Very distorted output on this amp (not vakves as a new set had been fitted). Speaker? Output Transformer? Low HT? Screen resistor? Take your pick before watching!
Stuart, you deserve a huuuuge "thank you sir". My blues jr had been making that noise since one of the EL84 blew. Somebody told me the noise was due to an underpowered output transformer or a faulty speaker. After a couple of years without being able to play it loud, I bit the bullet, ordered a new speaker, new OT and new F&T caps and did the job myself. It went smoothly except for the fact that it didn't solve the problem. And then... your video. I measured my screen resistors and there it was, bingo! In my case it was the left one. I added a new 100 ohm resistor and my god.... It sings loud now. Like never before. I got myself a new old amp thanks to you. And all for a couple of cents. Cheers to you.
Want to say THANK YOU for not only diagnosing this issue but providing such a clear video that I was able to take my amp with the same problem to a local electrician who was able to fix the problem for just $20 bucks! Without your channel it would've cost $100+ and taken a month or two! Keep up the important and worthwhile work!
Thank you Stuart. I'd acquired a non-working Blues Jr. Couldn't find the fault after checking capacitors, coltages (with the help of a schematic), and stumbled on this vid. Same problem, same solution. Same resistor had a *tiny* mark where it had blown. The previous owner of the amp had given it away, all for what turned out to be a component costing cents!
Thanks much for the tip on the screen resistor R35. My church has a blues Junior that one of the power tubes failed. So I put another tube in and amp was really distorted I quickly turned it off checked the screen resistor and viola that’s what it was. so all I did was go to R35 and it was open. I attached a 100 ohm resistor across it and it’s back in service. It is running really hot so I need to bias it now. You saved me a lot of time. Thanks again
thank you so much for that video. My Blues Junior had the same distorted sound. I checked the 100 ohms resistor, and it was dead as well! Replaced it and now I´m ready to rock again! :)
I have to thank you very much for this video. Turns out that we had the Blues Jr. amp with very distorted sound as yours. First tried fiddling with R21 R22. Still distorted. It turns out that the same screen resistor as in your unit (I don't remember if it was R35 or R36, 100 ohm) was open. Replaced it with another and voila. Quiet. What a great video. Thank you. From NY, Alan M Richards.
Hi Stuart, good job repairing the Blues Jr. I have one of these, and it is going to get a total thrashing with some mods. Thanks for the video as always.
This helped me find which track to the valve base had the issue I was searching for. It started with a power valve dying - sound went thin. Replaced them to get the sound in the video. Checked the screen resistors - both OK. Checked continuity from valve base to screen resistor - gone! The track had been broken by the replacing of the valves. Sorted!
Great video! What a coincidence! I just had exactly the same amp with exactly the same problem. Left hand screen resistor went . They are fusible 1/2 Watt. Did the same thing and a re-bias and all is well.
Nice work Stuart. I have the same amp on the bench at the moment with that same resistor open. I discovered the phase splitter, was oscillating at around 50kHz, which may have caused to the resistor to pop. Fixed with a 150pF across the anode resistor R30. Also new valves fitted by the owner but one has been roasted and has lost 25% of it's output already!
Good job I’m glad to see you jumping resistors instead of trying to remove the boards you my friend are one hell of a great amp technician great videos always
Hi sir! new subscriber from the philippines.i have question,what is the problem of guitar amp distorted sound when you switch in distortion? brand is kort cm15r are familiar in this brand?thank you!
Hello Stuart. First of all, congratulations for your videos, they have been very useful to me lately. But I have a "problem" with my BJr. The current between HT and the brown wire is around 40mA (I know it's too hot bias), but when I try to measure the current flow between the hot (red) and blue wire, the amp starts buzzing and my multimeter is not outputting any value... do you have any idea what it could be? I have tried all the resistors and they all work. The amp sounds correct, I just want to reduce the bias and remove that hum. Again, thank you very much!
Hi. Yes I've noticed that too, I'm not sure what causes it. So just do the red wire and assume the valves are matched. That's the best you can do without a bias neter.
@@stuartukguitarampguy5830 Ok thanks again! I've noticed that if you use the red wire that is located below the one you said I get some values. I get 40 mA and around 20 mA but I don't if that is correct... Any way... I will do what you suggest. Thanks again!
Excellent diagnosis. I have the same exact distortion from my BJ. Both power tubes are about the same temperature however. All three preamp tubes are illuminating and I have swapped them to see i there is a difference, which there was none. I still have the decaying distortion in the amplifier. What else might cause this issue? I've swapped a known good speaker with the onboard speaker and there is no difference. Any suggestions? Thanks for what you do. Cheers, Dave
I wonder if just ONE power tube in your amp is working? Try removing one, see if the sound changes. If it doesn't, put that one back and remove the other one. If the sound does NOT change in one of these scenarios, then one valve isn't working and you'll need to take it from there. If the sound changes significantly with either valve out, then it isn't this 'one valve not working' suggestion.
If I did, that is not correct. It is the primary which is about 100 ohms and that's what allows you to do the quick and dirty bias using a voltmeter. E.g. 2.5V = 25mA bias current.
Hi, same problem here. At 9:00 you checked the pin 9s on the valves. Am I right thinking that the positive probe was on pin 9 qnd the negative probe was on ground?
Thank you so much for your reply. I will attempt to check out pin 9 next time I am feeling confident enough ! My amp was crackling and popping when I was playing a little more aggressively. I put in new JJ power tubes but now the out put is distorted regardless of how hard or loud I play, not really bad distortion, but I prefer a clean sound. I checked and the right hand tube is way hotter than the left, but both of those 100 ohm resistors are reading pretty good. 97 and 96. I will be taking the amp to an amp guy, was just hoping to narrow down the possibilities. Thanks again for wayyyy out here on the west coast of Canada, .... Nanaimo, BC
Hi Ben, yes lots of old wives tales about valve amps and this is one. The thing to avoid is having the speaker unplugged and then putting a big signal into the amp (e,.g, strumming a loud chord). This CAN overstress the output transformer but I've never seen this actually happen or cause a problem. It's a 'possible'. Just having the speaker unplugged with no input signal is fine.
Thank you for this video. Can you help me fixing a problem? I have around 6.8V between red and blue from the output transformer and the same between red and brown ! Every voltages are good at the HT Caps and I also fixed the 100 ohms R35 witch was broken... Where should I search now? please help me there is nobody around me able to repair my amp! the sound comes but VERY distorted
Very hard to help fix by email! However, 6.8V means about 68mA of current through each EL84 and that's WAY to high. It should be about 25mA. I've done a video on biasing this amp so that's the first thing I would do.
@@stuartukguitarampguy5830 Ok, I'll bias it but i thought that such a big value shouldn't be biased and I thought that another problem had to be fixed first! By the way I found also 370V at the 470µF cap as the specs tell 329V why such a difference ? thank you
@@regisrenucci3441 Bias: No, I don't think you have a bias 'proble,m' that needs fixing. You just need to bring the bias down to about 2.5V the way you are measuring it. I doubt this will fix your distortion problem though. I guess you've tried replacing the tubes? I wouldn't worry aout the 370V, that's not your problem.
@@stuartukguitarampguy5830 Thank you, I'll do the Bias this week-end and I'll tell you. The tubes are brand new because the old ones were broken and the friend who asked me for help just told me: there is no sound. I checked also the txo 400V 0.22µF (C15 and C16) and they are right!
hello, I finally got time to bias this amp, the sound is OK to me but now, i've got two new problems: 1st the reverb is always on and full scale, no possibility to turn it down with it pot. 2nd problem is that the EL84 seem very hot to me as the current is about 25mA) and the smell is very hot, can you guide me please. Thank you !
I paused the video,, haven't watched it all yet, but, first of all, just because the owner installed new tubes doesn't mean that those new tubes are good or even the correct type of tube. Best not to take anything for granted! Honestly, the first thing I would have done after verifying the customers complaint, which you did, is that I would have done a resistance test of each leg of the output transformer primaries (to the centertap), and check the resistance of the secondaries as well, with an ohmmeter. And, as soon as you realised that one tube was running significantly hotter than the other, the obvious next step would be to swap the output tubes in their sockets and see if the same tube runs too cool, or if this current/ voltage imbalance occurs with either tube when inserted into that same socket.
There are many, many (many) ways of following a fault trail and I'm very happpy with the way I did this. Happy to (at THAT point) accept the tubes were ok. The amp had a fault, a brand new set of tubes and the exact same fault was there - I'm willing to bet the tubes are okay (for now) and move on to something more likely. There is no 'one way' of diagnosing an amp fault. Your way, you would have taken longer than me to find the screen resistor as you were busy checking output transformer resistances and valves! Not saying your way is 'wrong' it would just be one of the maybe dozen os so fault-finding trail possibilities all of which would eventually have led to the same discovery.
@@stuartukguitarampguy5830 , not having worked on these reissues (only on vintage Fenders), and not having looked at the schematic, I'm confused by something else; in most Fender-type bias circuits, decreasing the plate current requires *increasing* a resistor value to push the (negative) voltage divider upwards and throttle back the plate current. How did lowering a resistor value decrease the output tube plate current? Of course, if the customer was willing and if the budget was sufficient , I'd have installed an adjustment potentiometer, but maybe the circuit is weird and making it adjustable isn't so easy... The reason I would have checked the output transformer very early on is because it's the second most expensive component in the entire amp, and if replacement was required, it might have necessitated a phone call to the customer about the repair budget before proceeding.
@@goodun2974 Hi. Imagine a potential divider with say -60V on the top, two resistors in series, 0V on the bottom. Pick off the bias voltage (say -30V) from the centre. Now REDUCING the value of the top resistor will give more -ve voltage and hence reduce the current through the valve. INCREASING the bottom resistor will have the same effect. It's easier to reduce a value by tacking a parallel resistor across the existing one than it is to increase a resistor value. Makes sense?
@@stuartukguitarampguy5830 , yep, makes sense ---- considering that Fender didn't put an adjustment pot in there in the first place! Most of us are used to changing the resistor in between the bias adjust pot and ground, in order to centre the potentiometer around a reasonable adjustment range. Unrelated subject, but notice the spelling of "centre" above. When I use the voice-to-text function on this Samsung/Android phone, It often types the words with the British style spelling such as favour, flavour, colour, behaviour and aluminium. It didn't always function this way, it started after one of the periodic automatic updates to the phone's software......
He Lenn, yes, modern electronic device. Kind of a glass tube with electrons 'n' stuff. Quite neat. It might surprise you to know that you can get a thousand of these little beauties inside something only the size of shipping container!
![[4K직캠] BABYMONSTER(베이비몬스터) - FOREVER](http://i.ytimg.com/vi/PS3UAFto1z8/mqdefault.jpg)
Stuart, you deserve a huuuuge "thank you sir". My blues jr had been making that noise since one of the EL84 blew. Somebody told me the noise was due to an underpowered output transformer or a faulty speaker. After a couple of years without being able to play it loud, I bit the bullet, ordered a new speaker, new OT and new F&T caps and did the job myself. It went smoothly except for the fact that it didn't solve the problem. And then... your video. I measured my screen resistors and there it was, bingo! In my case it was the left one. I added a new 100 ohm resistor and my god.... It sings loud now. Like never before. I got myself a new old amp thanks to you. And all for a couple of cents. Cheers to you.
Nice one! Always very gratifying when it springs back into life! Well done.
Yes Stuart! Thanks again for fixing. Absolutely delighted with the amp, sounds better than ever.
Want to say THANK YOU for not only diagnosing this issue but providing such a clear video that I was able to take my amp with the same problem to a local electrician who was able to fix the problem for just $20 bucks! Without your channel it would've cost $100+ and taken a month or two! Keep up the important and worthwhile work!
Nice one!That's exactly why I do the videos.
Thank you Stuart. I'd acquired a non-working Blues Jr. Couldn't find the fault after checking capacitors, coltages (with the help of a schematic), and stumbled on this vid. Same problem, same solution. Same resistor had a *tiny* mark where it had blown. The previous owner of the amp had given it away, all for what turned out to be a component costing cents!
Nice! Well done.
Thank you for the great video. I learned more practical information from this video than so many others. I'm grateful!
Glad you enjoyed it Frank.
Thanks much for the tip on the screen resistor R35. My church has a blues Junior that one of the power tubes failed. So I put another tube in and amp was really distorted I quickly turned it off checked the screen resistor and viola that’s what it was. so all I did was go to R35 and it was open. I attached a 100 ohm resistor across it and it’s back in service. It is running really hot so I need to bias it now. You saved me a lot of time. Thanks again
thank you so much for that video. My Blues Junior had the same distorted sound. I checked the 100 ohms resistor, and it was dead as well! Replaced it and now I´m ready to rock again! :)
Nice one! Well done Oliver.
I have to thank you very much for this video. Turns out that we had the Blues Jr. amp with very distorted sound as yours. First tried fiddling with R21 R22. Still distorted. It turns out that the same screen resistor as in your unit (I don't remember if it was R35 or R36, 100 ohm) was open. Replaced it with another and voila. Quiet. What a great video. Thank you. From NY, Alan M Richards.
awesome! Well done Alan.
Hi Stuart, good job repairing the Blues Jr. I have one of these, and it is going to get a total thrashing with some mods. Thanks for the video as always.
Excellent. Some of the BillM mods are quite good.
Thank you very much for your clear and detailed troubleshooting explanation. I had the same exact open resistor problem (r35).
Excellent!
Thanks for the video you saved me hours of work and probably lots of money
Aesome. That's why I do them.
Many thanks, the fender amp I was repairing had exactly the same issue, both screen resistors were open so upgraded with the 1w. Now all fine.
Nice! Well done.
I had the identical problem. This video provided the solution. Thank you Stuart!
Excellent thanks Jim. This is why I make the vids.
This helped me find which track to the valve base had the issue I was searching for. It started with a power valve dying - sound went thin. Replaced them to get the sound in the video. Checked the screen resistors - both OK. Checked continuity from valve base to screen resistor - gone! The track had been broken by the replacing of the valves. Sorted!
Nice one Karn, well done!
Very intresting videos you put on you tube!
Thanks William!
Great video! What a coincidence! I just had exactly the same amp with exactly the same problem. Left hand screen resistor went . They are fusible 1/2 Watt. Did the same thing and a re-bias and all is well.
Wow amazing conincidence!
Brilliant. An excellent, informative video.
Thanks Mason I'm glad you enjoyed it.
Another great video, thanks
Thanks glad you enjoyed it
Very nice explanation. I'll put in an 82k 1W resistor since you said a half watt was a bit skinny. Thanks
Great, good luck!
great video!! thanx for sharing
Nice work Stuart. I have the same amp on the bench at the moment with that same resistor open. I discovered the phase splitter, was oscillating at around 50kHz, which may have caused to the resistor to pop. Fixed with a 150pF across the anode resistor R30. Also new valves fitted by the owner but one has been roasted and has lost 25% of it's output already!
Well done. You'd never find that oscillation without a scope on the output/.
@@stuartukguitarampguy5830 Not at that frequency! It's the 3rd one I've had with that problem, unstable amps
Good job I’m glad to see you jumping resistors instead of trying to remove the boards you my friend are one hell of a great amp technician great videos always
Thanks Ricky!
YES! this advice helped my tech avoid removing the board and instead fixed the issues in ten minutes or less!!!
Awesome … for us techie people this is video heaven… thank you! Beats Eastenders any day..
Your BANG out of awduh mah son....
Hi sir! new subscriber from the philippines.i have question,what is the problem of guitar amp distorted sound when you switch in distortion? brand is kort cm15r are familiar in this brand?thank you!
Tank you, your video, help me.
Great, I'm pleased.
Hello Stuart. First of all, congratulations for your videos, they have been very useful to me lately. But I have a "problem" with my BJr. The current between HT and the brown wire is around 40mA (I know it's too hot bias), but when I try to measure the current flow between the hot (red) and blue wire, the amp starts buzzing and my multimeter is not outputting any value... do you have any idea what it could be? I have tried all the resistors and they all work. The amp sounds correct, I just want to reduce the bias and remove that hum.
Again, thank you very much!
Hi. Yes I've noticed that too, I'm not sure what causes it. So just do the red wire and assume the valves are matched. That's the best you can do without a bias neter.
@@stuartukguitarampguy5830 Ok thanks again! I've noticed that if you use the red wire that is located below the one you said I get some values. I get 40 mA and around 20 mA but I don't if that is correct... Any way... I will do what you suggest. Thanks again!
i love you, exactly same error
Aqwesome. That's why I make the videos!
Good job. And I don' like power soaks either.
EVH would disagree! 🙂
Excellent diagnosis. I have the same exact distortion from my BJ. Both power tubes are about the same temperature however. All three preamp tubes are illuminating and I have swapped them to see i there is a difference, which there was none. I still have the decaying distortion in the amplifier. What else might cause this issue? I've swapped a known good speaker with the onboard speaker and there is no difference. Any suggestions? Thanks for what you do. Cheers, Dave
I wonder if just ONE power tube in your amp is working? Try removing one, see if the sound changes. If it doesn't, put that one back and remove the other one. If the sound does NOT change in one of these scenarios, then one valve isn't working and you'll need to take it from there. If the sound changes significantly with either valve out, then it isn't this 'one valve not working' suggestion.
Thanx
I'm liking that decade box Stuart, and was thinking of picking one up until I saw what they go for on ebay - ouch!
Yes I got mine (and aapacitance box) years ago from a used equipment dealer.
Hi Stuart
IYou mention that the secondary side of the transformer (feeding the speaker) should measure 100ohms. Is that correct?
If I did, that is not correct. It is the primary which is about 100 ohms and that's what allows you to do the quick and dirty bias using a voltmeter. E.g. 2.5V = 25mA bias current.
Hi, same problem here. At 9:00 you checked the pin 9s on the valves. Am I right thinking that the positive probe was on pin 9 qnd the negative probe was on ground?
Yes that's correct.
Thank you so much for your reply. I will attempt to check out pin 9 next time I am feeling confident enough ! My amp was crackling and popping when I was playing a little more aggressively. I put in new JJ power tubes but now the out put is distorted regardless of how hard or loud I play, not really bad distortion, but I prefer a clean sound. I checked and the right hand tube is way hotter than the left, but both of those 100 ohm resistors are reading pretty good. 97 and 96. I will be taking the amp to an amp guy, was just hoping to narrow down the possibilities. Thanks again for wayyyy out here on the west coast of Canada, .... Nanaimo, BC
Hi Tom. Those symptoms tell me that one of the valves is not working properly. Could be a faulty JJ. Your tech will sort it.
Thank you Stuart. All the best !!!
Is it really OK to unplug a valve amp speaker with the amp powered up? I thought that was a no no.
Hi Ben, yes lots of old wives tales about valve amps and this is one. The thing to avoid is having the speaker unplugged and then putting a big signal into the amp (e,.g, strumming a loud chord). This CAN overstress the output transformer but I've never seen this actually happen or cause a problem. It's a 'possible'. Just having the speaker unplugged with no input signal is fine.
Just did this today. Exact same resistor.
Awesome! Apart from the adulation of the masses, that's the main reason I do these vids!
@@stuartukguitarampguy5830 For some reason it always seems it's the one on the right that fails.
@@1Dougloid Hmm, I wonder why?
@@stuartukguitarampguy5830 Strange brew.
N/m my question. By watching, & the way you do your vid... I figured it out, it's a tube...right?
I do believe that some backward people in one of our minor colonies call them by that quaint name. Bless their simple hearts,...
Thank you for this video. Can you help me fixing a problem? I have around 6.8V between red and blue from the output transformer and the same between red and brown ! Every voltages are good at the HT Caps and I also fixed the 100 ohms R35 witch was broken... Where should I search now? please help me there is nobody around me able to repair my amp! the sound comes but VERY distorted
Very hard to help fix by email! However, 6.8V means about 68mA of current through each EL84 and that's WAY to high. It should be about 25mA. I've done a video on biasing this amp so that's the first thing I would do.
@@stuartukguitarampguy5830 Ok, I'll bias it but i thought that such a big value shouldn't be biased and I thought that another problem had to be fixed first!
By the way I found also 370V at the 470µF cap as the specs tell 329V why such a difference ? thank you
@@regisrenucci3441 Bias: No, I don't think you have a bias 'proble,m' that needs fixing. You just need to bring the bias down to about 2.5V the way you are measuring it. I doubt this will fix your distortion problem though. I guess you've tried replacing the tubes? I wouldn't worry aout the 370V, that's not your problem.
@@stuartukguitarampguy5830 Thank you, I'll do the Bias this week-end and I'll tell you. The tubes are brand new because the old ones were broken and the friend who asked me for help just told me: there is no sound. I checked also the txo 400V 0.22µF (C15 and C16) and they are right!
hello, I finally got time to bias this amp, the sound is OK to me but now, i've got two new problems: 1st the reverb is always on and full scale, no possibility to turn it down with it pot. 2nd problem is that the EL84 seem very hot to me as the current is about 25mA) and the smell is very hot, can you guide me please. Thank you !
We'll go figure. Same issue, but the resistor is supposed to be 100 ohm was 5 milliohms. Pin 9 voltage rolls though... not a steady output.
If you move the ribbon cables away from the blue wire a little it might get rid of the hum.
Yes I can;t recall if I tried that.
I paused the video,, haven't watched it all yet, but, first of all, just because the owner installed new tubes doesn't mean that those new tubes are good or even the correct type of tube. Best not to take anything for granted! Honestly, the first thing I would have done after verifying the customers complaint, which you did, is that I would have done a resistance test of each leg of the output transformer primaries (to the centertap), and check the resistance of the secondaries as well, with an ohmmeter. And, as soon as you realised that one tube was running significantly hotter than the other, the obvious next step would be to swap the output tubes in their sockets and see if the same tube runs too cool, or if this current/ voltage imbalance occurs with either tube when inserted into that same socket.
There are many, many (many) ways of following a fault trail and I'm very happpy with the way I did this. Happy to (at THAT point) accept the tubes were ok. The amp had a fault, a brand new set of tubes and the exact same fault was there - I'm willing to bet the tubes are okay (for now) and move on to something more likely. There is no 'one way' of diagnosing an amp fault. Your way, you would have taken longer than me to find the screen resistor as you were busy checking output transformer resistances and valves! Not saying your way is 'wrong' it would just be one of the maybe dozen os so fault-finding trail possibilities all of which would eventually have led to the same discovery.
@@stuartukguitarampguy5830 , not having worked on these reissues (only on vintage Fenders), and not having looked at the schematic, I'm confused by something else; in most Fender-type bias circuits, decreasing the plate current requires *increasing* a resistor value to push the (negative) voltage divider upwards and throttle back the plate current. How did lowering a resistor value decrease the output tube plate current?
Of course, if the customer was willing and if the budget was sufficient , I'd have installed an adjustment potentiometer, but maybe the circuit is weird and making it adjustable isn't so easy...
The reason I would have checked the output transformer very early on is because it's the second most expensive component in the entire amp, and if replacement was required, it might have necessitated a phone call to the customer about the repair budget before proceeding.
@@goodun2974 Hi. Imagine a potential divider with say -60V on the top, two resistors in series, 0V on the bottom. Pick off the bias voltage (say -30V) from the centre. Now REDUCING the value of the top resistor will give more -ve voltage and hence reduce the current through the valve. INCREASING the bottom resistor will have the same effect. It's easier to reduce a value by tacking a parallel resistor across the existing one than it is to increase a resistor value. Makes sense?
@@stuartukguitarampguy5830 , yep, makes sense ---- considering that Fender didn't put an adjustment pot in there in the first place! Most of us are used to changing the resistor in between the bias adjust pot and ground, in order to centre the potentiometer around a reasonable adjustment range.
Unrelated subject, but notice the spelling of "centre" above. When I use the voice-to-text function on this Samsung/Android phone, It often types the words with the British style spelling such as favour, flavour, colour, behaviour and aluminium. It didn't always function this way, it started after one of the periodic automatic updates to the phone's software......
can't help thinking that it would be less confusing for future techs if you clipped out that dead screen resistor
Nah. Let them wonder.... Bwhahahaha!!
If it's a blues junior, it's the screen resistor. You can lay money on it! 😅
Sounds like you are saying..."VALVES"...if that is what you are saying...what're valves?
He Lenn, yes, modern electronic device. Kind of a glass tube with electrons 'n' stuff. Quite neat. It might surprise you to know that you can get a thousand of these little beauties inside something only the size of shipping container!
Tubes, if you are Stateside.
You should cut the old resistor