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Strong distortion tube line driver?
Strong distortion tube line driver?
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Old 22nd October 2018, 02:55 PM   #1
Mario Pankov is offline Mario Pankov  Europe
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Strong distortion tube line driver?
Default Strong distortion tube line driver?

I bought a Marshall MG50cfx amp to play heavier music on it. Looking at the schematic, it does not seem to be that much designed for heavy sound so thought if I can fix this.

The amp has a very weak power supply, I doubt filtering is adequate and will rework it. It is also not shielded well, neither is the transformer. At such gain levels I think it is important to have this reworked too.

The 4558 opamps have a gain of around 500 which I believe will not produce the sound I want so will tweak them to at least 1, 000. However, I thought that adding a tube buffer at the guitar input might produce the sound I want. The output is a TDA7293 which sounds pretty ok, although mounted on a micro heatsink (I have a spare one from an amp project and will replace it). I believe this is why they put a fan.

So, any good buffer designs made for high levels of distortion?

Last edited by Mario Pankov; 22nd October 2018 at 02:57 PM.
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Old 23rd October 2018, 08:33 AM   #2
thoglette is offline thoglette  Australia
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<answer class="Standard">
First, read everything Rob Robinette has to say. Particularly about safety, start up and trouble shooting
</answer>
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Old 24th October 2018, 09:57 AM   #3
Mario Pankov is offline Mario Pankov  Europe
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Strong distortion tube line driver?
Thanks but:

- I have experience with tubes, have built several amps
- Have quality measurement equipment incl. a digital scope (hacked Rigol DS1074z unit)
- This guy`s website looks scary (the pictures section) )

I was looking for a proven schematic of a highly distorting amp line driver that can be used in front of the Marshall guts. I will modify the Marshall as the mod will cost me like EUR30 and should boost its performance considerably but I do not like the hard distortion of solid state so wanted to add a tube overdrive and possibly isolate the solid state overdrive.

one question that arises concerns the computer in it - I believe it senses various voltages across the board, if I throw away the overdrive section, how do I fool it its fine and working?
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Old 24th October 2018, 08:24 PM   #4
shanx is offline shanx  Canada
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I would consider putting the valve /tube stage after the 1st stage of opamp (with some gain) to drive the tube harder.
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Old 24th October 2018, 09:32 PM   #5
Mario Pankov is offline Mario Pankov  Europe
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Strong distortion tube line driver?
Do you expect any issues with the onboard computer, like error indicators or so? If I have to be honest, I would omit the whole solid state overdrive and go for tubes entirely but am not sure if the amp electronic won`t flash some error message (like a selfdiagnostics).
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Old 24th October 2018, 10:50 PM   #6
djgibson51 is offline djgibson51  New Zealand
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Post the schematic
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Old 25th October 2018, 12:41 AM   #7
Printer2 is offline Printer2  Canada
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4534FC0B-837E-4FC5-BC48-18EC687650BB.png

How do you know the filtering is a problem, how much will another power transformer give you? Will the chip be able to deliver any more performance with the new caps and transformer?

How will you double the gain of the IC's? What will that do for you and the sound you want to achieve? Why do you think the computer is sensing voltages across the board other than the effect send? How about just selling the amp and getting something better? How are you going to power the tube section?
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Old 25th October 2018, 02:14 AM   #8
thoglette is offline thoglette  Australia
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Quote:
Originally Posted by Mario Pankov View Post
I was looking for a proven schematic of a highly distorting amp line driver that can be used in front of the Marshall guts.
Two options -
Proven: something like Fred Nachbaur's classic McTube Or steal the first stage from an amp you like (perhaps Soldano stages 2 & 3 or V1 from the JCM800 )

Real question: what do you consider "heavy". I.e. which amps would you like to emulate?

Post script: the MG50 preamp linked above seems to do a pretty sucky copy a marshall. Have a look at the RunOffGroove Thunderbird (sound clips here here and here ), particularly what's going on between the second and third op-amps

Last edited by thoglette; 25th October 2018 at 02:29 AM.
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Old 25th October 2018, 03:51 AM   #9
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by Mario Pankov View Post
I bought a Marshall MG50cfx amp to play heavier music on it. Looking at the schematic, it does not seem to be that much designed for heavy sound so thought if I can fix this.
Donīt "listen with your eyes"
Just plug a guitar and play.
Look what kind of sounds did Johan Segeborn pull out of his (he had the 100W version but itīs basically same sound):
YouTube
Quote:
The amp has a very weak power supply, I doubt filtering is adequate and will rework it.
Power supply is adequate for a 50W amp.
That said, you can double main electrolytics and improve the sound, which will be tighter at higher volume or plain clipping (less residual ripple):
very little improvement beyond that.
The power transformer is adequate.
Quote:
It is also not shielded well, neither is the transformer.
At such gain levels I think it is important to have this reworked too.
Not a real problem, since sensitive inputs are well away of the power transformer, at the opposite end of the chassis.
Quote:
The 4558 opamps have a gain of around 500 which I believe will not produce the sound I want so will tweak them to at least 1, 000.
However, I thought that adding a tube buffer at the guitar input might produce the sound I want.
Maybe you can avoid that,there is more gain available.
Quote:
The output is a TDA7293 which sounds pretty ok, although mounted on a micro heatsink (I have a spare one from an amp project and will replace it). I believe this is why they put a fan.
yes, apparently a fan is cheaper than a heatsink.

Quote:
So, any good buffer designs made for high levels of distortion?
Read above
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Old 25th October 2018, 09:42 AM   #10
Mario Pankov is offline Mario Pankov  Europe
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Strong distortion tube line driver?
Thanks a lot for your comments. Here`s the story and the plan:

I am total novice, bought my first guitar (an ESP LTD EX-50) and asked a friend for advice on what amp would be suitable for rock and metal. He said get a Marshall, and I did, only later to read tons of bad reviews online.

So what I thought of doing:

- Not touch the transformer, just shield it with a pan, it will cost me like EUR3 here
- electrolytic caps are way too low and I think this amp will be noisy. I will replace them with bigger ones - 6800uF instead othe 1000uF 63V ones, 2200uF ELNA (I have some laying around) instead of the op amp supply, may do a CRC there if there`s space (EUR12 in total for all including some 47uF ones on the headphone output which will be replaced with a larger value to push down the corner frequency)
- Replace the coupling caps with Wima MKS (around EUR11 here)
- shield the case (free, have tons of sticky aluminium foil)
- increase gain of the opamps by tweaking Rf/Ri values and the feedback cap if amp starts to oscillate to something that will produce a gain of 1000 or so
- replace the heatsink with a big one, I have it lying from a past project

Then I read tube amps are much better for what I will do and thought, since I`ve built tube amps before, why not modify the section, have lots of 6N23P tubes (ECC88) laying around since my main ampis a hybrid that uses an 6N23P input. So, thought I can force it into the non-linear part of the curve but have zero experience of making a tube distort badly and thats what I need some input on

Hope that helps!
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