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new member, looking for help on my first amp mod project

Started by Drewpy Dawg, January 20, 2008, 06:42:48 PM

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Drewpy Dawg

Hello, I am a new member here and hoping this forum can help me on my first project. I am not looking to build an amp from scratch, but looking to modify an existing amp. My starting point is a Marshall 3203 Artist head. I've had this amp since I was 17 and have enjoyed it quite a bit. I have since begun using a pure tube amp and the Artist has sat in the closet for a few years. Recently I got interested in effect pedal modifications and noticed people using different opamps, diodes, and capacitors to improve the tone of effect pedals. So I wondered if the same things could apply to my Artist head? This model amp is far from a popular Marshall model, I've found very little information about it online and nothing about modifications. So the first question is, where to begin?

I have never liked the sound of the clean channel on this amp, it always sounded thin and weak. I would like to make the clean channel sound fuller and warmer.  I would also like the amp to have more dynamics overall and not sound so much like a solid state amp. Seeing as it has a single 12AX7 and two EL34 tubes I would think that a few changes in the preamp could make the amp sound more comparable to a full tube amp, such as a JCM800.

I have found the schematics online, however this raises another question as the schematics seem to show six op-amps, while I only see four op-amps inside the amp? They are all Texas Instruments MC1458P's. Is there another model chip that would help me achieve the changes I want to make in the amp, and what other changes should I make to go with them, such as different types/values of diodes and capacitors.

If anybody can offer some information or guidance on this project, or at least point me toward some good information sources it would be greatly appreciated. Thank you.

joecool85

If you could share the schematic that would help.  To fix the "thinness" of the clean channel it probably just needs a larger capacitor on the input going to it.  And yes, modification of amps is very similar to modding pedals.  You will have to be careful on this one though since it has tubes in it, tube amps require high voltages that are sustained within the amp for long periods of time after it has been unplugged- normally about 500v.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

sugrhigh

Quote from: joecool85 on January 21, 2008, 07:06:22 AM
You will have to be careful on this one though since it has tubes in it, tube amps require high voltages that are sustained within the amp for long periods of time after it has been unplugged- normally about 500v.

an off topic question, but you can use a capacitor discharger and it would be safe then correct? well givven that its unplugged and everything...

Drewpy Dawg

Thanks for the reply. I do know that tube amps and large capacitors can be dangerous, don't worry I won't be doing anything inside without taking proper precautions.

The schematics can be found here: http://www.drtube.com/marshall.htm#MS-3203

I'm still confused about the two "missing" op-amps. I could take some digital pics of the inside of the amp if that would help as well.

But first let me ask about the capacitor you mentioned. Which cap(s) in the schematic would be the ones to address, and what value would I change it to?

Thanks

Drewpy Dawg

Actually when I just looked at the schematic again I think I answered my question about the "missing" op-amps. The schematic has them labeled IC3a and IC3b, meaning a and b are each halves of the same dual op-amp.

joecool85

Quote from: sugrhigh on January 21, 2008, 02:05:33 PM
Quote from: joecool85 on January 21, 2008, 07:06:22 AM
You will have to be careful on this one though since it has tubes in it, tube amps require high voltages that are sustained within the amp for long periods of time after it has been unplugged- normally about 500v.

an off topic question, but you can use a capacitor discharger and it would be safe then correct? well givven that its unplugged and everything...

Not off topic at all, power caps need to be drained after the amp is unplugged and then it is safe.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

joecool85

Quote from: Drewpy Dawg on January 21, 2008, 11:51:09 PM
Which cap(s) in the schematic would be the ones to address, and what value would I change it to?

Thanks


C1 on the preamp is 8.1uF, try replacing that with a larger cap and see what happens.  Maybe a 20uF - 30uF cap.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

Drewpy Dawg

Good info. Now some related questions:

C1 is in the "input amplifier" which feeds both the clean channel and the boost channel, so am I right to assume that if I change capacitor C1 it will have the same (or similar) effect on both channels?

And if I wanted to change the sound of only the clean channel would I replace the capacitor just before the first op-amp of the clean channel which looks to be C17?

Thanks

teemuk

Yes, both channels share the first gain stage so the changes you make in it will affect each channel. But the effect may or may not be "similar". For example, hi-gain channels will typically "reveal" subtle changes much more likely than low gain channels.

I believe the value of C1 is 0.1 μF (100 nF) instead of 8.1 μF. (This schematic is so undecipherable). The hi-pass RC circuit is formed with the 1 Megaohm input resistor so even low coupling capacitances create a very low cut off frequency. 100 nF has practically no low cut effect whatsoever: For example, even with a value as low as 10 nF the –3dB point would still be at 16 Hz! We can barely hear that far. Tweaking this component to get more low-end just makes no sense.

Yes, you can replace only C17 to have effect only on clean channel but note that again the corresponding hi-pass RC circuit is formed with a 1 Megaohm resistor and the coupling capacitance looks to be 8.81 nF. Any changes of increasing the value are again extremely subtle – if not inaudible. However, besides tweaking those coupling capacitor values you could also alter the tone control circuits since they are individual for each channel. This will likely make much more difference for the overall balance between low and high frequencies. (Download Duncan's Tonestack Calculator to plot the frequency response of the circuit conveniently). To alter the low-end response you can also vary the values of the "unity DC gain" capacitors, as these also create hi-pass filters. Those caps are, for example, C3 in the first gain stage and C10 (I think)  in the "first" gain stage of the clean channel.

Drewpy Dawg

Funny that you mention altering the tone controls. Just last night I happened to find a review of this amp online and the user said to achieve a decent clean sound the bass control should be set at 10, the treble set at 0, and adjust the mid around halfway. So I guess changing the values of the eq controls might be the way to go in this case. I can always experiment with different coupling capacitors while I'm in there. Where would I find the tonestack calculator? Yes, the schematic is very hard to read in some parts, unfortunately this was the only one I was able to find online.