loving your work, and the "have fun and be safe in the new year", I have just made one of the other type bias test rigs, the socket on plug type with shunt resistors, in it which i bought as a kit , but may build your type also and love analogue meters I have an old AVO, and love it, my new years res is to finish my valve amps i started to make.
Ron Warren Hi Ron, you are wise to measure cathode current as that is so much safer and just as effective. For some reason I wanted to measure screen current back in the day when I built the bias test jig I have but it does somewhat expose the user to the HV from the screen and plate. Got to always work with one hand and the other hand in the pocket and never leisurely lean on the work bench with one arm while working on vacuum tube circuits. Some time back I reluctantly made a video on measuring plate current by shunting the meter across the output transformer. That is the most dangerous way to both the human and the meter. I have blown fuses in my meters too many times by accidentally touching one of the probes to the wrong place. I do not recommend that method or use it anymore.
ElPaso TubeAmps yeap that was the first thing I learnt doing valve tv repair at college ( back in the 1970's when i was at college), put left hand in your pocket, to save electrocution . was on the test paper to.
Hi David! One minor addition for the kids not familiar with bare-bones current and voltage meters like what you have built here: A good safety precaution is to *always* start the meter on the highest range, and dial down the range as needed. If you accidentally connect to a high voltage or high current source, with a sensitive meter movement you risk damaging it. I have seen pointers get wrapped around the end-stop. That sort of thing will ruin your whole day.
I love analog meters for "peaking" and "dipping" voltages and currents in tuned curcuits. Digital meters with all the flashing of numbers going on, makes it really difficult to concentrate.
Yes, be careful with your meter. I changed the setting on my DMM and forgot to move the lead from current to voltage, I came across 750V that had a 1000mF cap on it. It didn't only explode the glass fuse in my meter, but now the 20A setting on my meter no longer works, ooops. It was the best thing I ever purchased from Raidio Shack, and I had it forever. I loved that meter.
I understand, I smoked a Fluke meter because I did not believe the maximum RMS voltage was 750 and I measured something over 1000 volts and it sizzled and that was it. And I did a very similar thing with a Tektronix meter - probably what I mentioned in this video and smoked the current scale. We live and learn...
I love Yankee ingenuity (or in your case, Texan ingenuity). So, what is the resistance tally of each of the 100k ohm in parallel with the ni-chrome wire components? This measurement will be somewhat different depending on whatever meter is used, I suppose. Also, is the resistance consistent inch by inch in the wire so all one needs to do is measure off so much wire for an approximate amount of resistance? Finally, the 100k ohm resistors appear to be larger wattage, is that relevant... just the size for wrapping the wire? I greatly appreciate your pragmatic approach and thanks for the gift of teaching coming from you.
Mark Weber Hi Mark - Thank you - the 100K resistors are 2 watt as I have lots of them that's why I picked them. Anything over a few ohms would be OK as a form to wind the shunt wire on as the actual shunt is probably only a few milliohms. I have used 1 mA meters so many times I have the procedure down to a quick-and-easy fix. Meters like 50 uA are a bit of a different challenge and require experimenting with different shunt wire and large shunts like 30 A, again, are an experiment to make the shunt wire large enough to carry the current, etc. I have never seen the need to mathematically calculate the shunt resistance as measuring a few milliohms or a few mircoohms is just too tedious and not necessary as in-the-end you are going to have to manually adjust the shunt anyway. In the cases of 10 mA and 100 mA and even 500 mA shunts, it is just too easy to do it manually. Wire saved from large wire-wound resistors works good but a little bit of a challenge to solder to.
Making a dummy audio load,is it ok to have the amp neg output connected together meaning left and right channels I checked all neg black posts on amp output and the show that they are all connected with ohm meter making a 4 8 16 dummy load thanks
Another very cool video. But best to get a couple of diodes in there to protect your ammeter. (Use two 0.7v diodes, back to back to protect in both directions between your shunts and the meter. If you do mess up when connecting, that will then only destroy your diodes but will save your ammeter. They do this with all the school lab ammeters as kids always able to fry ammeters.) Yes better to use make before break, but diodes will give you the protection even in the worst scenario.
Kevin O'Brien Thank you for reminding me - I will add the diodes. No need in destroying a perfect 50 year old vintage meter over a single, "OOPS..." I once had a meter movement that I had connected to measure the HV (3KV) in my transmitter and even though I knew better, I used only one 2 watt resistor of the correct value - well, you know what eventually happened... the physical path was too short (I should have used five, 2 watt series resistors) and it arced and you can imagine what 3KV does to the inside of a meter movement. I wonder if in this case diodes would have saved the meter - maybe so. I still have and use that transmitter and the voltmeter (replacement...) still works after more than 30 years and I do have diodes across all the meters in that unit. Thanks again.
ElPaso TubeAmps If it arcs over (which it probably would do) then not even diodes would have saved it using 3KV. Ha ha! (Need a very fast acting fuse behind diodes maybe? but I am not sure even if that would have saved it?) Possibly using a sense resistor connected across a transistor to completely isolate the meter (small battery then needed) I think the old battery driven valve meters used this type of isolation setup. But again, I am not sure. But there are lots of circuits out there for high voltage protection methods, its a fascinating subject.) I can imagine that movement exploding into dust! HUGE BANG! Great videos BTW! keep them coming!
I think that's a good idea. All of the contacts should be closed all the time except for a reading. Maybe others will see this and choose to use (good quality) push button switches. Opening the plate switch without a DC path through the meter is assured destruction of the screen grid of the tube. Thanks...
You have to kind of look at this first from the 40,000 foot level. What I mean is, say you have a very sensitive meter like a 50 uA movement and you want it to measure 500 mA. This means you are going to have to shunt the vast majority of the current around the meter so you will need wire of pretty low resistance and not much of it. On the other hand, say you have a 5 mA meter and you want it to measure 500 mA. This meter is 100 times less sensitive than your 50 uA meter so you will need a higher value of shunt resistance. Another problem comes in with the length of the shunt. Ideally, you would want 3-5 inches so that you can trim it to read accurately but not so long that you end up with a huge spool of wire on the back of the meter or neither a 1 inch piece of wire that you cannot adjust for calibration because it is too short. Large wire like No. 16 or larger (like the wire in your AC motor) is probably too large to make a reasonable shunt as it will take way too much (several feet of it Might work for shunting a 50 uA meter to 50 A - just a guess). In general you can only make shunts for up to say an amp or so. Anything larger than that, you may need an external shunt and run wires from the shunt to the meter. Weston makes some really nice shunts and most are for 50 mV meters. You will find commercial made shunts for 100 mV meter harder to find and you may have to make your own as I did. I hope this helps. It is not hard but without some experimentation it may be hard to grasp. For example, if you take a 1 mA meter and shunt it with 1 foot of No. 16 wire it will read probably 30 amps as the shunt of No. 16 wire will have such a low resistance. This example is not an exact real life example but just to illustrate something realistic. Experiment with some low mA DC meter and a couple of batteries (3 volts) and an adjustable resistor in the 10 ohm range and you will get an idea of how to deal with it. I hope this helps.
Nice video - I have a bunch of 6L6's I want to measure. Is this pretty much the way to match them? It's only for guitar amps, not a McIntosh, if you know what I mean. I have a couple 100 mA analog meters I want to put in a box with binding posts - Thanks - keep 'em coming :)
cmguitar50 You don't have to have a test box to measure and match your 6L6's but it sure makes it easier and safer than doing it in your working amplifier. The goal is to measure the cathode current of each tube in a working unit. You can do this a few different ways. Let the tubes warm up very well and put a sticker on each tube with the value of cathode current and use pairs that are closely matched. It does make a difference.. Stay safe.
Hi David! I really like the idea of using an analog meter to measure current! Even though a digital meter says it clearly, I still prefer an analog movement. That's why I still use my old RCA VTVM...its especially good for high impedance circuits which is prevalent with vacuum tubes since they are high impedance devices. I'm also thinking about building a bias box using dual octal sockets and dual LED digital meters. I've seen these meters for as little as $13 each, and thought about putting them right into the box. What's your take on that? It will be a more sophisticated version of the Bias King since the meters will be LED and easier to see in low light situations. Another great video as always :) Frank Ferraro--Audio Craft Electronics.
very high voltage can kill.. !!!...i remember all the first experience. with the tubes.... and i feel from the finger.... all the european 220 volts comes inn to my body..... terribles but i'm still alive...cheers :)
loving your work, and the "have fun and be safe in the new year", I have just made one of the other type bias test rigs, the socket on plug type with shunt resistors, in it which i bought as a kit , but may build your type also and love analogue meters I have an old AVO, and love it, my new years res is to finish my valve amps i started to make.
Ron Warren Hi Ron, you are wise to measure cathode current as that is so much safer and just as effective. For some reason I wanted to measure screen current back in the day when I built the bias test jig I have but it does somewhat expose the user to the HV from the screen and plate. Got to always work with one hand and the other hand in the pocket and never leisurely lean on the work bench with one arm while working on vacuum tube circuits. Some time back I reluctantly made a video on measuring plate current by shunting the meter across the output transformer. That is the most dangerous way to both the human and the meter. I have blown fuses in my meters too many times by accidentally touching one of the probes to the wrong place. I do not recommend that method or use it anymore.
ElPaso TubeAmps yeap that was the first thing I learnt doing valve tv repair at college ( back in the 1970's when i was at college), put left hand in your pocket, to save electrocution . was on the test paper to.
Hi David! One minor addition for the kids not familiar with bare-bones current and voltage meters like what you have built here: A good safety precaution is to *always* start the meter on the highest range, and dial down the range as needed. If you accidentally connect to a high voltage or high current source, with a sensitive meter movement you risk damaging it. I have seen pointers get wrapped around the end-stop. That sort of thing will ruin your whole day.
You're a great tinkerer! Love it! I'm always doing things like that.
I love analog meters for "peaking" and "dipping" voltages and currents in tuned curcuits. Digital meters with all the flashing of numbers going on, makes it really difficult to concentrate.
Yes, be careful with your meter. I changed the setting on my DMM and forgot to move the lead from current to voltage, I came across 750V that had a 1000mF cap on it. It didn't only explode the glass fuse in my meter, but now the 20A setting on my meter no longer works, ooops. It was the best thing I ever purchased from Raidio Shack, and I had it forever. I loved that meter.
I understand, I smoked a Fluke meter because I did not believe the maximum RMS voltage was 750 and I measured something over 1000 volts and it sizzled and that was it. And I did a very similar thing with a Tektronix meter - probably what I mentioned in this video and smoked the current scale. We live and learn...
You should put a couple more binding post for plate voltage. Going to the plate and the cathode. Then maybe a switch to get screen voltage.
I love Yankee ingenuity (or in your case, Texan ingenuity). So, what is the resistance tally of each of the 100k ohm in parallel with the ni-chrome wire components? This measurement will be somewhat different depending on whatever meter is used, I suppose. Also, is the resistance consistent inch by inch in the wire so all one needs to do is measure off so much wire for an approximate amount of resistance? Finally, the 100k ohm resistors appear to be larger wattage, is that relevant... just the size for wrapping the wire? I greatly appreciate your pragmatic approach and thanks for the gift of teaching coming from you.
Mark Weber Hi Mark - Thank you - the 100K resistors are 2 watt as I have lots of them that's why I picked them. Anything over a few ohms would be OK as a form to wind the shunt wire on as the actual shunt is probably only a few milliohms. I have used 1 mA meters so many times I have the procedure down to a quick-and-easy fix. Meters like 50 uA are a bit of a different challenge and require experimenting with different shunt wire and large shunts like 30 A, again, are an experiment to make the shunt wire large enough to carry the current, etc. I have never seen the need to mathematically calculate the shunt resistance as measuring a few milliohms or a few mircoohms is just too tedious and not necessary as in-the-end you are going to have to manually adjust the shunt anyway. In the cases of 10 mA and 100 mA and even 500 mA shunts, it is just too easy to do it manually. Wire saved from large wire-wound resistors works good but a little bit of a challenge to solder to.
Making a dummy audio load,is it ok to have the amp neg output connected together meaning left and right channels I checked all neg black posts on amp output and the show that they are all connected with ohm meter
making a 4 8 16 dummy load
thanks
Another very cool video. But best to get a couple of diodes in there to protect your ammeter. (Use two 0.7v diodes, back to back to protect in both directions between your shunts and the meter. If you do mess up when connecting, that will then only destroy your diodes but will save your ammeter. They do this with all the school lab ammeters as kids always able to fry ammeters.) Yes better to use make before break, but diodes will give you the protection even in the worst scenario.
Kevin O'Brien Thank you for reminding me - I will add the diodes. No need in destroying a perfect 50 year old vintage meter over a single, "OOPS..." I once had a meter movement that I had connected to measure the HV (3KV) in my transmitter and even though I knew better, I used only one 2 watt resistor of the correct value - well, you know what eventually happened... the physical path was too short (I should have used five, 2 watt series resistors) and it arced and you can imagine what 3KV does to the inside of a meter movement. I wonder if in this case diodes would have saved the meter - maybe so. I still have and use that transmitter and the voltmeter (replacement...) still works after more than 30 years and I do have diodes across all the meters in that unit. Thanks again.
ElPaso TubeAmps If it arcs over (which it probably would do) then not even diodes would have saved it using 3KV. Ha ha! (Need a very fast acting fuse behind diodes maybe? but I am not sure even if that would have saved it?) Possibly using a sense resistor connected across a transistor to completely isolate the meter (small battery then needed) I think the old battery driven valve meters used this type of isolation setup. But again, I am not sure. But there are lots of circuits out there for high voltage protection methods, its a fascinating subject.) I can imagine that movement exploding into dust! HUGE BANG! Great videos BTW! keep them coming!
So would momentary push buttons be a better option for the switches in the bias box to avoid switching to an open circuit by accident ?
I think that's a good idea. All of the contacts should be closed all the time except for a reading. Maybe others will see this and choose to use (good quality) push button switches. Opening the plate switch without a DC path through the meter is assured destruction of the screen grid of the tube. Thanks...
question... I have enamel coated wire... from rewinding an ac motor.... could I use that.... since it is available?
You have to kind of look at this first from the 40,000 foot level. What I mean is, say you have a very sensitive meter like a 50 uA movement and you want it to measure 500 mA. This means you are going to have to shunt the vast majority of the current around the meter so you will need wire of pretty low resistance and not much of it. On the other hand, say you have a 5 mA meter and you want it to measure 500 mA. This meter is 100 times less sensitive than your 50 uA meter so you will need a higher value of shunt resistance. Another problem comes in with the length of the shunt. Ideally, you would want 3-5 inches so that you can trim it to read accurately but not so long that you end up with a huge spool of wire on the back of the meter or neither a 1 inch piece of wire that you cannot adjust for calibration because it is too short. Large wire like No. 16 or larger (like the wire in your AC motor) is probably too large to make a reasonable shunt as it will take way too much (several feet of it Might work for shunting a 50 uA meter to 50 A - just a guess). In general you can only make shunts for up to say an amp or so. Anything larger than that, you may need an external shunt and run wires from the shunt to the meter. Weston makes some really nice shunts and most are for 50 mV meters. You will find commercial made shunts for 100 mV meter harder to find and you may have to make your own as I did. I hope this helps. It is not hard but without some experimentation it may be hard to grasp. For example, if you take a 1 mA meter and shunt it with 1 foot of No. 16 wire it will read probably 30 amps as the shunt of No. 16 wire will have such a low resistance. This example is not an exact real life example but just to illustrate something realistic. Experiment with some low mA DC meter and a couple of batteries (3 volts) and an adjustable resistor in the 10 ohm range and you will get an idea of how to deal with it. I hope this helps.
Nice video - I have a bunch of 6L6's I want to measure. Is this pretty much the way to match them? It's only for guitar amps, not a McIntosh, if you know what I mean. I have a couple 100 mA analog meters I want to put in a box with binding posts - Thanks - keep 'em coming :)
cmguitar50 You don't have to have a test box to measure and match your 6L6's but it sure makes it easier and safer than doing it in your working amplifier. The goal is to measure the cathode current of each tube in a working unit. You can do this a few different ways. Let the tubes warm up very well and put a sticker on each tube with the value of cathode current and use pairs that are closely matched. It does make a difference.. Stay safe.
Hi David! I really like the idea of using an analog meter to measure current! Even though a digital meter says it clearly, I still prefer an analog movement. That's why I still use my old RCA VTVM...its especially good for high impedance circuits which is prevalent with vacuum tubes since they are high impedance devices. I'm also thinking about building a bias box using dual octal sockets and dual LED digital meters. I've seen these meters for as little as $13 each, and thought about putting them right into the box. What's your take on that? It will be a more sophisticated version of the Bias King since the meters will be LED and easier to see in low light situations. Another great video as always :) Frank Ferraro--Audio Craft Electronics.
Be careful, have fun and be save in the new year, nothing more to say....:)
oh this is really nice wow ! :D
very high voltage can kill.. !!!...i remember all the first experience. with the tubes.... and i feel from the finger.... all the european 220 volts comes inn to my body..... terribles but i'm still alive...cheers :)