Is this possible? (tight control)

[ju.stin]

Hi all,

First post here. Awesome place, with tons of info. I have been reading up for a while on amp modding and have a question or two that maybe someone could help me with.

The amp in question is a Mesa Dual Caliber 50.

I use a MXR 10 band eq to suck out some lows before it hits the amp. Doing this yields a very tight sound for higain, modern metal. It allows the gain stages to work on everything besides the lows, which gives it better clarity and cleans up the flub. I add in the missing bass later on with the on-board Mesa 5band eq, which is after all the gain stages. The frequencies I am targeting with the eq pedal looks like this:

31.25hz -6db
62.5hz -5db
125hz -3db
250hz -3db

What I would like to do is totally eliminate the need for the eq pedal, since it is so noisy when running it before the amp. My idea for solution for this is to place a high-pass filter(marked by the red X in the below photo, If I have it in the wrong place, please forgive and kindly or not so kindly correct me lol) only on the input of the lead channel, since the clean channel sounds like crud with the attenuated lows. I would like to make the range of bass cut adjustable, so any help on what variable resistor/caps to use would be much appreciated. Are there any drawbacks to this? Will it mess with my input impedance any or anything else I need to be concerned about? I have many extra holes on my amp that I could use for the pot, or I can even drill a new one.

Any and all help is appreciated!

[img669]http://i.imgur.com/Ync8n.jpg[/img][/img]

[Jana]

Since the place you marked is right at the input, you could test any circuit you come up without modding the amp.

If I was going to mod the amp to do what you ask, I would experiment with some of the values in the first two gain stages of the lead channel.

Try replacing the .022uf coupling cap on V1b with a .0047uf.
Change the 1uf cathode bypass cap on V2a to .47uf.

Tweak the values in the tone stack after V2a--that is a TMB tone stack with the values for bass, mid and treble at fixed values. That is a very high value for the "mid" (62K)--try reducing that value, that will also reduce some of the bass. There isn't much treble being used, increase that to, in effect, reduce the bass. If you are serious about modding on the amp, you could replace the resistors in that tone stack with trimpots and tweak it to your liking on the bench and then once you have your sound, either replace the trimpots with equivalent resistors, or, if you did a good job of putting the trimpots in, just leave them and close up the amp.

[John_P_WI]

The easiest thing to do would be to change the treble peaker on the output of V1b. Currently it looks to be 0.002 uF parallel with the 470k resistor. Changing this to a 0.001 uF or 470 pF (470 pF will have a 4 x higher break frequency at 720 Hz, think Marshall).

I would also try (lift one of the legs) of the 0.001 uF cap to ground on the input grid of V2a as that is dumping a lot of highs.

Try Jana's mods as well, looks to be lots of room in the tone stack to play.

[ju.stin]

Thank you both so much for the input.

Do you have in recommendations on what to try for my high pass filter?

[ju.stin]

Also, assuming I get the desired results from the filter, do I have the correct location for the install?

[John_P_WI]

It does not hurt to experiment, I would try something with a corner of 360 hz or higher.

IMO I would not put a passive filter in front of the the first gain stage, it will add noise to an attenuated guitar signal. Playing with the coupling caps and treble peaker after the first gain stage are working with an amplified signal.

[xtian]

Also, before you solder, have you tried your EQ pedal in the FX loop?

[Reeltarded]

It does not hurt to experiment, I would try something with a corner of 360 hz or higher.

IMO I would not put a passive filter in front of the the first gain stage, it will add noise to an attenuated guitar signal. Playing with the coupling caps and treble peaker after the first gain stage are working with an amplified signal.

Totally with John here on the couplers!

[ju.stin]

Also, before you solder, have you tried your EQ pedal in the FX loop?

I appreciate the suggestion. Thing is though, that will not have the same effect as sucking all the lows before the gain stages, instead of after, which is what putting it in the loop will do.

There is just better clarity when the gain stages are just working on the mids and highs, then introducing the clean lows with the onboard Mesa 5 band eq after all the gain stages.

[VacuumVoodoo]

You can begin with 2.2nF in front of V1B, before the 470k, this together with the 1uF cathode bypass will give you 260Hz hipass filter. Should be easy to implement and test.

[ju.stin]

I have a friend that suggested some things. Can any of you tell me if they are good ideas?

You can try messing with the resistor values in the 1st (fake) tone stack,
just after V2a.
- There is a 62K resistor where the mid pot would normally be, try
decreasing this.
- There is a 15k resistor where the bass pot would normally be, try
increasing this.
- There is a 15K resistor between where the bass pot should be and
where the treble pot would normally be, you can try removing this.
- Try changing the bass and mid cap to .033
- Bump either one of the treble caps (probably the one in the real tone stack) to 750pf or even 1000pf (this boosts the upper mids, but mid and treble pot need to be turned up for it to work)

There is a 22K resistor just after the treble pot that bleeds signal to ground, you can try removing this or changing the capacitor that is attached to it. If the circuit becomes unstable, just put it back.


Gain:
There is a 150K resistor in a voltage divider just before the 2nd stage. It goes from the .002/470K to ground. Change this to a 220K 1/2 watt

Just before the Gain pot, there should be a 750K resistor in series with the signal. I changed mine to a 470K, but might go up to 680K if I keep the
amp.

[jestaudio]

Try PM,ing Monsta on the boogie board at Grail tone, he has a load off great mods for DC series Boogies and may well be able to point you in the direction you want to go

[ju.stin]

Try PM,ing Monsta on the boogie board at Grail tone, he has a load off great mods for DC series Boogies and may well be able to point you in the direction you want to go

Andy(Monsta-Tone) is the one that gave me these ideas. lol