Any tricks to add a narrow midrange bump in a given range?

[12AX7]

I know of course i can alter the mid cap, but thats much more of a wide Q than i'm looking for. I just want a fairly narrow bump of a couple DB in the 600-800 hz range. Is there a way to accomplish this w/o anything too major? (don't want to use outboard EQ) I've has some success in other amps with a R/C combo of 470k/.0047uf between stages and that can do a similar thing in some amps but in this one it doesn't seem to for some reason. the preamp is like a marshall cascaded affair.

[PRR]

You probably need another stage.

The Fender tonestack is all about mid-dip, no way to bump.

A small peak in other existing stages pretty much means reducing gain all over except in the band that wants bumping.

[12AX7]

It's actually a marshall circuit with the typical marshall stack but 6V6 outout. 3 stages, CF, tone stack, LTPI, 6V6.

[drgonzonm]

If my memory serves me correctly, RDH4, has an all tube sixteen level equalizer.  For some reason, what sticks in my mind is most guitar amp tone stacks are subtractive. That is like PRR says, you selectively remove frequency gain, so the amp provides midrange bump. 

For some common tone stacks there is Duncan Amps, tone stack calculator, might give you some ideas.

[jjasilli]

The Ampeg V4 has a Baxandall-like tonestack with 3 switchable Q frequencies:  300, 1000 & 3000 Hz + TMB knobs which are flat @ 12:00 o'clock, so you can boost clockwise or turn down counterclockwise.  Maybe some ideas there.

[vibrolax]

The Ampeg V4 has a Baxandall-like tonestack with 3 switchable Q frequencies:  300, 1000 & 3000 Hz + TMB knobs which are flat @ 12:00 o'clock, so you can boost clockwise or turn down counterclockwise.  Maybe some ideas there.

The Ampeg midrange circuit is independent of the James tonestack that provides the bass and treble.  It indeed uses another triode stage and a custom multitap toroidal inductor.  It is a huge boost/cut range, +/- 20 dB if I recall correctly.

I reverse engineered and hand-wound the inductor for my Ampeg SVT preamp build, documented here:http://www.frontiernet.net/~jff/SonOfSVPCL/DIYSVTBassPreamplifier.html

I believe the V4 uses the same midrange circuit as the SVT

[PRR]

> It's actually a marshall circuit

A Fender with trivial differences.

The Ampeg mid-boost runs *three* triodes. It is more than you want to build, and does more than you need.

[vibrolax]

> It's actually a marshall circuit

The Ampeg mid-boost runs *three* triodes. It is more than you want to build, and does more than you need.

Yeah, it IS three triodes!.  The whole midrange takes about 6 inches of board space.

[HotBluePlates]

I know of course i can alter the mid cap, but thats much more of a wide Q than i'm looking for. I just want a fairly narrow bump of a couple DB in the 600-800 hz range. Is there a way to accomplish this w/o anything too major? ... the preamp is like a marshall cascaded affair.

1.  Start with adjusting the amp's existing EQ.  You should be able to get "all the mids" simply by turning the Mid control at/near max, Bass control near 0, Treble control to taste (but start ~half).  Overall EQ is actually near flat except where bass is rolled off.

Suggested because "a 600-800Hz bump" would be exactly where the mid-scoop is taking place.

2.  Above is "too broad"?  Just use an EQ pedal.  You can zero-in on exactly the frequency you want to boost, and probably narrow-enough for what you need.  EQ pedals with a parametric mid may also have a Q control to narrow the band of frequencies getting boosted.

3.  Above not suitable because you want the change inside the amp?  Look at all other places you're shaping the frequency response.  Every one of those bass roll-offs (due to coupling caps, bright caps around volume controls/voltage dividers) is cumulative.  You might have a more sensible impact by making one of those bright caps a little bit larger to roll off a bit lower (if they tend to result in roll-off in the 600-800Hz region where you want less-cut).

[12AX7]

Thats pretty much all things i've tried either recently or in the past. It's more a TYPE of mids i'm looking for. Much like turning the treble knob up adds brightness as does the presence, but they sound very different. Or getting less lows by turning the bass knob or adjusting it's cap vs bigger bypass caps in the preamp. In other words, there are a million ways to add or subtract treble or bass or mids but most sound/feel very different than the next. It's like the difference between two high gain amps, one with NFB and one with no NFB but a cut control. Both accomplish similar tone keeping the top end under control but the feel and character are different. In any case, i can't describe the type of mids i'm looking for with words but i've had it in the past with various setups and just thought if someone had a simple solution i would try it and see it it gave me what i'm after. 

I know of course i can alter the mid cap, but thats much more of a wide Q than i'm looking for. I just want a fairly narrow bump of a couple DB in the 600-800 hz range. Is there a way to accomplish this w/o anything too major? ... the preamp is like a marshall cascaded affair.

1.  Start with adjusting the amp's existing EQ.  You should be able to get "all the mids" simply by turning the Mid control at/near max, Bass control near 0, Treble control to taste (but start ~half).  Overall EQ is actually near flat except where bass is rolled off.

Suggested because "a 600-800Hz bump" would be exactly where the mid-scoop is taking place.

2.  Above is "too broad"?  Just use an EQ pedal.  You can zero-in on exactly the frequency you want to boost, and probably narrow-enough for what you need.  EQ pedals with a parametric mid may also have a Q control to narrow the band of frequencies getting boosted.

3.  Above not suitable because you want the change inside the amp?  Look at all other places you're shaping the frequency response.  Every one of those bass roll-offs (due to coupling caps, bright caps around volume controls/voltage dividers) is cumulative.  You might have a more sensible impact by making one of those bright caps a little bit larger to roll off a bit lower (if they tend to result in roll-off in the 600-800Hz region where you want less-cut).

[HotBluePlates]

... It's more a TYPE of mids i'm looking for. ... In any case, i can't describe the type of mids i'm looking for with words but i've had it in the past with various setups and just thought if someone had a simple solution i would try it and see it it gave me what i'm after.  ...

If you can't describe it, no one can tell you how to get it.

That said, use the Tone Stack Calculator (if you haven't already) to understand how your present tone stack is scooping mids.  There can be a huge difference in the apparent effect if you move the mid-notch to a different frequency.  That's because there will be different frequencies left over on either side of the scoop.

You can adjust that mid-dip frequency by changing component values in your tone stack.  You'll have to try & listen to see what you think of the sound.

I have an amp with a continuously-adjustable mid-dip, where both the depth of the mid-scoop and its center frequency can be adjusted.  However, that amp uses a somewhat different approach to tone control which probably wouldn't suit your amp directly.

[John]

If you haven't tried this already, look at the James tone stack. Replace the fixed resistor that goes between the 2 wipers with another pot. That's your "shift" control. Read that in Merlin's excellent book. :)


Just played with the Duncans TS calculator. Looks like with a 1M pot you could move the highest "hump" on the mids from ~500Hz to 1kHz. I have no idea if that would be useful or not, most of my amps don't have tone stacks.

[12AX7]

Thanks, but i was just looking for something more simple than ripping out my TS and replacing it. I'm just going to leave it as is.