Thanks this worked a treat! I also found if you turn the power straight off (after warming the tubes up) without going into standby the plate voltage dissipates rapidly through the tubes. I also put a few layers of duct tape over the capacitor connectors to reduce the risk of getting zapped.
I really enjoyed your biasing lab. I do have to say if people are searching for help regarding biasing tubes, they should already know about the operation of a high voltage supply, safety concerns, etc... Awesome lab from beginning to end. Thanks for your expertise!
so what do you do when you have one bias control, 4 tubes and the bias is off by a few mv/tube. say 32 - 38mv, like in my 2203 100w Marshall? Do I need new tubes or it dosen't really matter. Thanks.
Question, what is the point of having resistors on all tubes IF the bias control is for all 4. I know the value of my tubes as Ive had them checked with a tube tester and they are all the same strength. Wouldn't it make sense to only have one resistor as the bias knob is for all 4? I guess this would only apply if the tubes were all the same strength. Thanks
Thanks Terry, enjoyed the video........Been enjoying your videos for many years and the guys who play the guitar through your amps......Have a great day and thanks for your service in our Air Force.............Bill
KILLER INFO Terry. Us dummy guitar players need that on our old Marshalls etc. It only makes sense. Thats why I switched to playing Fargen amps. He puts a test point on the outside of the amp making it easy for fine tuning bias etc.!!
Excellent video just two thoughts, one you didn't mention where to connect the black leads from your two DMM's and also can you leave the one Ω resistors in place after you're done with the bias adjustment?
The black leads are connected to the chassis (0V) and so are the other ends of the resistors. One meter is measuring the voltage at the plate of the tube. The other is measuring the voltage across the 1 ohm resistor. The voltage across a 1 ohm resistor has the same numerical value as the current flowing through it, e.g. a reading of 37mV means that 37mA is flowing. (Ohm's Law: I = V / R, I = 0.037mV / 1 = 0.037mA.) Many amps have these resistors installed at the factory, so you can leave them in the amp. You won't notice any difference in the sound of the amp and they are already installed when you next want to adjust the bias.
Great video.. But I am not sure if you mentioned it Terry.. The 1 ohm resistor is total cathode current. YOu have to subtract screen current to get plate current draw. And some EL34 will actually draw a lot of screen current (~ 15%, Mullard datasheet).. Hence the large 2.2K 5W screen resistors seen on thin particular amp.. Most EL34 amps has 1K resistors which is good for screens run at ~ 400V but if you run the screen close to 500V a higher value screen resistor really adds a lot of protection. And the added screen compression might make the amp sound a little softer when played loud. EL34 draw much more screen current than 6l6GC beam tubes. For 6l6GC you can neglect screen current but with EL34 it can be optimal to see how much screen current is drawn to get the precise plate draw.
Thank you for the info Sir. I enjoy working on the tube amps. Usually, I am a Fender guy. This Marshall came in with melted heater wiring. Output tubes were flat dead. I was a bit taken back with no way to verify bias. I wonder how they initially set it? Maybe used a tube extension with current shunt? The end user suffers with no clear way to perform the task. Glad you liked the video
+Terry Dayton Take the amp into a dark room, turn down the bias until the tube plates glow red, then turn it up a bit! Seriously though, they set the bias voltage to their specified level, and just hope the tubes are close enough.
i am trying to check the bias on a vintage push pull amp that has no bias pot no test points and no standby switch. The tubes are red plating bad and I want to see if they are biased properly or if i have another problem but nobody seems to show this on vintage equipment im not understanding what exactly i am trying to look for. I see conflicting information.
Howdy. Another good thing with the cathode resistors is they help managing parasitics. If mounting the resistors is felt cumbesome one might buy a couple of biasing adaptors. They will allow to connect a mA meter in the cathode circuit. Regards.
Forgive me if it's been answered, but do you have a list of "go to" vendors for electronic parts or even better a common parts list for Gtr Amp Repair...?
Another interesting vid. I always put 1Ω resistors in and luckily I have never known any blow, although, as you say, they do act like a safety fuse. Thanks
Thanks bro! You’re great! I really do appreciate what u do! Really helpful channel... thanks to u I’ve got my old jtm45 roaring again and my 2 dsl40c are melting faces all around.... Always been doing pedals with my little business but thanks to u I’ve got plenty guitar amps done to many friends🙏🙏🙏 Thank you very much 🙏🤘👍
Hi seianniesei, I also have a JTM45 and 2 DSL40C Marshall's! Small world. Do you think the JTM45 with 2 5881 tubes are different to bias than the 50 watter shown here?
I’m not sure I understand the resistor you installed. So you clipped the jumper between pins 1 and 8. And then connected the resistor to ground and…. Pin 8?
Pins 1 and 8 are jumpered and pin 1 has a wire to ground. He removed the wire to ground and replaced it with the 1 ohm resistor. Voltage drop over the resistor in mV DC is the same value as total current through the tube in mA.
Thanks a LOT Terry. I just ordered new tubes (First time since I bought this amp) and would have never thought of adjusting the bias. I was just cruising YT to make sure I didn't miss this exact sort of thing. Yes, a scope demo would be way cool too! Thanks again!
So,What’s the standby switch used for,and which one of main sw n stby sw should be turn on 1st n turn off 1st, I know nothing about my guitar amp that bcz it works/no work (sound) eventually (Marshall JMD-1 model 501 2*12” celestion speakers).pls give me some detail,thanks
I also have a 1977 2203 Marshall mk2 master model that had new tubes throughout and caps also by the same tech who did the 73 super lead, in the mk2 the power tubes are quad set of 6550 JJs out of curiosity i popped the back off to look at the work and right off noticed that two of the power tubes had signs of running hot, hot enough to dark-n the lettering and the other two looks like they have have never been used, lettering was still bright red ,is there a bias problem?
Hi Terry, I have a question for you. I have a Marshall Bluesbreaker JTM45 amp that takes two KT66 valves. Unfortunately I wound up with very badly mismatched valves. They have a bias difference of 10 millivolts. My question is, can these valves still be used or will I have to toss this pair of valves in the dumpster? Thanks from Michael Newell from Niagara Falls, Ontario, Canada
There is comment by ps10iceman to this video from 5 months age where he wrote that it is possible to add bias adjustment to each tube separetly so you don't have to worry if tubes are mismatched. I don't know how ask him or search in the internet, i only know that in my dsl15c there was adjustments for each tube but when i was changing bias for one there was little change on the other, so it looked like that set bias for one tube then set for other then chech first and correct little bit then chech this second and on and on to the point when both (it has only two output tubes) had same bias value.
It looks like the C19 cap has been replaced by a much bigger cap. I know some people dont like the treble boost that this cap makes to the lead channels and often disconnect it, but why would someone put in a different cap. Is it a different value?
This is a variation of Gerald Weber's idea of the box with an octal socket, a 1 ohm resistor in series with pin 8, 2 banana jacks, an 8 conductor cable, and an octal male plug. I built one about 10 years ago, and it sees a lot of use.
Quick question I have a 50watt valve amp with 2 el34s both power valve base plates pins 1and 8 are linked in series to ground via the 6.3 V CT Brown winding from the Power Transformer can I put a meter on it without having to fit the 1ohm Resistors?
Got a question. I have a JCM900 4100 100W head that I've had for years and it has gone through tubes left and right. I finally tore it down to see what was the cause and found R8 R24, R25 & R36 burnt and open. I also found the bias range resistor that the schematic shows as 15K had been replaced with a 6.8K. I've replaced all those resistors and also C15 the bias cap (47nF/250V ClassX ) with 47nF/600V Sprague orange drop. I bought a matched quartet of EL34's and replaced all the 12AX7's with new tubes as well. While trying to set the bias using a socket style probe, the current is running between 1 and 2 amp's. I have read that the bias range resistor may need to be a lower value to achieve a higher negative reading, hence, the 6.8K I removed and replaced with the 15K. What are your thoughts?
Didn’t you do a video biasing KT88 tubes ? Do you recall what the bias voltage/current is supposed to be? Thank you for any help I’m asking for a friend.
Couldn't you use a tube tester socket that is used in tv repair??? that would go between the tube and the base, and it has pins that come out the side to test...just a thought....nice video..
Thank you for your effort and you video, BUT I have been reading and Learning to bias and this whole add resistors thing has got to be some BS. Why should you have to install anything to do routine things? again thank you, but there has got to be an easier way.
A weird comment about knobs...That Marshall has the same knobs as the Rogers Ravensbourne and Ravensbrook amplifiers which dates the Marshall to somewhere around 1973 or 4 at a guess...I'm a Hi-Fi man, not a guitar man..unfortunately for me...
sooooo, install a one ohm resister betwixt pin 1 to ground, connect probe to pin one across same resister, then read milliamps. set to about 35ma for el34. am I correct? next... neutralizing a pair of 6146... thanks Terry, Don
No, you need to set your meter to it's voltage range. You're reading voltage, but the number of mA flowing in a 1ohm resistor is the same numerical value as the number of mV across it.
Mr D-LAB or may I call you Terry ? I know this sounds ridicules to say but, have you ever done any repairs on super vintage 1973 Marshall 1959 ptp super leads? the reason i say this is when i turn down the volume pot to completely off, it still has sound like it's on three , most components - power tubes/ pre amp tubes/ filter caps and some resistors have been replaced by a supposed certified tech from a well known music store there is some other pixies lurking inside such as hum! I thought that you might be able to throw a few pointers in there, or you might take on this item to use in one of your vids,with a quote?
2 questions. What's percentage of plate dissipation results in 30mA current? Also 3mA offset between plate currents is 10%. In your mind, how much percentage offset is 'too much' before you look for different tube pairs? Thanks for the video.
OK, I'm working on a JCM800 today and was looking around for bias point recommendations. Some say 70% plate dissipation, others say 60%, while others say just adjust then play to ear then check to insure you're not over power dissipation. A lot of contradictory statements out there. I'll keep looking.
Hello, SOrry, I did not know if the first Q was serious. Can you measure the current thru the tube? Is there a cathode shunt resistor installed? 1 or 10 ohms by chance?
I'm currently working on a solution to verify your selection point of 30mA. I'll get back soon. I have made a video, but it has a mistake, correcting the mistake now.
+Craig Hollabaugh and I'm guessing you are the twit who gave Terry's video the thumbs down. It has been 2 months genius, still waiting on your video findings
I have a Marshall ma50h. Just added new el34 and one tube is reading about 35mv and the other tube is reading funny like 20+ volts. Ot mv. Any suggestions ?
To follow up with Robert, I think the whole group is biased (say Watt?) I'm sorry but I get amped over these current potential issues, we must resist getting her tubes tied! Lame jokes aside, thank you D-lab for this really nice instructional video! Very nice approach to adjusting the amp bias after I change our my tubes.
Hello Terry. I have a jmp 2104 50W combo from 1980 which has exactly the same problem, no testpoint. It has also EL34 tubes. The plate voltage is lower because of the power transformer. It's about 390V . Is there a formula that I can use to calculate a prober value for the bias setting? Thanks for the very educational video
The maximum plate dissipation of an EL34 is 25W. Calculate the plate current required for max. dissipation by dividing 25W by the plate voltage on pin 3, and then set the plate current to 70% of the result. In your case, 25 / 390 = 0.064 i.e. 64mA and 70% of 64mA is 45mA, which is the plate current for 70% of max. dissipation.
Is it not easiest to measure the DC resistance of each side of the primary, then measure the voltage across the same, then work out the plate current from that? No need for 1ohm resistors and and no need to measure the screen current and subtract that...
Hey question here.i got a tube amp a while ago,first tube amp ive had.it seems the tubes are dead because they wont turn on ,could i put any tube that i buy and put them in.Its a Marshall dsl40 with celestion classic lead speaker and it came with the stock marshall tubes .i have mesa boogie 6l6 str 440 tubes pair i would like to put in
Only thing I'd add would be to let the amp warm up for at least 10 min or so because the EL34's have a habit of drifting until they're fulling hot and you risk biasing too hot or cold. For EL4 rated at 25 watts and say a plate voltage of 460v, 25/460 = .054. Now times that by 60% is .054 x .6 = 32.4 ma. The screens draw around 4 ma so 32.4 + 4 = 36.4ma for 60% biasing. Now that is with a 1000 ohm screen, I see these in this amp are 2.2k so not sure what the current drop is across them in idle. Anyone measure it?
Terry Dayton Terry first just want to say I appreciate all the videos. I follow a bunch of folks on here. Takes a lot of courage to put videos on here. I'd also like to to see one with a signal being applied to the input and a scope on the out put so we could see when clipping starts. It would also be interesting to see the crossover distortion mark being displayed while adjusting bias in order to get rid of it.
Question, why can't you just read the plate voltage off pin 3 and then divide into max dissipation, then multiple by 60-70%. But rather, people/books use shunt or current draw like you. I don't understand why. Could you shed some light?
to know the power dissipation, you need to know both the plate voltage and the plate current P = V x I getting the plate voltage is easy, simply measure it from pin 3. getting the plate current is trickier. you can use the transformer-shunt method (dangerous) or you can measure it using a precision 1Ohm resistor (using Ohm's law) between the cathode and ground. just a note though, when you measure the cathode current, you're actually measuring both the plate current and the screen current. but typically the screen current is much smaller than the plate current and can be ignored.
E-Z-Hook probes/clips would be a lot safer to use than alligator clips. And, I'm surprised you don't have any "socket extenders" with exposed contacts so that you can measure any and all voltages at the socket(s) while the tubes are operating but without having to poke around inside the unit. I have several homemade ones, with the 1 ohm resistors built in as well. PS, the 1 ohm resistor method isn't 100% accurate because it adds in the screen current as well. I prefer inserting a series milliameter in the plate circuit (using a "socket interrupter") ; or the " transformer shunt" method....
good 'un shunt method is best. It’s dangerous, but just as dangerous as poking around the live chassis. However, I always use the method where I measure the resistance of each side of the output transformer primary side. Write it down. Then turn the amp on, let it warm up. Measure the voltage drop across both sides of the output transformer primary side. Divide the two to get your mA calculation. Measure your plate voltage then use that to multiply by the current to get your plate dissipation wattage. That’s the real figure we’re looking for. Especially since most modern amps push more voltage to the plates than any data sheet suggests.
Well if you measure the voltage drop across the screen resistor and subtract it from the total reading you'll be within a1 or 2ma difference with shunt without the worries.
Hi D-lab! :-) Thanks for the help! I have a question about the 1ohm resistor! If it's going to serve as a fuse, I would like to be sure I will put the correct wattage resistor! Would You please give me link to were I can buy the exact resistor, or even if You sell them, I would buy few from You! Also I found something online about grounding pin 1 separately! Would it matter based on Your opinion and experience? Thanks for the help, and Kind Regards! Steve:-)
One meter was on the cathode DCV setting but you stated the reading as amperage? Did I miss something? Only one bias adjustment for both output tubes? Also, please cut out the background music, it's very distracting.
The resistors are 1 Ohm, which means that the voltage reading in mV corresponds directly to the current in mA. Ohm's Law: I = V / R, so (e.g.) 50mV / 1 = 50mA.
Hi I have JMP-50 2204 amp with EL34’s! Well.... it sounds horrible! I have replaced everything I could think of and I could! Tubes, filter caps, trim pot,bias caps, pots, jacks tube sockets, a lot of other caps! .....still sound awful:-( On low volumes it is not that bad, but the more I push it, the worse it sounds! Well....with Marshall tube amps is supposed to be the other way around isn’t it :-) I was checking the bias from time to time and the last time I did I left the probe and meter on V5 , and found something that definitely didn’t look right to me!? Before that :My amp plate voltage is 400V! Most JMP’s are on 600V, but few run at lower : at 400V! My JMP is one of them! So I have set it around 37mA idle! There was around 0.4~0.5 difference between V4 and V5 at idle! I have one of these socket probes that goes between tube and amp socket with probes to be plugged in meter! I am biasing the amp, on mV setting on the digital multimeter! After about 15 min waiting for the bias settling in ,I left the probe and the meter on, and start playing! The readings on the meter went up, which it should! When I was swapping the probe back and forth between V4 and V5 , I found V5 was jumping a lot higher then V4! And as I said above,the more I was pushing the amp, the bigger that difference became! For example: The highest readings on louder lower notes on V4 was reaching 45mV, while V5 was going way higher to 130mV! If I stop playing, both will go back to 37.0 37.4! The lauded the amp pushed, the bigger the reading differences between V4 and V5 ! If I swap the tubes, the higher readings are still on V5! I have well matched pair of svetlanas, and I would guess, there is crossover distortion caused by the big difference in the current draw between V4 and V5! What possibly can cause such a dissbalance Between the two tubes when amp is pushed!?? All transformers are room temperature! No hum or any other signs of something being wrong?! I have Marshall JMP, and now i’am recording with amp simulators .......yuck ! Please Help! Any suggestions would be highly appreciated! Thanks for Your Help, and Kind Regards! Steve! :-) Sent from my iPhone
Hey man, Sounds like a project. Have you verified that the screen resistors are OK. Usually when an output fries, it takes out one screen resistor, see this often. TD
Great Info Right up to the very end ... now subscribed, thx Best Simple inExpensive Dummy Load ever ... ! ... would 10wt work for a 16ohm load~? ... I always use or select > 16ohm & speaker's to get full use of the transformer. 8>)
I wish Marshall would have easily accessible testing points and bias pots on all of their amps. And make them to where you don't have to take the amp out of the chassis.
John Simms I agree. It is fairly simple to install some banana jacks for the bias TP. Also, the little PC type pots they use for bias has to go. I install a standard Alpha pot, chassis mount
+D-lab Electronics i was even thinking to put a panel Millivoltmeter on it and a 3 way switch v3 v4 off a pot outside and set the bias without tools the pot is 50k linear ? or am i wrong
If you watch closely, you would crank the bias until the tubes started glowing red, and then back down until the red plate goes away. It's a crude, old timey method, like Grandpa adjusting the timing on his engine with a glass of water on the air cleaner. It'll work if it's all you have, but there are better ways. Most people today would not recommend it.
I have gotten zapped a few times. It is not a pleasant experience. Once it burned a pin hole in my finger that took a while to heal and confused me for a bit. I do not recommend!
Thank you Silas, I'm going to give it a try next days. Can you make a full guide to add a bias trimpot for each of the 4 tubes on my Laney GH100? If yes please send me PM so we can talk about the work and payment as well e.g. I can pay you through your PayPal account once the guide is ready to be sent to me.
If you are going to modify it with those resistors. Why not go all the way and drill 2 holes to install Test Points on the top next to the tubes? That is what I always do. Then the user can just use a DVM to check the tubes bias whenever he wants? He can match his own tubes then and know when 1 or both tubes should be replaced. If they are a Freak about keeping their amp original you need to get one of those sockets with 2 leads on it that you plug the tube in to and the leads into the meter. LOL 8^)
Not a fan if your music bed. It sounds like you have rambunctious neighbors. Also, you shouldn't use music that has vocals as it competes with your narration.
THE TRAINWRECK PAGES Read it every time you adjust bias. Every. Time. Ken Fischer was the Leonardo Davinci, Albert Einstein, and Nicola Tesla of guitar amplifiers...
The resistors will normally be dissipating 33mA x 33mV which is just over 1mW. A 1W resistor will be fine, and offer greater protection to your amp than a larger resistor case of a cathode to heater short (as described by Terry).
UptempoMusicLessons. Once he starts messing with the amp live, he does switch to a chopstick. He must have made sure at the beginning of the video that the voltage was zeroed out. But I’m guessing. I agree he should have used the chopstick from the beginning of the video.
You kept referring to measuring the current, but yet your leads were in the voltage jack of your meter and you were not in series with the load. You should bias to a certain mV value, not a current or mA value. I think this kind of terminology often confuses the average person who does not know anything about electronics.
Don't get me wrong, I love your channel and you have inspired me to do some awesome things with amps that I find. For that I thank you. You just don't know how often I hear from guys that they heard someone say this or that on You Tube. People hang on every word that some You Tubers say, without really understanding what is really going on. This leads to confusion and frustration with some of them. Sometimes they get so frustrated that they just give up trying to do things for themselves. That is what these videos are supposed to be all about. Showing what you have learned and to help others learn as well. Thanks again.
Actually Terry & D-Lab are correct. The leads are in the V of the meter, but it's actually measuring milliamps. One millivolt across one ohm equals one milliamp. Thats the whole point of using the 1ohm resistor. As an aside, Terry I didn't think you could get away with a small 10w dummy load on a 50w head?
