Biasing Double-Ended Amplifier Output Tubes, Part 1: Basic Explanation & Methods

greetings fellow members of AMPA holics unanimous today's video is going to be a sequel to the previous video in which I described the process by which we can buy us the output tubes and single-ended amplifiers today's video is going to describe the process by which we can accurately bias the output tubes and double-ended amps whether they are controlled by the cathode or by the grid first a real quick review of what we learned in the first video and I really recommend if you're a beginner to this type of project please please watch the first video to get all the basics without them I doubt that you'd be able to understand much in this video in the first video we discussed how to buy a single ended output tubes that were cathode biased and we saw that first step was to measure very accurately what is the resistance of the cathode bias resistor step two was what is the voltage drop across the resistor then we use that and Ohm's law to calculate the plate current then we multiplied it by the plate voltage to get the plate dissipation I hope that all sounds familiar the only difference being that in the double-ended amps at our cathode biased the cathodes which are pinnate on the tubes if they are 6 v 6 s 6 l 6s or 58 81 s they share a cathode bias resistor so to determine the bias value of these tubes we follow exactly the same steps number 1 accurately measure the resistance of that resistor number 2 accurately measure the voltage drop across the resistor now then we will divide the voltage drop by the resistance using Ohm's law and determine the plate current in the tubes now here's the trick since they share one cathode bias resistor your plate current value will appear to be double so you must divide it by two when you divide it by two you will then get the average of the plate current of both tubes you will not get individual play currents using this method but you'll get the average that'll be just fine for our purposes okay then you know the fourth step and that is going and accurately measure the plate voltage you'll find it's most likely the same on both tubes multiply the plate voltage times the plate current that you calculated by Ohm's law and you have the plate dissipation of the tubes if it's too high will increase the value of this resistor if it's too low we reduce the value of the resistor and if it's just right we leave it alone and put everything back together here's a chart to tell you what the maximum permissible plate dissipation values are okay this is the these are the values here if you are cathode biased and over here are the values if your grid biased we will talk about this later okay but I at least want to tell you what this was so let's stick right here for the cathode bias values 6v 6 or 6 bq5 unknown to some as el84 it's 12 watts the metal 6 l 6s or 6 l 6 G's which are the great big globe 6l6 is about 19 watts 6 L 6 GB which is a little smaller or 58 81 which is smaller yet 23 Watts maximum and 6l6 GCS can stand a whopping 30 watts now this is maximum okay you can actually buy us this hot if you want I generally go a little under it just to play it safe okay also one last bit of warning if you're biasing 6bq fives the pin designations are different you have to look carefully at this schematic to find your plate and to find your cathode it's not the same as 6v 6s and 6l sixes now for those of you who intended to bias double-ended cathode biased amps I guess you're done now you can use the procedure that we used in the first video and just remember to divide your plate current by two you'll get the average and then use that to do your biasing for those who would like to see a second and a little more sophisticated method to to bias and to learn the method for grid biased tubes then stay tuned and we're going to discuss it right now okay we're going to use Ohm's law again but in a different way instead of using Ohm's law on the cathode current down here as we did in the previous video and the beginning of this one we're going to use Ohm's law on the plate current here and what you'll need to do this is to first you look at your rectifier if it's a 5 y 3 a 5 you for or GZ 34 we're going to be looking at pin 8 of the rectifier because in almost every schematic that uses one of these rectifiers can eight connects to what we call the center tap of the output transformer look at your schematic and I think you'll see this I'm going to show you four or five schematics and we're going to trace this out so I can help you become more comfortable with reading schematics so pin 8 here is your B+ this is the highest voltage in the amp so for God's sake be careful when you're doing these measurements okay comes up here to the center tap now the center tap feeds first this plate then will feed this plate because these tubes are in a push-pull relationship and due to the phase inverter which we have an early discussed but just trust me on this these tubes are taking turns providing energy to the speaker to give us our sound so here is what we're going to do we're going to first we're going to very accurately measure what is the resistance of this half of the output transformer we're going to with the amp unplugged completely obviously turned off and the filter capacitors drained and I will demonstrate this for you we're going to put our own meter leads first here on the pin eight lead to the center tap one lead here and one lead to the pin three of the one of these 6v 6 s 6l 6s or 58 81 s we have to do them individually so one lead on pin three of the output tube one lead on pin 8 of the rectifier read this resistance then leave your lead connected to pin 8 of the rectifier and go to the other plate of the other output tube and accurately measure this resistance write it down I usually say outside tube inside tube if you say left or right you might get confused one of them is always closer to the outer right or left side of the amp and the other is closer to the centre and that's the way I designate them and the way I will in the demonstration now that we know this resistance and we know this resistance we will hook up our leads pin eight to pin three and using the DC voltage reading from our voltmeter measure the voltage drop across this resistance and the voltage drop across this resistance and we'll write it down now if this is at all confusing to you don't feel bad I think when you see the demonstration seeing it again will help clarify it for you for the rest of you that are probably one step ahead of me by now I hope you know that if we know this resistance and we know the voltage drop across the resistance we can use Ohm's law and divide the voltage drop by the resistance and find out what the plate current is to that plate and then do the same division process down here and calculate the plate current to this from this plate or through this plate now unlike the first procedure I showed you in this video where we get the average plate current of the two tubes you're going to know the individual plate current in this one say it's a 66 this one may be say 30 milliamps and this one may be 28 milliamps don't fret they're almost always different now you've all heard of tube sets where they say the tubes are matched what they mean is they will draw the same plate current when they are plugged in to the amplifier now this is very important if you have matched tubes you're going to get like a 30 or a 28 and a 27 or 29 the values are going to agree if you've got an old pair of used tubes even and especially if they're mismatched like an RCA and a Sylvania and you plug in in you're liable to find 32 here and 16 down here now that gets to be a problem because it there are ways to get around it but you can see that the tubes are not matched so I think we've pretty well described here term that we hear all the time matched output tubes and what they mean is when you plug them in and you do what we just did you will get about the same plate current for both tubes it makes the biasing procedure a lot easier remember now in biasing the critical value is the plate dissipation so we have to do step four here and that is measure the plate voltage for each of the output tubes and it's done exactly like we did in the first video red lead on pin three of this output tube black lead on the chassis voltmeter set to DC volts get your reading say it's 290 then do the same thing here red lead on three black lead on the chassis take your reading you're going to find almost invariably they are the same they'll be if this one's 290 so is this one then you know the next step is multiply this plate voltage times this plate current that we got from Ohm's law and you get plate dissipation multiply plate voltage of this one times the plate current we get from Ohm's law you get plate dissipation okay now no matter how the tubes are biased whether they are cathode biased or grid biased this method of measurement of plate dissipation will work no matter what this is really actually a more accurate method because we're actually measuring plate current when we do this when we measure the cathode current as we did in the last video in the beginning of this video we end up with a slightly too high of value it's it's elevated a bit in this one you will get the accurate plate current okay to get you comfortable reading schematics so that you can see exactly what the wiring scheme is for your particular cam let's look at several different schematics and I'll make them as varied as I can this is from a late 30s Gibson eh 150 okay ancient five you for rectifier we're coming here off of Penn eight of the five you for rectifier we're going to come along here we go to a speaker field we come up here to the center tap of the output transformer this end connected to this plate of the 6l6 which will be pin three this end down here connected to this plate of this 6l6 which is also pin three to calculate our plate current then we'll be hooking on pin three here pin eight down here and do our resistance and our voltage drop measurement then we will leave the lead connected to pin 8 of the rectifier and we'll go to this pin 3 of this sex l6 and do the same thing also let's look at how these tubes are biased look there's the cathode okay the little horseshoe it comes down here and this is cathode biased these two output to share a two hundred ohm resistor going to ground so our method of biasing these tubes will be to alter that resistor if we need to okay this is from a double-a 764 Princeton Reverb down here GC 34 rectifier can eight comes up here to the you guessed it Center top of the output transformer the upper end of the transformer goes over here to the plate of this 6v6 the bottom goes to this plate of this 6v 6 I think you see exactly how we would determine what the plate dissipation is then of these two 6v sixes in this case if you look the cathodes are bound together and go straight to ground therefore this is going to be a grid biased type of circuit remember if the cathodes are grounded the circuit is most likely grid biased we haven't discussed how to adjust grid bias but we will now let's take a look at the GA 40 Gibson amplifier circuit the famous Les Paul amplifier down here we have our five y3 pin 8 comes out here goes through a filter choke comes up and yes sir right to the center of the output transformer this end of the output transformer goes to this plate this end goes to this plate I hope you're starting to see a pattern here also ODG a 40 let's see how it's biased the cathodes are bound together like they were in my diagram and it's a 200 ohm cathode bias resistor to ground so if the tubes are too hot we increase this value tubes are too cold we reduce that value okay here's a Fender Deluxe reverb a a B 868 schematic it looks really complicated and scary but look down here here's the 5 u 4 rectifier pin a comes out comes up here it goes through the standby switch a little jog over and you guessed it right to the center of the output transformer this end of the output transformer goes to that plate this end of the output transformer goes to that plate you see no matter what make no matter what vintage amp it is they follow the same pattern and as we can see the cathodes of the deluxe reverb output tubes are bound together like they were in my diagram but instead of having a biasing resistor they go straight to ground therefore we know this is grid biased and last but not least let's take a look at a Valco made of Gretsch 6120 10:8 let's follow it we come out from penne come up here on what a surprise right to the center of the output transformer this end of it goes to the upper plate this end goes to the lower plate isn't it nice how consistent they are let's look at the biasing of the 6161 circuit well the cathodes are bound together and we come out here to a 250 ohm resistor to ground once again tubes to hot increase the resistance say to 300 tubes to cold reduce it to say 200 you see it's not as hard as people might lead you to believe well it does it for the part 1 of this video series on biasing of double-ended amps now in part 2 I'm going to actually demonstrate the procedure of biasing a via the output transformer rather than the cathode bias resistor also I will discuss with you how to adjust the bias on the grid biased amps and I'm going to present to you a much better way to cathode bias double-ended amps we're going to buy us each tube individually okay which is instead of accepting the average we're going to bias them separately okay if that sounds appealing then keep an eye out for the part 2 video which should be posted soon until then bye for now thanks for watching and don't forget to subscribe if you haven't already

Channel: Uncle Doug

Views: 128,322

Rating: 4.957159 out of 5

Keywords: Double Ended, Cathode Biased, Grid Biased, Tube Biasing, 6L6, 6V6, 5881, 6BQ5, 5U4, GZ34, 5Y3, Ohm's Law, Voltage Drop, Voltmeter, Ohmmeter, Measuring Tube Bias, Adjusting Bias, Center Tap, Output Transformer, Reading Schematics, GA-40 Gibson, Princeton Reverb, Deluxe Reverb, Fender, Gretsch, Valco, 6161, EH150, Plate Dissipation, How to Bias

Id: w9B0Rhr_Y-E

Length: 19min 15sec (1155 seconds)

Published: Fri Dec 06 2013