Preamp Mods?

Freddy G

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Hey guys!

Picked up this nifty little preamp kit. A dual channel....Fender-ish cleans and Marshall-ish drive.

Love to have a discussion on what you knowledgeable amp dudes think of this circuit.

I can say that the clean channel sounds fantastic. The drive channel? It's OK...stacks of distortion....more than I'll ever need.

Can anyone suggest mods to make it less compressed? It's really creamy, fine grit distortion, but I find it lacks punch.

I have already switched out the R11 plate resistor for V1A from 220k to 100k. That helped a bit. And I changed the slope resistor R18 to 50K because I wanted a bit more bottom.

I guess what I'm thinking about is playing with the various cathode resistors and bypass caps. What I find a bit curious is that all of the stages have bypass caps and a couple of them are large. I think a lot of Marshall type voicings have stages with no bypass caps and even a stage with just a high value resistor...I think that's referred to as "cold clipping"....but I don't see anything like that in this scheme. Do you think that's what I should go after? I'm thinking of hooking up a switch to try that...maybe a two or three different resistor and or bypass cap combos....what do you think? Maybe on V2B?


LP dual channel pre.jpg
 

redking

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Hey Freddy - is this a rack-mount unit?
 

Tubejockey

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Raise the value of R13, that will trim down the gain. You could also lower c10 a bit to cut some gain from the bottom. To get more punch, you want to keep the low end cleaner and the overall gain in check.
 

el84ster

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I’ve built and modded many tube designs searching for an ultimate tube drive. What I’ve found, for my ears, I really love the sound of tubes center-biased when distorting. It give the least crap (blatty , ratty, etc) sounds. That means 1.5k/25uf on the cathode and 100k on the plate (or something not too far off) Fender figured it out.
Another tip is that the best distortion is made up by a few different stages adding some gain, not adding lots of gain at once. This sounds more like an amp.

bias each stage a little different and you get a nice sound at the end. And yeah sounds likeyou need to lower the gain of each stage, too much makes things preampy sounding and not power tube sounding.

Have fun voicing this, one of my favorite things to do.
 

Freddy G

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First off....thanks guys for chiming in!

Raise the value of R13, that will trim down the gain. You could also lower c10 a bit to cut some gain from the bottom. To get more punch, you want to keep the low end cleaner and the overall gain in check.
Thanks for the suggestion. What is the function of that resistor...attenuator? I think a typical Marshall value there is 475k right? How about R12? should match that with R13? I'll try a different value for C10...maybe even throw a switch in to go between the 100n, a lower value and no bypass cap at all. Sound reasonable?
sounds like you need to lower the gain of each stage, too much makes things preampy sounding and not power tube sounding.
....
That's a good description of what I'm hearing.
 

V2

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KOC's LP pre has three gain stages before a plate-driven tone stack and one more gain stage after the TS. The centre two gain stages have 221k plate load Rs; the coupling caps are generally high value (10-22n), and the frequency shaping is done in the cathode circuts of those two middle gain stages. There is a tonne of gain on tap.

Do you have too much gain/compression when you have engaged that big voltage divider after the TS? I'm not sure how the three pads engage it, but a 909k series resistor and 121k shunt R will drop the signal to about 12%!

Things to try:
- Lower R16 from 221k to 100k (as you have done with R11).
- The stock values of R12 and C7 will boost frequencies above ~130 Hz, and so this is really a bass roll off. Even though it is a 'voltage divider that cuts the signal in half (with equal-value resistors), the large-value bypass cap boosts frequencies above the cutoff to max gain. So think of this as having max gain above 130 Hz and -3 dB below that cutoff (keeping in mind that the cutoff is actually a gradual roll off...). You can achieve bass reduction by lowering the value of C6, and then you can use the voltage divider to reduce the signal at all frequencies (by removing the bypass cap). Try changing C6 to 2n2 and removing C7. For more signal reduction, increase the value of R12 (e.g., to 220k, 330k, or 470k).
- I know it is tempting to try an unbypassed 10k cathode resistor for the Marshall mojo. But you'd probably loose too much bass attenuation and end up with flub. But if you do want to try it, then do it before the step above. Remove C8 and increase the value of R10 (to 6k8, 8k2, or 10k). Don't do it on the following gain stage, because C10 has a greater effect on tone.
- KOC has written about a 'low gain' (or perhaps low-noise) version of the LP-pre, where the distortion channel doesn't hook back up with the fourth gain stage (which is shared with the clean channel). Does the kit come with instructions for doing that? If so, that's probably what you want to do (in addition to lowering the value of the 221k plate resistors).
 
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V2

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Raise the value of R13, that will trim down the gain. You could also lower c10 a bit to cut some gain from the bottom. To get more punch, you want to keep the low end cleaner and the overall gain in check.
Increasing the value of R13 will increase gain. You could lower the value or increase the value of R12 (or do both)
 

V2

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First off....thanks guys for chiming in!



Thanks for the suggestion. What is the function of that resistor...attenuator? I think a typical Marshall value there is 475k right? How about R12? should match that with R13? I'll try a different value for C10...maybe even throw a switch in to go between the 100n, a lower value and no bypass cap at all. Sound reasonable?


That's a good description of what I'm hearing.

R12 and R13 for a resistive voltage divider and together form the 'load' for the previous gain stage. If you increase the values to stock Marshall values (470k each), you'll actually get more gain from the previous preamp stage (probably not what you want unless you make other changes to the circuit to lower gain). As noted earlier, try removing the bypass captor's lower gain by 3 or 4 dB for most frequencies (above 130Hz)
 

Freddy G

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KOC's LP pre has three gain stages before a plate-driven tone stack and one more gain stage after the TS. The centre two gain stages have 221k plate load Rs; the coupling caps are generally high value (10-22n), and the frequency shaping is done in the cathode circuts of those two middle gain stages. There is a tonne of gain on tap.

Do you have too much gain/compression when you have engaged that big voltage divider after the TS? I'm not sure how the three pads engage it, but a 909k series resistor and 121k shunt R will drop the signal to about 12%!

Things to try:
- Lower R16 from 221k to 100k (as you have done with R11).
- remove C16
- The stock values of R12 and C7 will boost frequencies above ~130 Hz, and so this is really a bass roll off. Even though it is a 'voltage divider that cuts the signal in half (with equal-value resistors), the large-value bypass cap boosts frequencies above the cutoff to max gain. So think of this as having max gain above 130 Hz and -3 dB below that cutoff (keeping in mind that the cutoff is actually a gradual roll off...). You can achieve bass reduction by lowering the value of C6, and then you can use the voltage divider to reduce the signal at all frequencies (by removing the bypass cap). Try changing R6 to 2n2 and removing C7. For more signal reduction, increase the value of R12 (e.g., to 220k, 330k, or 470k).
- I know it is tempting to try an unbypassed 10k cathode resistor for the Marshall mojo. But you'd probably loose too much bass attenuation and end up with flub. But if you do want to try it, then do it before the step above. Remove C8 and increase the value of R10 (to 6k8, 8k2, or 10k). Don't do it on the following gain stage, because C10 has a greater effect on tone.
Thanks! Lots of stuff to try....will report back.
 

V2

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I updated my original suggestions a bit after I remembered that the fourth gain stage is shared with the clean channel in the LP-pre. That means you cannot remove C16 without making the clean channel anaemic. So I removed the suggestion to remove C16.

I also added the following:
- KOC has written about a 'low gain' (or perhaps low-noise) version of the LP-pre, wherein the distortion channel doesn't hook back up with the fourth gain stage (which is shared with the clean channel). Does the kit come with instructions for doing that? If so, that's probably what you want to do (in addition to lowering the value of the 221k plate resistors).
 

Freddy G

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I updated my original suggestions a bit after I remembered that the fourth gain stage is shared with the clean channel in the LP-pre. That means you cannot remove C16 without making the clean channel anaemic. So I removed the suggestion to remove C16.

I also added the following:
- KOC has written about a 'low gain' (or perhaps low-noise) version of the LP-pre, wherein the distortion channel doesn't hook back up with the fourth gain stage (which is shared with the clean channel). Does the kit come with instructions for doing that? If so, that's probably what you want to do (in addition to lowering the value of the 221k plate resistors).
I switched the plate resistor to 100k. I like it. I removed C16 (actually I put it on a switch so I could a-b instantly)

I like it removed for the gain channel but I'm not 100% in love with it. It removes some of the "bloatedness" but leaves it a little too anemic...also it dropped the output by a whopping 3db! And just like you predicted....way too anemic on the clean channel. Perhaps there's a "split the difference" solution.

Does the kit come with instructions for doing that?
Yes and no...it says ....and I quote..."any lead sound from mild overdrive to 1970s saturation is easily set-up by changing one or two resistors or caps"

but then it doesn't say which ones! And when I called him to ask directly his answer was "you should buy my book"
 
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el84ster

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Mmm hmmmm! Dug the clean and dirt with c16. Man that’s sounding pretty cool.
 

el84ster

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That sucks though about his response, esp. after you already bought his preamp. I have lots of his books but he sounds like kind of a douche.
 

Freddy G

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Mmm hmmmm! Dug the clean and dirt with c16. Man that’s sounding pretty cool.
I agree....I just love the clean with the C16. The drive though....that's the prob....if I could just trim back some of the bloat but without losing the balls...(or affecting the clean sound)
 

Freddy G

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That sucks though about his response, esp. after you already bought his preamp. I have lots of his books but he sounds like kind of a douche.
Well, I don't blame him for wanting to sell his books. So I'll leave it at that.

But it IS a testament to how cool this community is that you guys are interested in helping me and there's nothing in it for you....I really do appreciate that!
 

NotScott

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Lowering the slope resistor added some of your bloat. I know when you drop that value that gets back a lot of gain that gets lost through the tone stack making the amp sound bigger and gainier, albeit at the loss of tone control responsiveness. Maybe try and raise it a bit and then readjust your tone controls. It will thin out the cleans a touch but based upon your samples, I think the cleans have a bit of meat to spare and with the added control response, you should be able to dial it back to something you like.

If you want to just tweak the OD and leave everything else, then V2's suggestion around the R8/C8 combo is where you should be starting.

Happy soldering!
 


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