08 Mar How To Bias Your Amp Yourself
Do you know what the bias setting on your amp is? If your tubes burned out, would you know how to get the same sound back as you had before?
The amps bias makes a critical difference and you really should know all of the bias settings for each amp you own and what the voltage number of the tubes that are in them, especially if you like how your amps sound.
You can check the number of the tubes by looking on the tube box (you did save the tube boxes, right?) it will be written on the end.
But, now you need to know how to bias your amp.
First you will need to purchase a bias probe tester. I own the QuadStage BiasPro from Asharpfretworks.com (it’s metered and easy to use), but a search for “Bias Probe” on Ebay will get you several options to choose from. For a Marshall be sure to select the 8 pin cable option (you will need two of them and they come with the tester, but you have to tell them you want 8 pin when ordering).
You will need to remove the 4 screws on the bottom of your Marshall and slide the chassis out the back to get to the bias adjustment on the underneath side of the chassis. WARNING: Even unplugged, parts in an amplifier can hold enough voltage to kill you. Keep one hand behind you and wear proper gloves when adjusting the bias and you will be fine. Just don’t be stupid and there is nothing to be scared of. I’m only putting this warning up so someone doesn’t get the bright idea to do this after a few beers
1. Turn off your amp, pull the AC cable out and allow the output tubes to cool.
2. Pull the output tubes out and insert the tester probe sockets into the amp’s tube sockets. (The tester sockets get inserted between your amp and the tubes and have a cable coming from them that attaches to the testing meter).
3. Insert the output tubes into the tester sockets (which are now inserted into your amp).
4. Make sure your amp is connected to a speaker and then power up your amp and let it warm up for at least 5 minutes
5. Read the numbers on the test meter screen (switch between tubes on the tester and always use the higher number). This is what your amp is currently biased to with the tubes you have in it. Write down this number and the number of your tubes for reference (if you like the tone) If you have done this reference test previously and are biasing new tubes, then skip this step.
6. Read the numbers on the meter with your new tubes installed. The bias setting will most likely be off from your reference numbers you took from the original tubes. Insert a screw driver into the bias level adjustment potentiometer on your amp (Located on the underside of most amp chassis, usually a little box or round pot sitting sideways or facing up with a screw driver slot, please search the web instead of assuming you have found it). Adjusting this clockwise raises the bias, counter-clockwise decreases. Adjust the bias control on the amp until the test meter shows the value of your reference test (or new bias setting if you are experimenting with different number tubes or raising the bias to increase distortion or lowering it to clean it up, but that’s for another discussion, Search the web to learn about how larger numbers on your tubes increase headroom, smaller numbers distort quicker, etc…).
7. Power down the amp, pull AC cable, let tubes cool down. Remove tubes from tester sockets, remove tester sockets from amp’s tube sockets. Now put the tubes back in the amp’s tube sockets and you are done.
bm229Posted at 14:51h, 20 February
Looking for some help. I recently bought a 78 JMP 2203 and an Aracom attenuator just arrived so ive finally been able to crank the master volume. When the MV gets to about 7.5-8, if i play a note/chord and let it ring out, i start to get crackling through the speakers. Could this be the power tubes not biased properly? or would you be looking elsewhere to fix the problem?
DriesPosted at 17:51h, 20 February
Let it service, the amp is 30 years old and has 90% sure the original power suppyl and bias filter caps. Also retube the amp. Many people with vintage marshalls should do this !
bm229Posted at 01:16h, 21 February
I totally agree with you and plan on doing so. Its just after buying everything im broke. Seller of amp told me it was only serviced by tech 12 months ago.(true or not, i dont know but he seemed a genuine guy). As you can imagine i just want to play the thing but have to take the gamble of not damaging the amp until i can afford to get it serviced.
cevapcicPosted at 02:26h, 21 February
Hey, I have recently bought a Marshall 2104 which is essentially a combo version of the 2204 and I’m having the same issue. My guess would be it’s probably the tubes that need replacing. I haven’t gotten around to that yet, but fortunately I’ll get it done in a week. I’ll let you know.
bm229Posted at 03:27h, 21 February
That would be great. Thanks
DriesPosted at 08:22h, 21 February
Amps get a 1000 times better after a (full!) service. Many resistor and cap values can be drifted, making the amp sound way different than it used to do.
Important things to check:
-Plate resistors ( esp carbon comp ones )
-Bias filter caps
-Power supply filter caps
fmaciasPosted at 14:12h, 16 January
I have a Mesa Boogie Mark I. I understand the bias is permanently adjusted. Is this so or do I need to bias it. I just put in new tubes.
rockabillybobPosted at 21:40h, 07 March
I have bias point installed in to the back of my JCM800 4104 2×12 combo…how do I attach a pic?
ps I’m new here so greeting brothers and sisters 🙂
AntPosted at 14:46h, 08 March
welcome dude 🙂
sorry i cant help you im no techy but some of the guys here might be able to help
oldmenPosted at 20:24h, 24 May
Why do you use a Testsocket?…..Isn’t there any Point where you can measure the Millivolts (Resistor or other). When you have an Schematic diagram, can you read where you can measure the Bias. But when you say i will have this Dound, then measure it and write it into the Schematics.
EmplexadorPosted at 19:45h, 10 October
To answer the OP: How often does one need to bias the amp? Depending on the place and situation, up to several times per day. Wall voltages fluctuate during the day and in some places the difference between peak and off-peak is enough to throw finely biased amps in critical settings (recording) off. Thus the need for expensive voltage regulators. Assuming your input AC voltage is stabilized, then it depends on the tubes and the frequency and intensity which affect tube temperatures and other variables (are you playing out, recording, etc). Measuring the bias of old Marshalls is easy with bias meters between the amp and tube. No need to open amp.
Dave4433Posted at 18:33h, 09 October
Silly question (as I’ve never had a tube amp not to mention never rebiased one): how often does one need to bias the amp ? Say if i change from el34 to el34 what should i look out for?
Thanks in advance for any help. =)
DriesPosted at 19:04h, 09 October
ALWAYS bias after a tube change.
And bias check every year isn’t a luxe.
Dave4433Posted at 19:55h, 09 October
Alright. Thanks for the heads up.
I’ve looked around and from what I understood biasing is just making sure that the tubes don’t exceed their voltage limit. Or is there more to it ?
DriesPosted at 21:08h, 09 October
No no, that’s not right.
A tube has alwas a certain B+ voltage on the plates ( 250 -400V ), and a rest current. So the tube is producing heat even when its in rest. For an EL34 this is 25Watts max. You’re always biassing around 60-70% of the max, so 16W max.
P = U.I so when you have a B+ of 350V for example, the bias current has to be around 45 milliAmps. There are certain ways to measure this current, if it hasn’t got bias measure points. The easy way is to get a bias tool.
And please always be aware of the high voltages in your amp!
If you don’t know what you are doing, let a tech do the work.
Dave4433Posted at 18:18h, 10 October
Alright that pretty much clears it. Thank You!
I think most of the bias tools come with a sheet of paper where the bias current for certain tubes is written down. Are those numbers reliable?
And thatnks again for the safety reminder. =)
DriesPosted at 18:57h, 10 October
Those numbers are indeed reliable, but its worth it to study a bit on the formulas on your own too.
The B+ voltage of your amp isn’t that reliable. That voltage must be measured, but a bias tool will probably do the job.
And after installing new tubes, check the bias again after playing a couple hours.
Dave4433Posted at 19:15h, 10 October
Ok. Thank You once again, I’ve written it all down and hopefuly will be able to do it correct when it comes to actually biasing the amp.
Though this topic here does infact cover most of all there is to know, a little more reaserch will never hurt.
Off to google search! 😀
OldSchoolRocker666Posted at 17:51h, 24 March
Is there no technology to ”suck’ voltage out of amps as to not risk getting electrocuted while working with the inner parts, like drinking water out of a bottle or something ?
06AngusSGPosted at 18:25h, 24 March
There is a way to drain the power out of the caps eliminating the shock risk. However, if you are Biasing the tubes then it has to be charged up.
To drain my 1987x here is what I do. (I’m not sure if other amps are the same though)
1) WHILE THE AMP IS PLUGGED INTO THE WALL (and the speaker cab.), LEAVE THE AMP POWER SWITCH IN OFF MODE, BUT HAVE THE STAND-BY SWITCH IN PLAY
MODE…THIS WILL DRAIN THE FILTER CAPS IN ABOUT 2 MINUTES
2) REMOVE THE POWER CABLE.
3) ONCE YOU REMOVE THE CHASSIS, USE A SCREWDRIVER OR OTHER TOOL WITH AN INSULATED HANDLE TO TOUCH THE HT FUSE TERMINAL TO THE CHASSIS, THIS WILL TAKE CARE OF ANY LEFTOVER VOLTAGE, THOUGH THERE SHOULD BE NONE (OR ALMOST NONE) AFTER YOU DRAN THEM USING THE STANDBY SWITCH.
AGAIN: These were the instructions from David Bray for my 1987x. If you have something other than a simpl circuit Marshall it might be a different process. 🙂
By the way, what are you wanting to do?
ar2619RobPosted at 19:15h, 18 March
Probably a daft question, but is any adjustment to new pre-amp tubes required? Have just bought a bias tester kit and will take some readings of how well my amp is operating, maybe get a full set of tubes.
sargePosted at 04:24h, 10 March
i really need a great amp that giv es great malcolm tone but idk if i should look into line 6 spider, mesa boogie, or marshall…..help?
kjellgibsonPosted at 09:18h, 09 March
Super Bass 1975 -Bias 36mV /// Super Bass 1976 -Bias 35mV
DriesPosted at 10:36h, 09 March
This wouldn’t make any notable difference. Every amp is also different, because the B+ voltage is different from every tranfo.
kjellgibsonPosted at 11:01h, 09 March
Just sharing settings on two PARTICULAR amps
acdckicksassPosted at 17:05h, 09 March
are you dutch?
sorry for this useless question
DriesPosted at 17:13h, 09 March
Yes I am !
SGACEPosted at 00:13h, 09 March
Nice article, if there are any photos would be welcome.. Even now I dont know where is the trim pot for bias in my JMP…
ghostwriterPosted at 02:33h, 09 March
I don’t know how to upload photos or I would post some. Anyone who can tell me how to post them?
06AngusSGPosted at 05:23h, 09 March
As far as I know you can’t post anything but links to pics here in the comments.
When you’re writing the post itself though, there is a button in the editing tools area for picture insertion.
Click the button and a new little window opens up. You’ll see where to put the pictures link. (the picture link does have to be from a hosting site; Flickr, Photobucket, ect… )
Hope that helps 🙂
ghostwriterPosted at 21:59h, 09 March
headwhop26Posted at 23:55h, 08 March
Damn, I wish I could experience the glory of tube amps. One day!
I love it when step-by-step instructions are available like this, I could use more of these if you have the time to write them out!
SoloDallasPosted at 23:57h, 08 March
You will. I had my first tube amp when I was 20 something. Before then, I simply couldn’t afford one 🙂
DriesPosted at 11:29h, 09 March
Fil, I’m still doing some research on what amps AC/DC used. You remember the ‘baby please don’t go vid’? From december 76. Angus then had that kind of ‘hollow tone’, thats what i’m looking for. I’ve looked good what they used, and in the flashlights of the camera’s you can see they’r allready using MV jmp’s. And Angus is still using his chord. Those amps MUST be modified to get such a tone.
bananePosted at 11:56h, 09 March
You mean this one?
DriesPosted at 12:02h, 09 March
Yes. I see a lot MV’s on the stage. They must be one of the first ( in the large box with big logo ) !
bananePosted at 12:04h, 09 March
Hm, what makes you think this are MV amps?
DriesPosted at 12:08h, 09 March
You see the large box heads with the large logo’s ? I can see only 2 inputs too. Will upload some more pictures.
DriesPosted at 12:25h, 09 March
Here you can see again 2 MV heads, and one old type box ( left ). Can’t see exactly how many inputs there are on this one…
bananePosted at 13:59h, 09 March
Yes, it’s hard to tell on both images. I can’t say if there were MV or NMV amps.
thearmedmonkeyPosted at 14:36h, 09 March
The one with the grey duct tape is the same he used for recording Let there be Rock i think. If you watch the original video (the one in the church), you see clips from the studio and Angus standing in front of it. I THINK it’s the same amp as it is the only one I’ve seen with that duct tape on it.
thearmedmonkeyPosted at 14:37h, 09 March
The one in the top left corner has the old style switches, maybe NMV?
DriesPosted at 14:41h, 09 March
But I think the main sound comes from the new box style heads. So it would mean there are not much ACDC recordings with ‘old superleads’.
thearmedmonkeyPosted at 14:51h, 09 March
Agreed. From the variable sound of the albums, I think they used new JMP’s every year. 100w’s and JMP 50’s for soloing. Malcolm said in an interview that for live shows, they moved the equipment from the studio onto the stage. And at some shows you can clearly see heads with the big logo and 2 inputs. So from 77 – 80 I’m guessing they used 2203 and 2204’s of the current years.
svh366Posted at 15:19h, 09 March
you guys wanna know something about angus’ amps? ask him via email! no, seriously, are the ones above the same amps as used at the apollo theatre, ” plug me in”-dvd?
DriesPosted at 15:41h, 09 March
Yes. But there’s he’s already using his wireless with a kind of boost. His guitar is heavely modified too in this era.
– His output jack is still there and plugged in, but till the end it’s plugged out and there’s still sound. So it could be only cosmetic, and the actual output is straight from the cavity to the transmitter.
– Badass bridge installed ( I hear much more attack and resonance here). But bending and vibrato might got harder, because there is very less string length behind the saddles to stretch.
Here is his ‘ hollow ‘ type of sound mostly gone. Now it sounds more clear, to me a very different type of tone than in the baby please don’t go vid.
SoloDallasPosted at 20:46h, 09 March
The one with the tape “should” be his older 1959, from the late ’60s or super early ’70s. I can’t prove it for now. I am still testing, and I bought (another) 1978 JMP 2203: the one I have now is too much modded to represent the tone of that era. I will probably sell it.
This “new” JMP (2203) I am getting is instead untouched, so it will retain its full tone of the era. I think it will sound closer to the 2204, but meaner.
I am almost convinced that Angus “told the truth” when he said he mostly used a 1959 (or a bunch of them). The more sound tests I make, the more I hear a ’59 driven by a boost. He did have live 2203s or 2204s, and they did state that they were doing the live shows with the amp they had in the studio, however, with the exception of Powerage, where it is clear that MVs have been massively used, I can not hear MVs elsewhere for Angus. Maybe – I say maybe – on For Those About To Rock and Flick of The Switch. But even on FOTS, videos of the rehearsals show Angus with two 1959s and one 1987. The thing is that, boosted, all NMV amps tend to sound closer to MVs. Still not being MVs. Will find out, in time 😉
adam10603Posted at 14:54h, 27 January
Wait a second. Are you talking about the grey piece of duct tape that AC/DC used on the front of some of their JMP heads? When I first saw that, I didn’t pay attention to that. But watch this:
(this is a hungarian band, called Hobo Blues Band)
If you stop the video at 0:14, you can clearly see a red JMP in the background, that has the SAME PIECE OF DUCT TAPE. But this band has nothing to do with AC/DC. It’s kind of a mistery for me. I’m sure it means something.
What do you think?
bananePosted at 22:29h, 27 January
Well, it’s just a very common tape used on stage for fixing cables or whatever. Most times it’s grey or black.I won’t give it a specal meaning 🙂
SoloDallasPosted at 20:40h, 09 March
Dries, I am too doing that research. COntinuously.
Now, I think that that “hollow” tone (which I love) was made by a single microphone, off axis on a 4×12 loaded with greenbacks, 25 watts. I should be getting back my 1969 (date) 1960A cabinet with pre-rola (the best ones) original greenbacks, fully restored to original (looks and function) condition. So the major point there is the microphones. The amp counts, too. I think the amp there was a 1959.
DriesPosted at 23:08h, 09 March
Fil, happy you understand! His ‘hollow’ kinda tone seems disappeared around the late 77’s – mid 78’s.
But it’s not really possible to overdrive this amps so far without a boost ( read: without his vega wireless ), and if you puch the amp’s volume too far, it starts to get fizzy.
I still strongly believe their amps were modified. At ALL times, even now. That evidence that Angus is using a JTM45 RI, is hard proof. But you can’t get such a rich tone out of a stock JTM, it’s just impossible.
I will continue my research !
bananePosted at 23:40h, 08 March
Would be nice to experiment with different bias values and how it changes the tone. Does anyone have experiences in this?
ghostwriterPosted at 23:53h, 08 March
Each amp is different, so there is no absolute.
But, biasing higher with a higher number tube can give you more headroom. You can harden the attack of amp. Great if you are going for a metal tone and your amp is a little too squishy for your taste.
Biasing higher with the tubes at the same number will cause the amp to distort quicker. Lowering the number of the tubes and biasing at your usual setting can also do this.
SoloDallasPosted at 23:35h, 08 March
Thanks very much for this. Having just burned (well, not sure, will go to the shop) a ’78 JMP 2204, this will be valuable!