Guys, I tried a 500K pot ahead of stage 3, as well as a 220K/330K divider ahead of a 500K pot. IMO, it's not a great mod. Reducing gain here takes the life out of the tone, stiffens the feel, and removes some perceived low end. More and more, I am thinking the gain on the express is just right. It actually can get pretty loud and cleanish.
For me, I stuck with a switch to toggle between .002/150K and .0047/220k/330k divider. Note, this does not reduce gain, but adds bass.
A good non-invasive mod to make the amp more bassy, or dark is to install a .0047/500K pot across the V1b plate. Cut the track on the pot to make it completely out of the circuit when turned all the way up. This was suggested to me by Mark Abbot a long time ago.
Jake
express pre mods
Moderators: pompeiisneaks, Colossal
Re: express pre mods
Sorry no way to take pics right now. I believe I tied the 330K resistor to the ground side of the 10K cathode. The other cathode values I used are in new turrets next to the 10 K cathode.straycat wrote:Can I just change the cathodes to 2.7k/.68uf on V1 pin8 and 1.5k/22uf V1 pin3 along with adding a 33k to the input without doing the voltage divider? What effect would this have ? What is the pupose of the voltage divider and how would you implment it on a turret board without adding turrets? Keithrick do you have any pictures of your amp?
You can surely try everthing you suggested but don't forget to at least try the Express variations cap values as well.
BTW-I tend to believe that Higain is correct about reducing gain. I was looking at the 500K pot as a voltage divider as a way to increase gain by increasing resistance to ground? If this is the case, I believe that I'll pass on that mod.
stock with a pot
Hi!
there's a way to implement a divider pot and keep the stock grid to ground resistor when dimed ( schem below)
No idea if it sound good in a TW or which pot log/lin is the best.
Blues Jr first stage volume use 1MA pot, 130k resistor & a 100pf cap across pot legs.
there's a way to implement a divider pot and keep the stock grid to ground resistor when dimed ( schem below)
No idea if it sound good in a TW or which pot log/lin is the best.
Blues Jr first stage volume use 1MA pot, 130k resistor & a 100pf cap across pot legs.
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Fischerman
- Posts: 819
- Joined: Thu Dec 07, 2006 3:47 pm
- Location: Georgia
Re: express pre mods
In my Express I ended up with a 270K resistor to ground and no 220K series resistor between V1b and V2a (and I still used a .0022uF cap)...IME it's the resistor to ground that makes the most difference and anything over 330K or so doesn't really add much...most of the range is between 0 and 220K or so.
FWIW, the first two cathodes now go 2K7/.68uF and 4k7/.68uF, I increased the tail resistor in the PI to 22K, and changed the 91K plate resistor in the PI to 82K. These two PI mods were crucial to getting the PI distorting right...with 10K and 91K it was way assymetrical and the attack was all thuddy/fuzzy...even at medium levels of dirt.
I also added a 270K resistor to ground after the third stage and then added a 'new' PI input cap of .022uF in an effort to slightly decrease the signal hitting the PI. This thing still has tons of gain though.
FWIW, the first two cathodes now go 2K7/.68uF and 4k7/.68uF, I increased the tail resistor in the PI to 22K, and changed the 91K plate resistor in the PI to 82K. These two PI mods were crucial to getting the PI distorting right...with 10K and 91K it was way assymetrical and the attack was all thuddy/fuzzy...even at medium levels of dirt.
I also added a 270K resistor to ground after the third stage and then added a 'new' PI input cap of .022uF in an effort to slightly decrease the signal hitting the PI. This thing still has tons of gain though.