JCM900 Bias Video

A detailed video of the JCM900 or similar biasing process. I also talk about my favorite tubes for this amp and why i chose them.

[@darthdurkelthewise320]

This is a fantastic video, and super helpful. I just bought a Marshall 2466 VM and I plan on making use of this tutorial. Too many of the few videos that are out there are either flat out wrong or they don’t provide proper warning ⚠️ and explanation of parts, tools needed, etc. Bravo sir, bravo.

[@hadley101]

Thank you for posting such an excellent video in biasing the JCM 900 amps. Used this to bias my JCM 900 2100 SL-X, excellent work man!

[@roycox4415]

thanks so much for taking the time to comment!

[@vertebralremains379]

Thankyou for making this man, I'm taking a shot at biasing my new tubes in my JCM900 4500 50 watt tonight, I feel more confident attacking it after watching this.

[@greghall8992]

Great video..very..very informative and explained.Thanks for sharing your skills and talents with us .

[@Mikem-mq2hh]

Spectacular video!!!

[@krudler406]

excellent vid, thanks. also yes, killer amp indeed.

[@1-800-NO-FAULT]

Hey I just changed tubes on my 900 combo. Your video was great. Unfortunately I watched it after I almost blew myself up. LOL All the things you mentioned I figured out by trial and error, one other thing is where is the 56K R28 resistor so I can replace with 47k cause EL 34 cant be biased properly the way mine is set up.

[@AndyK.23]

Very informative! I miss the 'old' days when you could actually get tubes at reasonable prices!

[@BrewersArcade]

Thank you for this!

[@brpadington]

I have a 2555x and was unsure about biasing. It has bias points which makes it a lot easier and safer. what I don't know is what I should bias it at. When they shipped it the bias was all out of spec. One side was 85 and the other was 65. I put them both at 70 and it sounded much better.

[@1-800-NO-FAULT]

I can only bias my up to 26ma. I also went out and bought a eurotube pro one just to avoid the aggravation cause I thought for an 75 bucks it was worth it.

[@richclayton5785]

Good safe advice

[@themathrock1607]

awesome!

[@xderiwx]

Awesome video! Do you personally drain your filter caps when doing tube changes/biasing? Thanks!

[@kenyonanderson5275]

Hey Roy, great video, best I've been able to find. But, I have a JMP 100W Super Lead Mkii which I'm going to replace the 6550s. Is the biasing procedure the same for this vintage amp? I know the voltage/current values will be.

[@Lymanization]

Another question if you have time,? I have many tube heads in storage for over a year most from early 2000's or late 90's,will they be ok in a few months more of storage? Thanks again man,realy, a great tech you are!

[@hushjohnny9543]

Great video man! I just bumped into a 94' JCM900 4100 100 watt. The amp sounded great at the sellers home, however I don't think i'm going to use it until I get done what your doing to this one! I'm a little confused about seeing the 7881's being replaced with the ones you put in this amp, and others using EL34's. Should I stay with 7881's.. are EL34's okay to put in? Is one set better for the amp or not? I play harder rock/metal. I do know you get a different sound with different valves. Again..'thanks for your video.. the best on youtube! \,,/

[@SongsForDamnation]

Really good video. Just a question: what current is your probe measuring? Is it connected to pin n.8 (cathode)?

[@Black-Maple]

Hi Roy, first, really cool video, usefull and carrefully explained. I was wondering about modifing it to 50w. What about removing 2 tubes, to have a 50w version : Same process with only two tubes?

[@Wardell43]

Tube prices have doubled recently since we are have trade issues with Russia and China. I use Mojotone for my parts and you can use them to check the tube prices. I think I paid roughly $25 each for the 2 Gold Lion ECC83 that are now $44 each.

[@MBString]

The best video for biasing on youtube!! i have one question though if you can answer, can i do this and how without a bias probe and only one multimeter? and the screwdriver you are using, they dont have the metal tip??

[@roycox4415]

Thanks so much! Sincerely appreciate that! You can bias with one multimeter only, you just have to use it to read the plate voltage and then use it to bias the amp instead of having them both hooked up. The screwdriver has a metal tip but completely incased in plastic and rubber.

[@MBString]

+Roy Cox Oh thanks for quick reply!! So i can measure bias current the same way as plate voltage? I dont need a bias probe for tube socket?

[@roycox4415]

+MBString I'm pretty sure you can measure bias by using using different pins but I'm not familiar with biasing without a bias probe . If you do some research you may be able to find info on biasing without a probe. Be careful!

[@roycox4415]

It's definitely pin 3 it's just that the PIN numbers fall slightly behind the actual pins so it just appears to be pin 4.

[@underpressureman]

so is it just the 1 bias pot that you have to bias or is there one for every output tube?

[@Albee213]

I have a JCM 900 MKIII that has all stock tubes from the factory and they are still good. I has EL34 tubes and I have not had it to a shop. I have read that it might be biased from the factory to run 6L6 tubes and that it might be over biased running a lower current, is there any way I can check that without a bias probe? My amp was made in 1990 serial starting with Y. Since I play metal I use only metal on my amp.

[@DennisBastian]

Great video, but too bad you didn't let us hear how it sounds after this adjustment. I've played one of these for a while at band practice and it would have been interesting to hear the difference since I belong to the group that thinks this thing sounds way too harsh and fizzy.

[@camillopillan8247]

nice video! what is it the voltage of the plate or anode ac or dc?

[@MrOzgooner]

This is awesome Roy..Have to ask though , did you notice the difference between the 5881 stock tube to the 6L6 ?

[@hushjohnny9543]

great question. I just found his video and asked a similar question,, I just bought a 94' JCM900. It has 7881's. Taking it in soon to have done what he's doing and i'm curious about the same thing!

[@vincent33195]

Great video! I'm curious- is it necessary to do this with each power tube in the amp? Or are we assuming that the tubes are matched to begin with? Any benefit to testing one vs. all four, except to ensure that the tubes are matched? Finally- what kind of bias probe do you prefer?

[@roycox4415]

They are usually done in pairs, meaning you can take a reading of the right two and then the left two, etc. if you are reading and biasing singles, then atleast check the first and then last to make sure they are way different.

[@camillopillan8247]

for measuring the current i have to se multimeter in mm current and put the red on pin3 and negative on the ground-Chassis?

[@scottm382]

I have a Marshall ma50h head. When I check the 3 pins for bias one side (tube) is 35mv and the other side is reading 18 volts. There's no buz. What am I doing wrong ?

[@shredding51]

what's the bias for the 50w?

[@rockstaradventures1]

Ive got an issue with mine, it works fine, but when reading the plate voltage, im getting 672 and not 470 like the average, what can I check or what could be causing this? Thanks!

[@billwilliams6338]

ROY COX, 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?

[@arrakkis666]

Is this a safety when working on tube amps video or a video on amp biasing?

[@dokdorian]

such a great video... thanks so much for ur work! if i saw this vid couple of days ago.. i could have saved nearly 100$ haha. hey btw! i have jcm900 sl-x. does sl-x has same form like ur, so i can bias mine same as what u've done to urs?

[@roycox4415]

absolutely!

[@dokdorian]

thx for reply! cheers mate!

[@MIKEROADDOG]

great video Roy , very detailed , what about the other 3 tubes , do you use the same adjustment pod for the other 3 tubes ? , my 900slx conked out , blew a output valve fuse no -2-3 valve , replaced the fuse with the same value , , fuse LED shows not lit now , so fuses are good , but no sound now , just hum , i'm going to replace the output valves with the same ones sovtek 5881 , so this video i'm using as reference , everything in the amp board looks good no blown caps no burnt resistors , since the output valve fuse blew im assuming its the tube 2-3 , they are 23 years old , i understand your whole video except the part after you did the first tube , does that one tube adjustment cover the rest of the tubes also ? , , i'm new to this , thanks for the great tutorial........Mike

[@roycox4415]

tubes are generally biased in pairs (power tubes). When you do one you can usually not mess with the other because it will be super close. The best method, if only using a single socket bias probe like mine, is to bias the first tube and then remove the bias probe, place it on tube 3 and check that side as well. This is a commonly done practice. I have found that this amp is good to go by biasing the first. The rest will be very close to the firswt tubes reading. Also, it depends on how closely matched your power tubes are. I buy mine from "the tube store" online and they match their new tubes extremely close. They even mark the numbers on the sides as to what each tube read on their machine so you can be guaranteed a close match. Hope this helps.

[@danielsaturnino5715]

I had a friend bring me one of those that was faulty. It had power transformer buzzz. Is that normal? And when I take it out of standby I could hear a little bit also on the speaker. Thanks

[@Lymanization]

Best vid yet,,I'm looking to buy a JCM 900 but he put tubes in it of another kind and did not bias it,what do you think?

[@roycox4415]

Sorry I. Missed your message! Did you buy the amp back then?

[@alexandergantikow1400]

First: Great Video that helps me further. But one question arised while watching different videos for biasing: Is there only one potentiometer to adjust the bias for all tubes, or is there one potentiometer per tube? Thanks in advance :)

[@roycox4415]

Hey there alexander, thanks for the compliment. Im glad it helped you out a bit! There is one pot for all the tubes in this amp.

[@alexandergantikow1400]

+Roy Cox Thanks for the fast reply. I have the Jcm900 too - so good to know that i have to look for only one poti :) I'm just searching for a good vid that explains biasing with just a multimeter. But probably a technican is more secure :D

[@roycox4415]

+Alexander Gantikow You'd be surprised at how simple it is once you do it once safely! It will save you a bunch of money also. Just be extremely careful when removing the chassis from the housing that you don't grab underneath of the electric, especially where you see the blue capacitors. you'll be just fine if you watch this video step by step!

[@alexandergantikow1400]

+Roy Cox I share your opinion about doing it by myself - not only because of the money but rather because im interested in how such an amp works :) Thanks for the safety-tip. I will grab the amp at the transformer as you said in your vid :) I found a vid by premier guitar that describes how to unload a capacitor. (Its vid-name is "DIY: Amp Safety 101") This should make the safety higher, don't you think so?

[@roycox4415]

+Alexander Gantikow sure it never hurts to take extra precautionary steps!!!

[@thejoyz1952]

Hi, great video! You said 30watts for the power tube, is it always 30watts by power tube? Else, how to check my power tubes rating? Thank you!

[@silasfatchett5693]

Each type of power tube has a power rating which can be found on its data sheet, available on the net.

[@jamescassidy4045]

So I'm confused. Can I ask two things? So, Isnt there a way to set the Bias of an Amp with just the Multimeter, without having to use bias probes? Like with just using your regular multimeter probes? Because when I tried biasing my 900 for the first time, I had some help, and I just remember having to have the black probe on the chassis, and touching the red probe to a leg of a certain resistor, or the bias test point, and then setting the bias until I got a certain reading on the MM. Like within the recommended bias number as per Marshall, and then I made small tweaks while playing to decide where I wanted it. Second, so is using bias probes just a more in depth elaborate way to bias the amp, or a way to get a better average if tubes are not matched or something? So the process is, If your using those Bias Probes, are you just getting the plate voltage first(with the normal MM probes), then sticking the bias probe in one of the Sockets with the tube in the bias probe, doing the math, and then setting the bias trim pot, while that same tube is in the bias probe, and then thats it? That's what I don't get, because it seems odd to me that you only need to use the Bias Probe with one tube, but then if you do for some reason use it with every tube, and each tube reads different, or is unmatched, then how do you set it since theres only one trim pot? You just take an average of all the tube bias readings first? Like what I mean is say you set the bias to 44 with the first tube, and then go to the second tube and it's not 44, how can you set it to 44, without the other one drifting? Also, don't you need to get the plate voltages for all the different tube sockets? I guess so I'm not confusing you by babbling, Im just confused, because in the video it looks like you are getting the PV off one socket, putting the probe in one socket, getting a Bias reading, doing the math, and then setting the Bias while the bias probe is in that same socket/tube, and thats all there is to it.

[@underpressureman]

Quick question. So you were touching the red probe to pin 3 with the black alligator clip on the chassiz to get the reading I understood that part, but when it came time to adjust the bias how were the probes set up, did you just have an alligator clip on pin 3 instead of the probe, or how was it hooked up to get the bias reading?Also can you bias an amp without knowing the values of each tube? Like do you only need to get the reading off pin 3 and thats it for this amp?

[@Black-Maple]

you unplug the alligators after reading the plate voltage, and plug it on the bias probe, to read mA. you only have to do it for one tube. Pay attention to buy paire quartet.

[@Albee213]

I am trying to bias a JCM900 MKIII 2500 (50watt) el34 and having some trouble. First off when I set my multi-meter to 200ma it gives me a super low reading (11.7ma). When I set it to 200mv them it gives me a low reading. I turn the pot all the way up and max I can get is 27.2mv. What am I doing wrong?

[@RDEL54]

Bias resistor is the wrong value

[@mohdtaufiqismail]

hi,im noob with biasing..do we need to bias all 4 power tubes by repeating the same process?or just 1 ?

[@Gutch220]

you just have to do it once. the trim pot is for all of the power tubes

[@timothyjay2012]

If you wanna make that JCM 900 talk to you!!?? Change out those original Big Blue Caps!! It will have better smoother gain and tighter sweeter bass response!!! I used the ARS after market Caps in my SL-X and DAMN!!!!

[@rareform6747]

Have same amp . What if I just changed out the power tubes new matching without bias ? Thanks

[@roycox4415]

if the new tubes are rated exactly or super close to the old one then you can probably get away without biasing but the chances of them being the same are usually not that good so I always like to at least check the bias. It just helps to make sure the amp sounds its best!

[@juceliodelmondes1755]

Where did u find that 30W information? Datasheet too? What field? Is it plate voltage or screen grid? I can't find the same information on the Sovtek 6550WE Tetrode.

[@roycox4415]

the tube wattage is from the tube manufacturer and follow the video closely and i talk about where i find the plate voltage, etc.

[@moparsnguitars]

Roy. Joe here. Wanting to know if I have jcm900 head I bought and it don't have the 4 power tubes in it. It has the 3 original marshall preamp tubes in it. It says it has 5881 power tubes.my question is I don't know it's history with previous tubes if I buy the new 5881 tung sol would it need to be biased

[@silasfatchett5693]

Yes.

[@johnsimms3957]

Don't be afraid to bias the tubes to 60% dissipation if they sound better.

[@roycox4415]

Absolutely! I just personally like this amp and tubes at 70

[@SnowLlama]

If you don't have any biasing equipment, can you just crank the bias pot up while the amp is unplugged and discharged and hope for a better sound?

[@roycox4415]

I'm sure it's possible but that adjustment screw is so sensitive that it could easily cook those tubes if turned too far which could easily burn the tubes up. New tubes cost way more than a bias probe so I recommend not risking it but rather getting the probe and multimeter. You'll get your money's worth many times over in the future when you have to replace the tubes. It's so worth having the probe and learning how to do it!

[@SnowLlama]

+Roy Cox Hey man, I cracked my amp open and learned that the amp is currently biased at 42, and when I did the calculations that you explained in the video, I learned that It should be set at 38. Any thoughts? is that too hot? Sometimes the amp starts crackling, and I don't know if thats a product of over-biasing and I wanted to get your opinion first.

[@roycox4415]

+Kelly Bouchillon that's a little hot but nothing too severe. I doubt the crackling is coming from the hotter bias but most likely your preamp tube socket of sockets being dirty of a bad tube. You may want to spray deoxit 5 on the pins of a 12ax7 and push it in and out of the preamp tube sockets to clean it, then put the tubes back in. If that doesn't fix it then try a new 12ax7 in each slot and see if it goes away. The hotter bias won't hurt anything but can wear the tubes prematurely.

[@silasfatchett5693]

At one time, amps were biased by watching the output tubes in a darkened room, turning up the bias slowly until the tubes' plates began to glow orange, and then backing off the bias until they stopped glowing. Not recommended!

[@ben156]

What screwdriver set are you using?

[@roycox4415]

i can't remember the brand but its the set they sell in home depot. high voltage models.

[@tshouse2782]

It states on the amp to only use 5881, how can you get away using 6L6s?

[@PeterDad60]

They are the same tubes, different names. They are in every way equivalent.

[@Charmander71025]

Is it just me or does he show him putting the test probe on pin4 even though he "says" pin 3…..

[@klepetar]

well..i don't have those tube thingnies for biasing..and i will not buy one..i remember biasing my amps just with my trusty multimeter.. that was it is there a video on youtube that shows where to connect the mA ? someone biasing the old way? please???

[@kameel68]

Do you only bias one of the power tubes?

[@roycox4415]

no, you are biasing each set of power tubes. the left two and right two

[@kameel68]

@@roycox4415 I've got a Marshall JCM900 SL-X with 2 power tubes(50W version). I just see 1 pot to adjust. So you can adjust 2 tubes with 1 pot?

[@roycox4415]

@@kameel68 you only have one pot to set the bias for all of the power tubes as a whole actually. This is why you want to find power tubes that are as closely matched as possible. This way all of the tubes are super close when biasing with only the one pot

[@kameel68]

@@roycox4415 Today I had opened my amp to fix some creaking potentiometers. While doing that I also took a look inside and noticed that there was only 1 adjustable pot on the PCB.

[@benseitz739]

Pin 3 is giving me 111mA every time. Not even close to the 460mA I’m looking for.

[@rareform6747]

Hi , attach a load or speakers .

[@roycox4415]

Rare Form the video shows the load box I'm using for that. There is one hooked up

[@shredding51]

I find 5881’s are better in this amp then 6l6gc’s. The 6l6’s take away some gain and mids.

[@roycox4415]

I agree. I prefer the JJ 5881 tubes in mine. Best in all the tubes I’ve tried in the jam 900 4100

[@Sorbzski99]

You are really Steve Stevens, aren't you!

[@Woundstring]

Haha! I was thinking he sounds bit like him too 😂

[@maselbac]

Rubber gloves would save my life???

[@rexmasters1541]

JJ and EH are the same tubes from the same manufacturer so are Ruby, TAD and every other cheap chinese made tube. The low end JJ tubes are MADE IN CHINA at the Shungyen plant. JJ tubes that are made in Solvakia are made with Chinese supplied parts and are made on Chinese equipment in Solvakia and when you buy cheap chinese shit you will have cheap tubes that never meet any specs.