Biasing Double-Ended Amplifier Output Tubes, Part 2: Demonstration and Discussion

greetings and welcome to my second of two videos in which I try to explain how to bias double-ended amplifiers in this video I'm going to actually demonstrate the biasing process of a double-ended amp using the output transformer method of determining plate current rather than the bias resistor method I will explain to you why I think it's a better method although the cathode bias resistor works this is actually a little better in cases especially when the plate current is different when the tubes aren't well matched and thirdly I will discuss with you briefly how to adjust the bias on amps that our grid biased and have the cathodes grounded if that sounds of interest then stay tuned we're about to begin first off I prepared a worksheet where I can write down my results in a orderly fashion so that I don't get mixed up as you can see from this chart I think this is a decent way to do it and then keep this in a file with the schematic of the amp I'm going to designate the two tubes either outer or inner on the outer one is one near the edge of the amp the inner one will be near the center if you say left and right that can change depending on how you orient the chassis on the on the work table first we'll measure resistance pin three to pin eight that will be the one half of the output transformer winding we're going to find out what the resistance is then we'll switch and find out what the resistance is for the other half of the output transformer winding will measure the voltage drop across each of those resistances will calculate the plate current by dividing the voltage drop by the resistance we will measure the plate voltage directly and I'm going to show you safe methods to do all of these procedures and then finally we will calculate the plate dissipation and compare it to this chart and depending on what type of app it is grid biased or cathode biased we'll see if the tubes are biased correctly if they're too hot too cold and then we'll also cover how to adjust that if they're not correct here's the Apple we'll be using for this demonstration it's a really nice ol 1954 national deluxe amp you just go to love this grill cloth and this is the national emblem it looks like a route 66 emblem to me okay so let's turn it around and I'll open the back and before I do I just want to remind you this would be a really great time if you haven't watched the first and second videos in this set the first one covers biasing single-ended amps and covers a lot of basic theory the second one covers the theory behind biasing this amp and you really would gain a lot if you haven't watched those videos to watch them before you watch this one alright let's proceed here's the back of the amp before I pull off the rear panel it's just such a nice amp it has a really nice dashboard also you know you can tell if you see an older amp and it's covered with this tweed material in this case this one doesn't have the diagonal lines like the fender does this is the smooth and the reason they did this and it's almost always in the 50s when they use this material is that's the way luggage was made and I guess in their minds we got a handle it's sort of like a suitcase well let's cover it with tweed because that's the way we covered luggage well here's the rear of the amp with the back door off as you can see the chassis is fairly simple and real clean also it has those 7 by 10 Roll oval speakers a pair of them output transformer mounted down here these speakers are virtually impossible to find in decent shape you can't really recall them so you operate amps like this at a very moderate volume to avoid any harm to these speakers and that will affect my decisions and the plate dissipation values that I bias they have bad okay let's take a look in the chassis and see what we're doing okay these are 58 81 tubes and the cathode is pin 8 so we look over at pin 8 we see there's a yellow wire that jumpers to pin 8 of the outer 50 881 and connect it to that is this large a cathode bias resistor now I could use the first method that I described you in the part 1 video and that is jumper on one side and the other of this measure accurately the resistance then measure the voltage drop and calculate the average plate current for the 250 881 now instead on this particular amp I'm going to individually check the plate current on each of the 58 81 s using the output transformer method first let me take a few seconds here to explain why I think that using the output transformer method to determine the individual plate currents is better than using the bias resistor to determine the average of the two okay let's consider this scenario first off let's say that 40 milliamps is the proper plate current for a 50 881 now what if this one were flowing 60 milliamps and this one's flowing 20 milliamps if we used the bias resistor method and get the average 60 plus 20 is 80 divided by 2 we would think that the plate current was ideal in these tubes at 40 milliamps but instead this one would be 60 and way too hot this one would be 20 and way too cool but we would not know that if we use this method and if we accepted this value and allow the amp to persist like this we probably end up burning up this tube and in the long run having really lousy tone from the app because of the gross mismatch between the two tubes during the push-pull process using the output transformer method however we would quickly see that this was blowing 60 milliamps this was flowing 20 and we could either change tubes or alter the bias of the tubes to balance them now how that's done is this instead of allowing them to share the same cathode bias resistor what I would do if I were confronted with a 60 20 split here is I would remove this cathode bias resistor because it really doesn't suit either of these and I would put in a much higher resistor on this cathode and a lower resistor on this cathode and come up it's there's a trial and error process here but I could bring these both back around to 40 milliamps of plate current and virtually identical plate dissipation one other consideration if you do that is if you have a cathode bypass capacitor say like a 25 micro farad at 50 volts that is in parallel with this bike the bias resistor you would then have to put one of those across each of your new bias resistors so you'd have to have 125 here and another 25 over here that said let's get started on the procedure this app has not been plugged in in several weeks it's definitely not plugged in now and it's not turned on so while it's in this state I'm going to use the method that I showed you in the single ended video and I'm going to drain these filter capacitors to be sure that they're not storing a charge if any of you think that I go to ridiculous extremes on these I remember there was an old cowboy movie called the quick and the dead meaning you're either one or the other okay well I would paraphrase that dealing with amplifiers and say you could have two types the careful and the dead and I prefer to be in the former rather than the latter group so now I'll demonstrate this once but I'm going to do it repeatedly throughout the procedure I'm going to clip one end of my jumper cable to a chassis ground and then I'm going to go in with my resistor and touch the lugs of the filter capacitors and hold it there for a count of 10 now that I'm satisfied that they are drained I will continue now since I've never been able to find a good schematic for this amp I'm going to use my own diagram here is the schematic and we're going to first measure the resistance between pin 3 of the outer 58 81 and pin 8 of the rectifier and that will give us the resistance right here of 1/2 of the output transformer then I will do the same with the pin 3 of the inner 58 81 and the pin 8 of the rectifier and determine the resistance of this half of the winding of the output transformer okay now I'm connected from the plate of the outer 58 81 to the 10 8 of the rectifier and it looks like I have around 220 ohms of resistance I'm going to write that down now I'm connected to pin 3 of the inner of 50 881 and the other tests latest remains connected to a pen eight of the rectifier it looks like I have about 195 ohms of resistance which I will write down now with the test leads in the same position they were on my second measurement that is pin a2 the rectifier pin 3 of the inner 50 881 I'm going to plug in the amp to a current limiting device which you must have for safety I have posted a video to show you how to make one of these if you don't have one you really need to have one if you're gonna do work like this then I'm going to set my whole meter voltmeter down to DC volts and switch on the amp let the voltage stabilize it looks like the inner tube voltage drop has settled down around seven point two two which I will write down turn off the amp unplug it and drain the filter capacitors now working one-handed with your other hand in your pocket move your test lead from pin three of the inner fifty 881 to pin three of the outer plug in the amp set your meter to DC bolts it should still be there and turn it on it looks like the outer tube voltage drop has settled down around nine point six five or so which I will write down turn off the amp unplug it and drain the filter capacitors now we're going to measure the plate voltage of the outer fifty 881 I'll leave my red test lead on pin 3 which is the plate and I take the black test lead off of the rectifier and attach it to the chassis to a good bare unpainted chassis ground then I'm going to set my meter to DC volts plug in the amp and turn it on play bolt ajaan the outer 58 81 is settled down about 356 which I will write down turn off the amp unplug it and drain the filter capacitors now using one hand with the other one in the pocket we change the red Tesla from the outer fifty eight eighty one to pin three of the inner fifty 881 plug in the amp turn it on and we will measure the plate voltage of that to the inner tube plate voltage is settled down around 360 which I will write down okay let's analyze the data that we got from our measurements outer tube the resistance through the output transformer was 220 ohms voltage drop was nine point six five volts you divide this by this and you get 0.043 nine amps or forty three point nine milliamps which is a actually a decent amount of plate current for this to remember we said probably around 40 to 45 is about right we found the plate voltage on this on the outer two to be 356 volts we multiply that times 0.04 39 we get fifteen point six watts let's look at the bottom tube 195 ohms of output transformer resistance voltage drop across at seven point two two volts when we divide the voltage dropped by the resistance we get point zero three seven O amps or thirty-seven milliamps a little bit low but still within acceptable limits plate voltage three hundred and sixty volts times 0.03 seven o amps gives us thirteen point three watts now these do not agree exactly but these are pretty close if this were 25 and this were 15 or 12 or 10 I would start getting a little concerned now these tubes are grief early well fifteen point six and thirteen point three are close enough that this would be an acceptable match the average is fourteen point five watts now when they come down here and we see that fifty eight eighty one s can handle up to twenty three watts with probably around twenty being okay seventy percent value on them is 16.1 this is definitely cold this would probably probably be better if it were around 16 or 17 so there's two choices to be made here leave it alone and because it's an old amp that's never used at very high volumes you would get probably decent tone at low volumes and you would get some early breakup at moderate volumes so cold bias might suit this ham the other possibility is if you want to goose it up to around 20 or so watts of plate dissipation then what we can do is go in here and alter the value of the cathode bias resistor and let's go from 250 this is 250 ohms go down to 200 and then repeat the process and I think you'd end up here with a value closer to 20 one last little interesting tidbit here compare the results that I just got we using the output transformer method with a 14.5 watt average I also ran just to see how they would compare bias by the cathode bias resistor method and this is the result I got for the average of the two tubes fourteen point six eight watts so it's around 0.2 Watts higher than the output transformer method and as I told you this method almost always yields a slightly higher result but the fact that they agree so well makes me believe that the results are accurate and reliable and finally let's briefly discuss how you adjust the bias on amps that our grid biased in which the cathodes are connected together and taken straight to ground now this is a rather complex issue and I'm going to suggest that you might check on the internet for specialty sites where they go into great detail on how to adjust the bias on your grid biased amp I will go over it they're superficially here but if you want great detail you're really going to have to check in to specific sites first off we'll look at a deluxe reverb Fender amp now as you can see the cathodes are bound together and go to ground so there is no cathode resistor here for us to adjust instead we follow the grids out here we follow them down and we see that there is a potentiometer provided in the chassis for us to adjust the bias of the deluxe reverb if you look at the layout of the deluxe reverb chassis and you can also look up layouts on other fender amps you'll see that this right here is the potentiometer it's right allocated right in the chassis you will do your biasing procedure take your measurements and determine if it's okay if it's too high or too low then you will adjust it using that potentiometer after you've adjusted it a bit you can recheck a little bit of trial and error is called for but you can get it to be exactly right using that potentiometer whether fender amps like the Fender Bassman also have the potentiometer located in the chassis to assist you in your biasing procedure now one real challenge presented itself recently when I was restoring a couple of Princeton Reverb now if you look at the Princeton Reverb cathodes are bound together go straight to ground we follow our grids out here they come to a resistor and network they drop down and it goes to not a bias adjustment but the intensity control of the tremolo so to be brutally frank there's really almost no way to adjust the bias of a Princeton Reverb amp without seriously affecting the tremolo so I came up with my own way of doing it and that is I came in right here and I ungrounded the cathodes and you can start off with like 100 ohm resistor to ground for each of your cathodes or you can have them share a 100 ohm resistor to ground if you wish but try that now I only suggest this if your 66s are biased way too hot and I warn you they probably are I have never seen plate current and plate dissipation values on any amp like I did on a Princeton Reverb am okay so in my case to save my tubes I actually converted it and turned it into a cathode biased amp by adding 100 ohm resistors to the cathode of the 6v sixes you can bring them down then to reasonable levels of plate dissipation and in my opinion the tone is still wonderful and your tubes are going to last I show this modification in detail on the two videos that I posted for the Princeton Reverb am well that about does it for this part 2 video covering biasing methods of double-ended amps we covered a lot of ground and you may have to watch it a couple times for it to all make sense please if you have any questions feel free to ask them but sit down and think about it there are some of these things require a little active study and consideration before they become clear so while you're doing that I'm going to bid you a fond farewell I appreciate your time and interest I hope you'll subscribe if you haven't already but no matter what I hope you stay tuned and that I will see you again in the near future thank you so much

Channel: Uncle Doug

Views: 84,863

Rating: 4.953917 out of 5

Keywords: Output Tube Biasing, Grid Biased, Cathode Biased, 6V6, 6L6, 5Y3, 5U4, Electrical Safety, Output Transformer, Ohmmeter, Voltmeter, Voltage Drop, Fender Bassman, Deluxe Reverb, Princeton Reverb, National Amp, Valco, Draining Filter Capacitors, Discharging Filter Capacitors, Cathode Bypass, 6X5, Plate Dissipation, Plate Voltage, Plate Current, 5881, Rectifier, Output Tube, Watts, Volts, Milliamps, Amplifier Repair

Id: pjKYiSr497w

Length: 21min 22sec (1282 seconds)

Published: Mon Dec 09 2013