cathode R value

[daz]

I have a question regarding cathode resistor value in cascaded gain stages. I raised the values in my amp from 1.5 to 2.2k on the first 2 stages. This of course lowers the gain, but this is what i don't understand....when i use a clean boost now i get what sounds like the beginning of blocking distortion. With the 1.5k's in there i didn't get this at all. Why would this be? It would seem to me like the opposite would be the case.

[Chuck H]

It's possible that what your hearing is the difference between more symmetrical clipping and a signal that is clipped more in the cutoff region. Those resistor values do set the bias point for those tubes closer to cutoff. It's a more compressed sound with a smooth but less defined bass character and lower dynamics. It's employed more aggressively in some cascade uber gainers for it's tone shaping properties. As a single stage, a cold clipped signal can be a good thing in the right amp. But it's all subjective, Amps that use an intentionally cold biased stage would include the 5150, the SLO100 and the TrainWreck Express. All popular designs. But it's a matter of taste.

[daz]

It's possible that what your hearing is the difference between more symmetrical clipping and a signal that is clipped more in the cutoff region. Those resistor values do set the bias point for those tubes closer to cutoff. It's a more compressed sound with a smooth but less defined bass character and lower dynamics. It's employed more aggressively in some cascade uber gainers for it's tone shaping properties. As a single stage, a cold clipped signal can be a good thing in the right amp. But it's all subjective, Amps that use an intentionally cold biased stage would include the 5150, the SLO100 and the TrainWreck Express. All popular designs. But it's a matter of taste.

Actually, since i posted this i went back to 1.5k cathodes. Not because of that, but i guess my ears were just tweaked because the next day the 2.2k cathodes just didn't sound good at all. I do find what you said helpful and interesting tho. I like when you reply to my posts because you seem to be one of the few if not only person who often describes things in simple terms and those which even non techs like myself understand and find useful. If i were to ask what values to use for better dynamics most will just point me to merlin's page which i usually just stare at like an idiot wondering wtf he's saying, or they try and get me to read between the lines. You just said that as you raise the cathode value the tone loses dynamics, one of my biggest concerns. So that was one of those bits of info that will stick with me. In fact, i was intrigued to the point i jumped up and turned the station on and immediately halfed the 1.5k's in there now to see what it did to dynamics. It did seem to enhance them by creating a wider dgree of clean to distortion when i have the guitar turned down to that in between area. But i felt the 1.5k's are better overall because while they seem a bit less dynamic, i much prefer the tone itself. But a good and interesting thing to know, thanks.

I just removed the chassis and began experimenting recently after close to a year of playing it and no tweaking. But i'm glad i got the itch because i feel i made strides in the last few days. I found that the 33k slope in the stack that i long ago canned in favor of a 47k because the 33k made it woofy and inarticulate actually now sounds great after dropping coupling cap values. Before i had feared smaller couplers because i felt the amp sounded too thin. So the 47k removed some of mud of the bigger couplers. Now i find it's better to do the opposite and the result besides better tonal balance is i'm rewarded with more fluid sustain, articulation, and complexity. If theres one thing i've learned in the 5 or 6 years i've been building and tweaking it's that theres are endless ways to fatten/thicken or brighten/thin the sound, and each does it in a different way. Some create a smoother more fluid and complex tone while others kill sustain and complexity. And the trick for me at least if finding the ones that work together well. Remove any excess mud and find a way to beef it back up elsewhere. For example, beefing up the tone with bigger couplers then removing the woofiness with the slope resistor is the opposite of what worked well for me. It's all about finding all the ways and places to beef up or thin out the tone that work best together. To me thats where the magic is. It's all good fun.

[Chuck H]

There's a reason I explain things in layman terms I'm pretty low tech compared to many here. Your already onto some things that are very practical. Like the slope resistor vs coupling cap relationship. Other more obscure relationships will reveal themselves now and then. For example, you may think that a cold clipping stage isn't the right thing for you. When it's actually just not the right thing for you in that amp, in that location. You may yet build an amp that uses one more effectively to compliment the rest of the circuit.

And, FWIW, I wasn't saying that a higher cathode resistor will result in lower dynamics. I was actually saying that a cold biased stage is lower in dynamics when clipping. And even that is subject to some debate depending on the actual implementation and ones perspective. If a stage is never clipped, or only slightly clipped, a cold biased stage can actually contribute to greater dynamics. This certainly wouldn't be the case with a clean boost pedal overdriving an input. So my observation was condition specific. Also, I'm sure that if you've ever played through an slo100 or a TW Express that you didn't find them lacking dynamics!

Please just take my statement regarding cold biased stages at face value, but add: 'when clipping hard'... It's a more compressed sound with a smooth but less defined bass character and lower dynamics.

[Enzo]

I'm pretty low tech compared to many here.

But...but... if you don;t cover the sub-neutron rectifiers and the ion induction valves, how can you TRULY understand this stuff?

[daz]

There's a reason I explain things in layman terms I'm pretty low tech compared to many here. Your already onto some things that are very practical. Like the slope resistor vs coupling cap relationship. Other more obscure relationships will reveal themselves now and then. For example, you may think that a cold clipping stage isn't the right thing for you. When it's actually just not the right thing for you in that amp, in that location. You may yet build an amp that uses one more effectively to compliment the rest of the circuit.

And, FWIW, I wasn't saying that a higher cathode resistor will result in lower dynamics. I was actually saying that a cold biased stage is lower in dynamics when clipping. And even that is subject to some debate depending on the actual implementation and ones perspective. If a stage is never clipped, or only slightly clipped, a cold biased stage can actually contribute to greater dynamics. This certainly wouldn't be the case with a clean boost pedal overdriving an input. So my observation was condition specific. Also, I'm sure that if you've ever played through an slo100 or a TW Express that you didn't find them lacking dynamics!

Please just take my statement regarding cold biased stages at face value, but add: 'when clipping hard'... It's a more compressed sound with a smooth but less defined bass character and lower dynamics.

Well, when turning my guitar low enough to get very clean, i seemed to get more distortion when lowering the R value. In any case, aren't trainwrecks so dynamic because you have to crank them fairly loud to get a good amount of drive? Or in other words, isn't it because thier OD is from the PA? I could list at least 7 or 8 aspects of tone like sustain, complexity, etc etc that are all important and when tweaking i am always trying to get all of them at once and in good balance. I would say while my amp IS fairly dynamic, thats one thing that has eluded me to the degree i would like it to be. I'd probably give it a 7 on a 1-10 scale. Good, but not droolworthy.

[Chuck H]

Dynamics, IMHE, are a relative thing. You need headroom, a tight power supply, highs and lows to get great dynamics. Of course too much of any of those things messes up an amps clipped tone!!! Some amps do balance better than others. I guess my point is that you can hear your way into that balance if you understand where the trade offs are. Then you don't need to wonder. You can know that you've done all you can. Example... You like the OD tone BUT you wan't more dynamics. Maybe try a stiffer power supply, higher preamp voltages or a different balance on the gain structures or whatever. You'll hear the change and decide "that's too dynamic and makes the OD harsh" or "yeah, that's going the right way now" and when you've tried a hundred things, as you have, you've probably done all you can with that amp! Now you can just play and enjoy!!! Maybe you want a different amp for certain types of tones. But that might be another amp entirely. No one amp does it all. The best you can do is understand the limitations and decide what an amp should do before you design it. Then you won't be disapointed that your new Marshall/Fender switcher doesn't do Robbin Ford and Malcolm Young both flawlessly!

[daz]

Dynamics, IMHE, are a relative thing. You need headroom, a tight power supply, highs and lows to get great dynamics. Of course too much of any of those things messes up an amps clipped tone!!! Some amps do balance better than others. I guess my point is that you can hear your way into that balance if you understand where the trade offs are. Then you don't need to wonder. You can know that you've done all you can. Example... You like the OD tone BUT you wan't more dynamics. Maybe try a stiffer power supply, higher preamp voltages or a different balance on the gain structures or whatever. You'll hear the change and decide "that's too dynamic and makes the OD harsh" or "yeah, that's going the right way now" and when you've tried a hundred things, as you have, you've probably done all you can with that amp! Now you can just play and enjoy!!! Maybe you want a different amp for certain types of tones. But that might be another amp entirely. No one amp does it all. The best you can do is understand the limitations and decide what an amp should do before you design it. Then you won't be disappointed that your new Marshall/Fender switcher doesn't do Robbin Ford and Malcolm Young both flawlessly!

I don't expect the amp to do anything like that. (Ford+Malcom) But i've had marshalls that sounded like this but a bit more dynamics i think. In any case, it's all good as is. The PSU is pretty tight and i don't play it loud either. (100uf/100uf reservoir, 20 H choke) One thing i don't understand tho is how some amps get big voltage on the preamp when mine which has 425 at the OT (can't recall the initial PSU V) ends up with only 125 at V1 with 1.5k/100k cathode anode and the typical 10k droppers at each node. I've wanted to try more DCV, but barring the matchless style parallel nodes i can't see how.

[Chuck H]

Some Marshalls (even from the same era) have widely different voltages. The Mark amps could be the same model and one might have 380Vp and another 470Vp!!! More typical is 420Vp for those amps. Still, the difference between 420V and 470V is 50V!!! And it makes a huge difference. I see no reason why your PSU rail can't be modded to get higher preamp volts if you want to experiment with that. It really doesn't take that much resistance to keep good isolation and decoupling. If you post a schem with voltages, even just the PSU schem, I think I can coach you on the changes.

[daz]

Some Marshalls (even from the same era) have widely different voltages. The Mark amps could be the same model and one might have 380Vp and another 470Vp!!! More typical is 420Vp for those amps. Still, the difference between 420V and 470V is 50V!!! And it makes a huge difference. I see no reason why your PSU rail can't be modded to get higher preamp volts if you want to experiment with that. It really doesn't take that much resistance to keep good isolation and decoupling. If you post a schem with voltages, even just the PSU schem, I think I can coach you on the changes.

Well, it has 425 on the plates and each of the preamp nodes is seperated by a 10k. Theres a seperate node for the PI, V2 and V1. I have another PT i had in there for a while that had 470on the el34 plates but it really didn't sound that much different. I also have run it with 2.2k dropping resistors but come to think of it i didn't like it like that.

EDIT: Man, it HAS been a long time since i upped the droppers to 10k, but i coulda sworn it sounded rather harsh with the 2.2k's....i was wrong ! After i posted that, as i often do i tried it. Popped three 2.2k 3 watters in place of the 10k droppers and man ! I suppose when it was like that before it didn't sound as good because the amp was just very different in other ways. Chuck, you know how long i've been screwing with this thing, so you can imagine how different it's been. Anyways, thanks for mentioning that because i would have never tried going back, and it sounds fantastic now. Theres 100 volts more at the plates of v1. The tone is just so tight now but still retains the squishy feel, so the tightness is not harsh or sterile at all. Best of all, the dynamics are SWEET. And the cleanup tone is much nicer and rounder whereas before there was some fizzy trash on the top end that kept it from sounding clean. I'll give it a good workout this weekend, but i think it may be ready to zip it up again and leave it for another year ! As always, thanks Chuck. Not something i haven't done before, but had you not mentioned it like i said i never would have gone back to that PSU setup.

[Chuck H]

I'm thrilled for you. Don't neglect experimnting with fine tuning the voltages on the preamp. You might find it can get even better. And yes, the amp may well have been a bit different the last time you tried higher preamp volts. Which lends to my post about balancing circuits and how they're handled. Differen't combinations lead to differen't results. Even on paper the actual tonal results can't always be predicted. So kudos. I'm glad we were able to take it to the next level... For now. I know you'll jones for even more "next level" soon enough

EDIT: Incidentally... The first amp mod I ever did was to a RI Plexi Marshall. I cascaded the channels and made my own additions to get it sounding right and under control. Then I added a master volume and bumped the preamp volts. I use to run it with a clean boost in front. Sound familiar!?! In the SF Bay Area where I played I got raves about my tone from all who heard it. I built my own franken strat from Warmoth parts with a SD JB mini humbucker in the bridge and Van Zandt singles for the middle and neck. I also had custom wiring to allow series options for pickup combinations. Other player would ask me how I got my tone and I would say: "I'm just playing a strat through a Marshall."

[daz]

I'm thrilled for you. Don't neglect experimnting with fine tuning the voltages on the preamp. You might find it can get even better. And yes, the amp may well have been a bit different the last time you tried higher preamp volts. Which lends to my post about balancing circuits and how they're handled. Differen't combinations lead to differen't results. Even on paper the actual tonal results can't always be predicted. So kudos. I'm glad we were able to take it to the next level... For now. I know you'll jones for even more "next level" soon enough

I've already started thinking about that. I swapped 2 of the dropping resistors at first because i couldn't find another 2.2k/3w. But i later did and swapped out the 3rd and last one and it seemed a bit TOO much towards that tighter tone. So i plan to play it a bit because my ears may feel different tomorrow. Bit late now to be messing with it.

[daz]

This is really bizzare, and it's not like i'm a stranger to this phenomenon. Not really ear fatigue because i wasn't playing and tweaking for long periods. In fact, just minutes. But last nite i'm thinking this sounds fantastic. Today i get home from work and plug in. Sounds great still for 2 minutes. Then i try putting the 3rd 2.2k back which as i said last nite seemed like too much of a good thing. It still was. But when i then put the 10k back it sounded hard and lost that smoothness and complexity to a gret degree. And it had just been like that 3 minutes before ! I've had my ears change in minutes before, but not to that degree ! So i looked hard at everything i touched and i'm sure it's all back the way it was when it sounded great minutes before. I'm just stumped and disappointed. I have to think something went wrong. A solder joint or something. I hit the solder i had hit when changing resistors again to reflow it, so i don't think it's that. Well, i'll continue with this over the weekend and hopefully figure out whats going on. Friggin tube amps....it's always something strange !

[daz]

Ok, i think i get it now. First of all, i do think there was a solder issue and that seemed to help because i felt there was less gain than usual. this was a completely different part of the circuit, not the PSU. But in any case that aside, i think i see how the higher voltage affects things and i DO like it a lot. But it brings with it some serious issues, and they are the ones that bothered me about it long ago when i experimented with higher voltages in the AX7 stages. heres what i'm hearing/feeling. The tone gets very dynamic as far as tone even more than volume dynamics. Dig in with a pick who's edges aren't completely smooth and that grind you get can at times (especially in the upper ranges of the plain strings) gets very bright to the point it sounds like it hits a wall and is hard and harsh. The overall tone is brighter and harder. But on the plus side the dynamics are nice and theres a in your face organic punch with sweet and aggressive harmonics that are bonerific. However, as you turn the master up that hard bright sound gets rather unbearable. Turn it down enough to living room at midnite levels and the brightness/hardness goes and the tone gets smooth yet retains all the beauty i described.

My feeling is that this IS the place to be, BUT, and it's a big but, it needs some sort of other tweaks to smooth it out or it'll be useless except at very low volumes. I feel if i can keep this tone but get that low volume smoothness it'll be absolutely killing. I'm thinking things like conjuntive filters and that kinda thing. Not that actually, as i don't like those, but something along those lines that takes the top off without taking the hi harmonics and articulation with it leaving a dull top end. Not sure what just yet, but i think this is my new objective. If i can't find a way i suppose i'll go back to the 10k droppers. It was sounding great like that so i can easily more than live with that. But this looks like there are big possibilities here for seriously amazing tone...the tone you have always heard in your head kinda tone. So i have to search for a way. Not gonna spend a hell of a lot of time, but i will try.

[Chuck H]

Well this goes back to what I was saying before. That is, you can't expect an amp to perform exactly right on all criteria. The thing to do now would be to experiment with different value rail resistors (maybe try some 4.7k's?) and find a good compromise. The amp can't really be expected to perform the same cranked as it does with the master down low. This has been a lamented snag throughout guitar amp history!!! So choose your trade off and live with the drawbacks. Another option would be to go utterly insane I suppose.