Please help in improving the sound of a gain channel.

[Alan0354]

I am experimenting. Right now, I have VR1=1M pot, C2 is 1500pF. I have a 100K resistor from C2 to VR1. I changed the resistor from 100K to 500K, it has less of a throaty sound, a lot better. I don't understand why as it is only a voltage divider effect to get less voltage to the second stage. Why does it change the sound?

[J M Fahey]

I am experimenting. Right now, I have VR1=1M pot, C2 is 1500pF. I have a 100K resistor from C2 to VR1. I changed the resistor from 100K to 500K, it has less of a throaty sound, a lot better. I don't understand why as it is only a voltage divider effect to get less voltage to the second stage. Why does it change the sound?

Everything is interactive.
I guess you still refer to the schematic on post #5.
Raising the series resistor not only changes the voltage divider ratio but also almost doubles voltage divider impedance.
You just added a full octave down, how's that?
As if you had replaced 1500pF by 2700pF !!!
Another "unseen" change, which in fact is in plain sight, is that now the effective grid stopper also rised, which cuts highs even more.
So now you lowered the stage passband considerably.
More than you bargained for, huh?

[Alan0354]

Everything is interactive.
I guess you still refer to the schematic on post #5.
Raising the series resistor not only changes the voltage divider ratio but also almost doubles voltage divider impedance.
You just added a full octave down, how's that?
As if you had replaced 1500pF by 2700pF !!!
Another "unseen" change, which in fact is in plain sight, is that now the effective grid stopper also rised, which cuts highs even more.
So now you lowered the stage passband considerably.
More than you bargained for, huh?

Actually, I have provision to switch change coupling caps to experiment, so coupling cap factor is eliminated totally. I know better than that!!!! Makes no different between as low as 820p to 0.022 on the throatiness.

Grid stop is a good point. Grid stop going from 100K to 500K roll off high end. But I don't see why it change the throatiness. Because in the original circuit, I can turn down the 1M volume pot and literally did the same. That cannot explain the difference. But maybe I should experiment with a real grid stop resistor after the volume pot. That might be good in rolling off a little highs.

The only other thing possible is my ears get used to the sound!!!

Still the question, what cause the throatiness? This is a very generic circuit every joe blow uses. Why other's don't have the throatiness? I can say the clean channel sounds very good, so it is not the PI or power stage.

I am going to experiment with the grid stop and see what happens

Thanks

[Chuck H]

Aside from the actual voicing... It might behoove you to keep the divider resistances as low as practical and raise the capacitance to suit. You don't have an uber gainer going there, but you do have a cascade preamp. More resistance and higher loads generally means more hiss. The difference will be small but it's worth keeping in mind as you design.

[Alan0354]

I double check, I think my ears are playing a trick on me. I put two pots as grid stop resistor and removed the 500K so I have adjustable grid stop for the second and third stage inputs. It dose not make any difference on the throatiness. It did make a big difference on the high freq as expected. So I was wrong on the hearing. So we are back to square one. the question is still why I have the throatiness or boxyness with my preamp. It is very standard run of the mill stuff.

About the voicing, I don't even know what to do, it is very simple. I tried increase and decrease the coupling caps, the cathode bypass caps and nothing change on the throatiness, only the bass response. I even change the mid cap of the tone stack between 0.022, 0.033 and 0.047, no difference on the throatiness.

Of cause, all the changes of the caps change the amount of bass and bottom. But that has nothing to do with the throatiness.

[Chuck H]

Perhaps what your hearing is a modulation issue. The particular shape of the waveform has a certain effect. Changing the bias, usually drastically, on one of the stages may do something good for you. Try changing R6 in your above schematic to something like 10k and then recover lost gain by decreasing R8. The difference is any clipping of that stage will now be asymmetrical. This is commonly done in some Marshalls and nearly all high gainers somewhere along the signal chain. Might be something to it.

EDIT: I just ran some figures and it looks like changing R6 to 10k will require that you bypass R8 completely to recover the stage gain.

[Alan0354]

This is the updated schematic:

[Chuck H]

Ok... New kettle of fish to fry. You've been busy. Don't go all daz on us (hope he sees this )

Here's what I'd try (with brief, non technical information why)

R3 150k You need gain here to start the clipping early. It's almost impossible to clip the second stage too much with the first. This get's the ball rolling.

R7 100k A common value and chosen mostly for familiarity with what it'll do with other changes. You don't need as much gain here because there is plenty of input signal for this high mu triode to pound the following stage. As you already know because you have a 50/50 divider on this stage.

R8 2.7k (second stage cathode resistor) This is a tad cold. Just enough to get a little asymmetry into the wave form. This helps promote 2nd harmonics and keeps things from getting too square and hollow sounding. You could try values up to 4.7k here. Higher than that and it the bypass cap shelf becomes really dominant and tricky to voice for IMHE.

C3 .022 Using two small caps in a row is something I've only seen on amps with four or more cascade gain stages. This is part of what's thinning your sound and forced you into other circuit oddities. A little lack of bottom end clarity is OK and a small part of the sound. It probably won't be objectionable because of the extra shelving effect of the raised value cathode resistor on this stage.

C12 jumper/remove You don't need the gain here. Cathode follower clipping is more of an effect than a quantified flatter/rounder waveform. It compresses a little and takes the sharp edges off the square wave. There's plenty of gain for that without the bypass cap. Besides, it's only bumping the top end and you had trouble balancing that in the tone stack. Removing this bypass cap should help.

R22 jumper/remove You shouldn't need it with the other changes. You'll be voiced too dark if you keep it.

VR5 25k This is a Marshall style circuit. They sound better with fuller mids and 10k keeps a lot of mid scoop in the tone.

That's my take. I'll bet six beer PNW that anyone following will back these changes. That doesn't guarantee you'll like it, but it should get you out of the box, so to speak. You may end up needing to change the R20/R21 divider values to restructure gain a little. Maybe not. If it sounds too dark at low to moderate gain settings add a bright cap of 220P to 1000p to the volume control.

[Alan0354]

Ok... New kettle of fish to fry. You've been busy. Don't go all daz on us (hope he sees this )

Here's what I'd try (with brief, non technical information why)

R3 150k You need gain here to start the clipping early. It's almost impossible to clip the second stage too much with the first. This get's the ball rolling.

R7 100k A common value and chosen mostly for familiarity with what it'll do with other changes. You don't need as much gain here because there is plenty of input signal for this high mu triode to pound the following stage. As you already know because you have a 50/50 divider on this stage.

R8 2.7k This is a tad cold. Just enough to get a little asymmetry into the wave form. This helps promote 2nd harmonics and keeps things from getting too square and hollow sounding. You could try values up to 4.7k here. Higher than that and it the bypass cap shelf becomes really dominant and tricky to voice for IMHE.

I can't find R8, I think I drew something wrong and erased it and somehow left "R8" behind.

C5 .022 Using two small caps in a row is something I've only seen on amps with four or more cascade gain stages. This is part of what's thinning your sound and forced you into other circuit oddities. A little lack of bottom end clarity is OK and a small part of the sound. It probably won't be objectionable because of the extra shelving effect of the raised value cathode resistor on this stage.

C5 is 470pF treble cap, I don't quite follow. Please explain.

C12 jumper/remove You don't need the gain here. Cathode follower clipping is more of an effect than a quantified flatter/rounder waveform. It compresses a little and takes the sharp edges off the square wave. There's plenty of gain for that without the bypass cap. Besides, it's only bumping the top end and you had trouble balancing that in the tone stack. Removing this bypass cap should help.

R22 jumper/remove You shouldn't need it with the other changes. You'll be voiced too dark if you keep it.

This is very strange, I have so much treble that without R22, it really sounds bad if I put the VR2( treble) pass 3 o'clock ( noon=minimum).



VR5 25k This is a Marshall style circuit. They sound better with fuller mids and 10k keeps a lot of mid scoop in the tone.

That's my take. I'll bet six beer PNW that anyone following will back these changes. That doesn't guarantee you'll like it, but it should get you out of the box, so to speak. You may end up needing to change the R20/R21 divider values to restructure gain a little. Maybe not. If it sounds too dark at low to moderate gain settings add a bright cap of 220P to 1000p to the volume control.

Thanks for all your time, I will try the mod one by one and see which one is the major one. I really appreciate your help.

Alan

[Chuck H]

Thanks for all your time, I will try the mod one by one and see which one is the major one. I really appreciate your help.

Alan

Well, some of them are meant to work together so maybe carry through the list and take notes along the way. Otherwise the intended end result will never happen. Think of these changes as one mod.

[Alan0354]

Well, some of them are meant to work together so maybe carry through the list and take notes along the way. Otherwise the intended end result will never happen. Think of these changes as one mod.

How about the questions I have in Blue

[Alan0354]

Ok... New kettle of fish to fry. You've been busy. Don't go all daz on us (hope he sees this )

[Chuck H]

Ahhh... There's that doctor like writing of yours (and I don't see as well as I once did).

C5 should say C3. The coupling cap between stages two and three.

R8 is written a good distance off the component but didn't seem to have another home. It's the second stage cathode resistor.

I edited my post above for the corrections.

As to the extra resistance on the treble side of the tone stack... When you increase the coupling cap between two and three you're going to add some bottom end. Removing the bypass cap on stage three will reduce highs a little. Increasing the value of the cathode resistor on the second stage will add some mids because it will lower the knee of the bypass cap. A bigger mid pot will add low mids. So you'll need a little more treble to balance everything.

[Alan0354]

Ahhh... There's that doctor like writing of yours (and I don't see as well as I once did).

C5 should say C3. The coupling cap between stages two and three.

R8 is written a good distance off the component but didn't seem to have another home. It's the second stage cathode resistor.

I edited my post above for the corrections.

As to the extra resistance on the treble side of the tone stack... When you increase the coupling cap between two and three you're going to add some bottom end. Removing the bypass cap on stage three will reduce highs a little. Increasing the value of the cathode resistor on the second stage will add some mids because it will lower the knee of the bypass cap. A bigger mid pot will add low mids. So you'll need a little more treble to balance everything.

Thanks, It will take a little time to do all that. I'll report back.

[Chuck H]

It's a handful of components really. The hardest part will be replacing the mid pot. It won't take long if you have the parts on hand. And it won't take long to reverse it if you don't like it.

I'm looking forward to your results.