Let’s Build a Low Voltage Vacuum Tube Audio Amplifier 

Thank you so much! I think high voltages usually associated with tubes scare a lot of people away from them, but they actually work surprisingly well even at low voltages. We actually expand this audio setup a bit more in the next episode. Also, one of these days, I would like to try to build a proper OTL amplifier, although that one will probably require a genuine HV power supply.

@@UsagiElectric i agree the plates should only need about 36 volts to charge them

nah the easiest way to conceptualize these caps for novices is that they get dc biased u have to mention that the signal after the input is actually an ac signal with dc offset and the capacitor when exposed to this signal slowly gets charged to a voltage because this signal is centered around a dc voltage and not 0 they get charged to the dc voltage, therefore only the ac passes since the ac is swinging around that dc bias, and the capacitor cant react/charge instantly, therefore passing the ac this is basically a high pass filter basically imagine the capacitor gets charged to an average value of the signal it gets (both sides), say if the capacitor has 5v charged because ur signal is centered around 5v, the resistor after capacitor has a voltage of 0 or the divider voltage, imagine the resistor is connected to ground therefore 0v when capacitor is charged, now the signal is 6v, the capacitor cant charge instantly, but since the capacitor already has 5v across it, a 6v on the capacitor positive terminal means 1v on the other terminal, the signal is forcing 6v on the positive terminal because it is a voltage source, since the capacitor has 5v, the other terminal must be 1v, and this 1v is also on the resistor since the resistor is connected after the capacitor, this is how the changing signal gets passed, now novices will ask, whos to say the capacitor wont charge to 6v and wont pass the signal, that topic is covered within filters and frequency response, in the same way, if the signal suddenly becomes lower than 5v, the resistor actually has a negative voltage, which many novices dont understand how u can get a negative signal from dc voltage, this is because capacitor discharges the other way through ground, and this is also why the voltage signal needs to be push pull, say if u have an emitter follower without a resistor to ground, u could measure a changing voltage, but a connected capacitor wouldnt discharge because when the signal becomes lower, there is no pathway to ground to discharge if u understand these, u can further understand dc bias in capacitors, diodes, varactors, extending into rf circuits saying they block dc but allow ac is like memorization, u cant conceptualize and u cant understand when i was a novice, i sure didnt get or conceptualize wtf u mean block dc but allow ac, i mean i heard wat u tryna say(but i would just be a yes man, it be like "it is wat it is" for me), and theory also says so, but there was no conceptualization but what i mentioned is the actual conceptualization now if u learnt about superposition in ac circuit, then u can also calculate it and model it, u can both see how it passes the ac and see the frequency response/transfer function in a simple rc circuit when u a novice, u also dont understand why everyone says only ac make sound when clearly dc also makes sound because their definition is ill defined, though dc is defined as current in one direction, ac as current in both direction as long as a signal is changing, no matter ac or dc, it will make sound, might be better to call it an ac signal with dc offset, in circuit model, u would put dc source series with ac source and another thing novices dont get is how a transistor is able to work with ac when it is a dc device, the reason is also the capacitor, u cant just apply a negative signal, but if u apply that negative signal to capacitor first which has a dc bias, u can offset that signal into the positive as long as its amplitude isnt bigger than the dc bias, thus the transistor is actually still working with dc (formal definition)

@@urnoob5528 You dive into the fine details, while correct, novices will just stop listening because it is much too much information at once. A novice starts with basics, not fine details. If novices dont get it, its because the explanation is too detailed.

Thank you👍

Some pedal manufacturers, like BOSS, use electrolytic caps for interstage coupling, due to cost. A 1 mfd electrolytic is about ten cents, where a nice polyester or mylar cap is about two dollars. With the electrolytic, it only conducts one direction. So, you receive only half the sine wave at your signal out. I can always tell a BOSS pedal because they sound like BOSS pedals. MANY manufacturers do this, so, check your pedals and replace any interstage electrolytic with a nice polyester of same rating. Replace ONLY electrolytics that are INTERSTAGE not electrolytics used for power or other functions. Make sure you use a schematic. If unsure, LEAVE IT ALONE.

Thank you so much! That's awesome that you got to learn and work on some proper tube equipment. I can't imagine the amount of power needed to drive a 4.5" turret, the bank of tubes must have been truly a sight (and a great heater in the winter)!

@@UsagiElectric In general, the motors in heavy duty drive trains such as gun turrets use DC torque motors, which have their current controlled by DC motor generators (MG's). The output of the motor generator provides the current to one of the torque motor windings. The input to the motor generator controls its output current. At this point the MG input could be produced by vacuum tubes or solid state circuits in more recent equipment. Using the output power of a large group of tubes to provide the current through the torque motor winding is possible, but not very practical.

Thank you so much! That's awesome to hear that you've gotten back into tube experiments! I absolutely love those tubes that have that mesh on the inside, like the 6AJ8. I have a few 6EJ7 pentodes that look like that and they're just such cool looking tubes. I would someday like to build a proper high voltage amplifier that can put out some serious sound, but I've actually gotten really interested in OTL amplifiers. I have something like 150 6CB6 pentodes that are decently linear when triode strapped, so something like a 10 or 20 tubes output stage using those little guys might be pretty interesting and give some wild sound!

​@@UsagiElectric I love my 6BX6 for you can see the cathode glow, but man, those 6CB6 are on a whole new level ! Can't wait to see an OTL with these

yep those valves give useful output at 100v ish ht or even less, theres a variant of it called 30PL14/PCL88 which may give more current and output for a given ht voltage, and is identical pinout to a PCL82 and near enough same heater voltage 15, PCL82 is 16 .. another alternative is a PCL85/805 , normally a TV frame output valve

There's something like strontium or thorium or whatever that they put into the cathode, to moderate emission because it's too efficient. Which also makes it into a wear out mechanism for planned obsolescence. The state of the art was advancing way ahead until rug suddenly pulled out from under them with new solid state.

Thank you! Nothing like a little CAD (Cardboard Aided Design) to really round out a project, haha.

It would be fun to build a high voltage tube amp with a variac as the volume control! Want more volume, literally crank up the power!

Thank you! And thank you for the information the capacitors. I really just tested a bunch of different values in a trial and error way until I found a sound I liked. There was very little actual science done here, I just threw parts at a board until it sounded good, haha.

Yep. That's exactly what the cap and resistors do. Block any D.C. from adding an offset to the next stage and then it also create an R.C. time constant that affects the bandwidth of the incoming audio signal. The output impedance of the previous stage, AND the input impedance of the next stage that it is feeding, MUST be accounted for in order to come up with the correct value cap and resistor that will end up doing it's job correctly and also pass the correct bandwidth of audio through to the next stage. The math can be rather complicated in order to get this correct. Get the values wrong, and you end up with really crappy sounding audio passing through, and you can really end up wasting your gain also.

​@@UsagiElectricAh, experimentation.... Yep, that's what helps a person learn what does what, as long as you learn why. 👍

Thanks for checking the videos out! I have some more low voltage audio ideas I definitely want to try out in the future too, so more to come!

You won't really get the tube running in its "linear" region as you put it with such low voltage. Cathode bypass caps really won't make much difference at such low voltage, with such a small output transformer

​@@EsotericArctosRight, the tubes are basically running in "space charge" mode here

Tube front-ends in front of a solid state output stage is the ultimate setup. No distortion from tubes that are driving a high impedance load such as a final output stage. Sounds incredibly good

If you put enough of them together, you can use them to heat the house too!

Magnetrons..... 🧲

❤ 12AX7, 6V6GT, I, I, Sob, I, ❤ you!. Cit, print, that's a wrap. What a story for a prime time movie of the.... Cut, print, that's a wrap. What a story of a story made into a movie that I talk about in my new book.... Cut, print, that's a wrap of rehearsals for today? of our new broadway play about a story about a story made into a movie that a book author describes in his new book.

@@waynemasters8673 No! No! No! He was NOT referring to those types of tubes. He was talking about magnetrons !

U can open teleport portals with them too.. use 5 and align them and confine the electron

Back in the 50s, portable radios had "B" batteries. It was a 90v battery that looked like a long 9v battery. It fell out of popularity because the batteries were carbon-zinc (non-rechargeable) and the radios were very inefficient because of the high power usage for filaments.

I hadn't ever heard of the 28D7, but that's awesome! Even in the datasheet for it they list a plate voltage of 28V, it was designed from the ground up to be a low-voltage pentode, which is super cool. I may have to hunt one down some day and build something out of it. It seems a lot of the old battery operated radios used between 30V and 60V and on the plates as that was what the batteries could put out. I actually have an old Airmaster battery powered radio hanging around that I've been meaning to restore, although it uses a 1A5GT, 1H5GT and 1A7GT (and one more mystery tube I can't place, but I think is a 1N5GT). There's a lot of really awesome old tubes out there and it's kind of mind-boggling to think of all the differences between them!

@@UsagiElectric Believe it or not, "B" batteries (high voltage) did exist back in the day. 22.5V, 45V, 67.5V, 90V, and even 120V did exist back then. I think you can still buy some B batteries today. A company called Exell battery makes 22.5V, 45V, 225V, and 300V batteries today. Some tools use 60 volt lithium ion batteries that are rechargeable today. I think the biggest maker of high current B batteries today is Tesla. 375 volt batteries are used in electric cars. Porsche makes a battery that is over 800 volts today for its electric cars.

The 35L6 and others in this catergory are NOT designed for low voltage... the heater is at 35 volts because it is designed to have its heater in series with several other tubes and then have them all add up to 110 volts and then have the heaters directly connected to the mains in order to avoid the need for a seperate filiment supply transformer. The plates in those tubes require 200 volts, just like any other tube. Just because you see a 35 volt heater in a tube, does NOT mean that the rest of it runs at 35 volts.

@@davelowets if you look some of the 32 volt radios that ran from wind chargers did use 35L6 tubes in push-pull parallel. While not specifically designed for this use, there were radio models that did use them in their 32 volt models.

EF97 by Mullard, Phillips etc. range 6 to 27V on anode with current 2.5mA used in car radios

It's so tiny! It does put out a decent sound though, despite how small it actually is!

Haha, that's like when I tell people I know how to program, what I really mean is I know how to copy and paste from stack exchange!

I would love to build an Output Transformer Less (OTL) amplifier one of these days, but doing so with vacuum tubes is notoriously difficult! Tubes are really supposed to be high voltage low current devices, but I'm using them as low voltage even lower current devices, which really puts me at a disadvantage. Even with higher voltages, OTL amplifiers tend to use six to eight tubes as push-pull driver tubes to generate enough current to drive a speaker. It's super difficult, but in my opinion at least, one of the coolest style of tube amplifiers out there! Now, if I could get ahold of some high impedance speakers, that'd make life a whole lot easier... hmmm, time to trawl eBay some more!

Piezoelectric speakers scream with modest voltage. 😮

Someone should invent a PNP tube that uses positronic emission, for a complementary symettry pair

Ooh, that's actually really smart, I never really thought much about what the phone is detecting!

I solder component lead clippings onto components that i want to stick into a breadboard.

That's actually really true! Though, I have very little experience with FETS, haha.

I always had a hard time with wrapping my head around capacitors letting AC through. There’s no direct connection inside a capacitor and we’re not inducing any voltage with magnetic fields like in a transformer, so how can AC pass through? What ended up clicking in my brain was that AC isn’t passing through so much as the capacitor is just charging and discharging in sync with the frequency of the AC. When the AC is on the positive phase, it charges up the capacitor, making the other leg of the cap act like a ground pin. When the AC is on the negative wave, the capacitor discharges, making the other leg of the cap act like the positive pin. (At least, that’s how I’m seeing it in my brain, that’s probably not the best way to describe it though).

@@UsagiElectric And isn't the electricity flowing one way on the charging and then the opposite way on the discharging? So that electricity never actually flows through a capacitor?

@@UsagiElectric in the process of trying to build a low voltage tube amp for a pair of old 1920s head set I’ve got, and remembered this video, hoping it will work, any how seen the comment about capacitors and I figured I’d add some more info the Charge is being stored within the dielectric of the capacitor, Benjamin Franklin explained this when the layden jar was created, and somehow it changed to the metal plates!

@@UsagiElectric I've always just accepted DC blocking caps without properly understanding until you explained it.

@@edgeeffect There's also AC-blocking capasitors, called capasitor dropper. Take mains voltage it is 120 ACv 50Hz or 230-240 ACV 60Hz. When the caparcitor charges on the rising voltage it passes current trough, but when it is fully charged the current stops to flow. So based on the capacitor value and the current flow on the load it drops the voltage as it only lets part of the wave in each "hump" through.

Thank you! I was pleasantly surprised at the quality of sound it could produce with such a low part count and such low voltage!

Thank you for checking the video out! The high voltages of tubes are definitely the scariest part of them, but you can still build quite a lot of really cool stuff at lower voltages that are still safe. I'm using in total, 36V here, but you can even push that up to 60V-ish and still be pretty safe!

@@UsagiElectric I think the voltage multiplier would be the key to get B+ voltages. You can use 24vac and multiply from there. I want to try building an amplifier using just ac power to start from. The output transformer will be the challenge, someone told me to try a Pa transformer probably the 70 volt type. You can only use it on single end audio amplifiers won't work on push pull amplifiers. 73

​@@ronb6182A voltage multiplier wouldn't have enough current to power a tube circuit. Today, one can purchase an adjustable boost converter, that will provide 100+ volts out of 12-24 volts, online for a few dollars. Sure, they have some high-frequency noise to them, but they're fine for playing with tube circuits. The best thing about those boost converters is that they're isolated from the mains. No issue with accidentally becoming a ground and a fatal shock from the mains. As far as expensive output iron goes, a car ignition coil is cheap, and works well in many lower power tube amp circuits. So, for less than $30, you could have an isolated adjustable high voltage power supply that runs off 12-24 volts, and a large output transformer.... Now, order it up, and get to experimenting. 👍

Thank you! This video has really picked up some steam lately!

Thank you! Yeah, the majority of this was built with random scraps from other projects I had laying around. I just wanted to see if I could get a decent sound out of random junk, and I was pretty happy with the results!

He could just series two of those transformers together, use the common input terminals as the center tap, and then parallel the outputs....... 😂 Imagine the math to try and figure THAT one out... 😳

That is some excellent information, thank you for letting me know! The resistor and capacitor values I ended up with here were actually just from about a week straight of trying different values until I found a combination that gave a sound that I liked and that didn't clip. Having said that, there's definitely a better way to go about building this, and I definitely think you're on point here!

@@UsagiElectric No problem, I am always glad to share knowledge with others about stuff like this. In fact when running tubes on very low voltage it may even be necessary to bias the grid positive to even increase the flow more simply by making a voltage divider from B+ and grid 1 ground preferably with a pot so that you can tweak the voltage to be just right for best sound. If this method is used and working you can omit the cathode resistor and bypass capacitor since you won’t need them anymore to bias the tube. The good thing to do is to measure the plate voltage or even better use an oscilloscope to check which part of the signal that starts to clip first. Then you know where to bias the tube to gain most signal before clipping :)

Thank you very much! Let me know how your project goes!

Yup, those are the coupling capacitors!

Thank you very much!

Thank you very much! For this one, I used a 12BH7 dual triode for the amplifier and a 12B4A single triode for the driver tube. They're actually not specifically low voltage tubes, I'm just running them at low voltages, which means almost any tube can work! All it takes is some tweaking of resistor/capacitor values and proper expectations that it won't be all that loud of an amplifier.

@@UsagiElectric Thanks for the information.

I always had such a hard time understanding when people would say caps block DC and let AC pass, and you can have both on a circuit. I think it's more of a terminology problem for me though. Fundamentally I understand exactly what's happening in terms of signals, just the wording always tripped me up. For my brain, it helped to think about the output on the plate being an oscillating waveform that moves from +10V to +20V for example. The capacitor shifts that signal to -5V to +5V. Still a 10V P-P signal, it's just shifted the center point to be around ground. It does this because as the voltage at the plate increases, it charges the capacitor. And then, as the voltage on the plate decreases, the capacitor feeds that voltage back (because it's at a higher potential). The result is that the other side of the coupling cap changes potential in accordance with the charging and discharging of it, which replicates the signal but eliminates the DC bias. That's probably not the best way to explain it, but thinking about it in that way is what made it click in my head!

Thank you! I've actually got an entire series dedicated to building a processor using the 6AU6 at just 24V (with -12V for grid biasing). The 6AU6 is a brilliant little tube and works surprisingly well at low voltages! I actually have no idea where the output transformers are from. I ordered them years ago on eBay and they were listed as just "Audio output transformers". I didn't look closely enough at the pictures because they were about 1/4 the size I thought they were going to be!

Thank you so much! That's awesome! You're essentially building low voltage tube Eurorack components, that's something I've been wanting to do for a while now. Do you have some links to your projects, I'd love to see them! The transformer is essentially to convert voltage levels. So, if we remember P=VI, our output power from the tube and the power going into the speaker are the same, but the tube is very low amperage and (relatively) very high voltage. However, the speaker wants very high amperage and very low voltage. Power stays the same, we're just dropping the voltage level to very low and bumping the amperage up to very high. So, in this version at 36V I think we get around 0.6W of output power (maybe, I could be totally off in calculating that), but on the tube side, that's actually 36V and 18mA. The transformer changes that to 2.2V and 285mA for the speaker. Still the same 0.6W of power, just at a different voltage with different amperage behind it!

@@UsagiElectric ah, that could be (one of) my missing link; it’s less eurorack, more art project at the moment - Convert this box of tubes into a little city of sound to install in a dead factory where they were originally made... looks like that old telephone handset I tore up to get my first capacitors will be getting a revisit, as I recall one in there somewhere - might need a hundred more though :) Thanks 🙏 ps I am sampling the heck out of them as I build/rebuild, not very scientific but learning a lot

@@UsagiElectric btw, I forgot to send you a link, so here's two :) soundcloud.com/christian-krupa-1/a-short-improvisation-on-dual soundcloud.com/christian-krupa-1/inherent-tension they're both being amplified through an iRig interface into my ipad, and processed / enveloped in AUM with other auv3 effects - I found my wee transformer but haven't yet figured out how to get it to drive a speaker directly though...

Hiya! Thank you! You're definitely on the right track to where I went in my next episode. More voltage means more power, so even a little more voltage above 24V makes a big difference. There's also a few other things playing against the overall sound output. The speaker and output transformer being the biggest bottlenecks in this one. Having said that, I do manage to get some decent sound out of it in the next episode, so definitely stick around for that! I have a few triode pentode combo tubes, so I may have to try tinkering with those for future audio projects! I don't have any ECL86s, but I do have quite a few 6GH8s, which are pretty decent little combo tubes, and the pentode can be triode strapped for even more linear response. I would love to build a proper high voltage amplifier someday!

Yup! But the main purpose of this was to see if I could build a tube amplifier with a super common low voltage. Granted, with limited voltage, we have very limited power, but it still produced a surprising amount of sound for such low voltage! This is definitely not the best way to build an amplifier, but it's certainly an approachable way for anyone wanting to get into tubes without being quite ready for higher voltages. You're definitely right on the output transformer, a properly sized one would probably make a much better sound!

@@UsagiElectric yeah I know there are a lot of fake tube headphone amplifiers out there . I bought one and it had a solid state amplifier for the output. Low voltage plate current can be used for headphones and with good sound. It's a shame they don't make high impedence headphones. I did not.comment.to take away from your idea of low plate current but to tell people we are not limited to 12 Volts or 24 volts. I just threw another option out there for higher.voltsges using only 24 vac. Power transformers are hard to find at a low price. I commented on another page how to use two transformers for isolation. There are many ways around using what you already have. 73

You're not being a negative nancy, I'm a total noob at audio stuff and any and all feedback is welcome! As for the negative feedback, I did test the circuit off camera both with the feedback and without, and surprisingly it actually hurt overall volume with the feedback hooked up. But, without it, I didn't notice any unwanted oscillations or sounds, so it could be that at such low voltages, the feedback isn't quite as necessary. As for being good practice to remove power before modifying the circuit, as you said this is a really low voltage circuit, so in this instance, there wasn't really any risk to me. At higher voltages though, well, I would certainly hope that no one is building a high voltage tube amp on a breadboard, haha.

Negative feedback ALSO steals gain.... He didn't have much gain to begin with, and certainly not enough to end up with oscillation issues.... 🤷 There are PLENTY of amplifiers out there that do not use a negative feedback scheme. Negative feedback IS a double edged sword, and is not always a welcome, good thing. A high gain, high power amp would definitely need it, but not something like like this.

​@@UsagiElectricMany people build tube amps with tag board or point to point "dead bug" style. I've even seen it done on a bread board... lol. BUT safety has to be of the utmost importance with high voltage tube circuits, and most veterans understand this. Im sure you understand this as well.

@@davelowets every possible amplifier circuit has been built and documented. It's already known what will work and what will not and the reasons. It's called circuit design and people have done it already. There's math proving these concepts. Poorly designed amplifier circuits no matter the gain will oscillate which was the point of my comment which I stand by. Sure anyone can throw parts together and sometimes it works sometimes it doesn't. That's *poor* circuit design! Experienced/knowledgeable circuit designers know these things well already

Thanks! Not Japanese, but I did indeed live in Japan for quite a few years though! And up until this year, I would go back about three to four times a year for work, so it kind of feels like my home away from home.

Thank you! I definitely think you're right! Those output transformers are a little too small to give proper good sound, but I have a handful of them, and I really wanted to use them on something, haha.

@@UsagiElectric Normally they have lower impedance , in my old book there is ST-32 which is 1K : 8 ohm they use for 9V battery , i did clone it with ferrite core in my youtube video. if u can find a small scarp transformer maybe u can wind urself..I have several 16K to 8 ohm and 32K to 8 ohm use for my vacuum tube experiment..

@@6p1p Just checked out your channel, you have a ton of awesome videos on CRTs! I'm going to have to check those out as soon as I get some free time to binge some RU-vid! The output transformer is one of the most difficult parts of a building a tube amplifier I think, because the rest of the amplifier can be amazing, but if the output transformer is mediocre, it'll totally kill the sound! There's a definite art to getting it all right. Your 6J6 push-pull hybrid amp actually sounds really good, that's an awesome build! I'll dig through your channel more as I get more time!

@@UsagiElectric Yes i agree with you , to make it sound good the output transformer is the most difficult one.. I tried wind my output transformer since 2004years and i wasted alot material for keep try and error.. If you just want it work and sound ok not as grade as hifi , you can use any power transformer core , the hifi grade they normally use Oriented core Z11 or M6 lamination core which the thickness is 0.35mm , typical transformer core is 0.5mm thickness. Normal core will work good as well. The only you need to wind it . The winding isnt difficult just need patience to do it. The ratio of primary turn versus secondary turn , square of it will be primary impedance versus secondary. for example 5K:8 which is ratio of 625 , and square root of 625 is 25 , so your primary need to have 25 time number of turn of your secondary by using this u can make all transformer u want.. the primary inductance impact your low frequency bass the more the turn the better the low frequency response. If u have any question is welcome to message me. good luck

@@6p1p Interestingly, I think my next proper audio project is going to be trying to build an Output Transformer Less (OTL) amplifier. Everything I've read about them makes them seem like the vintage Ferrari of amplifiers - super finnicky and difficult, but when running right, better than everything else out there. I think I understand it at a fundamental level. You've got the standard amplification part, then you've got a ton of tubes in parallel to provide enough current to actually move a speaker. There's a few different schematics out there, though this one seems to be the most common: www.bonavolta.ch/hobby/images/audio/6as7_2a.gif There's a lot of really interesting things going on in that schematic too, I particularly like the dual triode amplifier stage with the cathodes tied together, that's an interesting way to do it. I wonder if that was done specifically for some aspect of the OTL, or if that just so happened to be the preferred amplification method by the designer? At any rate, I love the idea of a tube amplifier that uses just plain brute force instead of an output transformer. One of these days, I'll get around to actually giving this a go!

Thanks! I actually really want to design a proper OTL amp someday. I built a low voltage OTL using a boat load of 12B4As and it put out a really great sound comparatively, so I think it would be a ton of fun to expand upon that idea!

@@UsagiElectric I saw the video, it worked well.

I'm using a cathode bypass capacitor on the driver tube, but not on the amplifier dual triode. The reason for this is because I was trying to build a low voltage replica of the famous Champ Amp, which doesn't use cathode bypass caps on the amplifier stage dual triode either. When it comes to audio amplifiers, there's a million ways to build them to make them fit whatever goals the listener has in mind. My goal here was to just take one of the most famous and simple tube amplifier designs out there and see how it did at low voltages!

This was totally just a spare parts build! I just grabbed whatever I had laying around. As for the tubes, I wanted to use the 12B4A because it looked like a pretty beefy single triode in a 9-pin package, and the 12BH7 was the only dual triode I had that used the same current for the filament so I could run them in series.

@@UsagiElectric you're a tube guy and don't have a 12AX7 around? Or a 12AU7 or 12AT7?

True, but I sure do find low voltage tubes fun!

In spite of above, rigjht or wrong. (It got a bit issue as the AC will be all positive ...). Still not sure how the cycle work. Frequency is not an issue as you can see that the outgoing amplified AC wave would follow the pattern of the input AC wave. How the large V of anode help the amplification. Say cathode is -0.1V, grid is AC change on +0.2 to +0.4V (use my model above), and the Anode is maximum 20V. Would it be 0-20V but how?

Audio stuff is super weird to me and so I sometimes struggle to wrap my head around it, haha. For computing, we sometimes want the input signal to stay high and the output signal to stay low, or vice versa, so coupling capacitors aren’t used at all. But for audio stuff, the input signal is always alternating, so a coupling cap is almost always used (thought the Champ Amp schematic here doesn’t use them, but we’ll talk about that in a second). So, the way I see a coupling cap in my head is that it takes whatever input signal is coming in and relocates that signal to be around 0V. For example, if we have a sine wave input that goes from +5V to +10V, that’s a level that’s too high for the grid. If we use a coupling capacitor, now the sine wave goes from -2.5V to +2.5V. It’s still an identical sine wave with the same amplitude, it’s just been moved a bit. Now, the Champ Amp here was actually meant as a guitar amplifier, and guitar pickups by nature are already oscillating around 0V, so a coupling capacitor isn’t strictly necessary, which is why the Champ Amp didn’t include them I believe. Another interesting thing to keep in mind are the cathode resistors on the tube. The Champ Amp uses a 1500 ohm resistor, and this actually elevates the potential of the cathode to above 0V. So, the input on the grid is always negative in relation to the cathode. This sounds like it might just totally stop the flow of electrons, but in practice, the 12AX7 (and most tridoes) are meant to operate with the grid always below 0V. If we check the 12AX7 datasheet ( frank.pocnet.net/sheets/127/1/12AX7.pdf ), we can see that the graph for plate current versus plate voltage and grid voltage gives us information on grid voltages from -5V up to 0V, but nothing above 0V. So, the 12AX7 used here is designed for the grid input voltage to be somewhere between -5V and 0V, which is why the cathode resistor is in place, elevating the potential of the cathode, so the grid input signal never goes positive relative to the cathode. Now, I believe this is because the 12AX7 is supposed to run on about 200V to 300V, which is such a strong attraction from the plate, it requires quite a lot of negative potential on the grid to slow the flow of electrons. For our low-voltage design here, the grid may actually need to go a bit positive to get proper control. This whole process was a bit trial and error since the datasheets don’t give hardly and information on really low voltage operation. Also, to answer your question about how the triode is actually amplifying the signal, it’s really just as simple as the grid being really sensitive to controlling the flow of electrons. We can think of it in percentages a bit. So, looking at the 12AX7 datasheet, if we have a plate voltage of 100V, a grid voltage of -2V will not let any electrons flow at all. A grid voltage of -1V will let enough electrons flow to give us about 0.25mA through the plate. A grid voltage of 0V will let a ton of electrons flow giving us about 2mA through the plate. So, we end up with something that looks like this: -2V = 0% flow -1V = 25% flow 0V = 90% flow That’s just a 2V change on the grid controlling the flow of electrons from near cutoff to near saturation. And when the tube is in cutoff, the tube essentially doesn’t exist, so the plate is pulled up to the same voltage potential as B+, which in our example is 100V. However, when the tube is in saturation and electrons are flowing freely, the plate is now being pulled down, to the same potential as the cathode, which is near 0V (internal resistance of the tube and the cathode resistor actually probably puts this a bit higher, but that can be adjusted by changing tubes or changing resistor values). The Fender Champ here is actually a pretty high voltage amplifier, I believe it uses around +450V on the plate. That means that with just a really small input voltage on the grid, the plate is moving quite a massive difference. The Champ Amp uses two amplifier stages, each giving a gain of about 18V I believe. So, if a 1V peak-peak-to-peak signal is input into the grid, the first stage amplifies that to 18V peak-to-peak. This then goes through a coupling capacitor to remove the DC component of the output and is then fed into the second amplifier stage, which amplifies is up to about 300V peak-to-peak! I actually simulated up the first two amplifier stages of the Champ Amp in TINA to see what the waveforms looked like, and this was the result: i.postimg.cc/8Cbsc2x3/ChampAmp.jpg It might be a little hard to read, but all the concepts are there for the massive amplification that the little 12AX7 produces at such high voltage! Whew, that was a long reply, haha. Let me know if you have any other questions or something doesn’t make sense!

That's actually a really smart idea! I never thought about it, but that would indeed elevate the cathode above ground and potentially get some more gain. It's certainly worth a shot anyways!

That's actually kind of a convoluted story, haha. In this video I use +24V, then +24V and -12V together to essentially get a B+ of 36V and then finally in the next video I use 48V. So, to start with, my choice of +24V. This is actually completely centered around my primary tube project - building the vacuum tube computer. I really wanted to see if I could build some good tube circuits at voltages low enough that anyone could get into it, even children! I tested tubes at 6V, 12V, 18V, 24V and 30V (pretty much the entire range of my bench power supply). I found that anything under 18V was starting to push it, and I was having a hard getting reliable results. The next problem was figuring out heater voltages. Since I was planning on using pretty much the 6AU6 tube exclusively, I figured I could get away with running the heaters in series. So, 24V meant I could build everything around 4-tube modules. Next, the -12V. To get the logic circuits working reliably and properly, I need a negative voltage strong enough to push the tubes fully into cutoff. Using a -12V supply gave me the perfect negative bias, and it meant if I needed to, I could run two tube heaters in series off the -12V. This gave me flexibility to build 2-tube, 4-tube or 6-tube modules. So, I built my power supply around those voltages, and since that's what I had on hand, that was what I used for this video. However, ultimately, I wanted to run four tubes, and since I was using tubes with 12V heaters, I needed to think about how to power them. I did a bit of searching on Amazon, and it turned out that 48V power supplies were pretty common and super cheap, and that meant I could run four 12V tube heaters in series and use that 48V for the B+ for a little more volume over the 36V version. A bit of a convoluted story, but that's how I ended up with the voltage you see here! As for a modelling software, I do tend to check everything in software initially. I use TINA TI with some imported SPICE tube models to get me in the ballpark, then I trial and error different resistor and cap values on the breadboard until I find the sweet spot for the tubes I'm using.