Tube Amp Design: Driver Tube Selection and Load Lines.

This is a video I have been asked to make for almost 2 years. I've been putting it off because I knew how much work this was going to be but I felt like this was a good time to dive into this subject, given all the comments about driver tubes and why certain types or chosen.
Hope you find this informative and I plan to do more of these as we work on this 12AX7/EL34 amp!
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[@debilthomes501]

Fantastic video and one I will be coming back to. Thank you for taking the time to put it together.

[@ericlane5949]

I guess that was my comment you referenced at the beginning (though “conspiracy” is your word, not mine, lol). I had always heard the 12AU/T/X7 tubes were all the same except for amp. factor. Thanks for your hard work showing this to not be the case.

[@bob_mosavo]

When Steph says, “You need to keep the grid negative,” the grid voltage is negative with reference to the cathode voltage and not to ground.

[@garagepunkfan]

Thanks for all the work that went into making this! Best explanation of load lines I have found!

[@SkunkieDesignsElectronics]

Glad it was helpful!

[@KUDJU]

Thank you very much for this video. can you dive deeper into it ? This video convern only imput stage, only triode input stage to be exact. What about outpust stage and pentode/tetrode input and uotput stages. What about phase inverters, internal tube capacitance, the connection between stages, bypass capacitance from one stage to another, how the signal changes from stage to stage and so on. And to top it all you can gide us through the design thoughts and calculations of one hole amp with all stages and interstages calculated and the dependancies between them. Thanks a lot if you consider this. It will be much appreciated so we can try and calculate for us different amps with different tubes at home for fun to see what combinations we can get and what results we can find :)

[@SkunkieDesignsElectronics]

Not in one video :P BTW It took 8 hours of work just to make this one! I don't think many viewers understand the time it takes to make a 30 minute instructional video.

[@JS-oq2cd]

There's also a bunch of Ohm's Law apps that you can use on a smart phone...very handy. And every RCA Tube manual has the charts for resistance coupled Class A tube circuits, including the tubes that were contemporary for the age of the manual. Nice work...good explanation.

[@el_wumberino]

Thanks, this - again - helps a lot!

[@nevillegoddard4966]

Excellent video Stephe! I really appreciate all the work you put in to assemble this video, & it's no surprise that it took you a whole day. But in the end it was very worth it. A great instructional video that could be easily used in a tech college. Thank you! 😃👍! Brings me back to the days when I was at TAFE college, about 30 years ago. Back then, we were looking at transistor load lines rather than those for valves. I shall need to watch this video again though just to really latch on to what you're talking about, as I'm a little out of practice at being an elec. tech student!

[@SkunkieDesignsElectronics]

Thanks!

[@belotan4218]

This is exactly the subject i was waiting for you to discuss. Probably consider more theories as part of your Monday monologues in the future.

[@SkunkieDesignsElectronics]

Well these types of videos are VERY time consuming and make my brain hurt trying to explain lol.

[@belotan4218]

Well, you did an excellent job in this part

[@jukingeo]

Great info! I was the guy, (or one of the guys) who inquired about the 6SL7. I see where it falls along the load lines and while it can still operate in a linear region, the operating point is a bit shifted to one end. This does have me concerned about early clipping. I know you mentioned this already, and there is the proof! The 100 gain tubes that you show, fall nicely in the middle. The 12AX7 does seem to be the best fit! I HAVE been following your prototype build of the 12AX7 / EL34 amplifier as I am most likely going to build that one. I am going to leave the door open by making the front end 'flexible', but it does seem that the first tube of choice would be the 12AX7. Thank you for the time and effort you put into this.

[@dtg474]

Thanks for the hard work and effort in expaining this to everyone Stephe. We have to do this quite a lot in RF with 6146, 3-500z, 4-1000, 4cx250, etc etc. Thanks again

[@SkunkieDesignsElectronics]

Glad it was helpful!

[@dtg474]

I have swapped a 12AX7 for a 12AY7. Although the mu was not the same and it was noticeable, the plate characteristic curves are similar. It was a nice sound. Check out the 12AY7 and tell me what you think. A 12AU7 is way too soft. It is true that the 12AT7 was really not designed for hi-fi. We ran 12AT7 as pulse generators and blocking oscillator in the 1957 ARSR-1E radar synchronizer. 100% conduct to cutoff.

[@SkunkieDesignsElectronics]

It looks like it could work. It's one of the other reasons I like LED bias, the bias voltage doesn't change when you change tubes.

[@dtg474]

I agree. I learned the LED bias from you in fact. I had done some engineering in the past, building some 1w transmitters for VHF. We would put BAS-16 diodes in series with emitter resistor biasing network. But it was for temperature compensation. I never thought of LED bias until I watched your channel. Thanks!!!

[@Cur_Tail]

My dear, this is a godsend. Ive been scouring youtube, reading diyaudio forum discussions, even bought the Morgan bible... And all that theory and engineering goes over my head. Your work putting this out in a way I can understand is muy apreciado. 😊

[@SkunkieDesignsElectronics]

I'm convinced some of these folks intentionally talk "over our head" just to try to make us think we are too dumb to understand what they do. They also WAY overcomplicate much of the math needed to make this stuff work.

[@user-zq8pp3ul2c]

I have an old RCA tube manual that has all if those special charts. I didn't appreciate how special the book is until this video. Makes me excited to go design some different stages with different tubes. My first home made pre amp was using a big 6J7 tube for a guitar pre amo using that book. A Very microphonic tube but nice to hear it creak as it warmed up for a fun 1st experiment

[@SkunkieDesignsElectronics]

Very cool!

[@RobertKohut]

Interesting and informative! 🙂

[@stevelangworthy9553]

Well done!

[@Pollys13a]

Another great vid, thanks.

[@SkunkieDesignsElectronics]

Glad you enjoyed it

[@whome8192]

Was doing this over Christmas looking at other driver tubes for the kt120 amp. Now I can check my work. Thanks!

[@cobar5342]

Stephe, this is a great video. I could listen to this sort of video all day!

[@SkunkieDesignsElectronics]

Wow, thank you!

[@Edgarbopp]

Great!! I really appreciate this “nuts and bolts” type of tutorial!!

[@SkunkieDesignsElectronics]

Glad it was helpful!

[@stevenjackson8226]

Brilliant stuff. Thanks for all the work you do. I'm not advocating anything here, but mentioning as a data point. In the range of my tubed audio ramblings, I've seen both Conrad-Johnson and Audio Research use the 12T7 in a number of their products. I can recall some of the C-J amps using the 12AT7 as an amplifier input tube and in both phono and linestages in some preamp models. C-J often used the 5751 as an input tube, rather than a 12AX7, because of the 5751s designed physical shock resistance and typically lower noise. The 5751 does have a slightly lower gain than the 12AX7.

[@kb6dxn]

Outstanding video, I have the old books that show charts.

[@airgead5391]

Excellent!

[@SkunkieDesignsElectronics]

Glad you liked it!

[@mfr58]

Thanks for all the hard work. It focused my mind on this important subject again and made more sense of it, great!

[@SkunkieDesignsElectronics]

Glad it was helpful!

[@carlespy7470]

Thanks for this explanation of load lines in a triode. Could you also show a load line example for a pentode?

[@SkunkieDesignsElectronics]

In another video in the future!

[@aso1493]

Excellent video as always - helped me a lot to draw the lines as I want to replace a 6N1P-VI with an ECC85 tube in my KT-88 SE amp. Seems to be a good choice if I drew my lines correctly 🙂 What would be an interesting video for me would be the biasing with LEDs. I did it in my amp with HLMP-6000 LEDs but to be honest I did not really understand what the benefit is - I know I have a fixed voltage drop but I do not understand, how the AC-signal passes to ground like in a R/C-solution like on the KT-88s. I just used the plans and played around a bit as my LEDs hat a lower voltage drop as expected... as I mentioned: perhaps it would be interesting for others also how you bias using the LED method. All the best & have a wonderful, healthy and successful 2024 🙂 Alex

[@SkunkieDesignsElectronics]

Great idea!!

[@debilthomes501]

I am also unclear about the use of an LED to bias the cathode.

[@gadget73]

The 12AT7 was designed for RF amp and oscillator duty. They aren't real linear, but for what they were meant to do, they don't need to be. If you want something kinda-sorta like a 12at7, a 6DJ8 gets you closer. Its also an RF amp tube, but much more linear. People know this though, so they aren't cheap. The 12AT7 and 6DJ8 both can deliver more current than a 12AX7 if you need that for some reason, but also need more gain than something like a 12AU7 or 6SN7 or whatever can deliver.

[@SkunkieDesignsElectronics]

The 12AY7 looks interesting for a lower gain audio tube.

[@gadget73]

@@SkunkieDesignsElectronics I have an old Knight preamp that uses a pair in the phono stage. Seems to work pretty well, about half the gain of a 12ax7 but capable of about 3x the current.

[@RobertKohut]

Well presented! :-)

[@SkunkieDesignsElectronics]

Thank you kindly!

[@bob_mosavo]

If anyone is interested in learning how to bias a triode but does not want to mess with expensive power supplies and high voltage, a n-channel depletion JFET is a good analog, where you can use a cheap power supply with 5V-12V. Also, you can use a solderless breadboard. Once you learn class A amp biasing and load lines with a n-channel depletion JFET, the learning curve with a triode will not be steep.

[@el_arte]

I agree with what you say, but obviously the 12AT7 could be used with changing the setup, different plate load, different B+, different cathode resistor, etc. Saying that it doesn’t fit in a 12AX7 setup is a red herring here. It’s not a good tube in this application because you are building a power amp and you need enough voltage swing to drive the EL34. The 12AT7 is mostly a preamp tube.

[@SkunkieDesignsElectronics]

My point was to show why I can't just use it as a driver tube for a power amp because there are a bunch of them cheap on ebay.

[@el_arte]

@@SkunkieDesignsElectronics That is clear. It's NOT enough that it exists.

[@samlysinger234]

Great video and I appreciate the visual drawing of the load line with the voltage swing relative to the bias point. Are you aware that the 6sq7, 6sl7, 12ax7, and 5751 are different versions of the same triode? (Which makes it easy to swap them in and out in this application.)

[@SkunkieDesignsElectronics]

Yes, absolutely they are similar, but each does have a different "tone" or sound signature.

[@miles5681]

Without the appropriate equipment (and the time to collect the data for load lines), how do we know how faithfully modern production tubes meet the tube specifications from 60 years ago?

[@el_arte]

You don’t. And very very few China manufacturers release data sheets for their products, so you have to assume they stick to the original specs.

[@SkunkieDesignsElectronics]

You get a good idea by simply looking at the idle current.

[@DrGIzmoBRad]

Quite the excellent load line triode characteristic tutorial :) Are pentode load lines handled the same way?

[@SkunkieDesignsElectronics]

Basically, that's for another video!

[@elminero49]

So the cathode resistor sets the bias point to 1.8 volts because the potential difference between the plate and cathode becomes negative 1.8 volts. Is that correct? So with no input signal we would have 205 Volts at the plate with .75 ma flowing through the plate resistor. Is that correct?

[@SkunkieDesignsElectronics]

It's negative 1.8V bias because of the potential difference between the grid, which is grounded and the cathode which is positive 1.8V. The plate doesn't have anything to do with it. Also understand these curve charts are guidelines to get you started in the right direction.

[@elminero49]

@@SkunkieDesignsElectronics I believe the plate current has a stabilizing effect on the grid bias. Why you ask? If the plate current were to increase, it would draws up more current through the cathode causing a larger voltage drop across the cathode resistor. This would cause a greater potential difference between grid and cathode that would reduce plate current.

[@elminero49]

If you were to short the plate to cathode, the current would equal (405V - 1.6 V) / (240K ohm + 160 ohm) = 1.6797 ma. The "405V / 240K = 1.6875 ma" is a good enough approximation, but doesn't help me visualize the short from plate to cathode.

[@SkunkieDesignsElectronics]

Sorry this wasn't helpful for you.

[@elminero49]

@@SkunkieDesignsElectronics Your video is very helpful. For me to visualize a short from plate to cathode I need to include the voltage drop across LED and the cathode resistor. I'm still working on completing the video and hoping to "Mu" a better understanding of vacuum tubes electronics. I will probably actually built the circuit to plot my own plate curves.

[@radik1969inboxru]

Hi. Can i use russian 6N2P instead of 12AX7?

[@user-zq8pp3ul2c]

You have to get the spec sheet and work it out from the lessons on this video.

[@radik1969inboxru]

@@user-zq8pp3ul2c Thank you!

[@SkunkieDesignsElectronics]

What they said, that's a great learning experience!

[@flyingmattyboy]

Heater wiring/pinout is different on 6n2p. The EV variants sre a nice tube though. Its simple enough to re wire 6n2p by using the datasheet etc though.

[@radik1969inboxru]

@@flyingmattyboy Thanks!

[@VyacheslavFuzik]

Glad to see you healthy and sound! Here's my shortest short on the idea for an amp cover. Maybe it can be found useful for smb as a very cheap-to-accomplish DIY idea. kzbin.infoPQ2h0vKWK1o?feature=share Since I'm from Russia I do not know the precice meaning of "Skunkie"but I feel it suits to call this idea as "skunkie" one... Let me know if some details wanted.

[@SkunkieDesignsElectronics]

Nice and simple for someone who needs one!