I have the TP24 power transformer which actually had a reading of 120 Ohms. I adjusted the math equation accordingly and set the bias for 25 milliamps. The amps sounds spot on. A very good video here. Thank you. Very well done.
Excellent work-through video... this is just what we need when we try to repair our gear... the Devil is always in the detail so being shown what to look for and importantly, where and how to find the measurements, is fantastic... I think I know what I'll be doing this weekend (!!??!!).
A power tube (purple lighting inside) took the OT and the PT,but left the boards alone ...put in replacement transformers by Hammond and 2 brand new JJEL84 (200Euro) and did the bias mod with 68k...tomorrow I'll switch it on the first time since it died and gone to heaven... Thanks so much Stuart, your vid helped me a lot!
@@stuartukguitarampguy5830 YES! :-) works good so far,...no hum,no crackling and it sounds as it should the bias is at 2,5V (outer power tube)...though the inner power tube is at 2,2V....I will see how it's gonna be when they're burned in (matched pair)
Hi. Great video. Thanks. Just FYI or for anyone else. I just bought the new one called Limited IV SE and checked. The circuit board is different now. Mine luckily is set perfectly at 65-67% and I’ve never seen 2 tubes matched so closely one was 22.4 the other 22.5 mV. Guess I was lucky. Great little amp with a huge sound! Thanks. PS - PV measured about 360-370 using 2 methods a probe and then at pin 3.
As you promised it was interesting and no wonder the amp was given away, after watching videos of burnt tube sockets that needed to replaced. I have the first series amp like on your bench which looks pristine inside with no speaker output. thanks for the disclaimer as I got a static buzz even with the power off on the reverb tank wire, so be careful.
Hey Stuart I’m curious, what was the fault with this amp that caused the bias to go wacky ? Was it a connection issue or a flaky cap. Thank you and you do a great job there’s not many folks that would spend that kind of time to find an issue. When I was serving electronic equipment I would do the same thing no matter how much time it took to find the fault I would keep going until I found it.
@@stuartukguitarampguy5830 oh my bad I didn’t think to look at how long ago you serviced this amp, that’s been some time ago. Never mind thanks for responding have a great day 😊
Great video. I'm getting ready to replace my EL84 tubes in my Fender blues Junior amp. Maybe I can put a quarter watt multi-turn trim pot on R51. Since your value was 120K, a 200K or 250K trim pot might allow me to bias the amp.
Great Stuff and much appreciated. I have a 1999 USA Blues Jr (green board 1995 Rev B P/N 049305) purchased new by me in ’99 which is different from all the cream board Jr’s all over YT. Even with limited use as a bedroom amp the hot bias has has resulted in a few power tube socket pins to loosen from the tube board, thus the desire to cool the bias and continue using the amp. As I understand this board revision schematic, R37 and R31 make the voltage divider circuit and a parallel resistor across R37 would cool off the bias. Values per the schematic are R37-33k and R31-22k. What would the proper ohm rating of the resistor to accomplish this? I hope I have described all this properly. Thank you…
Sorry for delay - you've probably done it by now. Anyway, start with a 220K and see what that does - you'll need a way of measuring the bias of course. Work down in value until you get what you need.
Do transistor amp have bias setting or only valve amps , and whilst replacing capacitors is it ok to fit exactly the same legen caps that at a third smaller any advice on this would be very much appreciated, thanks Craig Macdonald.
I've done the same thing a couple of different ways, one by paralleling a pot across across the 33k resistor and adjusting it to where I like as you did with your decade box and then installing a fixed resistor in parallel the other way one of my vendors hand selected some very low current draw EL84s which sometimes can be dropped right in. I've installed trimmer pots as well. Once I get it where I like it I mark the chassis so the next person in knows what current rating the power tubes should have. Now. Tell us what the fault was that you were talking about.
Can you just turn the amp off & measure the resistance between p2-red & p3-brown & p2-red & p1blue which are the anodes/plates of the 2 EL84's? If the resistance of both are similar say 91.0 & 91.1 respectively, then you could just turn the amp on & measure the actual anode/plate voltages respectively & then perform a simple ohm's law calculation of e/r=i? Then you would know what current they are pulling? Is it possible to do this way?
Hi Mathieu. Yes that's way too high. Email me and I'll send you a bias manual I wrote for this amp. That will get you out of trouble. Email address is on splash screen at the end of each video.
Great video Stuart - very informative. I'm wondering about the low level buzz / hum in that Blues Junior. In your experience is that normal for these amps? or can it be dealt with somehow.
I'm not 100% sure since it's been a while since I did the vid. The back is off and that would reduce buzz once it is back on. I haven't really noticed a problem with these amps in the past.
That blue wire passing close over the ribbons is notorious for noise. Think it’s become common nowadays to tuck them in a bit to get some distance as it can massively reduce hum
Trying to bias my 1 year old BLUES JR IV SONIC CREX 120V FSR2019. They seem to have again changed all the components around. R51 and R52 have been changed to only 266 ohms and 252 ohms? My readings from P2 red to P2 brown are 3.4 volts DC. And P2 red to P1 blue are also 3.4 volts DC. I wonder where I can find the correct schematic? I want to make sure I'm adjusting the right resistor.
Hi As it happens I can help you out with that. The FBJ IV has a different bias arrangement to all previous FBJ. The IV is cathode biased, which means it's not adjustable. The neasurements you took indicate that it's running a bit hot but it's probably designed to run at that current. So you can leave well enough alone, or if you really MUST adjust it you'll have to locate the cathode resistor and increase that. I don't know where that resistor is. It's likely to be R51 and R52 currently 270ohm. You could increase those to say 330 to start with and see what happens.
I didn't find an updated schematic on Fender's site. They should have an updated manual at the very least. To tell the customer they no longer have to have the amp biased by a technician. That would save them money on warranty.
I have a hoffman replacement tube board which has bias test points for each pwr tube, What would the vdc bias setting be ? not amps. This amp also had a bias pot added from original owner .
Hi Mark I have no idea I'm afraid as I don't know that board. The SENSIBLE place would be to put a 1 ohm resistor in the cathode of each tube and measure across that. I'd power it up and measure at the bias points. If you're getting something between say 20mV and 50mV then that's what they have done. In which case adjust to about 25mV - not critical. Let me know.
Thanks for the great video, very informative. Would it be concerning to use a 1/4 watt resistor vs. a 1/2 watt? And would this change with resistor value, say using 68K vs 82K or 100K? Some back-of-the-envelope math tells me it should be ok, but answers in forums seem to vary as to the safety of it.
fender blues junior IV : R52 : 27 kohm. R51 33 kohm. I put a 82 kohm resistor in parallel with R51. I measure 1,9 V between The Red and Brown. It's OK ? I think yes.
I got a question, when I measured my OT my brown and blue got 105ohms, my red and blue got 97ohms and my red and brown got 8ohms. Is my OT fried. Also my BJ wont turn on, could this be the reason why? Thank you, great vid.
Hi. Okay so assuming you have unplugged the BLUE and BROWN so that you are measuring durectly onto the OT, the RED-BROWN and the RED-BLUE should measure about the same resistance. So if you are getting 8 ohms on one and 100+ on the other I think there is a problem there.
That may be, I'm unsure. I rarely/never get a V4 in as they are quite new and not yet going faulty. Most of mine are the cream or green boards. The older versions.
Hi Stuart, another great clear and concise video. I was wondering re the crocodile test leads. Do these need to be rated at over 300v ? If so, any idea where I could buy them ? Most of the ones I have seen don’t seem to have a voltage rating. Many thanks
Thanks Paul. You only need a voltage rating when two things are in very close proximity to each other and might arc across. E.g. the voltage rating on a capacitor is defined by how close the plates are and the insulator between them. That's why high voltage capacitors are larger - the 'plates' need to be spaced further apart to withstand the potential arcing across.So, you can't really put a voltage rating on a piece of wire (e.g. croc clip lead). A current rating, yes, but not voltage. I hope that makes sense.
@@stuartukguitarampguy5830 Thanks a lot for the reply Stuart. To be honest it’s still not clear as the actual multimeter test leads have a voltage rating. I’ll see if I can find some more info via Google. Many thanks
Thanks for the informative videos! I like learning - and a few years ago replaced the Power Transformer in my late 60's Traynor YGM3. - it had a bodged in 220V replacement that caused the Heater to run at 7.5v and B+ at around 450vdc! Has the correct Hammond one in now. A couple of diagnosis questions if I may? I've just been given a dead Blues Junior that had a blown V5 valve and dead power transformer as a project - lost vacuum and has gone white at the top - which I assume may be the reason the PT went down. The traces around the base were pretty discoloured with some lifting. Have reseated, but I'm replacing the board. It looks like V5 has been quite a bit hotter than V4 for a while. Have replaced the PT and all up and running with correct voltages at the filter caps and heater at 6.7V - but low output and distorted. I measured the Output Transformer and have a massive mismatch it seems. I did add a 82k resistor to cool the bias - as per the recommendation by you and people like Psionic. When measuring the voltage either side of the centre tap, I have 2.1v and 0.8 volts. The resistance/impedance measurement either side of centre tap is 110 ohm and 45 Ohm. Is it likely the OT has gone south as well? All the other components seem to be at spec to the schematic. Many Thanks!
Hi. If you have that white at the top that means the tube lost vac. If the PT went down then the tube drew a lot of current. Surprised the HT fuse didn't catch it though. It's certainly possible that the OT went down as well. Despite my video I (and others) are finding it sometimes hard to measure the current on the BLUE side of the OT (i.e. RED to BLUE). I'm unclear why this is. If anyone reading this knows, I'd like to hear. It's fine on the RED-BROWN side. Your ohm readings on the OT are a fair way off. Each side should be about 85ohms. Disconmect the RED BROWN BLUE spade when you take the measurement just to be sure. Let me know how you get on.
@@stuartukguitarampguy5830 Thanks Stuart. I ordered a Hammond replacement and installed it and everything is fine now - so the OT was definitely at fault as well. The new one measures 92 either side of the centre tap. There is solid brown gunk oozing out of the old OT where it looks like it melted then set afterward, so I think one side of the tap was totally shorted - still measures 110 and 45 respectively. Hard to tell what the sequence of events were here - whether it was a transformer or valve failure. All the big IC caps were fine and showing correct vdc when I installed the PT prior to changing the OT as well. I've replaced them now anyway, as there was some yellow crustiness showing - so they were on their way. I've also wired the transformer to operate at 240v as it has a tap. We regularly get upward of 250v from the outlets here! The amp is still plenty loud and crunches nicely when you want it. Not sure why they set it all so high...maybe for the metal kids. The Valve board was a very brown on V5 and had some lifted traces, so that has been replaced as well. I was given the amp for free and all in it's cost £120 to get it back up and running. It's so great having gents like you on YT for self sufficient diy'ers like me, who still want to learn stuff well into their 50s...thanks so much
Great videos! Would like to know what the other issue was? I get proper voltage drop across red to brown on output Transformer; but meter is all over the place when checking red to blue voltage drop. Meter even beeps like a continuity check when under 200 ohms. Cream board, amp has painful high mid and ice picky highs it did not have before. A bad tube blew out a trace on the tube circuit board; replace it with an upgraded one. What might cause this? thanks for all you do!
Hi Bill. Glad you're njoying the vids. Actually I've had some issues sometimes measuring that blue wire but I'm not sure why! On your other issues, it's just too hard to diagnose over the internet - I hope you manage to get it sorted.
Yes you could do that if you can find a neat way to do it. A bias meter would also do the same thing but the small 8-pin bias meters are not cheap. Eurotubes do one.
I have a set of supposedly 'matched' JJ EL84s, but the voltage from Red to Brown is 3v (fine) but Red to Blue is just under 2v (bit low). They don't seem well matched to me at 50% different. Does this sound like a mod needed for separate bias?
One little point on my amp, is I have opted to use the 240v windings for the power transformer, not the 220v as was set originally, which does reduce the voltages subtly.
Hi Kam I've noticed this difference too. There's something odd about reading to the blue wire. I suggest you just do the broan and get the bias right on that. With a matched pair you can assume the other valve is ok bias wise.
Guru saya ingin tau buat travo OT tube EL84 itu untuk primer brp gulungan untuk sekunder ke sepepiker juga berapa gulung mohon bantuannya maaf gak bisa bhs inggris
Hi Stuart what are your thoughts of getting the blues junior to run at the uk 240vac i understand that at present they are set to run at 230 vac!! Is this wise to do, as its apprently a case of swapping the wires over on the power on switch !!
@@andrewmules5191 I've never actually done it. I don;t think you need a new transformer, just a change to which primary winding you use. If someone has detailed how to do this on,say, RU-vid, just follow that.
Hi Stuart ive just taken a reading off the brown wire on my blues junior the reading came in at 4.27 very high the meter was set at 20v Dc can you suggest a resister value to drop the voltage ?
Hi stu i havnt had your manual !! That would be a great help Thank you. Just one other point regarding my earlier question regarding running a jewel lamp off the heater leads. Does it matter which wire goeos where on the bayonet fitting fron the two greens ?
@@andrewmules5191 If you go to the splash screen at the end of all my vids you'll see my email address contact me via that for the manual. No either way around as it's AC!
@@stuartukguitarampguy5830 Thanks again Stu . god i wish i was closer to you i think i would have given you the amp will email you soon as Thanks again have a merry crhistmas👍
Top notch Stuart! Great explanation. One question, if I'm biasing a cathode biased 6V6, do I shoot for the 70% of total dissipation? Or should I just bias at 12watts? Thx!
So just to check, if you're biasing a CATHODE biased amp that means you're changing the value of the power resistor in the cathodes, right? If so, I'd go for the 70%.
If you connect a multimeter between transformer center tap in presence of high voltage it´s very likely that on one of the sides (red-brown or red-blue) you will read strange measurements due to an oscillation that occurs when extending those cables outside the amplifier. In this case it would be more convenient to identify the bad side and read on the good side twice by swapping the tubes between them. 33ma on a el84? Thats too hot! Why aren't you sharing the bias trouble? I'm working on the same amp and its giving me fits trying to get it right.
Hi. Okay here's the scoop. The best way to bias these amps is to use a bias probe between the EL84 and the socket. BUT... it's rare for people to own one of these small base bias probes (I do have one made by Eurotubes). So, how to bias? Well, the output transformer primary has a resistance of about 100 ohms. So if we measure the voltage across that primary half we can work out the current through it. so 2V measured across 100ohm gives 20mA current. We can measure from red to brown for the one half of the push-pull (one EL84). Ok so far? The problem comes with the other half (blue to red). You don;t REALLY need to measure this if you have a matched pair of valves. However, I too have noticed that if I attempt to measure Red to blue, I also get weird results. I have no idea why and if someone reading this knows, please let us all know as this has always mystified me. Take home? No bias meter - measure red to brown and use ohms law to calculate the current. If you want it more accurate, with the amp off, measure the ACTUAL DC resistance of that winding. You may find it's 94 ohms or whatever, and you can use this to get a more accurate reading. Hope that helps.
@@stuartukguitarampguy5830 Well the amp I'm working on just got weirder. The amp has a bit of distortion in it but I can't find nothing bad. I cut the 3000 volt flyback diodes out of the circuit and the distortion is gone. The diodes test ok. ???????????
@@rockinws6567 That is strange! Could they be breaking down at moderate vpltage, but read ok on the meter? Try replacing them with new, if you have any.
This is amazing for an electronics luddite like me. Thanks for making this video. Why would fender design the amp without a bias control? Or why not design it as a cathode bias so it does it automatically? 🤔
Hi Josh Two questions there. On the bias control, maybe cost? A few pennies more! You question about cathode biasing is a good one as many lower powered EL84 amps ARE cathode biased. You do lose a little bit of power with cathode biasing though, so maybe they wanted full power? Not really sure, it's never easy to figure out why something was desgned the way it was!
Informative as always! But when it’s a voltage that could / would kill you stone dead, I’m not going near it and leave it to the professional. A question as you’ve seen many amps come in: What is a good fender style amp build-wise and from a repair perspective? Any thing come to mind in perticular?
Hi Jut saw your other post. Stick with brown to red for the moment and ignore blue. For some reason I've never been able to find out, measuring broiwn to blue can sometimes gives weird readings.
Hello stuart with reference to an esrlier post regarding bias current !! Ive put new tubes into my amp and did a current test off the blue which reads 2.45 and out of curiousity it read -2.48 and the speaker started to hum a little, could there be a fault with tubes ive swapped them over and still the same outcome !! or could there be another problem . I do wish i was nearer to your workshop !! If you can shine any light on this much appreciated .
Hi Stuart thanks for a cool video. One question you ended up with plate voltage of 333 and bias at 26.5 ma. Doing a little math that is 8.8245 Watts for a 17 watt tube or running at 0.519088235294118 which to me seems to be way cold. Am I missing something here because my math says the tube should be at 11.9 watts and while the voltage will change going with 333 volts the bias should be set at 0.035735735735736 ma or 36 ma. Am I missing something or are we just trying to make the tubes last forever?
Hi. Interesting. I can;t fault your calc. I always use weber bias calculator app on the ted weber site. If I plug these figures in there it say 26mA. at 70% dissipation. I'm not really sure why there is this discrepancy.
@@stuartukguitarampguy5830 I think I figured it out. The question is what is the actual wattage of those power tubes. I thought the EL84 tubes were 17 watt tubes but it appears they could be 12 watt tubes. The amp itself is listed as a 15 watt amp but I would have to say the Weber site calculations are based upon a 12 watt tube.
@@stuartukguitarampguy5830 I was thinking being there doesn’t appear to be an agreement on what the tube power is in class AB operation that perhaps we should go with the Fender numbers? Fender says it’s a 15 watt amp so 70% of that would be 10.5 watts!? Hey, what ended up causing the runaway bias in that amp? What did you find? Thanks so much Stuart!
Hello. Thanks for your video, I enjoyed it. Although I know nothing about tubes, I'm confident enought in Electronics and safety to attempt to bias my own Blues Junior. To that effect, I was hoping you could suggest a value for the series resistor attached to the 50K pot (I'm thinking a trimpot), that replaces R52 (as per the comments below). And would this pot alone suffice? Because you mentioned two biasing resistors: R51 and R52, so wouldn't it be necessary to change R51 for a pot as well? Thank you!
Hi I don't want to give a huge long reply here, but this is the principle. First decide which biasing resistor you are going to swap for a pot. Let's say it's 33k. So you want that resistor to vary between say 25k and 40k or whatever. So choose a pot and series resistor to suit. E.g. a 25k series resistor and a 15k pot wouild give you exactly what we were aiming for in my example. I would use a 10-turn pot as this makes the bias much less fierce. You may have to play around with resistor and pot values. Check out 'BillM mods for Fender Blues Junior'. I think one of those mods covers the bias pot. Good luck!
@@stuartukguitarampguy5830 Hello! Thank you for your reply. Would you consider replacing BOTH biasing resistors with a pot + series resistor? I would assume this should make for easier biasing in subsequent changes of tubes.
One of the interesting things about the Billm mod is the trimpot takes the place of both resistors. The actual bias voltage is developed between the common point of R51&52. The Billm trimpot lowers one value and raises the other in the voltage divider network, keeping resistance to ground approximately the same. 55k to ground in the original circuit, (22k+33k) and the 50k Billm trimpot. It’s probably easier on the zener diode regulating the 15vdc going to the reverb IC. Just a guess.
Im thinking of changing the cheap looking power lamp for a jewel lamp on my blues junior, can i use the exsisting led wires to run the lamp or does this mod require other alteration to achieve this ?
Hi Andrew No you can;t use the existing leads as there is probably a series resistor there. Take the feed off the 6.3V AC heater windings and that should do you.
Hi Stuart. Any tip on how to measure the resistance of the primary winding of the output transformer? Need to check my biasing since I don't have a probe.
@@stuartukguitarampguy5830 I believe the transformer had the resistance around 100 ohms. It could be 95 or 98 ohms etc. I would like to measure my milliamps for el84 considering the exact resistance of the transformer. I believe I need to measure the resistance between the red and brown half of the transformer. Correct me if I am wrong.
@@user-uj7nj8zu9s Hi Again, sorry it's me who is getting confused! Yes you can measure each half. From memory it's RED-Brown and RED - BLUE then use your exact reading and ohms law to calculate the current. Sorry for confusion!
@@stuartukguitarampguy5830 Hi again Stuart, thank you so much for the response. The measurements will be done. I've seen lots of recommendations for UK Blues Junior owners to switch the transformer operation to 240 volts. The export transformer for UK and Europe Juniors has a dedicated terminals which could be switched with no soldering. This will make tubes running a bit cooler and reduce hum as well. Could you recommend doing it?
Excellent video Stuart. I'm replacing the EL84's in my Blues Junior because one finally failed. I've replaced the tubes and using your technique to measure the bias by measuring the voltage across the primary of the audio output transformer I'm getting about 15 volts which I assume corresponds to 150 mA of plate current. This is way too high. I tried putting a 100K resistor in parallel with R51 and I'm getting 13.4V of c- which should have brought the plate current down. I have tested all the resistors in the bias circuit and all resistors are within tolerance. I tested the capacitors from the phase inverter thinking some B+ was leaking through them but they test OK. The screen resistors and diodes across the output transformer test OK as well. I know you had an issue with bias in a different video that you made and I was wondering if I am having a similar problem with this amp. Any suggestions would be greatly appreciated.
Hi Ralph Very surprised at your reading as that is WAY too high. You should be getting about 2.5V. 13.4V C- also sounds too high. I'd expect about -16V. All I can suggest is that you decrease that 100K resistor until the bias comes into line.
Stuart, why are you poking at the board with a metal x-acto knife?!? This is a terribly dangerous thing to do, a poor example to be setting. People...ALWAYS use a non-conductive wood or plastic tool for this purpose. A disposable bamboo or wooden chopstick is an ideal and free tool.
11:30 I won't bore you with the details? 5 hours of debugging what? That's the most interesting part... don't hide your failures, other might learn from it. Lord know what would have happened if you didn't have the bias probe to set you straight! How hard is it to measure the exact resistance across both sides of the OT if your lead are already clipped to measure the voltage drop? Furthermore, I have never seen an OT as linear as this, was it really 92 ohms on both sides?
I have to keep the vids to a sensible length particularly the 'how to bias' series as people just want fast answers about biasing. The output transformer method (for the Blues JJunior) is a 'quick and dirty' test of the bias. If you've got about 2.5V -ish across each winding then the bias is about right. Yes the o/p transformers are very consistent resistance on this amp.
zjokka. I agree ! And even if it takes 5 hrs to track down the problem, it only takes a few seconds to say for instance, there was a cold solder joint or intermittent break in the ribbon cable to the grid of the tube that was having runaway current, or there was some leakage from one circuit trace to high voltage , or a cracked grid stopper resistor, or whatever the fault was. Usually the Fender Amps of the late 80's to mid mid 90's have issues of poor quality 1/4 watt resistors, incomplete cleaning of etching chemicals from the circuit boards, leading to bad solder joints or leakage across the circuit board, bad switches, and connectors, poor quality potentiometers, and on the Blues Jr. and Deville the very foolish reliance on ribbon cables for the tube board (if You work on those boards which has to be done from below the ribbon cable connectors can easily crack or pull off the traces, resulting in a horror show.. On later Fenders,- 2000's non-eutectic Euro Union solder becomes a major issue, as one or several connections become compromised by the thermal fatigue that sets in, creating poor conductivity or tin whiskers or film than manage to create extra circuit paths. Bias problems (runaway and redplating) run rampant in the newer amps for several reasons, mostly having to do with an inadequately stable bias supply , due to potentiometers that are too small to handle the current that should be in the bias adjust circuit to guarantee contact wetting , on the potentiometer sweeping arm, or switches placed in the bias circuit for an amp that uses switchable output tubes from 2 to 4. Stuart, i like Your channel, lots of good advice here , but there is absolutely no shame in disclosing a difficult problem, advancing knowledge not only helps those who are trying to learn how to repair amps, but also those who are designing the current amps, who often times either have little practical experience, or (seems like) can't wait for it to break so that You will buy a new one. Fenders before 1983 are built better than battleships, and the Marshall are all good until around 1986. Sensible bias supplies consist of an appropriate winding on the power xfomer, a diode, a filter cap. (or two), and a fixed resistor, and a variable resistor., and a couple 220k resistors to provide bias to the upper and lower halves of the circuit. A video showing the classic circuit fender used, and how it has been implemented in different amps, would be good, as well as a video showing how to bias with a scope, on the output and removing the negative feedback resistor.
I could not help but to notice you touching vdc points withing the amp while using a metal tool instead of a wooden chopstick..!! Do you also publish the videos of you being shocked on your ass..??? LOL! Definitely a no-no for newbees.......
@@stuartukguitarampguy5830 While my 84's are a well matched set, I am unable to equalize the voltage on each tube when metering from trafo to plate.. Any ideas..??
@@pjf7943 What is 'trafo'? The method in this vid is VERY 'quick and dirty'. If the voltages are not a million miles out I'd leave well enough alone. The correct way is to get a bias meter. A more accurate way is with the amp off and discharged, measure the ACTUAL resistance of each winding half, blue and brown. Say you get 87 ohms and 96 ohms. Then use these actual resistances to calculate the bias current in ach leg using ohms law.
Just replace r52 with a 50k pot. That’s all you need to do, I’ve done it on lots of these. And yes the board must be dropped down. Set your bias to 23 ma. Not what this guy says.
Pete, thanks for the input. I'm responding not to counter you but just to record useful info for future readers of this thread. Although it is always preferable to 'just' install a pot, in practice this is a far more difficult task for people than just soldering an extra parallel resistor across an existing resistor from the top side. Anyone who can solder can use my method - that's why I did it this way! You causually say " and yes the board must be dropped down". This is a MASSIVE job for the most people. I dread it and I've done dozens and dozens of blues juniors over the years. You then have to mechanically secure the pot somehow. I've fitted dozens of these bias pots (BillM mods etc.) and it's far from easy - well beyond most people's DIY abilities. Your advice to swap out R52 for a 50k pot is not good advice I'm afraid and I urge people reading not to do this. I'm not saying it won't WORK - it will. But that pot will be very hard to adjust and can be turned down to cause catastophic runaway current. E.g if the pot was turned to zero, you would have a short circuit where R52 used to be. The valve would draw huge current and blow the HT fuse at best, cook the valves at worst. If you are installing pots you need to know about ranging them by having a suitable series resistor. That needs thinking about and calculating. If you put a 50k pot in like you suggest, at least half of its travel would put you into 'dangerous current' territory. Adding a pot is complicated for the average person, hence my simpler advice. I'm sure YOU know how to do this, but someone reading your comment would asume they can just chuck-in a 50k pot and all will be well. On the bias current, the correct bias current for the 330V plate voltage I had is 26mA. I biased a bit hotter, agreed, at 32mA. But you cannot simply say 'Bias at 23mA, not what this guy says'. The correct bias current depends on the plate voltage. Sure 23mA is in the right ballpark (a bit cool, as mine was a bit hot) but the way you state it seems like 23mA is THE bias current to go for. It isn't. For people reading this, if you have the electronic skills to drop the board and put a proper ranged pot with a correct series resistor in place of R52 then Pete is right - it's a better solution. If you don't have those skills, my simpler method will get you the result you want. If I were doing the video again I would probably go for a lower bias current than 32mA. Maybe 28mA - 30mA. The Blues Juniors sound better running a bit hot.
When I tested my brown wire I read 3.23 volts but when I tested my blue it was 2.23 volts. Is this normal and if not, is it hot or cold or some larger issue?
Hi Michael. If you want to be really rigorous, with the amp off and everything discharged, measure the acual DC resistance of the blue to red and brown to red. Make a note of each. They should be around 100 ohms. The measure the voltage across each winding with the amp running and use ohms law to determine the current. If there is more than about 5mA difference between your valves you may have a mismatched pair.
@@michaelwbuncher7599 You can also try swapping the valves. If the reading moves with the valve, then it's a valve mismatch. If the reading stays the same regardeless of valve position then it's safe to assume valves are matched. You could then maybe go for an average of the two readings. This is a really quick and dirty method without finesse. It just tells you if you're in the right ballpark bias-wise.
@@stuartukguitarampguy5830 I tried to swap the valves and took the readings again. Similar but larger different now appeared with Red to Brown reading 3.8 (was 3.23) and Red to Blue reading 2.09 (was 2.23). I didn't expect the difference to be larger! The tubes were advertised as APEX matched JJ EL84s. I'll have to try that other method of DC resistance reading, but I want to be sure I understand how to fully discharge the amp. I'll probably just take this sucker to a tech. Just don't have one that I know of close by and was hoping to find a nice simple way to confirm if I need to bias or not before taking it to someone. Maybe I'll just stick one of those 68k Ohm resistors on it and it would be easy enough to remove if it mucked with the sound too much.
@@michaelwbuncher7599 Ok it seems from that that there is a few mA differnce in the matching. Not the end of the world. Honestly? My View? If the amp sounds ok, leave well enough alone. That bias is close enough.
