Thank you for breaking it down the simplest way, and for explaining how to use the bias voltage/current chart. You, Uncle Doug and Terry from D-lab are my professors on amp building
I love this stuff and I always come away with same realization.....take it to someone who knows what they are doing. Confirmation puts me at ease. ha !
Some people think it is better to use two tubes that are not perfectly balanced. And I am a fan of 5881's in Fender amps. To my ear, they have a cleaner, tighter bottom end. The current Tung-Sols seem to be good.
I'm doing my first bias job. A 73 traynor yba1. Your video seems very straight forward. Thanks. I have 6l6 in mine so I'm assuming that chart would work with my amp? I was following a tutorial online that said I need to measure center tap voltage and I don't know where to findnit in my amp. It doesn't seem lime you did that. Is it unnecessary... scratching my head here. Thanks for any insight
James Patillo, thank you. There is still no answer to John Miller's question "How can I measure current?" I assumet he means cathode current to set the bias. There is quite a discussion on the polarity and indirect reading of current through the DVM set to mV (due to the built in 1 ohm resistor in the probe), but no answer, so I must ask "How can I measure the cathode current directly, without using the bias probe?" I tried measuring current (yes very carefully) across cathode (pin 8) to plate (anode pin 3), but no reading showed. Please help.
Hi James, thank you for sharing this. I have a '65 Fender bassman that I love the sound of. I was recently playing it and the volume cut in half suddenly. I suspect (and hope) it's just the power tubes. It had some really old Raytheon tubes and I just purchased new 6L6GC's for it. I'm also researching the tools I need to do the bias myself. I noticed in the video you have what appears to be 2 volt meters, one for measuring the voltage of the amp without the tubes and one for measuring the bias of the tubes. I was wondering why you used two separate devices to measure the readings?
When I repaired amps for a living and rebiased a lot of amps, I just left my bias probe attached to one voltmeter and used it only for that purpose. It was just a convenience to me and I just continued that habit. But, no reason you can't use the same meter for both measuring voltage on plates ( or whatever) and also with the bias tool. Thanks for your question, and I hope you get your amp working 100%.
My 1963 bassman does not have a Bias pot. So, other than installing one, how would you adjust the bias on an amp that does not have a Bias trimmer (Bias pot)?
You could vary the values of the voltage divider resistors in the bias circuit, but this might require multiple iterations. So, it’s easier to install a potentiometer. Unless you’re knowledgeable of tube amps and electronics, I recommend finding an amp tech to do the work.
So if I am biasing both my tubes like on my Blues Deluxe, I have to double it correct? So if my plate reads 450v then 1 tube would be set at 35m, so I would double that and set the bias at 70m? Am I correct?
Thanks for this it is great .Do amps running below 30 watts need to be biased? Ive heard they dont.but dont believe anythingI just put Genelux Gold Lion tubes in my Deluxe Reverb and it sounds better than ever .I didnt bias it .
It’s important to measure the bias on all amps to get a baseline. Dr Z recommends Medium Mu EL-84 tubes in his amps. You should google the subject for more info than I have time to go into here.
First off, I love your vids and appreciate your knowledge. Please correct me if I'm wrong... This method doesn't seem accurate and will bias the tubes colder than expected. You are using UNLOADED plate voltage and CATHODE current (=plate+screen current). As you raise the current the voltage will drop. You know this... but I trying to understand the reason for biasing this way. Beginners notice that this graph assumes a 6L6GC with max dissipation of 30w...not all 6L6's are equal, know your true limitations. Many new production tube manufacturers use old data sheets... But new production tubes may not be as robust as old era tube manufacturing.
Blake, I believe by "unloaded plate voltage" you are referring to measuring voltage of power supply w/o tubes in place. I measured the voltage this morning with the tubes in place to compare with voltage measured in the video. The voltage with tubes is 445 volts, and without is 448 volts. So, not enough difference to matter. Also, this is an idle current method. This method of setting the bias as described in the video is an accepted way to set the bias of power tubes in amplifiers. I have used it on hundreds of amps and it works well. There are other methods, but this is the simplest and most useful to the average amp owner. I like it because it is easier and safer and gets good results. As far as the differences in tube characteristics, if you set to the middle of the "normal operating window" or center point of a general tube characteristic sheet, you should be fine. There can be debate about the best method of biasing an amp, my intention was to show in a simple manner that most anyone can successfully and without a lot of equipment set their own bias. Thanks for watching my video and for commenting!
The safest way is to temporarily install a 1 ohm resistor in the cathode to ground circuit and measure mV’s across that resistor. For a direct measurement of cathode current you would have to break the cathode to ground circuit and use your dvm (set to measure current) in place of the cathode to ground circuit. The dvm in the current measuring mode looks like a short circuit to anything you connect it to so you have to be careful. By trying to measure cathode current by connecting your dvm from pin 3 to 8 with the dvm in current mode, you were shorting the high voltage of the plate to ground. The probable reason you measured 0 is that you probably damaged your dvm. Be careful! These are 350-450 volt levels that can kill you. That’s why my video addresses biasing with a bias probe. It’s safer!
@@jamelectronics Yes. Thank you for that warning! You're right. I won't try to measure current like that again! I have been studying my schematic and see that my "65 Fender Deluxe Reverb Reissue" Amp, AB763 circuitry. The cathodes on the 6V6GT tubes go to ground with no cathode resistor. So, I measured (with amp off and caps drained) the resistance from pin 8 of the 5AR4 rectifier tube to pin 3 (plate/anode) of the 6v6. That ohms reading gave me the resistance between the center tap of the power txfrmr, then ,using those same test points, set my DVM to DCV and power up amp to read the voltage drop across that 1/2 of the transformer to the tube. Then, using ohms law calculate the 6V6 bias current indirectly. this is voltage drop divided by resistance, or plate current = voltage drop / transformer coil resistance, or ohms law: I=E/R, where I=current in amps E= voltage in DC volts R= resistance in ohms you don't measure plate current directly! Or if you have the money and want to be safer, buy a bias probe. Thank you!
Dennis Green Go to this website and use this calculator. Most commonly guitar amps are class AB. Just plug in the tube type and plate voltage and you get the median current in mA that you can use. www.tedweber.com/bias-calc
Hello, I have a twin reverb, several years ago one valve broke and I change it and just that; sounded always good. Now I have another valve burnt, so I am here with the new one...but I cannot get all this BIAS thing. Again, I never touched anything and the amp worked good for several years; also I recorded a demo on studio, played some gigs and still kicks (but with 3 valves instead of the 4) So, still do not get all this. I was lucky? Also, the simpler stuff I think is not measure anything, turn off all the knobs put the valve, turn on the amp let it warm a bit then listen the noises etc then adjust the screw....or not? Again, I did not do anything and the amp sounds great. Thank for any explanation about it.
TheReverb1 Yes, you can just install a new tube(s) and not adjust the bias and the amp may sound great. However, without adjusting to bias to a recommended level, the current passing from plate to cathode in the tubes may be more than the tube is rated for. This in turn will cause the tubes to fail sooner than they might otherwise. If the bias is adjusted such that the idle plate current is too low, the amp may sound distorted and/or lack punch. So, correct biasing ensures the best sound quality and longest life of the tubes.
@@TheReverb1I thought I had explained that in my reply, however, here it is again. I think what you did is fine. Many times it will be OK to replace tubes without resetting the bias. However, by leaving it to chance, sometimes that procedure will result in a poor sounding amp or short life of your tubes. You chose.
@@jamelectronics Hello; may be I was expressed it wrong but what I tried to ask in that second question is this: "Also, the simpler stuff I think is not measure anything, turn off all the knobs put the valve, turn on the amp let it warm a bit then listen the noises etc then adjust the BIAS screw....or not? Again, I did not do anything and the amp sounds great. Thank for any explanation about it." So I mentioned that I did not nothing; only put the valve and mentioned this other question too. If with this simpler procedure could be possible adjust it (mentioned to me by other) Then asked you about your opinion about it. Hope this time would be clearly enough. Thanks
Hello Mr. TheReverb", this gentleman answered your question twice. It is you not understanding his answer. That's why he did not reply a third time. He said "YES" you can do it your "simpler" way. Ok, that's the answer. Understand that he warned that you could burn out your new power tubes using your "Simpler way". That's the part that you did not understand or perhaps you are choosing to ignore. So he is saying yes you can do it the simpler way. BUT, it is smarter to bias the tubes to prevent them from burning out soon. Have a nice day. Enjoy your day -Peter
Pin 8 on my ab763 is grounded. Not getting any current readings but voltage is high close to 500v. How can i measure current? (2) 6L6 amp. Why is your current reading negative volts? Thats measuring the grid which is the negative bias adjustment not the current reading
The bias probe inserts a 1 ohm resistor between the cathode and ground of the tube. Then voltage across the 1 ohm resistor = the cathode current. You can do the same thing by disconnecting the pin 8 ground and connecting a 1 ohm resistor between pin 8 and ground. Use a 5 watt resistor. My current was reading negative volts because the bias probe came with the red connector on the black wire and the black connector on the red wire and I never noticed that I should plug the red connector into the ground of the meter and the black connector into the positive connection of the meter . The polarity of the reading was never a concern since since I knew the tube would not conduct backwards. I am not measuring the grid. There are no connections to the grid in my procedure. Measuring the grid would require a connection to pin 5 not pin 8.
@@jamelectronics thanks for the explanation. I think i need a good bias probe. I would rather not desolder pin 8 leads and mess with a resistor. Does a bias probe have a built in resistor to measure current?
@@jamelectronics well it looks like my ab763 is not cathode biased but fixed biased. The probe i'm looking at passes all the pins through except pin 8 which is running to each probe lead. I would be measuring the current running through pin 8 to the tube i guess. No resistors needed
@@johnmiller9219 Yes, the bias probe inserts the 1 ohm resistor internally in the probe, so need for an external one. Typically, AB763 amps are not fixed bias- meaning you can adjust the bias by varying the control grid voltage with a pot located on the chassis near the power supply. Just like in the video.
@@jamelectronics i hear what you're saying but i've watched countless vids on making a bias probe (or dissecting one) and have never seen a resistor in one.
I got mine from my Bias Probe owners manual, but I like the calculator on Ted Weber website. You can get bias settings using that. www.tedweber.com/bias-calc/
If you’re asking which should be used for amp biasing, you could use either. If you are using a calculator such as tedweber.com, they use cathode current. Someone else’s calculator might use plate current. Cathode current is easier to measure since you are dealing with lower voltages in the cathode circuit. If you measure cathode current but want to know plate current, you can subtract the screen grid current from the cathode current to get the plate current value. To get the screen grid current, measure the voltage drop across the screen grid resistor, divide by the value of the screen grid resistor and there you have it.
@@jamelectronics Yes, thanks so much....I'm slowly learning, I used to bias my amps but years and years ago and perhaps incomplete understanding...thank you for sharing
What can be a little confusing here is you are measuring in DC millivolts giving you a negative bias output, but don't most bias probes measure in DC milliamps which give a positive current value??
The bias probe measures the voltage drop across a 1 ohm resistor which is mounted inside the device. What we want to know is the amount of current that is flowing through the cathode circuit, but measuring the voltage across the 1 ohm resistor is more convenient than measuring the current directly. Current = voltage/resistance, so if the resistance is 1 ohm, then Current = voltage. Therefore, the probe reading in millivolts = the amount of current in milliamps in the cathode circuit. I hope I answered your question and thanks for asking!
Not exactly. If the 1 ohm resistor in the probe is than supposed make the current equal voltage, then why is the bias negative? My Eurotubes probe measuring the current in milliamps is a positive bias of 38ma. Plate voltage of 470v.
@@djefferson5669, OK I understand your question. If you notice at 8:56 in the video, the red lead of the bias probe is attached to the black plug and the black lead is connected to the red plug. They should have been red wire to red plug and black wire to black plug. This is the way it was wired when I bought it and since we are only concerned with the amount of current not the polarity, I never took the time to rewire it. I could also just plug the red plug into the ground of the multimeter and the black into the positive connector of the multimeter. The current will always be flowing from the plate to the cathode and then to ground ( positive sign convention) and should indicate + on your meter.
The plate voltage on this amp is 441volts with the tubes in and 447 volts with them removed. Using the Tube Bias calculator on tedweber.com website, this yields less than a 1 ma difference in cathode current. So, the effect of measuring idle cathode current with or without the power tubes installed is negligible. Also, the advantage of measuring with the tubes removed is that the inexperienced person avoids having to insert probes into the underside of the chassis thereby lowering the danger of electrical shock.
Before you decide what voltage you're dealing with, let your amp warm up for at least 10 minutes! You might get 450 volts initially and it drops down to 430 volts as it's warming up and when it's fully warmed up and stabilized it could be 460+ volts. If you bias it for the wrong voltage it won't be right. This is tube stuff. You can't make it hurry. Go slow...............................
It is always good practice to let the amp warm up for a minute or so before taking measurements as stated in the video. I decided to check the Bassman amp plate voltage (with the tubes installed) at various times from initial to 10 minutes. After a 1 minute warm up, the plate voltage was 442 volts, at 2 minutes - 443, at 5 mins. - 444, at 7 mins. - 444 at 10 mins. - 443. So, my conclusion is that the variation after a minute of warm up is insignificant.
