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Old 16th January 2009, 04:15 PM   #1
Joelg is offline Joelg  Canada
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Default Marshall guitar amp. Upgrade IC's recommendations

Hey,

I'm looking to upgrade my Marshall VS-265R guitar amp, the schematics are here: hxxp://www.drtube.com/marshall.htm#VS2 Here's a rundown of the chips, just wondering recommendations for replacements or if what's there are good...

Amount Brand Model
3 JRC NJM072BD (TL072)
6 Mitsubishi?? 5201A
1 Panasonic?? MN3007
1 Panasonic?? MN3101
2 SGS Thompson LM348N
1 Motorola MC74HC4316N
1 Motorola TL071CP
3 TI RC4558P

I think the JRC's are the main ones to replace. Also I think the MN3007 is for reverb, and the MN3101 is the clock for it, so I'm not worried too much about either of them.

What's the opinion on JRC Op-Amps, I've read that they aren't the best, maybe go with TI (Burr Brown)? I'm not a total electronics newb, I worked in an electronics service dept for a few years, but as a computer tech.. So any advice is appreciated.

Thanks!
Joel
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Old 16th January 2009, 08:35 PM   #2
lineup is offline lineup  Sweden
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JRC Opamps can be good.
In order to do a good replacement, we need to know what type the old opamps are.

These are all opamps, IC-s I can have seen before: TL072, RC4558, LM348, TL071

These I do not know about:
6 Mitsubishi?? 5201A
1 Panasonic?? MN3007
1 Panasonic?? MN3101


MC74HC4316N, is a digital logic IC and not to be replaced.


Marshall The Valvestates Mk2,
VS265R, 2x65W 2x12" combo with stereo chorus & reverb

http://www.drtube.com/marshall.htm#VS2

The 2 PDF schematics with those opamps are:
Chorus, Reverb: http://www.drtube.com/schematics/mar...v265-61-02.pdf
Preamplifier: http://www.drtube.com/schematics/mar...v265-62-02.pdf

Maybe some other members has some good advice.
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Old 17th January 2009, 12:32 AM   #3
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Hi there,

I own a VS265 although it hasn't seen much use recently. Many years ago I had a play at improving it. I didn't try changing op-amps because in a guitar amp, as long as they're reasonable I don't think it's worth the effort.

One thing that was very worthwhile was to fit some much larger power supply smoothing caps. The clean sound in particular improved significantly and became much more dynamic and responsive to transients.

If you are looking for upgrades then I suggest looking mainly at the overall build of the amp, interconnections etc to reduce noise rather than the small signal circuitry itself.
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Old 17th January 2009, 01:20 AM   #4
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Wassamatter? You don't trust Marshall?

Changing these opamps will make 6ugger-all difference.

As regards changing the amplifier sound, I thought the whole point of a Marshall amplifier is that you get that 'Marshall Sound'.

If you feel an overpowering urge to play with components, why not build a second amplifier of some description rather than interfering with the one you've got. If you break it, then watcha gonna play?

Every minute you spend messing with amplifiers is a minute you don't spend practising.

Gearhead or guitarist? I guess we're all a bit of both. Those of us who play guitars and come on here that is...

w
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Old 19th January 2009, 03:20 PM   #5
Joelg is offline Joelg  Canada
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Thanks for taking the time...

I'm not really looking to "change" the sound too much, just was curious if I could "improve" it a bit..

Lineup, do you have any recommendations for replacements to the TL072, RC4558, LM348, TL071?? The original brands and models are in the original post..

ceharden, so do you just mean increase the capacitance of the caps in the power supply? Is there a limit on how much I should? As I understand it, I should keep the voltage the same, but the capacitance can go higher right?

wakibaki, I of course have faith in Marshall, but I know that they are trying to make as much money possible while producing a decent product, so they might cut some corners on some parts.. Now, I don't really want to change the sound, MUCH, but I don't mind if its a bit unique.. That's why I replaced the pickups in my Les Paul (granted the P-90's were fairly unique, but noisy). It is a bit of a curse, I was always the kid that took the toy apart at Christmas BEFORE playing with it.. Just always want to know what makes stuff tick, and now for the cost of op-amps, thought it might be a fun, cheap upgrade..
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Old 19th January 2009, 10:17 PM   #6
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Regarding the PSU caps, the originals are 2200uF PCB mounted. I think I added somewhere around 4700uF or 6800uF caps in parallel but just mounted them in free space in the case. The voltage rating needs to be the same or higher than the ones fitted (50V I think) but otherwise not critical.

I really think you don't need to worry about changing op-amps. The difference in that amp won't be worth the effort if it's even noticable. It's a guitar amp so it has distortion anyway and there is far more noise introduced elsewhere. Also consider improving the smoothing of the HT supply for the valve or trying different valves?

I think you need to decide what you want to change about the sound before diving in! Just wanting it 'better' isn't a good place to start!
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Old 20th January 2009, 09:48 AM   #7
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I think that if there was a cheap, straightforward way of 'improving' the sound, Marshall would be the first to adopt it.

After all, what is more important than the sound when you take money out of the equation? They will have refined the amplifier to the point of diminishing returns. By this I mean that they don't think the tiny 'improvements' possible are worth the huge increase in expense.

If you want a bit of fun, try building a practice amp (combo) from components or scrap. You need a power amplifier, a tone stack (optional), a PSU (can be batteries) a speaker and a box. There are loads of chip amps from the LM380 @ 2.5 watts on up. You can figure out most everything by reading the datasheets.

I always get a kick out of mine beacuse it's quite useable and nearly everything in it was junk.

w
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Old 21st January 2009, 12:57 AM   #8
Joelg is offline Joelg  Canada
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Alright, I think you've talked me out of it.. I should try to build something like you suggest. Has anyone tried anything from here hxxp://www.ax84.com ? Or have any other recommendations for plans to start from for a basic amp? I'd like to go tube...

Thanks to all for taking the time.

Joel
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Old 23rd January 2009, 12:48 AM   #9
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I've built an el84/ecc83 (12ax7) single ended tube amp.

Basically it's a Fender Champ

http://www.schematicheaven.com/fende..._5e1_schem.pdf

..with an EL84 instead of the 6V6 and a solid-state rectifier. I even bought a vintage-style Jensen P10R to go with it.

In fact, looking at ax84.com it's virtually identical to their AX84 P1 but for a few details.

Go for it.

Only thing, in the end it cost me more than a new Fender 6V6 amp with SS front end...

A valve amp is a lot of work in the chassis and sourcing the components. If you can get a kit or part kit, so much the better.

A quick google 'guitar amplifier kit' got me a few, worldwide shipping.

With SS you can just hang components or even a PCB off the back of the tone controls, you can get the components anywhere and if you run from batteries it's never gonna kill ya.

w
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