Members Markymarc1979 Posted February 14, 2006 Members Share Posted February 14, 2006 I had my 5150 modded last year by a local tech (bias pot installed), now I would like to switch my 6L6's with a new matched set of JJ 6L6gc's. Here's the question, I just got a new Multimeter, and when I connect my bias probe up i'm getting no reading. I am pretty sure the amp was biased around 32ma when I had the work done to it. I am 99.9% sure i am doing it correct, but any help/pointers would be welcome. Here's the bias probe and multimeter in using I had it set on 200 DCV and yet nothing, I also tried all the tube sockets, and not a sausage. Oh yeah the tube was lighting up ok as well, so no problem there. Marc Link to comment Share on other sites More sharing options...
Members onehourburn Posted February 14, 2006 Members Share Posted February 14, 2006 Thats strange!!! You will want to bias it again for the new tubes cause it wont be the same as the old ones... onehourburn Link to comment Share on other sites More sharing options...
Members Markymarc1979 Posted February 14, 2006 Author Members Share Posted February 14, 2006 That looksjust like my Multimeter, I shall give it another try! Fingers crossed! Link to comment Share on other sites More sharing options...
Members Phrostbit Posted February 14, 2006 Members Share Posted February 14, 2006 Your picture of your multimeter is kinda fuzzy, but when you are measureing ma (milli-amps) you want to make sure your multi-meter is set to the 200u on the "A" side. Take a look at this one, it's the closest I could find to yours on google images: See the 200u on the "A" side ... thats your spot for measuring milli-amps. Now if you are trying to meaure milli-volts, go for the 200 on the "V" side. I wanted to get this in my post too, from Randall's amp website: Quote From Randall Amplifiers BIAS READINGS: 6L6/5881 28mV to 35mV EL34/6CA7 30mV to 38mV E34Ls 35mV to 45mV 6550 35mV to 45mV Meter displays differ from one meter to the next. Some may indicate for example, 30.0 for 30 millivolts. Others may show .030 for 30 millivolts. Advanced theory (for those who care): Those of you with electronic knowledge may notice we are referring to current draw but are making measurements in millivolts. Ohms law states that I=E/R or current (I) equals voltage (E) divided by resistance ®. Inside the amp are one ohm resistors in the cathodes of the output tubes. The external test points allow access to those resistors. When you measure across those resistors at the rear panel test points, you are reading the DC voltage drop across a one ohm resistor. Referring to ohms law, if R=1 in the formula, then I = E or current equals voltage. So when you read for example, 30mV you are also seeing the equivalent value of current or 30mA. WARNING: Do not be tempted to run your tubes hotter than the maximum values in the chart. You may find it sounds really cool as you destroy your expensive tubes and possibly damage your amp, of course voiding your warranty! Also, in case you haven Link to comment Share on other sites More sharing options...
Members Peter Diezel Posted February 14, 2006 Members Share Posted February 14, 2006 Originally posted by Irish_whiplash I had my 5150 modded last year by a local tech (bias pot installed), now I would like to switch my 6L6's with a new matched set of JJ 6L6gc's. Here's the question, I just got a new Multimeter, and when I connect my bias probe up i'm getting no reading. I am pretty sure the amp was biased around 32ma when I had the work done to it. I am 99.9% sure i am doing it correct, but any help/pointers would be welcome. Here's the bias probe and multimeter in using I had it set on 200 DCV and yet nothing, I also tried all the tube sockets, and not a sausage. Oh yeah the tube was lighting up ok as well, so no problem there. Marc Marc ! You have to measure the current, not the voltage. 200 milliamps DC current, DCA ! Do You have a current meter ? Peter Link to comment Share on other sites More sharing options...
Members 65_289 Posted February 14, 2006 Members Share Posted February 14, 2006 Originally posted by Phrostbit See the 200u on the "A" side ... thats your spot for measuring milli-amps. Wouldn't 200u be 200 microamps, and 200m be 200 milliamps? Or am I missing something? Link to comment Share on other sites More sharing options...
Members Phrostbit Posted February 14, 2006 Members Share Posted February 14, 2006 Originally posted by 65_289 Wouldn't 200u be 200 microamps, and 200m be 200 milliamps? Or am I missing something? Yep, you're right ... it is microamps ... but he'll still get a measurement off it, just one decimal place off. My bad . --Phrostbit Link to comment Share on other sites More sharing options...
Members Markymarc1979 Posted February 14, 2006 Author Members Share Posted February 14, 2006 Originally posted by Peter Diezel Marc !You have to measure the current, not the voltage.200 milliamps DC current, DCA !Do You have a current meter ?Peter Peter, Yeah I have a current meter, 200m DCA right?, alas after a quick check (which I know I should have done) I have a blown fuse, and will not be able to get one till tomorrow!! That explains alot! Marc Link to comment Share on other sites More sharing options...
Members Peter Diezel Posted February 14, 2006 Members Share Posted February 14, 2006 Originally posted by Phrostbit Your picture of your multimeter is kinda fuzzy, but when you are measureing ma (milli-amps) you want to make sure your multi-meter is set to the 200u on the "A" side. Take a look at this one, it's the closest I could find to yours on google images: See the 200u on the "A" side ... thats your spot for measuring milli-amps. Now if you are trying to meaure milli-volts, go for the 200 on the "V" side. I wanted to get this in my post too, from Randall's amp website: Quote From Randall Amplifiers BIAS READINGS: 6L6/5881 28mV to 35mV EL34/6CA7 30mV to 38mV E34Ls 35mV to 45mV 6550 35mV to 45mV Meter displays differ from one meter to the next. Some may indicate for example, 30.0 for 30 millivolts. Others may show .030 for 30 millivolts. Advanced theory (for those who care): Those of you with electronic knowledge may notice we are referring to current draw but are making measurements in millivolts. Ohms law states that I=E/R or current (I) equals voltage (E) divided by resistance ®. Inside the amp are one ohm resistors in the cathodes of the output tubes. The external test points allow access to those resistors. When you measure across those resistors at the rear panel test points, you are reading the DC voltage drop across a one ohm resistor. Referring to ohms law, if R=1 in the formula, then I = E or current equals voltage. So when you read for example, 30mV you are also seeing the equivalent value of current or 30mA. WARNING: Do not be tempted to run your tubes hotter than the maximum values in the chart. You may find it sounds really cool as you destroy your expensive tubes and possibly damage your amp, of course voiding your warranty! Also, in case you haven Link to comment Share on other sites More sharing options...
Members Peter Diezel Posted February 14, 2006 Members Share Posted February 14, 2006 Originally posted by Irish_whiplash Peter, Yeah I have a current meter, 200m DCA right?, alas after a quick check (which I know I should have done) I have a blown fuse, and will not be able to get one till tomorrow!!That explains alot!Marc Yes, You made a short like me everyday :-)) Link to comment Share on other sites More sharing options...
Members Phrostbit Posted February 14, 2006 Members Share Posted February 14, 2006 Originally posted by Peter Diezel Please excuse me : The cathode resistor is not common, cause it Link to comment Share on other sites More sharing options...
Members JerryP Posted February 14, 2006 Members Share Posted February 14, 2006 If that bias probe is like most of them you'll want to measure DC voltage, not amps. You'll be measuring the voltage drop across a 1 ohm resistor inside it which will give you your bias reading. Beacuse of ohms law the voltage you read will be equal to the current draw of that tube.You get no reading on all tubes or just one or two? If you get no reading on some, but you do get a reading on other then I'll guarantee you have an open screen grid resistor on any tube that has no reading.Jerry Link to comment Share on other sites More sharing options...
Members Markymarc1979 Posted February 14, 2006 Author Members Share Posted February 14, 2006 Originally posted by Peter Diezel Yes, You made a short like me everyday :-)) Yeah that sums up my day, I have loads of 1 amp and 10 amp fuses but no 2 amp fuses, god dam it. Anyway thanks for your help. Link to comment Share on other sites More sharing options...
Members JamesPeters Posted February 14, 2006 Members Share Posted February 14, 2006 +1 to Jerry. Also, keep in mind that "200V DC" is too high a range for reading mV. Set it to 200mV instead. Good luck! Link to comment Share on other sites More sharing options...
Members Markymarc1979 Posted February 14, 2006 Author Members Share Posted February 14, 2006 Originally posted by JerryP If that bias probe is like most of them you'll want to measure DC voltage, not amps. You'll be measuring the voltage drop across a 1 ohm resistor inside it which will give you your bias reading. Beacuse of ohms law the voltage you read will be equal to the current draw of that tube.You get no reading on all tubes or just one or two? If you get no reading on some, but you do get a reading on other then I'll guarantee you have an open screen grid resistor on any tube that has no reading.Jerry So Jerry when I replace the fuse, just so i'm clear....200m DCV not 200m DCA, by the way the bias probe is the Groove tubes one. Link to comment Share on other sites More sharing options...
Members Markymarc1979 Posted February 14, 2006 Author Members Share Posted February 14, 2006 Originally posted by JamesPeters +1 to Jerry.Also, keep in mind that "200V DC" is too high a range for reading mV. Set it to 200mV instead.Good luck! so 200m on the DCV part of my multimeter is ok? Link to comment Share on other sites More sharing options...
Members JamesPeters Posted February 14, 2006 Members Share Posted February 14, 2006 Originally posted by Irish_whiplash I may need another multimeter as this one does not have 200mv as an option. 200m on the DCV option is as low as it goes. Um...that is actually the setting. Add the "m" to the "V" (of "DCV" because it means "DC volts"), and you get the "mV" I was referring to ("millivolts"). Link to comment Share on other sites More sharing options...
Members Markymarc1979 Posted February 14, 2006 Author Members Share Posted February 14, 2006 Originally posted by JamesPeters Um...that is actually the setting. Add the "m" to the "V" (of "DCV" because it means "DC volts"), and you get the "mV" I was referring to ("millivolts"). Thanks mate, sorry its late i'm tired, got to get up early and get new fuses, thanks for you help. Marc Link to comment Share on other sites More sharing options...
Members JamesPeters Posted February 14, 2006 Members Share Posted February 14, 2006 No problem Marc! Good luck! Link to comment Share on other sites More sharing options...
Members Markymarc1979 Posted February 15, 2006 Author Members Share Posted February 15, 2006 A quick bumpage to say thanks to the guys who helped me, evrything is running fine now, and I am now happily biasing my amp right now:wave: Marc Link to comment Share on other sites More sharing options...
Members Peter Diezel Posted February 16, 2006 Members Share Posted February 16, 2006 Do guys really use cathode resistors for thebias setting ? Link to comment Share on other sites More sharing options...
Members JamesPeters Posted February 16, 2006 Members Share Posted February 16, 2006 Originally posted by Peter Diezel Do guys really use cathode resistors for thebias setting ? Some amps have 1 ohm resistors installed from the cathode to ground of each power tube for the purpose of making quick bias readings. In this case the "bias meter" device itself has a 1 ohm resistor installed on the cathode. "Bias meters" of this kind are generally made as such. Link to comment Share on other sites More sharing options...
Members Negative K3 fan Posted February 16, 2006 Members Share Posted February 16, 2006 Originally posted by Peter Diezel Do guys really use cathode resistors for thebias setting ? is it so bad?? Link to comment Share on other sites More sharing options...
Members Peter Diezel Posted February 16, 2006 Members Share Posted February 16, 2006 Originally posted by JamesPeters Some amps have 1 ohm resistors installed from the cathode to ground of each power tube for the purpose of making quick bias readings. In this case the "bias meter" device itself has a 1 ohm resistor installed on the cathode. "Bias meters" of this kind are generally made as such. Dear James ! Thank You very much for the introduction I worked in a repair shop for many years, so don Link to comment Share on other sites More sharing options...
Members Peter Diezel Posted February 16, 2006 Members Share Posted February 16, 2006 Originally posted by Negative K3 fan is it so bad?? Don Link to comment Share on other sites More sharing options...
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