Any Midboost layouts for AB763 preamp?

[RobBozic]

Does anyone know of a good midboost for the Fender AB763 preamp?

I was thinking of installing a 25K mid pot with a pull switch on my amp (which is based on a AB763/6G16) and paralleling a cap over the treble cap.

I don't know where to start though. which kind of cap value would sound good and do I need a resistor in there somewhere...etc

I want to fatten up the tone for lead playing.

Any help would be appreciated
Rob

[txstrat]

I like the way it's in the Fender Champ II circuit.
(http://www.schematicheaven.com/fenderamps/champ_II.pdf)
Put a .0027 (I've used .003) cap in series with a 10M resistor and parallel to the bright cap. Short the 10M (I tried 8M and it works just fine) resistor with the switch and you are there. I've used this in almost any Amp with a AB763 circuit I built. It's just right for the reason you mentioned (lead playing). You don't even need a mid pot.

[pdf64]

I prefer to use a 50k log pot for the mid control. Between about 1 to 7 it covers the same range as the regular 10k lin 1 - 10, then from 7 to 10 there's plenty fat mids available, along lines of early Santana when it's up full.
I've altered the function of the Reverb footswitch to bring this mid boost in / out, by putting an 8k2 resistor in parallel with the mid pot and switching it's ground return.
txstrat's method gives a more aggressive tone, so try both and use whichever suits you best. Peter.

[RobBozic]

Thanks guys, I'll use the one from the SuperChamp (thank you Mr Rivera!).

I've tried the 50K Mid pot on other amps of mine. But I'm not a fan of tone stack bypasses on AB763 preamps. To me the 25K pot is max. as it is.

I just need something to give a mid high boost so the treble strings have more fat & sustain on them, especially when your bending high up on the register.

Thanks again
Rob

[HTH]

how about switching in a 1000pF cap across the treble cap in the tonestack? at the same time, switch in a 47k resistor to ground at the bottom of the tonestack in place of the existing 6k8 one (or is it a 8k2 or maybe 10k already?)

[olddawg]

Depending on what style you are playing, have you considered using a Rangemaster clone pedal with a mid accentuated coupling cap? A Rangemaster might be more effective than modding the amp and is very easy to build.

[chipaudette]

+1 on the parallel treble cap. Mesa Boogie does this for their "Treble Shift". They bring in 750pF cap in addition to the stock 250pF treble cap. That's 1000 pF (0.001 uF) total.

The Rivera circuit brings in a 2700pF (0.0027uF) cap. That's really big. Wow. I think that might be too much. I would suggest that you try various values between the Rivera 2700pF and the Boogie 750pF.

I definitely agree with the trick with the 10M resistor. It is totally the correct way to implement this feature. The alternative is just have the cap and a switch (no resistor). The problem with this method is that, when you close the switch to bring the cap into the circuit, the cap suddenly sees a very large DC voltage across it. As a result, there's a quick surge through the circuit to charge up the cap to support that DC voltage. This surge doesn't hurt anything, but it does make an unpleasant popping sound. Booo. Using the 10M resistor allows the cap to stay in-circuit so that it can always be charged with the right DC level. As a result, there's no popping sound when you engage the switch. Sweet.

Like I said, don't just blindly choose the 2700pF cap. I'd suggest that you try a few values other (smaller) values and pick the one that sounds best to your ears.

Chip

[txstrat]

Like I said, don't just blindly choose the 2700pF cap. I'd suggest that you try a few values other (smaller) values and pick the one that sounds best to your ears.

Chip

That's right. You should definitely try different values. Depends on what you want to sound it like. I decided to go for the .003 to have it sound more tweed'ish when you turn down the treble pot a bit.

[chipaudette]

That's right. You should definitely try different values. Depends on what you want to sound it like. I decided to go for the .003 to have it sound more tweed'ish when you turn down the treble pot a bit.

What setting do you usually have on your Bass and Mid knobs? If you run the very common 6-6-6 configuration, Duncan's Tone Stack Calc says that your 0.003uF cap nearly flattens the frequency response...ie, makes it so that the tone stack is doing very little.

That really would be a very different sound than the typical blackface scoop. Cool!

Chip

[txstrat]

Ususally I got it set at 6-5-4 with no "mid boost". With the .003 cap switched in I change to 4-7-4 to give it a bit more mid bite while smothing down the highs.
My mid pot (btw) is 25k. The response of the pots is quite effective, even with the mid cap "in".
I can smooth down the "effect" of the mid cap with the mid pot and still the treble and bass pots are very effective, although not as effective as with no mid cap. Hope you get what I mean...

[RobBozic]

I tried the .0022 cap which parallels over the top of the treble cap. There's way too much midboost in my opinion.

I'll try the 750pf tonight! And if that doesn't sound right I'll just bite the bullet and get an eq pedal.

Thanks again

[thumbs]

You might try a 5000pf cap on the bright switch. I put it on a Bandmaster's normal channel and used an A/B box for switching. If I remember correctly, I also had a 22k resistor on the NFB loop and a 5F6 style presence control on the back panel. A few simple mods really opened that amp up for me. I know it sounds like an awful big cap, but remember, it's effect will diminish as you increase the volume. I thought it sounded pretty good in the 4-7 range.

[Ben N]

I like the way it's in the Fender Champ II circuit.
(http://www.schematicheaven.com/fenderamps/champ_II.pdf)
Put a .0027 (I've used .003) cap in series with a 10M resistor and parallel to the bright cap.

Just to clarify, there is no "bright cap" in the schemo you posted--rather, that is the treble cap. What is typically called the bright cap goes across the volume control, with or without (as in the Deluxe/DR) a switch.

BTW, this might go nicely with a relay, especially if your amp already has a useless "Boost" jack, like my late '70s DR.

[txstrat]

Just to clarify, there is no "bright cap" in the schemo

That's right, I was talking about the treble cap.