6V6grid stooppers and other questions

[daz]

The way i run is to use a amp that gets just enough drive to get that classic rock thing only to the degree that turning the guitar down to 3-5 gets a really sweet chime, then use a pedal to drive the input with a clean boost for singing OD. This amp with the CF bypassed is even better then it was, and it was really good before. Now it;s just some of the most beautiful cleans i've had. Only thing i can't figure out is why boosting it with a clean boost doesn't sing like almost all other amps i've had and built. I use a tube screamer with the level up high and distortion knob at zero. Doesn't work too well here and i can't figure out why. Not a big deal, it works ok and i don't use that sound a lot anyways but it IS a bit disappointing. Most amps i have built have a 820R/.68uf cathode on the 1st stage so maybe i'll parallel a resistor on the current 2.7k/.68uf to get around 1k and see if that makes a difference. I can always drop some gain later on.

[Chuck H]

Only thing i can't figure out is why boosting it with a clean boost doesn't sing like almost all other amps i've had and built.

Most amps i have built have a 820R/.68uf cathode on the 1st stage so maybe i'll parallel a resistor on the current 2.7k/.68uf to get around 1k and see if that makes a difference.

This is worth experimenting with. The bypass frequency of the .68 cap is much lower with the 2.7k cathode resistor than it is with an 820R. Basically the higher the cathode resistance the lower the knee frequency of the bypass cap. And the more local NFB is bypassed. So with the higher cathode resistance you'll get more boost at a lower frequency. Like the previous mention of changing from a 150k load to a 1M load this has to do with circuit impedance.

But your woes could also have to do with how the second stage is receiving the signal from the first. Since this is likely where preamp clipping will begin when the amp is pushed. So perhaps compare the second stage of amps that are overdriving well with a boost with what you've done in this build too.

[daz]

I dunno what's wrong with the attachment function but i tried to upload a schematic twice and it ends up with 2 images. I'll try one more time after closing all browsers and see if that fixed it. Anyways, heres a updated schematic tho i did NOT update the change i did last nite where i bypassed the CF and fed the tone stack directly from V3 plate. It's just tacked in for now but I will change the schematic to reflect that if i make it permanent but not sure yet.

[Chuck H]

I've done some work in ltSpice to see what the changes you've made are doing. Then I did some tweaking to further the effects in that direction and should improve the amps tone with a boost pedal. Some of the values may look a little odd, but what I'm seeing in the simulations looks really good. I worked exclusively with the schematic format so you wouldn't have to change anything in the lead dress or layout. The only exception being the added grid stop at V2A. That could be placed right on the socket.

Basically the changes you've been making have made the clipped wave form more symmetrical. But the amp as shown in your original schematic does become much more asymmetrical when a boost is applied. The changes shown below will improve symmetry and stabilize the way clipping is responding to different input signal levels. As well as restore most of the original EQ profile so the amp "sounds" the same, but behaves better. I did EQ it for a small bump in the low mids but tried to keep the HF profile and location of the mid dip area the same to preserve the jangle

Changes in red.

[Mick Bailey]

I built an experimental amp with 4K7 pots instead of fixed cathode resistors and ended up with a really good sounding amp that cleans up nicely and is easily pushed into a smooth overdrive with just a little bit of boost from a pedal. or when engaging the internal preamp in my Tele. Once I'd arrived at how I wanted it I measured the pot values and replaced them with fixed resistors.

[Chuck H]

I built an experimental amp with 4K7 pots instead of fixed cathode resistors and ended up with a really good sounding amp that cleans up nicely and is easily pushed into a smooth overdrive with just a little bit of boost from a pedal. or when engaging the internal preamp in my Tele. Once I'd arrived at how I wanted it I measured the pot values and replaced them with fixed resistors.

I've done that sort of thing in several circuits. Well, before LTSpice anyway. Pots are a great way to eliminate multiple component changes when a specific resistor value is the target. In fact I used the pot as a cathode resistor thing to choose what ended up being a critical circuit in my favorite build. Which, FWIW, ended up being a "cool" clipping stage (rather than cold, as it were) so a 10k pot was used rather than a 4.7k. Final value being 6.8k. Not that it matters out of context. Just mentioning it since the cathode value of cold clipping stages can be targeted with higher value pots.

Now, having examined the behavior of amps I've liked I can use Spice to see much of what an amp or specific circuit will do before I even build it. There's always some ear tweaking in the end, but it's real handy to know about things like wave form symmetry/asymmetry, what's clipping where and approximate EQ at the different stages.

[daz]

Wow, thanks fr that. I'll give it a try. Last nite i realized i have been tweaking myself out of what at first was a great tone, so i set it back to the way it was for the most part. Not that it was bad but just kinda got away from what i loved about it at first. But i'll give this a go and let u know how it goes. By the way, i realized that while the grid stop and v2 was super low when i first dusted it off, it also was the biggest contributor to what i liked. SUPER responsive dynamics. More than just about any amp i've had.

I have tried that adjustable cathode thing too years ago but i found myself not sure where i liked it best ! LOL

[daz]

Oh, and i decided after some time with the plate fed stack that i prefer the CF, so thats back.

[Chuck H]

I did my simulations with the CF in circuit. Designing to get more clipping symmetry with just a guitar plugged in AND when a boost pedal is in use may affect what you are perceiving as the amps dynamics. So make changes carefully in case you want to revers them.

[Chuck H]

Last nite i realized i have been tweaking myself out of what at first was a great tone, so i set it back to the way it was for the most part. Not that it was bad but just kinda got away from what i loved about it at first.

It's been many years but I do remember that you tend to do that. It's hard to stay chill and objective when modding. Just remember it's not a race. The amp isn't going anywhere and taking time for reflection is also important.

[daz]

The dynamics were a product of low enough gain that i could still get a good classic rock gain tone on the lower side but because it was lowish it had great dynamics. And that was due to the super low V2 grid leak. It was actually lower than i told u because there were 2 resistors in parallel and before i removed on it was around 60k. Obviously any time you up he gain dynamics get less good. So i'll the changes then if i like it but want that lower gain dynamic i can always swap the 1M in the switchable V2 grid leak to whatever will parallel with the 220k to get the gain low as it first was.

[daz]

It's been many years but I do remember that you tend to do that. It's hard to stay chill and objective when modding. Just remember it's not a race. The amp isn't going anywhere and taking time for reflection is also important.

Tell me about it ! Thats why i didn't want to get back into this. While i do enjoy tweaking, at some point i just have trouble stopping because i have an ear that detects every little imperfection and leaving those bothers me. Gonna try hard this time to leave it alone once i try your changes and leave it or go back to the original, whichever i prefer. Maybe i'll put the chassis back in the cabinet with JB weld. LOL

[Chuck H]

Gain in the modified circuit I posted using the 220k load will be a tad less than the circuit you posted when using the 1M load. On that note, it's the load as it relates to the plate of the previous stage more than the grid leak value seen by the following stage that makes a bigger difference here.

Loading a tube down definitely has an audible charm in the right circuit. My own favorite amp has about 90k loading V1B. Most people don't notice but the Fender BF circuit loads the mix stage for the vibrato channel (the preferred channel) to lower than 50k. If you wanted to lower the 220k value as a tonal experiment that should be fine. It may necessitate increasing the value of the .0047uf coupling cap to maintain the low end. Maybe not.

[daz]

Ok, did the changes tho i used two .022's in parallel on V1B cathode because i couldn't find a .047uf. Shouldn't matter i'm sure. I also made the load on V2 switchable between 220k and what amounts to about 70k to still get that low gain classic rock tone. Well, theres good and bad. It lost some of what i loved about it in getting that tom petty sorta voxy or BFDR crunch, but overall it sounds more "right" if u will. But that comes at a price. Overall it now sounds better balanced and more "perfect". But sometimes perfect is boring if you know what i mean. The clean sound also isn't what it was. It was utterly glorious but now is just good. This circuit gives a sound i would have preferred several years ago but i'm not quite sure at the moment how i feel other then to say it's great now but it doesn't do that low gain classic rock thing like it did before even when i switch to the 60k V2 load and the cleans aren't amazing as before. (the cleans gotten by rolling the guitar down) At the moment my thought is to leave it like this because it;s more of an all around amp that is more likely to fit well in a mix and be pretty happy with my tone all nite. The loss of those other things i liked about the tone before may be worth it, but i need more playing time. Maybe i will come to find those other things in it with some more knob twiddling. I'm thinking the 220k/500pf in place of the 470k/500pf may be the reason the cleans aren't as good. Those peaking filters are really important to my ear and i may have to try removing that other 220k on the grid and changing the filer back to 470k/500pf and see if that does the trick.

[Chuck H]

Well any amp is "a package" of circuits that work together. It's interesting that your reception of the change doesn't surprise me. Removing the 220k grid stop resistor at this point would likely make it so it won't take the boost pedal as well and may also make the amp too bright. But you don't have to fall into that trap where you end up trying to reengineer from a new starting point again and again. I kept the changes minimal and mostly within the existing design topography so they could be reversed easily. So...

The two principal changes I made were the change on VB cathode from 4.7k to 1.5k and the addition of the 220k grid stop at V2A grid. All other changes were made to compensate for gain or EQ differences due to those two changes. And it's those two changes that make the amp more pedestrian in behavior. In other words, the changes were a sort of package deal. I wouldn't suggest changing any one thing. Rather, just put it back like it was because it can do things more ordinary sounding amps can't that way. But,..

I'll see what might be done to improve the dynamic response using where it is as a starting point anyway.