Please help in improving the sound of a gain channel.

[Alan0354]

Please listen to the first minute, Is this called gritty?
Marshall DSL15C and DSL40C Combo Amps - YouTube

[Alan0354]

I redo the amp, all I have to do is using 1uF cathode bypass cap and 1.5nF coupling cap for the second stage. The amp comes alive. Now I just need to get more bass.

I tried increase the cap C6 from 0.047uF to 0.1uF in the tone stack and increase the bass pot VR3 to 1M. That help a lot. But I still want more bass, any suggestion? I want to have the option of sounding like the metal type of "thumping" sound when they hit the lower strings.......Bassy by not farting.

[Chuck H]

When you say that you "is using 1uF cathode bypass cap and 1.5nF coupling cap for the second stage." does that mean that you have a 1.5k/1uf cathode circuit on the second stage followed by a 1.5nf cap feeding the grid of the third stage? For a cascade amp it's usually best to gradually introduce bass. What you have done is generated a lot of LF gain from the first stage to the second and then thrown it away between the second and third. You would do better to move that 1.5nf coupling cap in between the first and second stage. Then use a bigger cap between the second and third.

[Alan0354]

When you say that you "is using 1uF cathode bypass cap and 1.5nF coupling cap for the second stage." does that mean that you have a 1.5k/1uf cathode circuit on the second stage followed by a 1.5nf cap feeding the grid of the third stage? For a cascade amp it's usually best to gradually introduce bass. What you have done is generated a lot of LF gain from the first stage to the second and then thrown it away between the second and third. You would do better to move that 1.5nf coupling cap in between the first and second stage. Then use a bigger cap between the second and third.

I'll try that. Right now, it sounds pretty good.

Yes, 1uF with the 1.5K on the second stage.

[R ski]

A classic high gain issue often overlooked is that 10mf capacitor across the cathode resistor on the input on stage one. That value will give your gain a lot of ump, however I learned that reducing that cathode bypass to a lower value of 1mf or 0.68 mf keeps the low frequency from mudding up the amps voice. A single coil axe into that configuration will drive, but not as well as a humbucker axe.

Another area of concern is the output of stage two looking at a divider with 220 k then 1meg finally 220k to ground . If traced right, changing that resistor divider network so 1meg goes to groung from where the 220k taps from the annode capacitor will yield some more gain going into the stage three of that configuration.

[Alan0354]

So far, I use small coupling caps and result is very good, no more farting and sounds really good. I still have a problem with the tone stack. After using small caps to cut the bass in the first few stages, I don't have enough bottom. I increase C6(bass capacitor) from 0.047 to 0.1 and reduce the mid cap(0.047) down to 0.022 to increase the lower mid. Also I reduce the treble cap from 470p to 180pF.

I change the treble pot to 250K and Bass pot to 1M already.

I still don't have enough bass and I have to turn the treble almost all the way down to get the better sound. If I crank the treble pass half, it is squealing. I have to have the bass pot to full all the time.

I even tried using a 470pF from signal path to ground to cut some highs, I don't like the sound.

How do you change the tone stack to increase bass and reduce treble?

[Chuck H]

First, increase the treble cap back up to at least 250p. For a Marshall type circuit 330p would be better, and the lowest value that will approach sounding "right". 180p isn't going to have the right frequency range to give definition to the attack on lower notes. Also, the frequency knee of this cap changes with the size of the pot. A 250k pot and a 180p cap is going to sound horrible if you ever decide to overdrive the power tubes. I'd change the bass cap back to .047 too. .1 is not really increasing bass for a guitar, just extending the frequency beyond the useful range.

If you want to change the balance of high to low in the tone stack you can decrease the value of the slope resistor. You're already pretty low but try a 33k

Also, try larger coupling caps! Yup, raise them back up again, a little. If 1500p is in between 1 and 2 try a 2200p. Usually, for bluesy to hard rock crunch the first coupling cap is the only one that needs to be small. If you have a small value cap between 1 and 2 AND 2 and 3 try increasing the value between 2 and 3 to something more like .01 or .022.

[J M Fahey]

Agree and add: download a copy of Duncan's TSC , tone stack simulator, and start playing with cap and resistor values until you get a curve you like.

That said, I'd leave the amp as is and drive it balls to the wall into a 4 x 12" .
The "lost" bass will reappear

[Alan0354]

Thanks both of you. You two are of big help.



Hi Chuck, I learned really fast the 180p did not work!!! I kept trying, ending up back to the 470pF!!! I tried increase the coupling cap, but I ended up increase the cathode bypass cap. That gives me less farting. In fact, no farting. But still, I have to turn the treble very low, like 2 to 3 o'clock only ( 12 is minimum and 10 o'clock is max).

[Alan0354]

I usually test the gain channel at about 4 o'clock ( 12 o'clock is minimum). I am quite satisfy with the sound at this point. BUT I tried to crank it last night to 8 o'clock, it was loud!!! But it seems like the amp got cleaner sounding, too much treble. How can the distortion lessen when cranked. If you look at the topology, the preamp never change!!!! the volume is the final step before the power amp, the distortion is all set up in the preamp and never got touched. Is this my ears is too painful or is there any other reason.

How do I deal with the sound getting brighter at higher master volume. I already lower the master volume from traditional 1M to 500K to avoid treble loss when cranked down. Do I need to have a small cap to feed forward like the bright switch circuit in the old Fender amps?

[Chuck H]

How do I deal with the sound getting brighter at higher master volume.

Turn down the treble and presence controls Master volumes usually invoke some need to adjust other parameters when the master is at different settings. You may also find that different power tubes make a big difference. At higher MV setting the power tubes get more involved so how a particular tube clips in your amp can change the tone beyond a certain threshold.

Getting brighter seems unusual though. I think it's more common for an amp to start mellowing as the power tubes clip. I'm suspecting a possible oscillation issue or maybe, if the tone seems buzzy, you're getting crossover distortion as the power tubes begin to clip. You could check this on a scope.

[Alan0354]

I'll check for oscillation. As you see I am quite tied up in the noise stuff!!!. can't get things done the last two days.

[Tooboob]

I get frustrated with most amps, partially because I don't want to crank them up loud, and I'm depending on a lead preamp for my lead sound. This doesn't work well with most amps. I adjust the preamp gain for a nice distorted single-note lead tone, then bang a six string chord and get no definition. You can't really hear all the notes.

I did an experiment. I recorded a six-string chord with that high gain setting. Then I recorded each string separately and mixed them together. Comparatively speaking, angels came out. I've got a MIDI capable guitar, and I figured I could build a box with six tuned distortion circuits, mixed at the end. Roland beat me to it: GR-D: V-Guitar Distortion | Roland U.S.

The distortion in the video isn't to my liking, but you get the idea.

You can think of a very high "gain" preamp as sort of a tuned comparator. Let's feed it a combined low frequency square wave and a slightly lower amplitude higher frequency square wave. Looking at the result, the higher frequency disappears. The same effect happens to a lesser extent with sine waves. New harmonics appear from the clipping, so it can still sound harsh. A "high gain" preamp does this too, only to a lesser extent, generally with asymmetry. Reducing the bass before a distortion stage helps, and you can boost it after the distortion stage. You can also achieve this with cathode capacitors etc. inside the preamp. You can filter between stages and make the effect assymetric.

[Alan0354]

I have another question. I tried the Legend GB128 type of British speaker, the clean is good, but the high gain channel in this thread sounds terrible. It is very throaty. How do I get rid of this? With Utah, it sounds quite good as the Utah is very open sounding.

[Tom Phillips]

...I did an experiment. I recorded a six-string chord with that high gain setting. Then I recorded each string separately and mixed them together. Comparatively speaking, angels came out...

That's really interesting. Do you have sound files you can post so we can hear the results?
Thanks,
Tom