How to Measure Tube Amplifier Plate Current (Biasing) and Actually Understand What You Did

BG280 - How to measure the Plate (Anode) Current in most vacuum tube amplifiers and actually understanding what you just did. This theory should work for Audio Hi-Fi amplifiers as well as most RF amplifiers.

[@KUDJU]

I love those type of videos. High level knowledge for the common man! Keep them coming 🥰

[@AndJusticeForAll...1985]

you heard the man, keep em coming

[@paulj0557tonehead]

Finally! I looked everywhere for instruction A to Z of biasing, not the 20 second versions that teac nothing. Thank you! 😃

[@hestheMaster]

Great explanation of tube amps and how electricity works in them. No BS examples anyone can appreciate Mark.

[@tomsworkshop5856]

I very much thank-you! This series of videos has been both a refresher and, enlightening. Knowing what a part does, and what it's referred are two seperate things that the older gents who taught me weren't big on.

[@patprop74]

Thank you so much! For the last few days, I have been watching your videos and i learned more in a few days than I have trying to make sense of it elsewhere.

[@sparkey1958]

at last simple to understand video on tube biasing without any bullshit...well done!

[@jesseyasaitis9036]

This video is getting saved for future reference. My hobby is buying guitar pedals that dont work, fix them and re-sell them. I'm looking into attempting guitar amps in the future. This will be super helpful in trouble shooting. Love these type of videos!

[@gregeoryl]

That was part of our setup for every tube Amp that came through our shop. Set up for minimum idle current while maintaining minimum crossover distortion. They left running cool and sounding good.

[@pbaemedan]

Great video Mark, brings back electronics training in the Signal Corp. I wish those instructors had your understanding. Again thank you.

[@robnic52]

Really clear explanation, esp the distinction between DC and AC. Thanks for another lightbulb moment. Please keep doing these.

[@dennislarson9560]

This is, without a doubt, the best video on the subject to date! Congrats!

[@wadereynoldsgm]

Great explanation! Thanks for the video, Mark. I appreciate it.

[@michvod]

I bought an octal "tube saver" plug from ebay for $3, took it apart and broke the cathode connection, like you demonstrated. I mostly do fixed bias EL34 amps so these come in real handy. Also found out that I can test the cathode current on some tube TVs using 6DQ6 as the cathode pin is the same.

[@unboxedtravels]

Thank you so much, that was the best video on biasing I've ever seen !!! I finally understand 🙂

[@GORF_EMPIRE]

I found my Dynaco 35 on a junk heap.... plugged it in and it worked! Still have this 30 years later!

[@divadgnol67]

Perfect timing as I am looking to buy a prima Luna Evo 300 amplifier and know little to nothing about tube amplifiers

[@garyhinson3876]

As always, interesting and well put together, thanks, Going back to my collage days … Better Be Right Obout Your Grid Bias, or Valve Goes Wrong

[@bigmac4948]

Very informative and well presented video. Thanks!

[@jimnewman5596]

Another good educational video.

[@hugobloemers4425]

I just found this channel, watched the video and subscribed :)

[@killingstone2]

Thank you sir for an exellent video, especially for the matched tube part!

[@hondacxrider]

Great video, just the ticket for measuring in different scenarios.

[@lmt200ish]

I was thinking of getting one of the bias tools at the end to put in my tube tester as a socket saver and able to read current. Wouldn't be a permanent socket saver though I see from your video.

[@hadireg]

Awesomely explained!! Thanks sir!!

[@edwardhannigan6324]

Great Video and explanation, been stydying all you electronic amp guys and gleening load of info...Many thaks for sharing your knowledge sir, I understand bias now so much more..Ed..U.K.

[@1974UTuber]

I wonder if you look at your youtube stats.... How many people fast forward through the safety warning when you say "Please pause..." I am guilty of it. Always just excited to get to the content. Another great video

[@jdmccorful]

Good learning session,thanks.

[@bennoheritt8154]

Nice lesson, thank you

[@garysewell5465]

great teaching video

[@twincam825]

Thanks for posting learned a lot.

[@jyotindrajariwala5816]

Your video is fantastic

[@Ricobass0]

If you are going to access the inside of the amp to measure plate current often, it's probably simpler to insert a 1 Ohm resistor in series with each cathode and measure the volts across it. V=IR and if R=1, then V=I . Job done. Measuring the volts across 1 Ohm, which has one end grounded, is also very safe. (Always keep one hand in your pocket when probing HV with the other)

[@2tallB]

Yes, he mentions that method in the video.

[@robzecc]

Great video, thanks!

[@jatza07]

I Love this video..thank you Mark

[@tommyfoley]

Great job, Mark. Measuring plate dissipation video next?

[@jordanrenaud-pq7rx]

as always...excellent!!

[@EngineeringVignettes]

Thanks Mark, hope you are doing ok where you are. Cheers,

[@W1RMD]

Thanks for the great video. Perhaps, if it's not done already, make a test socket with the 1 ohm resistor inside it and disclaim to the customer to read the current in millivolts. That way only low voltage leaves the chassis.

[@KallePihlajasaari]

This is a interesting idea if it is on the cathode pin. On the plate pin both terminals would still be at 300+V above chassis but you would reduce the risk of shock between the two terminals and the chance of open circuit. It still has the problem with selecting the correct plate pins. You could even have a 1 Ohm resistor on every pin and have jumpers across the filament pins and then read the current in any terminal. Putting a ring of 2mm test sockets at the common jumper spacing around the socket could work but might not always fit into every chassis with close spaced tubes when bringing out the test leads

[@carlespy7470]

I really appreciate it when you do these instructional type videos. And I would like to see more trouble shooting videos. I learn a lot from those. Also, as a beginner, I want to know more about how to set the bias. It is my understanding that bias should be set to achieve the most linear operation of an audio tube relative to plate current versus grid voltage rather than just heat dissipation or hotness or coldness. Could you please do a video explaining how to operate a tube in the most linear region? Thanks, Carl Espy

[@pirobot668beta]

Well, from one point of view, a tube has no linear region, just a semi flat-spot in the transfer curve! In practice, negative feedback widens the flat-spot a bit while proper biasing avoids distortion at the low and high ends. The real art of tube 'balancing' is matching the slope of the linear regions, not just the bias! Two tubes may have the same B+ current with the same grid voltage, but might have very different 'slopes'.

[@carlespy7470]

@@pirobot668beta Greg - thank you for your response. I guess I'm somewhat geeky about all this. All the KZbin videos I've seen so far are arbitrary in setting the bias at some level of heat dissipation or at some level of plate current below the max in the tube manual. But it seems to me that there should be a more precise means of setting bias so that any given tube can operate in the most linear region of its characteristics (grid voltage versus plate current). I finally found a pretty good source for non-tech types like me: Department of the Army Technical Manual TM 11-662, "Basic Theory and Application of Electron Tubes", February, 1952. I am slowly working my way through it, and it speaks to my level of understanding. Hopefully, I can get some answers there. Thanks, again, Carl

[@Ngabaongta422]

Hi Mark, Change the mouse cursor to black (color) and the size to large (or larger). Definitely it will help to follow the cursor during presentation.

[@jordanrenaud-pq7rx]

Merry Christmas Mark

[@matthewf1979]

I prefer the plate to center tap method for fixed bias amps. I understand why you didn’t show it, having full B+ on your DMM leads can be deadly if you’re inexperienced.

[@pauldow1648]

🎉 thanks mark

[@gilbertocruz1683]

Thank you so much sir

[@frankgeeraerts6243]

Total cathode resistance for direct heated triodes with hum trim pot ( mostly 50-100 Ohm) = cathode bias resistance plus 1/2 of the value of the trim pot. Maximum cathode current is not at idling bias value ( without signal ), but max current on the left side of the load line when the signal has max negative input on the grid ! 60 mA on a EL84/6BQ5 idling is killing the tube under use....... Nevertheless a useful and good video. Thanks

[@Dave_____]

What about the small current (0.5 - 1.25 mA) flowing through the screen grid? Only measuring the current through the cathode resistor makes the plate dissipation reading a bit low.

[@timburle4980]

I’m curious why you didn’t mention plate dissipation when calculating the bias?

[@91octa]

Hi Mark, really like your videos. I have built the 807 SE following your video series anda have to say i sound great and really enjoyed building it. Sorry to ask but could you make a vid showing how to connect a amp meter to follow the amp plate current? Many thanks Regards Octavio

[@anthonysilva5312]

Well explained! Many manufacturers say that cathode bias guitar amps are “self biasing” and therefore do not require rebias when replacing power tubes. Is this accurate?

[@dastave]

love this video! subscribe! I'm thinking about making a single ended triode amp (je lab's simple 2a3/45 schematic). if i wanted to have a built-in meter to a display plate current, how would i go about doing that?

[@TheTrueVoiceOfReason]

Brad the Guitologist explains those very shortcomings in his video. But if you are concentrating on a specific tube or tube family, then these make sense for certain applications.

[@NINEWALKING]

I do not care much about mA numbers when biassing tubes. I am giving the plate dissipation the priority. Sure I obey maximum DC plate current rules as well. So, when biassing I am measuring both plate voltage and current at the same time and then I set it up at 60 to 70 percent of the maximum plate dissipation for the given tube in fixed bias circuits. Sometimes plate voltage in the old amps is way higher than what it was designed for due to the higher mains voltage. Then checking just current numbers might result in way to hot biassing. In such cases your plate current stays lower than typical but the high plate voltage makes it reach desired plate dissipation percentage. Most of my stuff are guitar amplifiers and sure there are triode pentode or triode quatrode switches in some amplifiers but they are not there for quality of sound. They are used to lower the power levels. So we use EL84 in pentode mode. Tube grids are voltage regulated and indeed grid leaks and such are not influencing biassing. On the other hand screen currents are few mili Amperes. This also goes true the cathode to the ground as well. Hence cathode current is plate current plus screen current. Hence to calculate exactly the plate dissipation we need to deduct those few mA of screen current from the measured cathode current. Then we have correct measurement of the plate current. Multiplied with the plate voltage and we have the actual plate power dissipation. Small asymmetry in push pull guitar amplifiers is ok. Guitar amplifiers do not mind a little distortion anyways. Sometimes people even disconnect NFB loop to make it overdrive more.

[@Dennis-mq6or]

6BQ5 is a Pentode, not a triode! Screen current flows through the cathode resistor along with the plate current. Your Ohms law examples were good because you showed actual circuit applications.

[@saurus738]

Very helpful! I notice an error at 13:00, it was correct at 57mA or equal to 0.057A.

[@tjminasi1442]

I measure the quality of information by the measure of careful detail in the info given. This was not careful.

[@alejandropino1115]

Really love your videos, I have watched his one 100s of times. I do have one question not covered in this video. If the current is too high or too low, do you just change the resistor value? Is there a way to control the current with a potentiometer? You showed how to measure it, but how do I control it or change it? Thank you in advance.

[@stillastillsfan]

I have the same question

[@KallePihlajasaari]

Pretty much. In most cases a good tube in a circuit with correct supply voltage and within tolerance resistor the value should be within working limits already. If you are balancing channels or a push-pull arrangement you would trim one resistor. You can easily reduce the value of a resistor by placing a resistor in parallel. To reduce the resistance by 10% you add a resistor of about 9x the existing measured value. You can do this by adding 8 x the resistance and then use a 2x resistance trim pot to add 8-10x the resistance and this will reduce the existing value by about 9 to 11%. Investigate the changes using the parallel resistance calculation. Adding the extra resistance is often more practical than changing the cathode resistor because you only have 1/10 of the current flowing in the parallel resistor so the required power rating is one tenth and you do not need to have a big box of close spaced values in multiwatt resistors. In some designs there is a balancing potentiometer between the legs of a push-pull stage but this is not good for power amplifiers as the full plate current flows through the wiper of the potentiometer and having it float even when adjusting the amp is not ideal. Adding a small variable resistor to the parallel resistor in the trim design will only cause a 10% deviation if the wiper looses contact during operation or adjustment. If you need to increase the resistance you can disconnect one end of the resistor and insert a small series resistor. If you add a value 1/10 of the original it will still need to handle 1/10 of the power dissipation so use a 0.5W resistor in series with a 10x bigger 5W resistor.

[@michvod]

I am actually interested which is the bias point that the amps should be biased to? I once read 68% of the rated output power. So if one has a 2 tube 50W amp, it should bias it to 34W so that the current would be around 43mA per tube if the B+ is 400V in this case (85mA through both tubes).

[@samuel_towle]

Why would the tech introduce a short across the filament of the 300B along with the 1 ohm resistor @13:24 ? Is that for job security?

[@Noah-gq7pq]

what about the cap in parallel with that cathode resister,wont that effect the reading

[@KallePihlajasaari]

Great video and background info. In your example with the added 1Ohm sense resistor you inadvertently simplify a bit too much and may mislead someone. I wonder if you can insert a note that the filament does not get shorted when you add the resistor. I expect you simplified the filament hum reduction potentiometer into a short circuit but that I believe is a connection too far.

[@OIE82]

I was just going to point that out also @13.45. BTW, for clarity, it is a 1 ohm resistor that is used.

[@walterwoodworth6078]

Do you take in work?I have some tine amplifier (RF)

[@MrMannakin]

Awesome - I've wanted to check the tube bias on my KT88 for a while. Is there any easy way to check whether the tubes are still "balanced"?

[@KallePihlajasaari]

Compare the plate currents at DC and then the same again at AC (not part of this video).

[@lupoal4113]

Hi, what about SRPP configuration? I mean when the upper tube cathode is connected to the lower tube anode? where/how to measure the current? thks in advance

[@moodyga40]

fixed bias fit a 1 ohm resistor in line with the cathode 1 volt through 1 ohm = 1 amp

[@eetechtom]

You meant = 1A, yes?

[@jedi-mic]

Have you seen the Euro tube probe one bias probe has a LCD screen for voltage amps that's a good bit of kit better than these ones

[@scottpeters4401]

I have one of these Bias Meter probe/switcher devices….Mine as a 1ohm resistor built into the cable,so I assume that’s the same as adding the resistor inside the amp?

[@silasfatchett7380]

Yes.

[@jjohnson67sg]

Mark, how do the voltages get so large like 425v and 300v and so on. where do they get stepped up. is that the output transformer doing that. I'm confused, sorry. I know its off topic, just curious.

[@silasfatchett7380]

The mod at (13:24)to measure cathode current in a 300B is incorrect. As drawn, it is shorting out the 5V filament winding of the power transformer. The 1 Ohm resistor should be connected from pin4 to ground, with no connection between pins 4 and 1.

[@chromaticmusic]

So I missed something, how do you actually correct the bias once you have the numbers?

[@richarddegannes2928]

Hello, Can you advise if PrimaLuna Evo 400 Tube Integrated amplifier and Evo 100 Dac are any good? I seen a gentleman on KZbin at Upscale Audio talking about them, but I am not sure what to believe as he sounds like a salesman, hence asking someone like yourself. Thanks.

[@nonsuch]

EL34s (8ET) and 7591s (8KQ) are 8 pin sockets. I'm assuming you meant EL84 when you said you need to buy a 9 pin socket around 18:18

[@kwacz]

I have 7868EH tubes and I cant figure out what the maximum plate dissipation wattage is supposed to be. I dont see it in the datasheet. Measuring from pin 3 to ground I am getting 0.054 MA and 340 volts on the plate. that equates to 18.36 watts. What is it supposed to be?

[@kwacz]

I think I found it. Since I measured cathode current on pin 3 to ground according to the datasheet maximum cathode current is 90ma so i am at 70% which I assume is where I want to be for push pull amp with two tubes.

[@jyotindrajariwala5816]

Old valve redio wich out put that start

[@waynegram8907]

Blueglow Electronics, when a push pull amp has one cathode resistor, how do you know that both power tubes are equal drawing the same amount of current? is there any test you can do to check if both tubes are drawing the same amount of current to be matched?

[@silasfatchett7380]

You can measure first the DC resistance and then the voltage drop between the OPT primary center tap and each tube plate. You can then use I = V /R to find the current flowing in each tube..

[@Mrguitarcraze]

Can you take measurements between the cathode resister by connecting between the cathode lug and the chassis? My Marshall astoria’s resistor is buried somewhere under the board. Thanks

[@silasfatchett7380]

Yes.

[@Mrguitarcraze]

@@silasfatchett7380 Thanks!

[@armoth6958]

How much I wish you could help me with my Cayin amp... one of the tubes biase set screws won’t move the meter, brand new amp 😞

[@frankgeeraerts6243]

Measure the negative voltage on the grid of the tube.... then turn or adjust trim pot and read if the voltage changes....( compare the reading with readings of the other negative ) voltages on the grids of the other tubes ).maybe also possible that the socket has problem and doesn't connect correctly to the tube pin(s) or to the negative bias circuit.....in that case the tube will normally red plate and self destruct.....Check the output tube(s) also. .

[@mrGoesto11]

How do you identify the primary side of the OT? Is it a certain color wire? Is it attached to the same spot in every amp?

[@silasfatchett7380]

In a single ended amp, one wire is connected to B+ (the positive terminal of the reservoir/first filter capacitor), and the other wire to the plate pin of the power tube.

[@natetolbert3671]

you said it yourself. If push pull was like you showed last video, with only 1 tube running on each half-phase, you wouldn't need to match tubes. The reason that they need to match is because They are both amplifying all the time. Instead of being off while the other tube pushes, The tube that is in the negative portion is on, but the negative signal is pulling from the other end.Therefore, with matched tubes, one pulls exactly what the other pulls. Otherwise you end up with low efficiency and distortion that is anything but harmonic.

[@stillastillsfan]

If you have slightly unmatched tubes in one channel, with one drawing a bit more current than the other, is there any way to carefully bias besides replacing with perfectly matched tubes? I think it is just a few mA difference.

[@DennisSantos]

But in a cathode biased set, you're measuring the resistance of the cathode resistor only (and its voltage drop) and calculating the current - what about the DC resistance of the OPT? In my el34 stage Rk is 500R with a 22V drop, allegedly giving me 0.044A. But if i take into account the OPT's DC resistance of 208R with it's 12V drop that gives me so far a total resistance of 708R? What about the tubes' own resistance? Also, my B+ is 339V, minus the 12V drop of the OPT and 22V of Rk leaves me with 305V left over (presumably the voltage drop of the tube)? But (22v/500R=0.044A) and (12v/208R=0.058A) and (305v/?=?) Where's the common series current? Damn! I'm so confused!

[@silasfatchett7380]

The OPT primary, the tube and the cathode resistor are connected in series so the same current must be flowing in each of them. In other words, the current flowing in the cathode resistor IS the current flowing in the tube.

[@willburrito9710]

Don’t mean to be picky but the last example of 66.4V / 771 ohms equals 76ma, not 86ma. Is my calculator busted?

[@walterwoodworth6078]

Tube

[@vaccarioou22]

0.57 A is 570 mA !

[@frankgeeraerts6243]

Electrons flow from cathode to the plate.........the convention of direction of a current flow is inverse to what happens in reality.

[@Blueglow]

For sure

[@PeterMilanovski]

One thing that I don't understand is that, how is it possible to run a vacuum tube off low voltage? I was getting really board with this Corona thing giving me way to much time to be on KZbin to the point where I have literally watched everything from everyone I follow. I found myself watching a video by Dave over at EEVBlog about one of those cheap Chinese vacuum tube pre amp things, this video was a follow up to an earlier video and he decided to take a closer look at the circuit to see if the tubes actually did anything. In his drawing of the circuit, I noticed that the tubes were being powered by plus minus 12v from a 6v plug pack. Now I have one similar except that mine came with a clear acrylic case and needed a 12ac plug pack to run. I had never questioned how it worked until I saw the video, as far as my understanding was regarding tubes, they needed hundreds of volts to operate and yet here was a tube operating on plus 12v and minus 12v, but how? And if that's possible, at what voltage can I run an output tube? Generally speaking, getting my hands on a high voltage transformer other than a microwave oven transformer in Australia is impossible at a reasonable price, it's almost like Australia doesn't want me to have anything to do with vacuum tubes and is forcing me to stick with solid state. But 24v PP is definitely achievable, hell even 48v PP I can do. But 300v or greater, forget it. My little cheap Chinese vacuum tube pre amp might not be as good as the real thing but to me! It sounds ok and it has so much gain that I can connect the headphone output of my phone to it and then to my cassette deck to record tapes with. Android headphone output suck for this sort of thing but this tube pre amp works wonders. Now if I can only understand what's the lowest possible voltage that tubes can operate at, maybe I can build a point to point wired tube amp, in Australia lol..

[@Blueglow]

very few tubes were designed to work off low voltage, almost none. it is possible to use low voltage (12V) to feed a power supply that raises those voltages up to vacuum tube levels, but that's really more about the power supply than the tubes operating at a low voltage. In general, vacuum tubes are high voltage devices. Thus why solid state became so popular, among a few other dozen reasons.

[@PeterMilanovski]

@@Blueglow yeah but from what Dave over at EEVBlog drew on the schematic, it showed plus 12v at the plate and negative 12v at the annode.. it really has me stumped because as I said earlier, I have only ever known that tubes need hundreds of volts to operate but this seems to contradict my understanding... There's plenty of these on eBay and they are not expensive... Could it be possible that if you have the correct heater voltage, you should be able to operate the tube at a lower voltage? If you can find the video on Dave's channel, it should be the second video on the same tube pre amp, it's one of those little aluminium case with the tubes sticking out of the top that are led backlit... Have a look at the schematic that he draws up on the whiteboard, he even marked the voltages but he somehow missed how it's able to work with the low voltage... I fully understand that tubes work on high voltage and low current and solid state works with low voltage and high current... But this is something else, now I'm tempted to purchase some tubes and sockets so that I can experiment with lower voltages... No one has made a video about this! Am I the only one who has picked up on this? Apart from the Chinese whom designed the circuit?....

[@PeterMilanovski]

@@garyswift135 funny you should say that, it's the next morning here for me and I'm still thinking about how no one seems to know this, I was just thinking about those korg tubes and decided to jump back on and try to explain myself better and then I see your post. No it's not one of those korg tubes and it's not a vacuum fluorescent display being used as a tube either... What I will do though, I will get the link to the video and also a link to something similar on eBay..

[@PeterMilanovski]

kzbin.info/www/bejne/maC2pWh-jLeprJY That's the link to Dave's video, it's from 4 years ago, man... He has made a lot of videos lol... Check out the schematic diagram that he draws up and check out the voltages at the tube! I don't think that it's a mistake on his part as I'm sure that he put the meter across it to get those figures, somehow he missed the fact that tubes usually run on hundreds of volts, not 24v peak to peak... He even showed the datasheet for the tube and I was able to see that the tubes maximum voltage rating was much higher than 24v... So what gives?

[@PeterMilanovski]

rover.ebay.com/rover/0/0/0?mpre=https%3A%2F%2Fwww.ebay.com%2Fulk%2Fitm%2F401765923735 Here's a link to the one that I have from eBay, there's voltages marked on the back of the PCB but they aren't the hundreds of volts that we normally see, this one uses the 6j1 tubes and the whole thing runs on a 12ac plug pack. Generally speaking, when you see inductors, you know that they are for the power supply and you know that it's either stepping up the voltage or stepping it down but I don't see any inductors! So is there black magic going on or is there something else that we don't know about? Have a look see, I would love to know what someone else thinks!

[@erin19030]

It's much easier to insert an ammeter in series with the plate and B plus

[@Blueglow]

And so much more deadly :-) That's typically not easy without unsoldering something unless you buy the socket adapters I mentioned.

[@hiseminencetheholymacdiarmada]

V = Voltage ✅ R = Resistance ✅ I = Current/amperage 🚫 wtf???

[@anrque]

:) I had to look it up. From Wikipedia: "The conventional symbol for current is I, which originates from the French phrase intensité du courant, (current intensity). Current intensity is often referred to simply as current." So, today I learned Amps is "I" 'cuz France.

[@erin19030]

Why are you showing current Flow from positive to negative? Electrons flow from negative to positive. In the study of electronics , current flow is electron flow, negative to positive. I know old Ben Franklin stated the opposite. He was mistaken and it did become accepted theory until modern day electronics.

[@shanebatherson9197]

Conventional current is often used in describing circuits because people find it easier to picture it in their heads

[@Blueglow]

We all know this, not worth debating. conventional vs. theory debate is decades old

[@silasfatchett7380]

Neither electron flow nor conventional current is correct. Each is a simple ‘model’ which allows those of us without a degree in quantum physics to see or predict what will happen when components are connected in a circuit. According the Merlin Blencowe (the Valve Wizard) who holds a Masters’ degree in electronic engineering: ‘current has no direction, only sign’.

[@mefirst4266]

i beg to pardon you at time 9:32 that capacitor keeps the voltage and current steady across it.