Any amp modders around?

J

JonWormwood

Member
Jun 16, 2007
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Jax, Fl
I've been getting into it recently with the lee jacksons. They are extremely easy to understand.

I've been experimenting with gain stages. Grid, plate, cathode, coupling, etc etc. It's great learning how to read schematics as well. They are starting to make sense.

Here's the schem for the lee jackson gp1000.

http://wormwoodprophecymetal.com/wp-content/uploads/2012/01/PC-GP1k_schem2_Rev33.jpg

Very simple.

And yes I understand the dangers and how to safely discharge caps.
 
That schematic looks a LOT like an 800/jmp!...

edit, well the input stage and some of the tone stack.
 
greyskull said:
That schematic looks a LOT like an 800/jmp!...

edit, well the input stage and some of the tone stack.
Click to expand...

Just about! This tone stack is pre gain while the marshall is post gain.

Most fender/vox/marshall amps use the same idea. It's weird how similar amps are. Here's the pre amp to most popular marshals

http://www.drtube.com/schematics/marshall/jcm800pr.gif

Though Jubilees are different iirc.

The Mesa Marks are much like the fender idea with tone stack before gain. But they have the graphic eq after to do most of the tone shappage.
 
I don't doubt the Lee Jackson stuff is marshall related, he made his name by modding the hell out of them for famous peoplez. I really want to try some of his Metaltronix stuff.
 
Sloan said:
I don't doubt the Lee Jackson stuff is marshall related, he made his name by modding the hell out of them for famous peoplez. I really want to try some of his Metaltronix stuff.
Click to expand...

Well actually it's more like a fender circuit. I was trying to emphasize how similar (but different) they are. Lee used to make/deaign amps for fender. Jim Marshall used a 59 bassman as his inspiration.

Where's Wolfman? I think that's his name...
 
Which mod where you attempting? The one with the blue caps in the tonestack? What did you want out of the mod and what did you get.

Real quick on tonestacks, the value of the caps and the slope resistor more or less have more of an effect on the mid frequency's Q more than anything by changing the center frequency of the high shelf and low shelf. The closer the shelves are to each other, the narrower the q of the mids. There really isn't that much you could do to a tonestack to make it more usable. Small things here and there to get more effective mid range cut, wider q, and you could throw in a mid sweep pot to really control the center frecuqncy of the mids themself, but this does effect how the bass works to some degree. The fact that the lee jackson you posted is a common cathode driven tonestack, I wouldn't really fuck around with a mid sweep.
 
TheWinterSnow said:
Which mod where you attempting? The one with the blue caps in the tonestack? What did you want out of the mod and what did you get.

Real quick on tonestacks, the value of the caps and the slope resistor more or less have more of an effect on the mid frequency's Q more than anything by changing the center frequency of the high shelf and low shelf. The closer the shelves are to each other, the narrower the q of the mids. There really isn't that much you could do to a tonestack to make it more usable. Small things here and there to get more effective mid range cut, wider q, and you could throw in a mid sweep pot to really control the center frecuqncy of the mids themself, but this does effect how the bass works to some degree. The fact that the lee jackson you posted is a common cathode driven tonestack, I wouldn't really fuck around with a mid sweep.
Click to expand...

There are no mods on the schematic, just different versions.

I've been understanding/tweaking the cathode resistor/bypass cap, plate resistor, coupling cap, etc and how they shape the tone/distortion. The tonestack is fucking useless in this thing.

And that's pretty much what i've gathered about tonestacks as well. Thanks
 
JonWormwood said:
There are no mods on the schematic, just different versions.

I've been understanding/tweaking the cathode resistor/bypass cap, plate resistor, coupling cap, etc and how they shape the tone/distortion. The tonestack is fucking useless in this thing.

And that's pretty much what i've gathered about tonestacks as well. Thanks
Click to expand...

If there is anything you want out of the amp, let me know. I can look through it and give some values as well as explain what the values are doing. For example, if you want tighter distortion, decreasing the bypass cap on V1 and the high gain tube will help a lot.
 
TheWinterSnow said:
If there is anything you want out of the amp, let me know. I can look through it and give some values as well as explain what the values are doing. For example, if you want tighter distortion, decreasing the bypass cap on V1 and the high gain tube will help a lot.
Click to expand...

Done that. Sent from 22uf to .68uf. It really didn't do what I was electing it to :err: Still kind of 'mushy' so I did it to v1b and v2a as well. :err: But an EQ or tubescreamer still cleans up the lowed :err::err: Originals drifted to nearly 29uf.

I think I might try something stupid low like .01uf see what that does.

Lowered the cathode resistor as well to get more gain. I raised the plate resistor (100k to 220k) on v1a and v2a.

I sort of want to move the tonestack to the end...
 
Eq on the first stages are tricky to get right as hell!!! If you need a tight sound you need to use very few low end on the first stages, and then add it later!!
 
Here´s what i´ve done to an old jcm900! Sample without tube screamer!!
 
Last edited by a moderator:
i didn´t like very much the sound of jcm 900!! So i´ve decided to remove the preamp and start from scratch!! It doesn´t look like that anymore. After i´ve decided what components and what values to use, i´ve made a new pcb and instaled all the components. The pcb costed me 35eur and all the components 13eur!!!!
The goal was to have a tight sound with a nice attack, kind of what a tube screamer does in front of an amp
 
covil said:
i didn´t like very much the sound of jcm 900!! So i´ve decided to remove the preamp and start from scratch!! It doesn´t look like that anymore. After i´ve decided what components and what values to use, i´ve made a new pcb and instaled all the components. The pcb costed me 35eur and all the components 13eur!!!!
The goal was to have a tight sound with a nice attack, kind of what a tube screamer does in front of an amp
Click to expand...

Can you elaborate any of the key features you used in this?
 
There´s 4 gain stages( 2x12ax7) So it´s Input--- V1-- Gain----V2-----V3-----V4----Eq--- Output. The tricky part was how to remove almost all low end from the first two gain stages but add it again later!! That is why i really like the eq on the end!! You can abuse the initial stages but control it later!!
 
covil said:
There´s 4 gain stages( 2x12ax7) So it´s Input--- V1-- Gain----V2-----V3-----V4----Eq--- Output. The tricky part was how to remove almost all low end from the first two gain stages but add it again later!! That is why i really like the eq on the end!! You can abuse the initial stages but control it later!!
Click to expand...

The trick with this is to kill the low end gain in all the gain stages, remove some high end in the 2nd and/or 3rd stage (usually just the third with the SLO based amps) and get back your low end by achieving high amounts of attenuation in the tonestack. What the attenuation does is provide an apparent "boost" to the highs and lows. Generally in high gain amps, if you want more bass you can increase the value of the bass pot. Further bass boost is achieved through more attenuation in the power amp section's Global feedback circuit in Phase Inverter driven power amps.

To Jon, the Lee Jackson you posted is probably not the best amp for a beginner to learn to mod on, it strays way too much from the basic, classic and historical design. I would say you are better off building something like the gain stage of an SLO or something similar and modding things to taste. With that you could even throw a clean channel in there as you would have half a tube not being used. A switch or foot switch jack, a relay and a connection to a low voltage would be all you would need for basic channel switching.

Also Jon, the reason that you are not getting huge results with decreasing the cathode bypass caps is because you said it, you increased the gain on the stages by decreasing the cathode resistor. This mitigates the effect of the bypass capacitors. Its a long explanation with some equations, I am more than happy to explain, but simply put, when a cathode bypass cap is being used, the gain of all signals above the resonant frequency will be as if there was no cathode resistor in terms of gain. All you are doing when you mess with the gain via the cathode resistor is increasing the gain on the low end, the gain on the highs never changes. Try going back to the stock cathode resistor values, and decrease the bypass caps on V1a (C1) and V2a (C3) to 1uF.
 
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