SL+ to BE mods

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CrazyNutz

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Fist off I'm new (and late) to MTS, but i'm loving it. I'm a relentless modder/ tweaker.

The brown sound, we love it, and its been made even more popular by the wonderful Brown Eye amps.

But what is the secret you may ask? Variac right? sure, but you don't need one because you can achieve the same effect in the circuit.

All you have to do is get one of the early stage triodes to give up that brown sound, and the way you do that is by increasing the anode/plate resistor from 100k to ~330k. This lowers the plate voltage, and you're headed to brown sound town.

On the SL+ this change would be made to R24. You can replace R24 with a 330k, or lift one end of the 100k and add a 220k in series.

Also this increases the gain on a already overly gainy module (IMHO). To rectify this you can add a 100k resistor to R7.

That's a good starting point Enjoy.



Going deeper into the abyss.


I know this is a mess, but this is just testing, it will be nice and clean when I settle on the values.

Going deeper:
1). Add a 250pf cap in parallel to the 100k resistor on the tube riser board. This snubs some of the extra noise, and unwanted high frequency content that will be added when you raise r24.

2). Add a 0.68uf cap to C13, this will add more balls, and face melting powers.

3). Add a 500pf to c10 (or just tack it across R26 if room is too tight). This gives a more authentic Marshally bite.

4). Add a 500pf cap across R30. This shapes the mids alittle differently.

5). Change R16 to 2.7k. This gives even more gain, and noise, but you should be ok if you add the 100k to R7, and add the 250pf to the 100k on the tube riser. Touch response, and gnarly overtones begain to pop out.

6). Change all the film caps out to something more Marshall esq like SoZo's or Mallory 150's
 
Sweet....ever start with the Plexi or a Brahma as base platform?

I think the Brahma provides a better base for it....every modded Brahma I heard sounded fantastic.
 
JKD said:
Sweet....ever start with the Plexi or a Brahma as base platform?

I think the Brahma provides a better base for it....every modded Brahma I heard sounded fantastic.

No, I need to grab some more mods. I only have two ATM :D Been trying to find base modules for cheap to do mods on but **** hard to find, and when I do find them on ebay they really get bid up pretty quick.

I did look at some pictures of the Brahma, basically just one less gain stage like the Plexi. With a little hackery I could make one of the SL+ gain stages switch in/out on a toggle.

EDIT: if you wanted to do this to a Brahma I "think" you would change R20 to a 330k, judging by pictures I've seen.
 
CrazyNutz said:
JKD said:
Sweet....ever start with the Plexi or a Brahma as base platform?

I think the Brahma provides a better base for it....every modded Brahma I heard sounded fantastic.

No, I need to grab some more mods. I only have two ATM :D Been trying to find base modules for cheap to do mods on but **** hard to find, and when I do find them on ebay they really get bid up pretty quick.

I did look at some pictures of the Brahma, basically just one less gain stage like the Plexi. With a little hackery I could make one of the SL+ gain stages switch in/out on a toggle.

EDIT: if you wanted to do this to a Brahma I "think" you would change R20 to a 330k, judging by pictures I've seen.

yeah...I changed a couple off things around once upon a time.....looks like you're having fun.

Sold all my basic mods and I think JF and Salvation bought up the last of the NOS stuff.....sometimes come across a gold mine when people don't know what they have..scored an amp and three mods for $400 once upon a time....of course these days it's near impossible to sell stuff too.
 
JKD said:
CrazyNutz said:
JKD said:
Sweet....ever start with the Plexi or a Brahma as base platform?

I think the Brahma provides a better base for it....every modded Brahma I heard sounded fantastic.

No, I need to grab some more mods. I only have two ATM :D Been trying to find base modules for cheap to do mods on but **** hard to find, and when I do find them on ebay they really get bid up pretty quick.

I did look at some pictures of the Brahma, basically just one less gain stage like the Plexi. With a little hackery I could make one of the SL+ gain stages switch in/out on a toggle.

EDIT: if you wanted to do this to a Brahma I "think" you would change R20 to a 330k, judging by pictures I've seen.

yeah...I changed a couple off things around once upon a time.....looks like you're having fun.

Sold all my basic mods and I think JF and Salvation bought up the last of the NOS stuff.....sometimes come across a gold mine when people don't know what they have..scored an amp and three mods for $400 once upon a time....of course these days it's near impossible to sell stuff too.

Right on, I scored this RM100 with the SL+ and a foglifter for $200 locally from a kid that played It without a cab. Long story short it was non-functional, I replace all the fuses and power tubes and now it's roaring. Haha, I was going to flip it until I realized how rad the **** thing is. I have some high end amps, and this thing is every bit as good or actually better because of the flexibility. Such an underrated system, It's hard to believe Randall didn't market MTS well enough to make it more popular. I have a feeling the stock modules were just too mediocre to outshine other amps.
 
Nice CrazyNutz and thanks for sharing.

It looks like you're running a couple of caps on the tube board in C1 and C2 so you'll be adding their respective capacitance at that point. I've found 250p to be alot in that position on even the highest gain modules I've built. Something around 100p is typically enough to stop unwanted squeal (or the stage going into oscillation) without losing too much top end sizzle.

Have you experimented with some lower values there?
 
CrunchBunch said:
Nice CrazyNutz and thanks for sharing.

It looks like you're running a couple of caps on the tube board in C1 and C2 so you'll be adding their respective capacitance at that point. I've found 250p to be alot in that position on even the highest gain modules I've built. Something around 100p is typically enough to stop unwanted squeal (or the stage going into oscillation) without losing too much top end sizzle.

Have you experimented with some lower values there?

I was running the stock 100pf + a 250pf there. There is a lot of nasty high fizz, and trash being generated when the first triode is ran with a 330k plate. The 100 + 250pf snubs it out very well, still had plenty of highs.

As of right now I have remove the 100 + 250pf, and replaced it with a 500pf, this is the value the brown eye has in the cathode follower, adds a little more towards the brown sound, still plenty of highs

This is what it looks like now


I removed c12 500pf as this is normally across the r24 100k plate resistor. When I added the 220k in series with the r24 100k the c12 was across both. I now have a 500pf sliver mica across only the 100k resistor. This is exactly how a real brown eye is setup.

Actually everything right now is very close to an actual brown eye, the main exceptions being the coupling caps at 0.022uf where the brown eye uses 0.0022uf. However I do have a 0.0022 in c3, so it sound like a BE100 with the fat switch on right now.
 
Excellent. For a lot of amps you can get pretty close just replicating the circuit then tweak the coupling / grid bypass caps and tone stack to taste. A brown eye is definitely on my to do list. Thanks again for sharing.
 
You might also want to try changing R29 to 820Ohm (just like the BE).

Also I reckon the saturation switch would be worth trying. I've been intrigued by that the use of transistors in that arrangement...
 
CrunchBunch said:
You might also want to try changing R29 to 820Ohm (just like the BE).

Also I reckon the saturation switch would be worth trying. I've been intrigued by that the use of transistors in that arrangement...

I did contemplate changing R29, but 820 and 1k are so close anyway. I might just try it to see If it makes much difference. 820 is marshall spec there too, looking at a BE you can see it started life as a 1968 spec plexi, Dave added mods to it, and later refined it with a vox style clean channel, etc.

As for the sat switch, I'll alligator clip some npn's in and see if it's worth it. I've modded od/fuzz pedals by swapping the clipping diodes with MOSFETs in the past with good results. MOSFET's sound more like tube clipping, not sure what general purpose transistors sound like, first thought would be much like a plain old Si diode, but that would be a naive thought.


Interestingly I've heard (videos, clips of) several "BE" and "HBE" mods built by Dave Friedman himself. I did not think they sounded much like a BE, they sounded brown but not like the "Brown Eye". The judge GTO module is supposed to be a BE with the Sat transistors in, and while it sounds good it does not sound like a Brown Eye. I found a good picture of the GTO, and it does not use the 330k plate resistance, it has the standard 100k there. It does however have a large looking silver mica in C1T, bet it's a 500pf.

I know there are other things that contribute to the "Brown Eye" sound, like the low main, and screens filtering in the real amp, etc.. But I've modded several amps to BE, and that first stage plate configuration is the largest contributing factor to the BE tone. It's like the 39k cold cathode configuration you find in Soldano's, Recto's, and 5150's
 
Our big problem with a true BE clone is that on the Randall system we have an initial gain stage which is very neutral in character. It's just providing a clean signal boost in the front end with its pretty much centre biased 100k/1.5k setup.

The real BE has two 220k+100k/2.7k+0.68uF to begin with. So we're really only able to start simulation from the second.

At full gain, a single 220k+100k/2.7k+0.68uF stage will be giving us the "brown" aspects of the BE but we're probably losing some harmonic richness by not running two of these. It will also mean the way the BE cleans up on the guitar volume will be different with the MTS version.

Good to hear though that from your experience a single 'BE like' gain stage is enough to get some of the character going.
 
CrunchBunch said:
Our big problem with a true BE clone is that on the Randall system we have an initial gain stage which is very neutral in character. It's just providing a clean signal boost in the front end with its pretty much centre biased 100k/1.5k setup.

The real BE has two 220k+100k/2.7k+0.68uF to begin with. So we're really only able to start simulation from the second.

At full gain, a single 220k+100k/2.7k+0.68uF stage will be giving us the "brown" aspects of the BE but we're probably losing some harmonic richness by not running two of these. It will also mean the way the BE cleans up on the guitar volume will be different with the MTS version.

Good to hear though that from your experience a single 'BE like' gain stage is enough to get some of the character going.

Yes that first built in "Randall" stage throws things off a bit, but not too much it's much like a clean/buffer/boost.

And yes in a Brown Eye amp there are two, (for lack of a better word) "Brown Stages" However both are only used in HBE mode, BE mode uses one. Therefore one is all we need to get that sweet brown sugar :)

I converted my 68 replica plexi to BE, sounds like pretty spot on. I have since added another 12ax7 to implement the clean, and HBE. I have it shelved ATM because I am having too much fun with this MTS stuff :)

Probably the more limiting factor in getting the feel right is the high filtering in the RM100, but in all reality I don't mind the stiffer feel too much, and with the foglifter the NFB resistor is 100K and that allows the amp to be a little more looser/open. I could go in and replace the filter caps, but that big mess of wiring hovering over and around the power board is not giving be incentive to poke around there :shock:

I have fresh ears today so I plan on firing up the amp to see where I'm at, probably going to try subbing that 1k cathode resistor out for a 850.
 
ok so R29 sounds better with the stock 1k and r16 sounds better with the stock 4.7k

Lowering these value only lends itself to more gain and more edgy stiffness.
 
CrunchBunch said:
You might also want to try changing R29 to 820Ohm (just like the BE).

Also I reckon the saturation switch would be worth trying. I've been intrigued by that the use of transistors in that arrangement...

Ok got some time to alligator clip in the sat components, and DAYUM! Sounds and feels rad! That one is for sure going in.
 
CrazyNutz said:
CrunchBunch said:
You might also want to try changing R29 to 820Ohm (just like the BE).

Also I reckon the saturation switch would be worth trying. I've been intrigued by that the use of transistors in that arrangement...

Ok got some time to alligator clip in the sat components, and DAYUM! Sounds and feels rad! That one is for sure going in.

So this works by basically adding clipping to the signal (you're using one gate of the transistor right....thought I've seen Zeners used for this) ?
 
CrunchBunch said:
Awesome Nutz. What transistors did you use?

2n3904, it's what I had on hand, I'm going to order some MPSA06's

Pretty simple, and should probaly work well on any marshall type module
 
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