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
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