts to change headroom....i don't know..
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Since this has been one of the most viewed videos and has generated the most comments, I feel the need to add something overlooked. First of all I want to mention that is was never meant to be a "How to Video". If you feel the need to ask questions about what was done, you may not have the ability to do it correctly. Now, that being said, this ONLY WORKS on the PCO112 JMP50 Board. If you choose to attempt it, be sure it is that board only. Similar boards have the same resistor numbers in the same places on the board but are entirely different. In some cases R26 and R27 are 150k bias feed resistors, not grid resistors as in the video. Just a heads up and thanks for watching!
Great video. It's kind of hard to see what you did with the R24, R25, R26 and R27. It looks like you removed them all and added jumpers across R24 and R27. What kind of bridge work was done underneath? What value resistor did you put on the tube socket?
i got a pretty old jcm 900 100 watt head that has been sitting in a smokey basement for years…original problem is that it kept blowing fuses, just really never had funding to get it properly fixed. Sadly, i think i need to sell it to pay for other life stuff…. anyone have any idea of what i should attempt to sell it for? …truly wish i could use it again, but i no longer play with live bands
I've got one on the bench right now and spent a whole day trying to figure out and studying all their schematics (all versions, 5881 and EL34) on the grid resistor issue. Makes absolutely no sense whatsoever. I know this is old news for you, but thanks for the collaboration. Replacing the "X' class bias cap with the 600+ volt is the right move also, in my opinion. That's what I do.
Thank you, someone who agrees with me! The grid resistor issue is simply a mistake on their part. Thanks for checking out the channel.
I'm having a problem with a JCM 900 4100 with the same version circuit boards. The two outer power tubes are drawing idle current but the two inside tubes are not. I have good bias voltage and plate voltage on all tubes. Any suggestions?
Check screen voltages. The screen resistors have a bad habit of the solder joints failing. If the amp has failed previously and new tubes were installed, the screen resistors could also have failed. The more likely scenario is that the 500ma fuse on the cathodes has failed. If so the tube failure led should be lit up. There is one fuse and led for each pair of tubes, center and outside pairs. Just guessing though.
I gather the grid resistor cock-up is peculiar to the 100 watt. In learning the "language" of the amp so I could work on it after coming from a black face Fender perspective I found my 50 watt bias system is as it should be and it's a pretty early example. First thing though, I replaced the volume and sensitivity pots which were shot. I then eliminated the resistors on the board the way you do and put 5.6k 1/2 wat on the tube sockets as well as 1 ohm 5 watt biasing resistors. Actual "mod" mods will at a later date.
Yes the main problem with the grid resistors being in series is only on the 100 watters. However, the grid resistors should not be on the board and that is common on both the 50 and 100 watt models.
Good catch on that grid resistor eF up. It's wrong on all versions of JCM 900's according to the drawings. Thanks for posting this very critical mod.
Some think I am wrong but I know i am not. Thanks for the comment, and viewing.
GUITAR CPR>
Is the Chokes storing current like a reservoir like the filter capacitors or is the Choke doing something else to the AC ripple, not sure if the Choke is regulating the current?
Most Power tubes 6L6,EL34 use a B+450vdc on the plates, if you increase the B+ to +600vdc would this increase the AC ripple but why would it increase the AC ripple? i would think it would decrease the AC ripple because more B+ voltage mean less current draw from the power tubes.
When you're biasing the power tubes in an amplifier circuit how do you bias the compression of the power tubes the 1dB compression point of when the power tubes start to compression?
because its say to bias tubes at a 30mA bias current per tube to be safe but how do you know when the tube is compressing?
Yes!!!
I do not see the correlation to the video but I will try to answer some of the questions.
First up, a choke does not store current or voltage. A choke is used in series with the power supply usually between the Main B+ and the screen supply to smooth and filter out 60 and 120 Hz hum and ripple. Think of it a a large wire wound resistor with an expanding and collapsing magnetic field. As the tubes are driven the screen current goes up. The choke's IR drop during power demands reduces voltage going to the screens and creates a natural compression factor on the output tubes stabilizing their operation. The input choke on tweed style amps is a typical PI filter. This type of filter drops the voltage of the entire amp under current demand resulting in compression commonly known as "sag"
Second, raising B+ voltages is not a solution to ripple, and for some reason the industry has been leaning to stupid high voltages.
Third, The -1db compression point is a point on the linear operation curve of an amplifier. This is the point that the amp no longer responds linearly to input signal saturates, and gain drops off. It has nothing to do with biasing.
As for biasing, The dumbed down catch all of 30 ma. is a overly repeated falsehood. Bias involves Plate Current ratings, plate load, Power supply and Screen supply voltages, and other variables. An amp with a plate load of 3.5 k will draw more current than with a 6.6k load with all things being equal, therefore it will need more negative bias voltage. Likewise a low PC# tube will run cooler than a higher PC# at the same voltage. Hope this helps.
Thanks for watching!
@@guitarc.p.r.9675 Thanks for the help. When you Decrease or increase the B+ voltage it changes the screen voltage and plate voltage which is changing the bias operations of the preamp and power tubes which is voltage biasing?
1.) How much current does one power tube draw at its maximum? because each filter cap in the power supply can only holds & stores about 1 amp of current because the formula is Capacitance X rectified voltage = current stored in filter capacitor
4700uf X 450vdc B+ voltage = 1 amp of current stored in filter cap
2.) When using very low capacitance guitar cable when the amps input Z impedance is 1Meg it will "roll off high frequencies" but when you change the amps input Z impedance to 3 meg or 5 meg it will not roll off the high frequencies, any reasons why?
3.) When using a Variac like eddie van halen & Randy Rhodes did by setting it to 90VAC this lowers the B+ voltage on the amplifier. Does the ripple voltage increase or decrease when you use a Variac set to 90VAC?
Because most amplifiers in general used a B+ voltage at +450vdc so lowers the 120VAC to 90VAC the B+ voltage is I'm guessing +250vdc to +300vdc. Biasing is balancing the voltage & current through the power tube. Because when you lower the plate voltage B+ voltage to the power tubes this is also biasing the power tubes?
4.) If an amplifier is designed for 6550 and you install KT88 or KT66 tubes. The power amplifier output impedance is designed for 6550 or EL34. What sets the power amplifiers output impedance is it the tubes plate DC or AC impedance? Because you would have to change the output transformer because the output transformers primary impedance was designed for 6550 or EL34 tubes and not KT66 or KT88?
@@billwilliams6338 OK, You are messing with me right?
@@guitarc.p.r.9675 no i'm serious I'm not messing with you at all
Hey buddy!! Many thanks for your video! I had the same problem with JCM900 and solved it with your help!! Thank you very much!!
Glad to help.
Great video! I just recently picked one of these really super cheap, but it has some problems. I have one of the power tubes getting extermly hot. Any ideas what this could be? Wished you was in NC I would let you work on it... Not many people work on amps around here. I do little work on them, but not much
If it is a 100w I would venture to say you have 1 bad tube. But you should use a "good" matched set. If it is a 50w the same still applies except you may have a bad Phase Inverter coupling cap. I have even seen PI tubes cause a current problem. Check the Blues Jr. video. You could always ship it to me, a lot of people do.
OK Guys...for those of you who keep questioning the replacement of the .047 250 volt bias feed capacitor....I replace it with a 630 volt cap because it is connected directly to one side of a 375 volt AC winding. Wonder why the originals fail?
Not familiar with this 'box' shaped capacitor. What is this, and where can I get one?
@@robert2948 OK, first it is a polypropylene film capacitor. There is also a similar version known as a "safety cap." Mouser Electronics has high voltage ones. Now, forget about these caps and get a good quality .047 uf 600v-630v poly cap, ie; orange dropetc., I use the Xicon here because it fits the circuit board holes easily. Thanks for watching.
That .047 cap is a class x cap. Designed to short if they fail blowing a fuse and not electrocuting anyone. Should be replaced with class x. Your thoughts?
Love these videos
Where are you located?
I am in West Virginia.
Hello, I've just bought a Mk3 2502 50W combo. Checking the power tube sockets on mine, R24 and R27 are connected through green and orange wires only to pin 5. Pin 6 doesn't have any connection whatsoever. I see in the video that the resistors are jumping pin 5 and pin 6, so I assume if I'm removing R24 and R27 from the board to install them in the tube socket, I would have to jump pin 5 and pin 6 too. Or is the 50W different than this 100W from the video, and I can leave pin6 alone and only add the resistor to pin5 (in series with the wire)?
To add to my comment, doing a bit of research, EL34 datasheet doesn't even mention pin6, and also checking my EL34EH tubes, they don't even have a physical pin for pin6, so I guess it is safe to jump pin5 and pin6 with the resistor. I was measuring my bias current, and it was 38mA in the outer tube (the one next to the filter caps) and 28mA on the inside tube. These are supposed to be matched tubes, but I didn't check with another matched pair (I'll do that later), but would you account this 10mA difference to the resistor placement?
You are correct Pin 6 does not have a connection in the tube. Connect 5.6k grid resistors directly to pin 5 alone. There is no need to connect them to pin 6. Stand them straight up. AND change that bias cap! As far as the current on the tubes, They should be within 5 ma of each other to be matched. I like to measure plate current only so if you have a screen problem it does not reflect on the plate current. You may have a screen resistor problem on the low side. I also highly recommend looking through this channel for the other 900 videos, watch them, and read the comments as well. There are a lot of questions answered there. Thanks for watching, I hope this has helped.
@@guitarc.p.r.9675 Thank you for the quick answer. By bias cap you mean that one that's 250V but should be 600V, right? Mine currently has a 47nF 250V. I don't have a replacement currently, so I'll order one online. The unmatched tubes, I will test tomorrow with a pair that I know for a fact is matched because when I biased my plexi-1987 last week they were only 2mA apart. If those also give a higher difference in the 900, then it might be a problem indeed. Cheers!
my jcm900 head has been out of commission years before u posted this video, ( and im just stumbling across it!) ...my head was never modified in any way, it just kept blowing fuses left n right...and RadioShack was never any count as far as fuses go...ive replaced the tubes ONCE because someone 'professional' advised it...no dice!. the last shop i took it to wanted to charge $300 just to look at it!...so here it still sits, almost 15 years since last used...really wish i had someone with your expertise nearby, because of all the gear ive let go of, THIS one is one i just dont wanna part with
Where are you located?
@@guitarc.p.r.9675 Good ol Tennessee
@@andrewlee3037 You are not that far, you could always ship....
@@guitarc.p.r.9675 , while i very much appreciate the offer, i probably couldnt even afford shipping charges, let alone your cost to fix it and send it back! LOL...im top-notch broke at the moment, and my wife would definitely have objections. LOL
Do you have a dvd on doing repairs
I am sorry I do not. This channel is purely for publiic srevice.
Hello sir, I could use your help - it appears C15 has given up the ghost - oddly this happened when I had the chassis out to bias new tubes - everything else looks to be ok. In order to replace this component the entire board has to come out right? I was hoping to not have to go to that length but I don't see any other way to get at that cap. So not only is it prone to faiure, it's a pain to get to also ;)
Yes the board has to come out, but it is an easy one. Be thankful it is not a TSL, or SLX! Be sure to replace it with a 600v cap.
@@guitarc.p.r.9675 Thank you!!
@@KurtLauerman actually I use a Xicon 630v
I ordered a couple safety caps that fail to closed, rated 650v and up - I think that'll do the trick? 47n is the same as .047uf right?
@@guitarc.p.r.9675 and you're right, the board was trivial to get loose :)
I have a 92 50 wat JCM 900, that I love. Great video, thank you!
Glad I could help, thanks for the kind words.
Thanks for the comment, I'm just a guy trying to do his job.
Excellent work. For the love of God, why would anyone change an MkIII over 6L6's? They aren't bad in a Dual Reverb or SL-X, but it just seems like heresy to convert in MkIII.
I don't see how that relates to this but, thanks for watching anyway.
@@guitarc.p.r.9675 You verbally and visually pointed out how a previous tech installed 6L6 output tubes when you turned the chassis towards the camera to show us. That is the relation to my comment.
Ok, It has just been quite a while since I did that video, and I was not sure exactly which 900 video it was. Actually, I sometimes use the JJ 6L6s in the 5881 models for the really heavy guys. They do not "mush out" as much as the 5881, the bottom is much tighter and defined. They handle gain better, and they "chug real good". In this particular amp he did not like them as he said they were just too much in the bottom, and wanted to put 5881s in plus take care of some age issues. Turned out his bass issue was actually a WAY off spec bass pot. Thanks for clearing that up, and thanks again for watching.
Understood. I too have 6L6’s in my Dual Reverb. The amp in this video is an MkIII and all of those were manufactured with EL34’s. So someone (previous owner and previous tech) went out of their way to convert it. At any rate your videos are great and a wonderful learning tool. Thanks for making them.
I must apologize, I had completely forgotten about that video, and was thinking of another. I am a repair guy not a cinematographer. I have a few 900 videos. I had forgotten how messed up that amp was. I may have to refresh my memory on this one ....hummmm 2017. Old one. Thanks again for watching, and I am glad you are finding them at least a little helpful.
That JCM900 model 2100 is using a JMP50B PCB.....it looks like you are looking at a JMP50C PCB version which is why it doesn't make sense. The JMP50B version should have 2k2 resistors at the sockets in series for all 1k5 resistors to all tubes but I'm not seeing them here in the 'before' images so perhaps they were removed prior by somebody(?). So they should all be the same. Check here: drtube.com/schematics/marshall/cd0192-iss7.pdf . I could be wrong, I'm not an expert. Marshall can be confusing sometimes by using different types of PCB's for the same model depending on the year. That mod for the resistor at R28 for the bias pot range makes a huge difference though. I've done this before and I no longer need to special order tubes
Thanks for the comment. And you are correct with the schematic you are showing, but that was not the case with the amp in the video, and it was also dead stock when it came into the shop. Yes, it was an early one with a wrinkle to be worked out. The series resistors you mentioned are still a bad idea, in this case adding up to 3.7k. Also a reason I mention changing these to 5.6k and moving them to the sockets for stability.
Man, I thoroughly enjoyed the video my friend... I haven't had the time to play in a couple of years due to chasing my 3 yr old and 2 yr old twins around being daddy....hehehe but I plan in the future to teach my boys how to play guitar... I started when I was 15 around 1980 after being so lucky to have the once in a lifetime opportunity to strum Paul Stanley's original Ibanez Iceman while it was on loan for display at Strings and Things in Memphis... Paul was using his new broken mirror Ibanez during the Dynasty Tour at that time and he was buddies with the guitar shop owner and the guitar was still in its fligh case when he showed it to me. I was blown away... so, a couple weeks later I bought my first guitar due to Ace's favorite axe the Gibson Les Paul Custom... mine was brand new 1979 GLPC wine/gold with chainsaw case... I wish I still had it bit like a dummy I took to college with me back in the 80's and some corksucker sticky fingered it... took me until 2012ish to decide to get another 2008 GLPC Oxblood... nice but nowhere near the 79... Anyways, if I ever need any work done on my dual reverb 100w 900 amp I will ship it to you... I tried to get someone to install some jj 5881's in it but it immediately made sizzling sounds... so I told him to put some new jj el34 back in it like it came originally... sad to say the only amp man near me passed away a few yrs ago.... I think there's one at Backsatge Music in Starkville, MS about 70 miles away... but I doubt he can even come close to your knowledge on Marshall amps. Enjoyed and subscribed and rang the bell my friend... peace
I am so glad you liked the video. Funny thing the 900 videos ( I believe there are 3 of them ) are the most viewed videos, and they have had mixed reviews. Regardless I stand by thier info. Thanks again for the comments.
have you done many mods on 900's? What are some mods you would reccomend? Ive heard of people bypassing a so called(crappy) tone stack to essentially make the amp into a jcm 800?
The 250 volt cap it too low and it fails, a lot, taking out the bias and the output tubes. Yes, I have also bypassed the faux tone stack the other channel and it does help immensely.
Can I get your info and ship my 900 100watt to you for future repairs.
Sure, PM me at gtrcpr.r58@gmail.com
Also what do you find is the benefit of changing that blue cap from a 250 to a 600? just reliabilitty?
Yes, It keeps the amp from blowing up!
does replacing that blue bias cap to a 600 v from a 250v change the sound of the amp at all?
Absolutely not! Unless you prefer the tone of an amp that is blown up. The cap change is to make up for the under voltage bias supply cap.
I have a jcm 900 mkiii 2100 would you want to take a look at for me? Give me your contact info so we can discuss prices and the issues I'm having
PM me at Guitar C.P.R facebook.
facebook.com/Guitar-CPR-158557297511711/
Where are you located? I’d like to send you my JCM900 head that has the same humming problem.
I am in Fairmont WV
I am also @ Guitar C.P.R. face book
I have a 1993 JCM 900 SL-X and I noticed that r24 and r25 are mounted on the pc board. should I put them on the tube socket as you did here and remove the resistors in r24 and r27? what do I do with the resistors from r24 r27? Thank you.
Looking at the schematic, looks like the very same situation. Replace R24 and R25 with a jumper, and move these resistors to the tube sockets. I would also use 5.6k resistors instead of the 1.5ks at the tube sockets on pin 5. IF YOU HAVE ANT DOUBT have a competent pro do it for you. Be cautious and thanks for the comment.
@@guitarc.p.r.9675 why 5.6k? I used the resistors that I removed from 24 and 25. Will they work? Also my amp is the 50 watt el 34 version.
@@rustyh62 OK, with the 50w version you do not really need to move the resistors, although the rightful place IS on the tube socket. 5.6k is the typical Marshall value and it will also limit the slap to the el34s as well.
What is the function of these integrated circuits and these diodes? here in Brazil many think that this head is hybrid because of these components!
Much of the actual pre-amp and gain circuitry of these amplifiers is done with ICs in an attempt to get larger amounts of gain with less noise and micro-phonics from tubes. The diodes in the pre-amp are also for clipping the signal.
ie....... Modern Tube amp. Believe me, it gets worse!
I’ve got a jcm 2000 tsl 122 on the bench and I suspect that class x safety cap to be the reason my bias is starting at 90mV per side after changing out the motherboard to the new revision 20 one. I see it is also 250v, should I replace it with a 600v? Kinda strange the two caps by the HT fuse are 22n 630v which are on the same board as this 47n 250v? Why would those two be 630v with this 250v on the same board?????? Why wouldn’t they all be 630v?
So, I take it the main board was changed due to thermal runoff? Regardless yes, change the cap to a 600v. I use the Xicon 630v it fits the board well. Take a good look at the bias board as well. As for the why the 250v cap, that is one of the great mysteries....boggles the mind.
@@guitarc.p.r.9675 yep, already changed both bias trim pots and hit all of the solder connections. Thank you so much for you reply, you’re the only one so far who did out of four videos I commented on 👍🏽.
You may want to check out my other later 900 video it clears a few things up concerning that bias cap. Thanks for watching and I hope it helps.
@@guitarc.p.r.9675 just checked them out, super informative. Thank you sir👍🏽
I have a 50 watt jcm 900 too, and the EL34 power tube next to the transformer visibly is redder then the one next to it. thanks for posting the video, my amp has the same resistor configuration. it looks like you jumper-ed R24 and soldered the resistor directly on the tube socket. is that correct?
That is probably the same model I found the problem in originally quite some yrs ago. Get back to me w/ amp model #. I'm pretty sure your problem is an uneven PI drive. .....And please explain "redder".
Thanks for responding to my post. It looks like I may have more issues then I thought, I realize now that my tubes are red plating, and one is red plating more than the other one. The model is a jcm900 50watt 4500 that was made in 6/26/90, I couldn't find a serial number, but the board revision number is a JMP50B. I can only find the schematic for JMP50A, and JMP50C. In my amp V5 has two resistors in series connected to pin 5 of the el84. The values are (r27) 1.5k ohm and a 2.2k ohm resistor that looks like someone before me soldered directly to pin 5. V4 has 1.5kohm resistor connected to pin 5. I measured the plate voltage (pin 3), and it stabilized to 294 volts. This voltage seems pretty low when compared to what values that I've seen from other peoples youtube videos. The tubes that I have are tesla el34 with a 37 code, and I have no idea how long that they were installed.
Definitely check the bias cap I mentioned. Your voltage is low due to the outputs pulling too much current . I will look into this more for you. Your tubes are already toast, pull them to check the amp, you will need a decent set of matched tubes. Look for my facebook...Guitar C.P.R. PM me.
So 3 weeks ago I took my Marshall jcm 900 4100 to an amp repair guy. I had bought it on reverb and it was shipped from Italy. When it arrived i plugged it in and it had really low volume, crackling when i turn the knobs, and reverb did not work in both channels. After that there was no sound coming from the amp. I called the repair guy last week and he said they had to order a transformer for the amp. I hope i get to play the amp soon.
Very informative video btw!
Thank you it has had mixed reviews.
@@guitarc.p.r.9675 what’s your input about the problems my marshall has been dealing with ?(:
Good question. Given it is a used amp sold with problems it is impossible to make an educated guess of what the problems are, and I said problems because it sounds like there are several different ones. The crackling in the pots is usually dirty pots or broken solder connections on the board or even a damaged pot. The transformer in question may be the reverb return transformer, though I have never had to change one, it could fail That would explain not having reverb on either channel, or it could be a problem with the IC circuitry and there is pleanty of it in these amps. The no sound thing cold be as simple as a dirty effects loop jack or as ugly as an output transformer. However I tell people all the time, until it is in front of me on a bench we are just guessing and actual trouble shooting is the only thing that will give solid answers.
@@guitarc.p.r.9675 I appreciate the response! I will update soon when it’s fixed.
nice video...is it possible to put varicac to supply the final valves(5881) with less voltage for changing headroom and dreaming have a better sound without high gain-volume (similar like van halen)?
or put variac between output of jcm900 to cabinet 1960a is it dangerous? i'd like to have sound like when i left maximum value of gain and volume is amazing and reach of armonic frequency, but of course i can't do it cause neighbor..how can i have that sound with low volume?
Thanks for the nice comment! OK, first thing! Do NOT put a variac between the amp and cabinet. A variac is to vary AC voltage, I firmly believe the only place one should be is on a tech's bench and sometimes not even in that case. Yes you could use a variac to drop the power supply voltages, but keep in mind that it is also dropping the 12 and 6 volt heater voltages which may cause a few issues, but most importantly the 4500 preamp is mostly IC driven which runs on a +/- 15 volt supply. I have seen a LOT of amps damaged by such things. A variac varies all the voltages in an amp. It also lowers the plate voltages for the preamp tubes which also reduces gain and browns out the tone some. Possibly more than the output tubes. Lower preamp voltages compress and grit up quicker too. This is why power scaling is becoming more popular. The thing with the VH sound is you only get it loud. Even variaced to 90v it is still a non-master Marshall dimed.
@@guitarc.p.r.9675 thanks for your attentions
my idea was put the variac between plate and ground without touch the chatode .but i fear about condensator and transformer what would have been happened?is the plate supplies in dc or ac ? jcm 900 have a lot operational amplifier and other components supply in dc like you had showed me.(+-15 volts).may be it had been better
if i do with old model amplifier like 1959 plexy or up with only valves tube circuits it would be better to supply the plate only adjust 70 to 90 volts to change headroom....i don't know..
@@guitarc.p.r.9675 i'd like to change the headroom by lower the dc or ac i don't know the voltahe supply the plate