Found it

[daz]

The reason for the bright hard sound i get when i turn up. Voltage....just too high voltage in the preamp. Before you tell me all about that, let me say i already know and in fact this amp spent the first few years of it's like with very big dropping resistors and voltages as low as 100 VDC at V1. I had some tone back then that i felt was missing today, and while looking thru a folder of old schematics i found one i had labeled as being the circuit i had when i played a particular party which i recall i was thrilled with the amp. Even my bass player loved it and he never commented about my tone. So it hit me i may have lost that certain tone along the way when i at one point decided i should copy marshall's rail and dropped all resistors after the screens to 10k. Apparently that was a big mistake i've been living with now for the last few years. So tonite I dropped the entire B+ AFTER the screens WAYYYYY down with one big resistor to start the PI on forward from. (33k) I may go even lower, as thats how it was in that old schematic. Now when i turn the amp up it's worlds better. No more harsh bright hard tone. Just the same sound louder tho with the added cut of the volume that our ears perceive. But a far better and different thing altogether. Of course now the never ending tweaker in me (that i'm beginning to hate !) wants to start trying the big voltage drop only at certain nodes. But i hope to just leave it, as it's just sweet as hell. I played it for 3 hours off and on after work and i felt like a new guitar player. I never thought I would conquer this issue but it's done done done. I still have the issue of a huge signal into the PI tho, so i may still keep working on that. But thats the last real issue this amp faces. Scares me tho.....if i ever DID get the entire amp right, should i just lay down and die then, or if not what will i live for?

[JHow]

...if i ever DID get the entire amp right, should i just lay down and die then, or if not what will i live for?

No, you don't die... you have to make several more copies to prove that your theories are correct. Work never ends!

[daz]

No, you don't die... you have to make several more copies to prove that your theories are correct. Work never ends!

I suppose, tho I'm not a builder like most here. Unlike most who spend a lot less time on a amp then look forward to the next, i have always just been about getting one marshall style design to sound and feel like i want. I have no real aspirations past that. Tho if and when i feel i have what i'm sure is the final version i will likely build a new one to have a clean new one not riddled with unused holes and slop from endless experimenting.

What I really wanna know tho is why i need to drop the voltage so much to get the same feel and dynamic i had in the marshalls i liked. I wonder if the Hammond PT is the reason. It has 100Ma more current (250 vs the CT's 150) than the classictone marshall replacement i used for a while so i wonder if thats it. Could it be the lower voltages cause a similar effect to current thats closer to the minimum needed? The marshalls i owned i believe had voltages as high as mine was b4 i did this change.

[Chuck H]

The current limiting effect of a lower rated PT rating COULD be why you need such low preamp voltages to simulate the sounds of yesterthought. There are other things to consider though. You were probably playing louder then. For example, you mention the tone of the amp at a gig. Your power tubes are responsible for 90% of the current drawn from the HV rail. If you push them, even a little, the voltage will sag. Sometimes considerably. Also, a hotter bias will induce this effect more pronouncedly. So a possible combination of higher rated PT, cooler bias and less current draw from not pushing the tubes could well have added up to a hard sounding amp. It's also worth noting that simply lowering the preamp voltages won't give you the same feel as an amp where driving it drops the voltage. The difference is compression, or envelope. Just lowering the voltages won't offer the same touch sensitivity and dynamic as an amp that's "sagging" analogous to current.

You could probably get better results by bumping the preamp voltages back up a little and then adding a big resistor to the front of the HV rail. This would induce an effect more similar to an amp being pushed.

One more thought. If you idealize the amp for quiet play WRT voltages, as it seems you have been doing with your PI experiments as well, without the consideration of extra current that will be drawn when the power amp is working harder, you may end up with an amp that has no huevos when it comes time to take the amp out of the home and play another gig. You can't have it both ways, exactly. That's why players and builders are always toying with and offering devices to make better low volume tone or high volume tone or cranked amp tone, etc.

By it's very nature a tube amp will, and should sound different for better or worse at different volume levels.

You've been going back and forth and back and forth with issues that are absolute juxtapositions and sometimes your results directly negate good design considerations. My suggestion is that you literally scratch every single notion you have ever formed about circuit analysis and design. Start fresh and design the amp to sound "right" to you at gig playing levels. Then build a good attenuator and live with the small concession in tone at the lower levels. And BTW, the Hot Plate isn't a particularly good attenuator for low level playing so lose any thoughts you have about how attenuators sound based on that model. Consider that because your low volume circumstances are dictating your goals, and you're working with a fifty watt amp, everything you believe you have learned must be questioned. Consider that the amps you loved in the past were not designed the way you are tuning your circuit and they sounded fine. Much of what you've done has been masking symptoms instead of fixing problems. That's a big part of why your circuit values often seem highly irregular. You're not discovering anything better than what's gone before. You're discovering ways to make your somewhat compromised circuit sound better for your very concise criteria. The moment you get away from the ideal, like turning the volume up one digit or moving the amp to another room, you're going to find yourself at a loss and wondering why the amp sounded so good before!?! JM2C on some of this.

[Alan0354]

I am still working on mine, but I did specifically tried switching the dropping resistor to drop more voltage just on the preamp. I did not hear the magic of lowering the voltage by like 50V!!! Yes, the sound is less in your face with lower voltage, but it's not better. I just ended up leaving it at higher voltage for now.

[daz]

Much of what you've done has been masking symptoms instead of fixing problems. That's a big part of why your circuit values often seem highly irregular. You're not discovering anything better than what's gone before. You're discovering ways to make your somewhat compromised circuit sound better for current, low volume your criteria. JM2C on some of this.

Thats absolutly true, and i'v always known it. BUT, i totally disagree with the last sentence because if you notice i have often described certain things as "right". What i mean by that is often even when the tone seemed great in many ways i knew it wasn't "right". Like theres something missing that i couldn't put my finger on but from years of playing marshalls i knew it wasn't right. Then at other times it HAS been right. But i'd keep going because i wanted to perfect the things i didn't like about the marshalls i used to use. When i mentioned that party i did not mean to insinuate that was the one time it was right. I meant that ERA, when the amp's PSU was like that and i only mentioned the party because it showed that even at stage volumes it was great, not just lower levels. And i don't believe low voltages mean it's a bandaid. The badcat shows voltages and shows 99v at V1 ! Mark Sampson designed it, and he's hardly a hack i think.

I feel now things ARE right, and the preamp is better now than it was last time it was right. Whether or not i'm done I do know that this was not a bandaid. It's a legit feature that has it feeling and sounding right. That cap at the start of the tone stack i mentioned....that WAS a bandaid and i knew it. But it was all i had till i realized i need to go back to lower voltages.

[daz]

Well, this is rather surprising to say the least. Well, for me anyways. I found voltage charts for a number of marshalls and it seems the preamp voltages are indeed low like mine now are ! Some much lower, as low as 100v at V1a. I always thought they were much higher than that, on the order of high 200's or even 300. No wonder it sounds so much more right now. The Free Information Society - Marshall Voltage Chart Electronic Circuit Schematic

[Chuck H]

That's true. Some of the really high power models with over 500Vp have over 160V on the first preamp tube plate, but most are under that with some of the under 400Vp, like one MK series model having only about 140V on the first preamp tube plates. This would be idle voltage. I've never seen a Marshall with less than about this at idle voltage but I'll take a look at the link.

It's worth noting that a popular "mod" for some lower HV Marshalls is to jumper one of the series 10k resistors in the HV rail that serve the preamp. This tightens up the tone in otherwise anemic sounding Marshall models. This could be why you first moved to the higher volts. Perhaps you read it somewhere. Maybe from me?!?

[daz]

That's true. Some of the really high power models with over 500Vp have over 160V on the first preamp tube plate, but most are under that with some of the under 400Vp, like one MK series model having only about 140V on the first preamp tube plates. This would be idle voltage. I've never seen a Marshall with less than about this at idle voltage but I'll take a look at the link.

It's worth noting that a popular "mod" for some lower HV Marshalls is to jumper one of the series 10k resistors in the HV rail that serve the preamp. This tightens up the tone in otherwise anemic sounding Marshall models. This could be why you first moved to the higher volts. Perhaps you read it somewhere. Maybe from me?!?

No, i experimented with voltages lot right from the beginning. But the reason i ended up with high voltages in the last year or 3 is because looking at Marshall schematics i always saw 10k droppers. So i did that which jack the volts way up. But i figured it must be right because for one thing i had seen a JCM voltage charts showing what i now realize were insanely high voltages, over 300, and i took that as gospel. I think it may have been some guy's modded marshall or something and didn't represent most marshalls. But at the time i had no idea. And also, i couldn't see how a marshall with 10k dropping resistors could end up with voltages much different than mine unless the PT was much lower voltage. But i knew they weren't judging from the power tube voltages. In fact they were usually higher there. So i assumed at one point i should lower the droppers to up the voltage and i left it like that since. Yesterday I thankfully found my mistake ! Even the controls all work better with more useful range. The one thing i still wish i could find a way to do is lower the signal into the PI. I just don't need so much that before 12:00 it's already insane the the output is distortion max. Might also make the tone even better since i'd be able to turn it up higher to achieve the same volume thereby raising the load at the master.

[Chuck H]

Roger that. What I've seen in many, if not most Marshall amps is two 10k resistors in series to serve the preamp. Not sure what you're using now.

[daz]

Roger that. What I've seen in many, if not most Marshall amps is two 10k resistors in series to serve the preamp. Not sure what you're using now.

I basically added another 23k before the PI. It was 10k so now it's 33k. Left the two 10k's for V1 and 2 the same.(10k)

[echuta13]

Very cool daz! I never realized that many Marshall amps had preamp voltages that low. I've always felt that voltages are part of the tuning of the amp. That being said... there are just so many things that can be tweaked be it for voicing, feel, headroom, ect... that I think it's very easy to not see the forest through all of the damn trees!

[daz]

there are just so many things that can be tweaked be it for voicing, feel, headroom, ect... that I think it's very easy to not see the forest through all of the damn trees!

Thats it. There are so many things and they are so interactive it's sooooooo easy to lose your bearings. You tweak one thing and then it throws all these other things off and you end up changing the entire amp to compensate for that one change, which may or may not be a good change. If it wasn't, you just veered off the trail and got very very lost ! It's a very tricky business. I think the trick is once you know it's right, at that point don't touch anything thats a major part of the circuit like the PSU. In fact, i've often felt the PSU voltages and filters are maybe the biggest key to getting the amp "right". After that it's all fine tuning that may change things somewhat, but won't throw the whole amp off the beaten path.

[eschertron]

Thats it. There are so many things and they are so interactive it's sooooooo easy to lose your bearings. You tweak one thing and then it throws all these other things off and you end up changing the entire amp to compensate for that one change, which may or may not be a good change. If it wasn't, you just veered off the trail and got very very lost ! It's a very tricky business. I think the trick is once you know it's right, at that point don't touch anything thats a major part of the circuit like the PSU. In fact, i've often felt the PSU voltages and filters are maybe the biggest key to getting the amp "right". After that it's all fine tuning that may change things somewhat, but won't throw the whole amp off the beaten path.

Daz, you've been blazing trails guided by only the sense of hearing, and moving fast enough that some of us, novices and DIYers, are breathlessly trying to keep up I have followed each of your threads on this amp of yours, and will continue to follow... learning quite a lot along the way about theory, aesthetics, and how some people are truly passionate in their pursuit! Thanks for all that's been shared and discussed, and I believe there's more than a few followers that have had their horizons expanded by your journey!

[daz]

Daz, you've been blazing trails guided by only the sense of hearing, and moving fast enough that some of us, novices and DIYers, are breathlessly trying to keep up I have followed each of your threads on this amp of yours, and will continue to follow... learning quite a lot along the way about theory, aesthetics, and how some people are truly passionate in their pursuit! Thanks for all that's been shared and discussed, and I believe there's more than a few followers that have had their horizons expanded by your journey!

LOL! Yeah, and a few that get very irritated by it too ! But it's just the way i am. When i am interested in something as much as i am guitar tone, i tend to be helpless to cease and desist. I've learned a lot doing this, but the most amazing result has been what i suppose is really NOT a good thing....my hearing sensitivity. It's become so sensitive that i'm super anal about tone to the point it's very hard to please me. I've always been a complete tone hound, but this have made me 100 times worse than ever b4.