Preamp Gain Stages - The Plate Resistor: Values, gain, tones and theory incl load lines!

A deep dive into the humble plate resistor in a 12AX7 gain stage. What is it's purpose and what impact is there on gain and tone with different values. This video examines a stock Marshall 100k value against a 330k in 3 ways, measurements, tone and theory.
headfirstamplification.com
0:00 Intro
1:16 DMM & Scope measurements
6:20 Tone test
7:54 Theory
22:22 Conclusion

[@Frankentoane]

You are the modern Uncle Doug for high gain people!

[@HeadfirstAmps]

Extra for experts: Notice the two pk-pk voltage readings 12.5 and 15.7. Difference of 3.2v, exactly 25% more! Theory never lies.

[@erichemard7066]

I added a 6 way selector with various values and a digital meter. This lets me add or subtract resistance to the 100k and monitor the voltage drop across all the while being able to sample the effects without having to break out the soldering iron. I use to connect a decad box, much easier now with a selector switch

[@brucewaldrop2933]

I've been building amps as a hobby for about 15 years. Looked at all the Valve Wizard stuff online as well as his books (and other books). I've never been able to wrap my head around the load lines until watching your video. You did a great job of explaining the theory to those of us who are not "educated" in this field. Thanks for posting this video...I'll definitely be checking your other theory videos as well.

[@HeadfirstAmps]

Thanks for this comment, it makes the effort worthwhile!

[@jayceecee1]

That was the best explanation of valves that I have ever seen.not just playe resistance. I have been looking at this stuff for years and had a mental block. I feel that the penny has just dropped. Thank you.

[@TheTrondro]

Sitting here, looking at the loadlines and smile (actually think I get it! for the first time ) Thanks Jason !!

[@RDamps]

Thanks for all of this Jason. I'm just a "mechanic" of amplifiers, but you're a Doctor!

[@HeadfirstAmps]

Thanks brother! Loving your channel, so great!

[@mookytc]

Dr. Jason...has a nice ring to it!!!

[@SeQFreQ369]

Thanks mate, I appreciate that you explain the techy stuff in a way that anyone can understand, also the feel difference, which to me is equally as important as tone. I'm waiting on some more parts for the 2 builds I'm doing and looking forward to doing some tests/subtle tweaks, between you and Uncle Doug I think I will be able to get the results I'm after a lot quicker now, knowing a lot more than I did on my 1st scratch build. Cheers.

[@HeadfirstAmps]

Glad to help!

[@GTPJoe]

Fantastic explanation of a complex subject. Really get how the load lines work now! Thanks

[@KleyDeJong]

Great video! Love both the playing tone demo and the excellent explanation of load lines. Thanks!

[@hogie1259]

Excellent video Jason! I can see your channel blowing up exponentially if you keep sharing lessons like this. Very easy to grasp the info . Thanks for your effort!

[@HeadfirstAmps]

More to come!

[@prook2001]

Thanks for this Jason. It's certainly helping my book knowledge coalesce with my practical knowledge. These practical demonstrations are really appreciated (I'm sure not only by me).

[@HeadfirstAmps]

Very welcome!

[@kirkgreenman1386]

Thank You. Amazing. I will re watch.

[@Frankentoane]

Can you make a comparison of a plate fed tonestack vs cathode follower tonestack? And anything in between haha

[@ramilrodriguez8340]

Very nice!! Specially the theory explanation. Sound wise the difference is barely noticeable but, besides what you said, I believe that with the 330k sounds tighter too.

[@remco613]

Great video, looking forward to the follow-ups.

[@HeadfirstAmps]

Thanks 👍

[@DavidBrown-ez5jc]

Just have to say what a excellent tutorial I am loving all your channel so impressed brilliant sir

[@RozsaAmplificationLLC]

Looking forward to checking this out Jason. After the last one, I was going to suggest that since you have everything set up to do these videos, to do just this and change plate and cathode values, bypass caps on the cathode, etc. and watch what happens to voltage and gain and frequencies. Very informative! :) Also, yes, the 330k in this example demonstrated that is was more saturated, compressed, and tonally complex. Oh, and if you could....try more values for plates, cathode, etc. For plates, try...82k, 91k, 100k, 220k, 330k!! :)

[@lousekoya1803]

Your channel is amazing Jason ! You'll soon become the Johan Segeborn of amp mod tutorials ! Hope to see you on Tone-Talk some day !

[@HeadfirstAmps]

Thanks Lou!

[@rangerdoc1029]

Legend! You helped explain why & how it works, not just how to calculate. I actually understand it now, not just how to do the math.

[@FedericoMK]

these tech videos are a godsend! Please keep 'em coming!!

[@kjons6897]

Eye opening video! I just watched one about "hot rodding" an amp but they didn't add any more tubes. The way you explained the process and theory behind it cleared up the confusion. Thanks!

[@TheGriff7628]

That was awesome Jase. I finally understand those graphs now!

[@icepick859]

Favorite channel on youtube now!!

[@utahprepper8925]

This is excellent. You helped demystify a somewhat elusive topic - those load lines. Thank you. You have a new subscriber.

[@digitalchris6681]

Fascinating stuff. I've used it to go the other way, and tweaked the first 100k plate resistor (the 'clean' input, or input 1 on my Plexi) to 50k (easy- just solder a 100k resistor in parallel to the existing one, so the pcb can remain in situ and it can be done in 10 seconds). This gives me a lower cleaner input whilst leaving input 2 as dirty as ever. And mixing them is so much fun.

[@HeadfirstAmps]

Great idea!

[@digitalchris6681]

@@HeadfirstAmps I've thought of a better way which I hope to try: replace V1 (12ax7) with a lower gain 12at7 or 12ay7, and then increase input 2's plate resistor to compensate. This should also give me exactly what I need (leave the dirty stuff alone but have more clean range) but be lower noise. I think....

[@greg4272]

You are a LEGEND!!! Thank you sooo much for this lecture! I was looking for this kind of knowledge for so long, when someone describes what is going on electrically, but also shows the effect from a guitarist point of view! 🤩🤩🤩👌👌👌🧠🧠🧠

[@HeadfirstAmps]

Thanks Greg, glad you got something out of it man

[@ericharbauer]

love this! cant wait for the cathode video!

[@MAURIIIII2009]

Thank you very much for teaching so much I really appreciate it.

[@HeadfirstAmps]

Glad it was helpful!

[@michaeltoren]

Absolutely love this. Keep up the good work!

[@HeadfirstAmps]

Hey Mike! Thanks man, I plan to keep the theory content central to the channel!

[@bretfuzz925]

I tried increasing plate resistors while looking at linear p-p voltages on my 2203 decades ago. I was able to get considerably more gain. The thing I observed is that it sounded more muddy as well. I suspect you need to reduce blocking/coupling caps as well to not decrease the high frequency content. In the end I went back to stock 100k. Some of the asymmetry when the p-p voltage exceeds a certain point is what helps flavor the sound. I now just hit the front end with a boss sd-1. Case closed…

[@Skman22]

Another great vid and another piece in the puzzle explained beautifully mate, thanks. On a side note, the neck on the bitsa looks smooth AF.

[@HeadfirstAmps]

Glad you enjoyed it man, it was inspired by one of your requested ideas! Yeah that neck is sweet, unfinished roasted maple.

[@Samy-hy5yf]

Great video, thanks so much!

[@PawlTV]

Awesome, thank you! :) Very useful examples.

[@scotthall8502]

thanks Jason very helpful

[@dantahoua]

Really love those videos! Hope you will make some about the "power" part of amp, still misterious to me vs the preamp I well understand now...

[@HeadfirstAmps]

Will do!

[@user-yb5gy7qs5j]

yes i most liked that study on the preamp / i would like to know more about voltage swing and into second stage thanks GKL

[@PhuketMyMac]

Really useful. Thanks!

[@tubehead4345]

Thanks Man, nice vídeo!!! 👍

[@Jonathan_Doe_]

Thanks for teaching me how to interpret those grids, they’ve always baffled me slightly. I’m guessing those -V lines are determined by the bias, and that the next video’s gonna explain that!

[@jasonstone1046]

Really helpful and concise info. Cheers mate!

[@HeadfirstAmps]

Hey Jason thanks man. My mates tell me I blab on about this shit and it sends them to sleep, so great to hear a contrary view! Lol

[@jasonstone1046]

@@HeadfirstAmps nha mate, enjoy it very muchly 😁

[@HeadfirstAmps]

Are you in Perth?

[@jasonstone1046]

@@HeadfirstAmps I am 🙂

[@HeadfirstAmps]

Ha, I remember. I bought the Matrix Q12's of you in 2017!

[@DavidBrown-ez5jc]

So Jason your channel and uncle doug and Dlabs it's like valve amp technology school

[@SeQFreQ369]

As I'm more of a classic gain guy rather than high gain I tried a 270k, and one thing I really liked that's not immediately noticeable with the amount of gain used in the vid, (although Jason does mention it towards the end ) - is that it is very dynamic to play, enabling me to go from a rich fairly clean tone to a nicely driven crunch and singing sustain just with picking dynamics, I don't need to pause my right hand in the middle of a lead up run to fiddle with volume pots, or worry about being within reach of a boost pedal, which is a big plus for me. I've since gone down to 220k for a cleaner clean, but still very dynamic and controllable. Thanks again Jason.

[@HeadfirstAmps]

Nice. This is great mate, tinker with these values to get the amp how you want it.

[@SeQFreQ369]

@@HeadfirstAmps Cheers, I've got a coupla really useful bypass switching options thanks to you, and it sounds and feels excellent to me. Now you've got me thinking about relays... I may be paying your store a visit soon. Thanks for all the gems!

[@HeadfirstAmps]

Ha ha, awesome.

[@matthewridgeway9250]

Great video. Inyeresting the voltage gain. But the current reduction is also a matter. With the output peak to peak increase, is the chance of signal clipping on the next stage likely? Also, it would be interesting to apply a potentiometer to switch the values on the fly so to speak. Maybe with a rotary switch with a ranhe of values. If the scope of an amplifier can be increased with such changes, might be interesting to see if there are any real benefits to having these variations on an amplifier. This actually explained a lot for me, and as I am new in this field of electronics, it has opened my eyes to matters about power transformers and using a generic one for various amplifier circuits, trimming the B+ to suit various amplifier applications for tone and responce. I can imagine switching whole circuits in and out using a relay set to switch between JTM and JCM circuit styles for example.

[@gil_evans]

Can I imagine it this way? When Ra is smaller, more voltage swing from the plate will be "lost" because of more current flow through Ra toward the HT supply, which can be seen as AC or signal ground.

[@Turboy65]

The plate resistor is the easiest one part change gain mod that can be done. I've run as high as 330K on a drive stage which gets really nuts. 220K is a good high drive value, one used by Mesa on V1 of their dual and triple Rectifiers. But I've found that it can be better to use larger than stock values on two consecutive stages, for a more controlled overdrive with (to my ear) a nicer harmonic spectrum. Say two stages each with a 180K instead of one with a 330K.

[@gil_evans]

Nice, would have expected a much bigger difference soundwise.

[@steakncheddar]

That's a great topic! I wonder about the cathode R, does it have a similar tonal impact?

[@HeadfirstAmps]

Yes, cathode video will come I promise

[@przecpany]

Digitech oscilloscope. I can’t believe !!!!

[@northernamplification8894]

Awesome and super interesting video mate! Personally I don't like going over 220k, if I need even more gain it's better to get it from other places imho. Guessing the plate values is a big reason why I like the Wizard MC circuit so much more than typical Jose circuits, it is kind of a "Jose modded Jubilee" in many ways 😎

[@HeadfirstAmps]

Very true!

[@maraschem7259]

If i wanted to mod a marshall dsl1 and hot rod it I would have to change that one resistor to a bigger resistance?

[@OperationEndGame]

Is there a way to reduce the muddiness when the gain/volume is set at 8? This is regarding a Marshall Plexi circuit.

[@Rustinpeace117]

Can you do one on the Cathode resistor and bypass cap?

[@HeadfirstAmps]

Yes, I will definitely do this.

[@chrisstevens4680]

I just can’t hear the difference, maybe it is my headphones. I guess I will have to try the mod for myself. When doing the comparison, do you sample the guitar and then play it back through the amp and then do the comparison in the edit?

[@Amp_Expert]

22:14 in...you show that there is a 25% increase in gain. You could have done this really fast without even looking at the load lines, though informative and just look at the delta (change) in voltage. For 320k you had 15.7V. With the 100k, you had 12.5V. Divide them 15.7 / 12.5 and you get 25% increase!! Easy!

[@Amp_Expert]

Oops. I see you did cover this in comments 2 months ago, but different calculation, but achieving the same outcome!! lol

[@HeadfirstAmps]

Yep!! I like to make it complicated!! lol

[@f1badude]

Did you make any amp adjustments when you were A/B playing other than swapping out the plate resistor?

[@HeadfirstAmps]

No adjustments, just the plate resistor change

[@gil_evans]

Was the other probe of the multimeter grounded or at the cathode?

[@HeadfirstAmps]

Was set to chassis ground

[@voxpathfinder15r]

So I don’t know if many guitars can reliably achieve a 1 volt peak to peak signal, but I imagine using humbuckers and a boost pedal, you should be able to reliably get there. My question is, if you have somewhere between 12 to 75 volts coming off the plate depending on initial guitar signal from the first gain stage, how do you not overwhelm the next gain stage? At 300 volts, the grid can only accept 3.5 volts peak to peak before you get clipping. Won’t a 12 volts to 75 volts signal hitting that next grid cause massive square waves from happening?

[@JonathanHerrera_Bass]

Remember that the next tube's grid is connected to the previous gain stage's plate via a coupling capacitor. This capacitor blocks DC, but allows the amplified AC signal to pass.

[@voxpathfinder15r]

@@JonathanHerrera_Bass I am talking about the AC voltage

[@patrickwilliams2530]

Weird question how would a 10k resistor affect tone ? because that's apparently the resistor that dave friedman got on his own personnal 68 plexi instead of the usual 100k he also got a 10k instead of a 100k linked to the tone slope 33k 500pf

[@HeadfirstAmps]

A 10k plate resistor rather than 100k? I don’t think so, that way too low

[@patrickwilliams2530]

@@HeadfirstAmps thank you there is a video of his 68 personal plexi getting biased on the fryette channel and there were some strange 5 band resistors so it turns out those were 100k. he plays this amp exclusively with a variac and at the end of the video you can clearly hear it having a crazy brown sound.

[@audiomsg]

You get 25% more gain, but you're operating in the triode's non-linear nether regions, right? I guess this is all good for higher gain sounds.

[@batpigguitar]

New Guitar?

[@HeadfirstAmps]

Thanks the Warmoth partscaster I put together with the Bareknuckle VHII pickups.

[@batpigguitar]

@@HeadfirstAmps very nice

[@Wim37u]

One load line has the wrong plate voltage.

[@johnmiller9219]

ok . going with a 220k . 330k too much gain

[@johnbuell8035]

Thanks for the explanation - very helpful. One thing, though - it would be good to hear the difference when playing with less gain. Can’t really hear much difference with the amount of gain you’re playing at. I imagine the changes you’re talking about would be working nicely when going from clean to edge of breakup.