Hi Pete You're half right and half wrong (but I'm willing to be corrected). I believe tubes 1&2 are a pair and tubes 3&4 are a pair. So... if you remove the middle 2, you have removed ONE from each PAIR - which is correct. If you remove the outer two, again you have removed ONE from each PAIR - again correct. BTW Marshall caution against doing this and say it may damage the amp. Also, doing this will give you barely any noticeable volume drop, so it's not worth doing in my opinion. If you want a 50W head, I'd ... buy a 50W head!
Yes on most 'fixed bias' amplifiers the cathodes are tied to ground. Sometimes theres a 1 ohm resistor in series across which you can handily measure the bias as a voltage. E.g. 45mV = 45mA. Of course with cathode biased amplifers (usually the lower powered amps) the cathodes are not grounded but go via a resistor to ground.
Stuart, excellent video and explanation of the 1 ohm shunt resistor to measure the bias. I would like to email you to discuss the bias on the new Fender Pro Jr 1V. That is a different scheme of it. Thanks in advance for your reply. 😀
Hello Stuart ! We talked through a slight problem with my 1959 SL 100 shocking me. I was actually just adding a master volume kit from Mojotone and pulled the chassis out and put it on my chassis stand and noticed the two inboard tubes where the JJ logo was faded almost off the glass and the two outboard tubes still look like new. When biasing these two had about 2mA higher than the two outboard tubes or the two just in front of the power transformer. I biased them as I said in and earlier post about 40mA but the two outboard are about 38mA. Any ideas why this is happening as the amp has been doing great ?
What pins do the Bias probes attach to on the tube socket that allow you to measure the Plate voltage (Pin 3 I think) and the plate current simultaneously? Does the Bias probe calculate the bias current via the I=V/R equation with internal resistance?
Thanks for this! very clear and easy to follow video. Gonna use this when i go take my amp back for service, I've been using a voltage converter to run my SL at 100v but with the bias that was set at 120v with the tech, I think he set it at 36ma, does that mean my amp has been running cold this whole time? It sounds good to my ears but then again I've only ever owned 1 100w super lead haha
Yes if the bias was orriginally set at 120V main in and you've been running it at 100V then it will probably be running cold. I say 'probably' because the HT will be well down as well, of course.
@@stuartukguitarampguy5830 - Hi Stuart. I've just recently landed on your channel after scouring the web for good information and practical advice in relation to valve amps. ( I fortunately was 'reared' on valves,.... and my nephews now being band members are always on my case to sort their gear out. - I did manage to get some (vintage) MARSHALL head circuits,... but wasn't so fortunate for the more modern 'hybrids' which have some digital processing/switching in the front end. - Do you know of a 'source' of schematics,... even if they are to be purchased ??? (Marshall & Orange are the kids favourites these days - H H electronics were my era LOL) - Many thanks in advance.
@@stevemacbr Hi Steve. Glad you found the channel. I have hundreds of schematics. If you need one, email me and I'll send it if I have it. You can contact me by looking at the very end of any one of my vids where I give the website address. Contact me through that. Stuart
Hello Stuart, i just took your advise and added a 1ohm resistor in all four positions off pin 1 to 8 to ground and i got a good shock off the on off switch, when I took them back out the shock was gone. This does not sound right to me but it happened. This was on my 1959 super lead.kit by Mojotone. Which also works and sounds great.
Thanks I have the ground off the chassis to pin 8 with a jumper to pin 1 verified. I have more resistors ordered so I will try this again as soon as they come. Will let you know what happens. I have doublechecked all ground tabs everywhere and have checked with a meter for continuity. Wish we ere not across the pond or I would bring it to you. No way to send you a picture or I would !
@@Phoenix20086fly Hi Ok make sure the resistors are 1phm and not 1k or something! Even so, I can;t see how adding a resistor in the cathode circuit would cause any kind of electroc shock issues/ Let me know if you find out what's going on.
@@stuartukguitarampguy5830 Hello Stuart, well I ordered more 1 ohm resistors and put the chassis back on my bench and went through it with a fine tooth comb looking for a loose ground or something disconnected. I knew I had everything in the correct place so reinstalled four resistors and with my wife on the switch on the surge protector....Just in case !!!, so I powered the amp up with all controls down guitar plugged in as well as a cabinet impedance selector set on 16 ohms. This time I took my meter on AC and DC checking for voltage anywhere it shouldn't be....all was well. Flicked the switches quickly and felt nothing. Had my bias meter under the two outside power tubes and re-biased again just a touch on the hot side where I like it and all is well ! I did put the resistors exactly in the same place as last time between pin 8 and ground tab with a short jumper from pin 8 to 1. But when resoldering the resistors in, I reheated all the connections on pins 1 and 8 and the little jumper. There might have been a cold joint in there somewhere or since I built the amp a component that I clipped after soldering might have got loose and shorted but that I would doubt as I am pretty careful about that as these things are not cheap ! But all seems normal and working fine now. Thank you for being there !!!
@@stuartukguitarampguy5830all turned out well reinstalled four new one ohm resistors after checking al grounding points. When soldering the new resistors in made sure and flowed the solder nice around all points from pin 1 to 8 jumper and resistor to ground ! Installed the bias checker under the outboard tubes to recheck that and turned on and used a meter to check switches just in case. I do not want to get zapped again. All was well and I reset the bias just a tad High as I like it better. Now it is doing great and sounds fantastic !
Read; THE TRAINWRECK PAGES By Ken Fischer. The safe operational range of EL34 is between 10-40mA per tube. You set it at 35mA... maybe this particular Marshall sounds better at say, 15mA ? I always have the customer play through the amp, using their speakers... I then adjust the bias within the safe operational range... until they find the "sweet spot" they like. You can then repeat this same bias adjustment every time you change the customers tubes, and save yourself time and money. Regards 🎸
Hi Scott. Nice idea but I couldn't possibly do that. The customer would have to drive over and hang around whilst I did all that, put the amp back together etc. And... the living nightmare of twiddling a control whilst the customer attempts to locate the 'sweet spot' as you say, whilst playing his favourite riff over and over. Spare me! I've never once been asked to adjust bias live for a customer, in thousands of bias jobs. My guess is that 75%+ of my customers don't even know what bias IS, let alone want to twiddle with it to achieve fine nuances of tone! I would also estimate that 99% of valve amplifiers out there are not biased correctly. I have a pretty good ear (for years I was a professional recording engineer) and I canot hear ANY difference between say 20mA and 40mA. Not saying others can't though. I also cannot hear a difference between quite widely mismatched power tubes - say 10mA difference, which is a lot. To look into any of this (inclduing preamp tube makes etc.) you would have to set up a carefully controlled double-blind trial with, say, 100 participants. I've often fantasised about doing that, but its not easy. My strong intuition is that when the results were in, it would all be in the noise - no statistically significant differences between one valve brand and another, or (for the same customer) one 'sweet spot' bias versus another, totally different one! But I haven't done the test and I could be wrong. There's a huge amount of 'Emperor's New Suit of Clothes' about all this. Test Subject: "Woah man, that NOS MUllard has this warm, mellow sound with just a hint of smoked oak and maybe a touch of raspberry on the highs...." Me: "Sorry man, I left the old JJ in, I haven't gotten around to swapping it for the NOS yet..." Getting cynical in my old age! For others reading this, just bias at 70% of maximum dissipation like most people do!
@@stuartukguitarampguy5830 With the bias higher (lower current) the amp will tend to sound crunchier, albeit thinner, with a lower bias (higher current) the amp will tend to sound beefier with more gain. A lot of the thrash metal guys running the 6550 powered 2203's back in the day loved to have their bias set really high... like 10-15mA because it sounded so crunchy. On the other hand, a blues player might like running a high current so the amp would break up much quicker. I like a bias somewhere in the 25mA range... plenty of clean headroom so my delay/reverb doesn't distort, but still enough current for the amp to get more oomph. Of course, every amp is different, and will prefer it's own "sweet spot" on the bias pot. I've had the same problem of not having the customer in person... in that case I go by the speakers, pickups, and their style of playing to best gauge where they would like their bias set.
@@scottdunn2178 Thanks Scott, interesting. It sounds like you have a higher level of client than me and more discerning! As I say, most of my customers don;t even know what bias is, let alone want it set at a certain level or in accordance with their string guage!!
@@stuartukguitarampguy5830 Google; The Trainwreck Pages I think you'll find lots of information on bias, and much more. You'll be glad you read it, and I can almost guarantee you'll use the tips and pointers Ken Fischer talks about. 🎸
@@stuartukguitarampguy5830 The sound goes hand in hand with feel. You don't even really need to be a guitar player to tune an amp into a magical zone. There's a place where the spank/quack from plucking strings is more expressive, and this can be done before the client comes by. All techs should tune for tone, not just for the numbers. When the player feels a good connection to the tone, they play in a way that captivates the audience. There's a lot more to the tuning aspect, but bias is the icing to a long and detailed process.
Hi Stuart, I tried to email you, but to no avail, I don't have the correct address. However, have you tried to rebias the new Fender Pro Jr 4? It may make an interesting video. Thanks Stuart 😊
Hi Ken You can contact me via the web address at the very end of each video. No I haven;t yet had a Pro Junior 4 into the workshop, probably because it is fairly new.
I think the power tubes are "paired" like this: the two inner tubes is one "pair", and the second "pair" is the two outer tubes. An example to prove this is you can remove the inner pair or the outer pair to bring the amp down to 50 watts. Nobody removes the two power tubes on the left or the two power tubes on the right - nobody! I doubt that the amp would even run if you did.
That's my understanding as well...and I can tell you for a fact because my 1971 Super Lead has a push pull pot that changes it from 2 Output tubes to 4. As you said, the 2 inside tubes act as a push pull pair when I'm only using 2 output tubes and when I fire up all 4 tubes, I see the 2 outer tubes light up.
Great video Stu, and a beautiful amp too!
Hi Pete You're half right and half wrong (but I'm willing to be corrected). I believe tubes 1&2 are a pair and tubes 3&4 are a pair.
So... if you remove the middle 2, you have removed ONE from each PAIR - which is correct. If you remove the outer two, again you have removed ONE from each PAIR - again correct. BTW Marshall caution against doing this and say it may damage the amp. Also, doing this will give you barely any noticeable volume drop, so it's not worth doing in my opinion. If you want a 50W head, I'd ... buy a 50W head!
Yes on most 'fixed bias' amplifiers the cathodes are tied to ground. Sometimes theres a 1 ohm resistor in series across which you can handily measure the bias as a voltage. E.g. 45mV = 45mA. Of course with cathode biased amplifers (usually the lower powered amps) the cathodes are not grounded but go via a resistor to ground.
Stuart, excellent video and explanation of the 1 ohm shunt resistor to measure the bias. I would like to email you to discuss the bias on the new Fender Pro Jr 1V. That is a different scheme of it. Thanks in advance for your reply. 😀
Really good video - thank you!!
Thanks glad you enjoyed it!
Excellent tutorial... many thanks. Are the cathodes always tied to ground?
Hello Stuart ! We talked through a slight problem with my 1959 SL 100 shocking me. I was actually just adding a master volume kit from Mojotone and pulled the chassis out and put it on my chassis stand and noticed the two inboard tubes where the JJ logo was faded almost off the glass and the two outboard tubes still look like new. When biasing these two had about 2mA higher than the two outboard tubes or the two just in front of the power transformer. I biased them as I said in and earlier post about 40mA but the two outboard are about 38mA. Any ideas why this is happening as the amp has been doing great ?
What pins do the Bias probes attach to on the tube socket that allow you to measure the Plate voltage (Pin 3 I think) and the plate current simultaneously? Does the Bias probe calculate the bias current via the I=V/R equation with internal resistance?
Yes pin 3 for the plate, then it breaks the cathode feed to ground and outs a 1 ohm resistor in series and measures across that.
@@stuartukguitarampguy5830 Ty!
Thanks for this! very clear and easy to follow video. Gonna use this when i go take my amp back for service, I've been using a voltage converter to run my SL at 100v but with the bias that was set at 120v with the tech, I think he set it at 36ma, does that mean my amp has been running cold this whole time? It sounds good to my ears but then again I've only ever owned 1 100w super lead haha
Yes if the bias was orriginally set at 120V main in and you've been running it at 100V then it will probably be running cold. I say 'probably' because the HT will be well down as well, of course.
@@stuartukguitarampguy5830 - Hi Stuart. I've just recently landed on your channel after scouring the web for good information and practical advice in relation to valve amps. ( I fortunately was 'reared' on valves,.... and my nephews now being band members are always on my case to sort their gear out.
- I did manage to get some (vintage) MARSHALL head circuits,... but wasn't so fortunate for the more modern 'hybrids' which have some digital processing/switching in the front end.
- Do you know of a 'source' of schematics,... even if they are to be purchased ???
(Marshall & Orange are the kids favourites these days - H H electronics were my era LOL)
- Many thanks in advance.
@@stevemacbr Hi Steve. Glad you found the channel. I have hundreds of schematics. If you need one, email me and I'll send it if I have it. You can contact me by looking at the very end of any one of my vids where I give the website address. Contact me through that. Stuart
Hello Stuart, i just took your advise and added a 1ohm resistor in all four positions off pin 1 to 8 to ground and i got a good shock off the on off switch, when I took them back out the shock was gone. This does not sound right to me but it happened. This was on my 1959 super lead.kit by Mojotone. Which also works and sounds great.
Defintely something wrong there! MY best guess is you put the resistors in the wrong place and so effectively putting voltage on the chassis.
Thanks I have the ground off the chassis to pin 8 with a jumper to pin 1 verified. I have more resistors ordered so I will try this again as soon as they come. Will let you know what happens. I have doublechecked all ground tabs everywhere and have checked with a meter for continuity. Wish we ere not across the pond or I would bring it to you. No way to send you a picture or I would !
@@Phoenix20086fly Hi Ok make sure the resistors are 1phm and not 1k or something! Even so, I can;t see how adding a resistor in the cathode circuit would cause any kind of electroc shock issues/ Let me know if you find out what's going on.
@@stuartukguitarampguy5830 Hello Stuart, well I ordered more 1 ohm resistors and put the chassis back on my bench and went through it with a fine tooth comb looking for a loose ground or something disconnected. I knew I had everything in the correct place so reinstalled four resistors and with my wife on the switch on the surge protector....Just in case !!!, so I powered the amp up with all controls down guitar plugged in as well as a cabinet impedance selector set on 16 ohms. This time I took my meter on AC and DC checking for voltage anywhere it shouldn't be....all was well. Flicked the switches quickly and felt nothing. Had my bias meter under the two outside power tubes and re-biased again just a touch on the hot side where I like it and all is well ! I did put the resistors exactly in the same place as last time between pin 8 and ground tab with a short jumper from pin 8 to 1. But when resoldering the resistors in, I reheated all the connections on pins 1 and 8 and the little jumper. There might have been a cold joint in there somewhere or since I built the amp a component that I clipped after soldering might have got loose and shorted but that I would doubt as I am pretty careful about that as these things are not cheap ! But all seems normal and working fine now. Thank you for being there !!!
@@stuartukguitarampguy5830all turned out well reinstalled four new one ohm resistors after checking al grounding points. When soldering the new resistors in made sure and flowed the solder nice around all points from pin 1 to 8 jumper and resistor to ground ! Installed the bias checker under the outboard tubes to recheck that and turned on and used a meter to check switches just in case. I do not want to get zapped again. All was well and I reset the bias just a tad High as I like it better. Now it is doing great and sounds fantastic !
Read; THE TRAINWRECK PAGES
By Ken Fischer. The safe operational range of EL34 is between 10-40mA per tube. You set it at 35mA... maybe this particular Marshall sounds better at say, 15mA ? I always have the customer play through the amp, using their speakers... I then adjust the bias within the safe operational range... until they find the "sweet spot" they like. You can then repeat this same bias adjustment every time you change the customers tubes, and save yourself time and money.
Regards 🎸
Hi Scott. Nice idea but I couldn't possibly do that. The customer would have to drive over and hang around whilst I did all that, put the amp back together etc. And... the living nightmare of twiddling a control whilst the customer attempts to locate the 'sweet spot' as you say, whilst playing his favourite riff over and over. Spare me! I've never once been asked to adjust bias live for a customer, in thousands of bias jobs. My guess is that 75%+ of my customers don't even know what bias IS, let alone want to twiddle with it to achieve fine nuances of tone! I would also estimate that 99% of valve amplifiers out there are not biased correctly. I have a pretty good ear (for years I was a professional recording engineer) and I canot hear ANY difference between say 20mA and 40mA. Not saying others can't though. I also cannot hear a difference between quite widely mismatched power tubes - say 10mA difference, which is a lot. To look into any of this (inclduing preamp tube makes etc.) you would have to set up a carefully controlled double-blind trial with, say, 100 participants. I've often fantasised about doing that, but its not easy. My strong intuition is that when the results were in, it would all be in the noise - no statistically significant differences between one valve brand and another, or (for the same customer) one 'sweet spot' bias versus another, totally different one! But I haven't done the test and I could be wrong. There's a huge amount of 'Emperor's New Suit of Clothes' about all this. Test Subject: "Woah man, that NOS MUllard has this warm, mellow sound with just a hint of smoked oak and maybe a touch of raspberry on the highs...." Me: "Sorry man, I left the old JJ in, I haven't gotten around to swapping it for the NOS yet..."
Getting cynical in my old age!
For others reading this, just bias at 70% of maximum dissipation like most people do!
@@stuartukguitarampguy5830 With the bias higher (lower current) the amp will tend to sound crunchier, albeit thinner, with a lower bias (higher current) the amp will tend to sound beefier with more gain. A lot of the thrash metal guys running the 6550 powered 2203's back in the day loved to have their bias set really high... like 10-15mA because it sounded so crunchy. On the other hand, a blues player might like running a high current so the amp would break up much quicker. I like a bias somewhere in the 25mA range... plenty of clean headroom so my delay/reverb doesn't distort, but still enough current for the amp to get more oomph. Of course, every amp is different, and will prefer it's own "sweet spot" on the bias pot. I've had the same problem of not having the customer in person... in that case I go by the speakers, pickups, and their style of playing to best gauge where they would like their bias set.
@@scottdunn2178 Thanks Scott, interesting. It sounds like you have a higher level of client than me
and more discerning! As I say, most of my customers don;t even know what bias is, let alone want it set at a certain level or in accordance with their string guage!!
@@stuartukguitarampguy5830 Google; The Trainwreck Pages
I think you'll find lots of information on bias, and much more. You'll be glad you read it, and I can almost guarantee you'll use the tips and pointers Ken Fischer talks about. 🎸
@@stuartukguitarampguy5830 The sound goes hand in hand with feel. You don't even really need to be a guitar player to tune an amp into a magical zone. There's a place where the spank/quack from plucking strings is more expressive, and this can be done before the client comes by. All techs should tune for tone, not just for the numbers. When the player feels a good connection to the tone, they play in a way that captivates the audience. There's a lot more to the tuning aspect, but bias is the icing to a long and detailed process.
That bias adjuster, it’s only on the reissues correct? Because I don’t see it on my ‘72 Super Lead.
Hmm not sure tbh
Does adding the 1 ohm resistors change the tone of the amp in anyway?
NO,... that's why he choose 1-Ohm. ( a piece of copper wire of the right CSA and length would easily have such a low value )
I have a 1964 AC30 that may need your help, would you be interested?
Hi Ian. Sure. You can contact me via www.fixguitaramps.co.uk
Hi Stuart, I tried to email you, but to no avail, I don't have the correct address. However, have you tried to rebias the new Fender Pro Jr 4? It may make an interesting video. Thanks Stuart 😊
Hi Ken You can contact me via the web address at the very end of each video. No I haven;t yet had a Pro Junior 4 into the workshop, probably because it is fairly new.
Does it need to be a 1ohm resistor? Or can any value be used?
USe 1ohm. Anything higher starts to interfere with the cathide current. 1 ohm also makes calculation easy.
@@stuartukguitarampguy5830 what about wattage?
@@astro2069 Use a 2W.
@@stuartukguitarampguy5830 metal film, carbon film, metal oxide?
@@astro2069 Not important. Very little voltage across this resistor.
Hi Zack, Short and easy answer - absolutely not!
I think the power tubes are "paired" like this: the two inner tubes is one "pair", and the second "pair" is the two outer tubes. An example to prove this is you can remove the inner pair or the outer pair to bring the amp down to 50 watts. Nobody removes the two power tubes on the left or the two power tubes on the right - nobody! I doubt that the amp would even run if you did.
That's my understanding as well...and I can tell you for a fact because my 1971 Super Lead has a push pull pot that changes it from 2 Output tubes to 4. As you said, the 2 inside tubes act as a push pull pair when I'm only using 2 output tubes and when I fire up all 4 tubes, I see the 2 outer tubes light up.