Express tone stack

[marcoloco961]

I have been doing research on the FMV tone stack to try and gain a better understanding of how they work. After reading Merlin's section on tone stacks and experimenting with the Tone stack calculator (courtesy of Duncan Amps) I have a few questions that I hope someone can answer.

According to Merlin's section on tone stacks a guitar's crossover frequencies are in the range of:

Bass below 200Hz, Mid from 200 to 1000Hz and treble above 1000Hz.

When I punch in the variables on the Tone stack calc. what I see is the bass only raising the very lowest notes possible on guitar and the mid not doing much either, so most of the express' tone flavoring comes from the treble pot which basically adjust mid and treble.

My questions are:

1. Does the fact that the TS on the Express doesn't connect the open terminal on the bass and mid pots change the frequency responses?

2. Would it be advantageous to change a few components in the TS to gain a better control of tone, or since the TS is early in the Express circuit, has the rest of the circuit been "shaped" to adjust to this issue where changing the values in the TS would take the entire amp out of whack?

Looking at the TS calculator and Merlin's info it looks like switching the bass and treble pots to 500 k, the slope resistor to 82k and c3 to around 6.8n might give a shaping pattern closer to what Merlin describes in his book.

Don't get me wrong, I love the sound of the Express, it is one of my favorite amps, and I am by no means trying to second guess Ken. These are just observations that formed into questions.

[gearhead]

It's been a while since I looked at it, but believe the cap between stages 2 and 3 (C8) has a substantial effect on the bass output. It's part of a HPF and Ken's default value really chops the bass.

[guitarmike2107]

Its not been called a one trick pony for nothing

The tone stack gets rid of some of the low mids and bass which = mud

The coupling cap after the second stage gets rid of the low bass = boomy mud

Leaving a signal that makes good sounding distortion

[marcoloco961]

It's been a while since I looked at it, but believe the cap between stages 2 and 3 (C8) has a substantial effect on the bass output. It's part of a HPF and Ken's default value really chops the bass.

This is what I am talking about. It chops the bass so drastically that the bass pot has very little effect on the output. Almost to the point of not being able to add any lows at all. I turn it from 0 to 10 and only hear a very slight difference in tone.

I am tempted to try and change something to get some more lows out of it. But I figure it will probably take away from that sought after Trainwreck tone.


Has anybody here tweaked the TS on one of their Express builds already?

Any insight to whether it is a plus or a minus?

[JamesHealey]

470pF/47k Slope & 500k Treble
500k / 47nF Bass
25k /47nF Mid

[gearhead]

Some good threads on the coupling cap (and other tweaks):

https://tubeamparchive.com/viewtopic.ph ... g+cap+bass

https://tubeamparchive.com/viewtopic.ph ... g+cap+bass

[marcoloco961]

470pF/47k Slope & 500k Treble
500k / 47nF Bass
25k /47nF Mid

That looks more like what I would expect to see for a guitar amp TS. Makes the bass pot look more usable and the mid scoop more where I would like to see it.

You have tried this in yours?

Opinions?

Clips?

I would love to hear some clips of this amp if you have made these mods to it.

[marcoloco961]

Some good threads on the coupling cap (and other tweaks):

https://tubeamparchive.com/viewtopic.ph ... g+cap+bass

https://tubeamparchive.com/viewtopic.ph ... g+cap+bass

Interesting info in those threads. TY. They pretty much confirm what I had feared, that a little bass boost can skew the Wreck tone.

Since the tone stack is early in the circuit, I had wondered about blocking distortion, etc. Sounds like that is the case. Changing the tone stack would probably mean changing many value post TS to compensate.

Jackie Treehorn had an interesting idea with the PI later in the circuit to increase bass response without adding any "flabby" sound. This might be a better way to go.