Finally got rid of the fizz!

[Matt72]

Here's a couple of slight things I've done to my Express copy (I'm not fond of the "clone" word).

After I built it, I thought is sounded great. Then after a while, it wasn't really quite what I was after. It was 90% there, but something was missing for me. That "something" was mainly a little warmer sound and some more beef to the tone. So if anyone wants the same, here' what I did.
1) No bright caps on the volume pot, I just disconnected the switch there (how does anyone stand that on this amp??)
2) changed that little .0022uf cap to a big fat .022
3) probably most importantly, changed that 100k slope resistor to a 10k (!) I used a 100k pot to dial in what I thought sounded the best, then measured with my meter.

Now it sounds great. More like a beefy vintage marshall. The stock Express seems to have a lack of lower mids...more of a Vox type sound. I really like some girth/grind/growl down there and this really did the job.

These mods would probably be a little dark with some speakers, but I'm using some fairly bright g12h30s and it sounds just right.

**I'm editing this post to say that this really was only half way there...see my post below for what I ended up doing**

[joe6v6]

Sounds like a winner to me, Tweak it untill its how you like it. Which 100K are you calling the slope resistor?
JOE

[clarkydaz]

i would like to know also. i'm familair with the marshall layout, but not the trainwreck. which caps/resisors are the tonestack etc?

[dobbhill]

The slope resistor is commonly the 100k resistor connected between the "hot" end of the treble cap and the bass and mid caps. On Marshalls, it can be 33k to 56k.
HTH,
D

[clarkydaz]

what is the 0.002 cap having an affect on?

[Jack]

what is the 0.002 cap having an affect on?

Filtering lows.

A 10K slope resistor indice a bigger insertion loss for the tone stack (in other words lower gain overall). The marshall/bassman 33/56K res works because of the cathode follower. IMO, you'd be better off playing with the cap values instead to acheive the same tonal results.

[novosibir]

Use the Duncan Tone Stack calculator, to see what happens with another slope's value. Use the 'Fender' tabliza there, since in TW's the source impedance of the tone stack also is 38.5K

Larry

[gcenker]

If it's big, fat mids you're after while being able to curb the bass, try this:

Use the TSC (Fender) and plug in the stoc TW values, then move the bass to 30%, mids to 50% and trebble to 30%. Take a snapshot.

Now sub the 5009f cap for 680, C2 & C3 for 0.01µf

Notice what happens to the mids without turning up the highs?

Anyone besides me try this?

-Greg

[gcenker]

If it's big, fat mids you're after while being able to curb the bass, try this:

Use the TSC (Fender) and plug in the stoc TW values, then move the bass to 30%, mids to 50% and trebble to 30%. Take a snapshot.

Now sub the 5009f cap for 680, C2 & C3 for 0.01µf

Notice what happens to the mids without turning up the highs?

Anyone besides me try this?

-Greg

[Matt72]

Well, I ended up tweaking a bit more. Set up like I had it, it was a little boxy sounding. So I changed the 500pf in the tone stack there to a 250pf...hmm little too bright/thin now. So then I settled on 350pf. Perfect! (I always thought of that cap as the "treble" cap, but I think it does as much to change your midrange!)

Then I screwed around with the slope resistor again. I used a 100k pot to dial in what I liked and then measured the pot to see what resistor to put in. 10k there ended up being too dark really, too midrangy. So I futzed with that pot for about 2 hours (!) dialing in just the right sound. I had it narrowed down to 2 sounds I liked. One was with a 48k slope and the other was 54k. Yes there was a difference between them! Ended up going with the 54k, 48k still had a very slight boxy sound to it. I tried a 56k (Marshall superbass/JTM45/tweed bassman value) and it was a hair too thin. So 54k it is (I had to put 2 different resistors in series to make up this "weird" value).

That slope resistor really seems to kind of be the "gatekeeper" regarding the character of that whole tonestack.

This was really sounding cool now. But it was still too...scratchy sounding. So I put a 68k grid resistor in series with the input wire, like most amps have. That tamed down some of the wildness and made it just right for me.

Also, I have a 12AT7 in the first preamp socket, this smooths things out/warms things up a bit too.

So after all that, I ended up with a tone just like this (which is what I was shooting for):
http://www.youtube.com/watch?v=24SOBX4PIx8

Alright!

So here's a tidy recap incase any wants to try this:
1) Add 68k grid resistor in series with input cable on tube socket (like every other amp on the planet has)
2) Change the 500pf cap in the tone stack to 350pf (try 250 too, that's the commonly used, but was a hair too thin for me)
3) Change that little tiny .0022uf cap to .022uf
4) Change the slope resistor to probably slightly under 56k (54k in my amp's case; but this will probably vary some with each amp/speakers)
5) For a little smoother/warmer, change one of the preamp tubes to 12AT7

These mods also help quiet hiss/noise of the amp too. Good all around for me.

[gearhead]

Grack, when I use TSC (Fender) with the Express values, the 1M bass and treble pots are useless?

Edited- Never mind, restarting worked.

Amazing how much a drop in the mid cap value (such as to .01) has on whole tonestack frequency profile.

[Matt72]

Well, it still wasn't quite there. Sitll a little too fizzy and buzzy. So I did a few more mods that I thought I'd share. I actually took pots and hooked them across where every reisitor went to see how the sound changed by adjustment, then I measured the pots when I thought it sounded just right and put in that value resistor. It took time, but now my amp is MAXED OUT!!

So here's what I did in addition to the mods above:
1) I wired up V2 to have a shared cathode, I really like how this smooths things out.

2) With V2 now in parallel, I clipped in a pot to see what cathode resistor gave me the smoothest sound: 14K

3) I adjusted that 150k voltage divider (what Paul Ruby refered to as sort of a 'master volume') to 330k. This made a really big difference in getting a smoother more singing tone.

4) I bumped up the 25uf cathode cap on V1a to 175uf. Just fattened things up and made the amp seem less treble and high-mid focused.

5) I changed the PI tail resistor to 80k. ! that really helped smooth things out. It dropped the gain a little, but #3 above made up for it.

6) Changed the cap in the tone stack back to 500pf. This is important to the fat juicy sustainy mids.

7) Then I ended up going with a 12AT7 in the PI position, to smooth things out one last bit.

So now I have a really nice, singing, smooth amp that is voiced a little differently than a TW, but is what I wanted from the get-go. Now there's more lower mids, and a rounded sound. More like a nice old vintage Marshall now.

Anyway, I hope this helps someone out. If you're having the fizz trouble you might want to give some of these mods a try.

[drz400]

Have you tried your setup in a band situation?
I always found that I needed more High end than I could ever imagine in a live situation and less bass.

Well, it still wasn't quite there. Sitll a little too fizzy and buzzy. So I did a few more mods that I thought I'd share. I actually took pots and hooked them across where every reisitor went to see how the sound changed by adjustment, then I measured the pots when I thought it sounded just right and put in that value resistor. It took time, but now my amp is MAXED OUT!!

So here's what I did in addition to the mods above:
1) I wired up V2 to have a shared cathode, I really like how this smooths things out.

2) With V2 now in parallel, I clipped in a pot to see what cathode resistor gave me the smoothest sound: 14K

3) I adjusted that 150k voltage divider (what Paul Ruby refered to as sort of a 'master volume') to 330k. This made a really big difference in getting a smoother more singing tone.

4) I bumped up the 25uf cathode cap on V1a to 175uf. Just fattened things up and made the amp seem less treble and high-mid focused.

5) I changed the PI tail resistor to 80k. ! that really helped smooth things out. It dropped the gain a little, but #3 above made up for it.

6) Then I ended up going with a 12AT7 in the PI position, to smooth things out one last bit.

So now I have a really nice, singing, smooth amp that is voiced a little differently than a TW, but is what I wanted from the get-go. Now there's more lower mids, and a rounded sound. More like a nice old vintage Marshall now.

Anyway, I hope this helps someone out. If you're having the fizz trouble you might want to give some of these mods a try.

[tubedogsmith]

Great thread. It's cool to see you trying this stuff on your own and coming to your conclusions based on what your ears are telling you. Great way to learn.

[Matt72]

Have you tried your setup in a band situation?
I always found that I needed more High end than I could ever imagine in a live situation and less bass.

Yeah, I play in a band, so I've been hearing it live as I make the changes. You're right though, you always do seem to need more treble than you think at home. But set up like this it still has plenty. With the treble control at 1-2:00 it's about all I need.

The cool thing is I learned so much. That 150K voltage divider resistor...I it seems like that was really key to adjusting the amount of pre vs. power amp distortion. Increasing that was really key.