Marshall MG15CD power supply

[kubik369]

Hi,

I want to build Marshall MG 15 schematics here as my first guitar amp after small 1/2W amps. But I live in Slovakia and local shops don't have 1N5245B zenner diodes in the end of the power supply. I can order them from Ebay, but I want to know if there is some substitute, becost I don't know what am I gonna do with 100 diodes while I need only two. My second problem is transformer, my friend says that it is probably 2X15V transformer, but I want to know your opinion, just to be sure. And my third problem is ground. Until now, I was doing things only with one common ground connected with - of the battery. I have found out that in 4 pins transformer connecting 2 and 3 terminal you'll get ground. But in this schematic is connected ground sign from cord ground and in rest of schematic is GRND in letters. I don't know which connects to which. Thanks, I really appreciate your help and if you don't mind, correct my english.

[52 Bill]

Welcome to the forum. I don't know what things are like in your country, but I would think that it would be easier to purchase a used or broken small amp to repair than to build one of these from scratch. When you add up all of the parts and labor involved in making an amp, I think that it will be far cheaper to purchase one.

If you can order parts online or by mail from other countries like Germany or the UK, you should be able to get 2 Zener diodes with no problem. You could also replace the Zener supplies with 2 three terminal voltage regulators, if they are easier for you to get.

The power transformer would probably be 30-35 volts center tapped, or as you friend suggested 2X15v.

[Enzo]

Those are 15 volt 1/2 watt zener diodes. ANY sismilar zener will work. And if they don't have 15v ones, 14v or 16v will be close enough. And if they can't find 1/2 watt ones, the 1 watt ones like1N4744 will be OK too.

[Jazz P Bass]

You could also run zener diodes in series.
The wattage rating is indicated by the lowest component.
The voltage adds.

[imaradiostar]

A small tube amp would be orders of magnitude easier to make. Just a thought.

Start with a Tweed Fender champ or maybe an 18 watt marshall or 5E3 Fender deluxe. Simple circuits and you'll get lots of mileage out of them.

jamie

[Sowhat]

Just putting my 2 cents in but the transformer should probably be a 24 or 25 volt center tap type at maybe 1 or 2 VA... (a cute little thing!) I'm assuming you have 240V/50hz mains there where you live so make sure to get a 230/240 primary. Once you run through the bridge rectifier that +-12 volts becomes almost 17 volts per rail: (24*1.414)/2 with 2,200uf main filtering per rail it will probably float about 18 or 19 volts... perfect for 15 volt zener clipping at somewhere around half a watt per rail. Just got done building a small run of 100 watt SS guitar amps and they cost me more than what I could pickup in the bargain bin at Guitar Center of equivalent power. Building it yourself don't save you any money anymore and it will probably cost more than just going out and buying a new Marshall MG15... then again you will not get the pride of cobbling it together yourself.

[Sowhat]

I may have spoken out before checking all the facts. National Semiconductor states:

The LM1875 delivers 20 watts into a 4Ω or 8Ω load on ħ25V supplies. Using an 8Ω load and ħ30V supplies, over 30 watts of power may be delivered

Marshall rates the MG15 at 15 watts so it's not being driven to it's max but it is not being driven at +-15V either. A little math shows it needing 18.75 volt rails to get about 15 watts out of it. My earlier assessment of the transformer specs is still correct as far as I can see but you will want a little more Volt/Amps (VA) to give the supply resistance from brown out. Maybe a 30 or 40 VA version would be better suited although the design only calls for a 1/4 watt fuse. Of course you could hyperwarp this thing a bit and up the output voltage to maybe a 30 volt center tap type which would yield about 24 volts per rail which would produce somewhere around 20 watts. Still maintaining a safety margin but the zeners, to be safe should probably be 1 watt to give you that regulated +-15 volts the op amps use. I've found that a higher than required VA for a transformer is never a bad thing but you have to watch the power rails to the amplifier chip closely... exceeding the design limits will give you a bad day.

[Steve Conner]

Come on guys, how are you going to get 15W of audio power out of a 4VA transformer? 20VA would be more like it.

In Europe the 1Nxxxx zeners aren't too popular. You usually see part numbers like BZX85C15, which is a 15V, 1 watt zener.

[Sowhat]

Come on guys, how are you going to get 15W of audio power out of a 4VA transformer? 20VA would be more like it.

Absolutely right Steve, I changed the incorrect post. Originally without looking up an MG15 I thought the original poster said it was a 1/2 watt amp, thus the low VA but when I saw it used an LM1875 I put up my addendum to my original post. Unfortunately I got brain freeze! I meant that a 30 or 40 VA would give nice headroom.

[kubik369]

Thanks for all of your replies, especially Steve for that part number I have found that BZX85C15 is in one of our e-shops, but they probably don't have it in near future, but they have 1N5351BG which is 14V/5W and 1N5353B which is 16V/5W. Can I use one of these? And how do I have to connect all of these grounds? And it is better to use toroidical or normal transformer? Again thanks for your answers.

imaradiostar : I was looking at these but I can't find half of the tubes in stores and I want to build something smaller and ,,cheaper" becost as I know UPS delivery, they are able to break half of the tubes and I'm okay with that this will probably get more expensive than buying one, but I will be so proud saying to friends : ,, I have built this one"

[Sowhat]

And it is better to use toroidical or normal transformer? Again thanks for your answers.

Toroidal transformers have a smaller electromagnetic footprint than IE core (normal transformers) and are easier to use in tight places, mounting is easier too, just a bolt through the doughnut hole and couple insulating washers which are usually supplied and that's it. Everyone is migrating to toroidal transformers these days, they are better in several ways be it reduced magnetic flux, reduced footprint or resistance from sympathetic vibrations and oscillations.

[kubik369]

Thanks, today I think I have solved all of my problems with schematic of this amp and by the way I was wondering why there isn't any amp with ,,built in" MP3? I think that it would be great for begginers and maybe for intermediate players too to have MP3 with songs they play normally in their amp becost I think that I'm not the only one who knocked mobile off the amp . I mean, if there would be some cheap or better MP3 next to the controls of the amp and from the front or back of the amp USB for charging and moving songs from PC to MP3, and this MP3 would be connected on the switch of the jack of the line in. What do you think of that?

[imaradiostar]

Thanks, today I think I have solved all of my problems with schematic of this amp and by the way I was wondering why there isn't any amp with ,,built in" MP3? I think that it would be great for begginers and maybe for intermediate players too to have MP3 with songs they play normally in their amp becost I think that I'm not the only one who knocked mobile off the amp . I mean, if there would be some cheap or better MP3 next to the controls of the amp and from the front or back of the amp USB for charging and moving songs from PC to MP3, and this MP3 would be connected on the switch of the jack of the line in. What do you think of that?

I would imagine it has to do with price point- someone buying a solid state amp for a low price might not be able to afford the same amp if it included 50 to 100 dollars of hardware and software dedicated to making it play mp3's reliably. This also doesn't account for wma and aac licensing fees, which would probably be needed to make the thing useful to a wide audience.

Why not just use the mp3 input? Many amps have them nowadays.

jamie

[kubik369]

Yeah, it surely has something with price point in solidstate amps, but what about high-end amps with price 1500$+? If someone buys kind of amp of that price, he surely has equally expesive guitar and other stuff so he wouldn't mind giving 100-200$ more on this. And of course we can use regullar mp3 input but it would be more estetic and good looking to have something like this.

[J M Fahey]

Dear kubik.
Congratulations on your idea, which I embrace, but start with something simpler, go step by step.
That Marshall model is about the most complex 15W starter amp I have seen, with lots of tweaks and add ons (considering its size) which will drive you crazy.
Worst will be designing the PCB!!!
I suggest you go step by step.
No kidding.
Start by building a "Ruby" or similar LM386 amp.
Even if you "do not need it" ... you'll love it.
Those 1/2W can be pretty annoying when somebody else is trying to sleep, study, or watch TV in peace.
Then build a simple , no preamp, TDA2003/2006/2030/2050/LM1875 one ... which already be the power amp of the next more complex one.
You can find a PCB layout suggestion in the datasheet.
You can drive these 12/30W amps straight (clean) or using a pedal for distortion.
Later you can build a preamp and the amp is complete.
I suggest you build the preamp from the Marshall Lead12/5005 .
It is so good, it was used by ZZTop to record, go figure!!

OR: as suggested, get a burnt 15W guitar amp, so you already have a cabinet, chassis, speakers, transformer, etc. and build your own inside.
The *big* problem is not the "Electronics" part but what I call the "mechanicals": chassis/transformer/cabinet/speaker/etc.
Good luck.
PS: and yes, obviously you'll have easier access to "European" parts .... I wonder why