Preamp Gain Stages: the Cathode Resistor. GAIN, TONES & THEORY!!! 

Howdy. Great. The cathode line was new to me. Credit shall go where it is due for taking the time and making the effort to make the video. Regards.

Great video, thanks for your work. The visual on the scope really helps you see what's actually happening to the wave.

Thank you. This is difficult to explain to another. I knew this from High School and College and technical experience building guitar amplifiers. So I get everything you spoke of. Still, I admire your effort to take this explanation on for your viewers.

Subscribed , load lines have been mysterious graphs for me for years, finally some one explained it in a way I can understand. Thank you for making this video.

Thanks Jason, more great information, carefully explained as always.

great video. Thanks for slowly stepping through this,- very interesting

Very good content, thanks! I did some preamp tunning on my mesaboogie .50 + e other bootique amp with very good results, to archive the preamp drive sound I had in mind. Preamp cathode resistors are very important to the final tunning of the amp.

Very cool vid mate, super informative.

Greatings from the UK! Just discovered your channel today!! Subscribed straight away!! Bloody marvelous videos!!!

Thanks Jason! Another great explanation video.

Thanks for that Jason, fantastic explanation 👍. Catch up soon bro

Merlin's site and books are GREAT!

Outstanding video. We’ve all heard asymmetrical clipping in our pedals through the years and it is an interesting sound. By my figuring, the waveform is a mixed one, a bit more complex dynamically than the traditional mostly symmetrical one. There is clean tone mixed in and it sounds like brilliance. I’m going to try this on my 1970 #1987 50W head to see if I like it. I already love the amp’s sound but this will be a simple mod to make variable. Thanks for all the gain charting. That’s a lot of time spent. I’ll report back.

Very helpful, thank you!

THANK YOU FOR THIS VIDEO >>>>> I just started digging into trying to understand this better today and then what do I see on RU-vid? Funny how things align sometimes... I have tinkered with the MC2's preamp stage the other day but I tried to cut the bass out from the first and second stage... Somewhere during all this I lost the function of my fx loop switch and my lead volume quit working... it was just on all the time and the gain only worked kinda as a volume and gain combo.... lol. Something was goofy from the get go though when I built the amp. THe rhythm cahnnel is TOO dark and the bright pot doesnt work at all to change it. Gotta look it over YET AGAIN!!! but its all good and fun as far as im concerned. Frustrating at times but worth it.

Awesome video! :) Thanks man!

Hi Jason, I love your channel and enjoyed this video along with your plate resistor video. Your explanations are clear and easy to understand. Could you enlighten me as to when to use audio or linear pots? I see variations on different amps and would like to understand why? Thank you for your time.

Tanks, Jason, for the awesome stuff you do! You could/should start holding online classes.

So funny - I just learned to draw load lines and was watching and realized Jason didnt add the cathode resistance to the load resistor value when determining current. I thought for a moment there were other ways to do it. Then as I am watching, the ticker came across the bottom saying to add the csthode resistor value. Whewwww.

can you make a video that show us individually all gain stages, phase inverter and power tube clipping? I would love to see how one side of the PI clipps and how a power tube clips :) and how PI and Power tubes clip in conjunction (how it diferes from preamp tubes)

What awesome work you've done on this educational series of videos, so thank you! Now, I have a question about the cathode follower resistor that is typically 100k. I know from the ampbooks calculator that using a 68k there (left over from trying some previous mods) will slightly lower the impedance seen by the tone stack, and a slightly larger cap is needed to attenuate the right frequencies for the JEL mods, but do you know of any other affects that using a 68k there would have on the preamp's performance in theory or otherwise?

ha, just had 'Designing Preamps' out yesterday to look something up.

It's worth looking at an unbypassed Cathode resistor as what is referred to as degeneration. This is local negative feedback and actually tries to linearise the stage - all other things being equal. The interesting thing about the so called ''çold clipper' stage (IMV not the right name for it) is that the 10k R being so large, reduces the gain and tries to reduce the distortion of the stage (by large degeneration)- BUT because the stage is so far off 'centre bias' due to the very low current it overloads very asymmetrically. We all tune these things by ear, like baking a cake, but it's an interesting rabbit hole understanding objectively what is actually going on.

Thank you Jason. Your recording tone is great. May I know how you record the amp? via reactive load and mic up with real cab?

Do you think asymmetrical clipping in the power amp sections valves (caused by slight phase inverter imbalances) could be reflected in the negative feedback sent to the preamp, and actually shape the distortion of the preamp? It’d be a tough one to test without an attenuator because even a 50w is ear bleedingly loud by the time you hit power valve distortion, but it’d be interesting to see if it does.

Hi Jason wonder if you could do a video of using a isolation transformer when working on vacuum tube amps with oscilloscope

Awesome video Jason. They ALL are man !!! The more I watch the more I want to try my own mods. For instance, ... I have the SC20H matched with a 2x12 with Vintage 30's. Very articulate and super distinguishable insofar as, you can hear each and every note that gets picked, even under full gain. Great setup !!! However, ....... It is soooooo BRIGHT that it is really almost unbearable. I hear you talking about "arresting" the bass frequencies going into the preamp section, on a lot of your videos. I seem to have the polar opposite situation. There's wayyyyy too much Treble in the amp - overall. To the point that I have the treble pot on absolute 0 and use the Presence for the high end. Does this sound familiar to you regarding the SC20H?? If I could "correct" that I would be a super happy camper. Any ideas would be greatly appreciated. I have not performed any mods to any of my amps before, but have been inside a multitude of many different models. Thanks for sharing your knowledge with us. Super cool stuff man !! - Cheers !!

Hey brother ive got a, well, a statement and a ? In one 😂 i learned this after plate loading/4 staging my old 800 2204. This thing is a beast in terms of clip but not the fizzy shit u get from Jose style stuff. I found that by shorting, bypassing, whatever, the 1st, 2nd and 4th cathodes with caps the amp became whisper quiet. No cross talk no oscillations just extremely quiet. Im talking 1st stage 100nf/6.8k, 2nd 470nf/4.7k, 3rd 2.2k and 4th 1uf/1.5k with 6550s at maybe 55mah which i would say is just asking for trouble. When i started all cathodes were unbypassed and it was noisey. You ever notice a reduction in noise by simply bypassing cathode resistors?

What I struggle with in understanding this is the difference in tone between symmetric and asymmetric clipping. Symmetric clipping is all odd-order harmonics, and I get that those will have more dissonance with the fundamental and string overtones which are even. And I get that asymmetric has both odd and even order harmonics. What's confusing is will that asymmetric clipping sound more pleasing because it has some even order harmonics, or will it sound more aggressive because it is more complex, having both even and odd order harmonics? Then what's the effect of clipping that cathode follower going into the tone controls? Is that making the overall clipping back to more symmetric and its the combination of the two that establishes the amount of saturation and overtones?

So when you draw a cathode load line does it mean you have the start the end of it from 0v the continue the line until it intersects the plate load line?

On the cascaded preamp stages, is it better to have the first stage with the 2.7K, 0.68mf and the second stage 820, 330mf or the reverse?

Hey Jason do you remember what clipping mode you used in the playout?

Diamonds....and dust...

i have some switches setup (2204 build). One switch changes the V1b cathode resistor from 10k to 2.7k. The other switch adds a .22uf cap. When the cap is engaged i cannot hear much difference when switching between 10k and 2.7k. Also i have tried many caps and have found the small 50v polyester box caps have the lowest noise.

So, is this also called "starving the plate voltage?" I've always heard that term in amp modding discussions but never really knew what the techs were talking about.

Great as ever sin city song by acdc 🤘🏻🤘🏻🤘🏻😎

Very good stuff bro. I have a question.what the small resistor value I can use on 12ax7 cathode ?plate resistor 100 k. Thanks.

How was I not subscribed?! Unacceptable.

cathode resistor in series with a cap to ground. what does this accomplish? (bogner ecstacy R12 mod). also, how can this be tweaked for different results?

The supply voltage on the Node is so low compared to a JCM800! a JCM800 has around 330v

I have a Friedman pt20 and it has the 3 way toggle switch on it for different gain stages and I was wondering if there was a way to make that foot switchable?