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Author Topic: Rebuilding Marshal Amp  (Read 14346 times)

CraftyZA

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Rebuilding Marshal Amp
« on: March 25, 2015, 02:39:11 AM »
I'm rebuilding a marshal valvestate S80 amp. The one with the 2 x 12" drivers.
I'm going to rip out the current amp portions and only use the chassis and drivers.
I've got the new power stage sourced. I can decide between a veloman kit amp, or lm3886 based amp designed by a friend of mine.
However...
The preamp and overdrive is still a headache. I need to understand the dynamics before i can design it.
The idea is to use an 12ax7 valve for that. possibly 2.
The input signal on the power stage is set to max 2v. This i quite high. I can adjust to about 1v
Do i need 2 stages before that? IE normal preamp, that will be overdriven by a 2nd preamp? Or do i just give one preamp a very large gain range from clean to crunchy?
Any one got simple circuit diagrams for it?
I've bought an extra 12ax7 tube. Plus I have the one that can be salvaged from the original valvestate amp.

CraftyZA

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Re: Rebuilding Marshal Amp
« Reply #1 on: March 30, 2015, 09:30:31 AM »
So I've decided on this:

www.instructables.com/id/Guitar-Tube-Pre-Amp/?ALLSTEPS

Seems like this will do the trick. Do you guys know this design? know what it will sound like?

Roly

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Re: Rebuilding Marshal Amp
« Reply #2 on: March 30, 2015, 03:04:48 PM »
Hi CraftyZA, welcome.


Quote from: CraftyZA
I'm rebuilding a marshal valvestate S80 amp.

Why?
If you say theory and practice don't agree you haven't applied enough theory.

CraftyZA

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Re: Rebuilding Marshal Amp
« Reply #3 on: March 31, 2015, 06:36:56 AM »
Hi CraftyZA, welcome.


Quote from: CraftyZA
I'm rebuilding a marshal valvestate S80 amp.

Why?

It is fried! Replaced the 2 chips, and a bunch of zeners, then some resistors blew up, and some tracks melted.
I rescued the valve, and some other parts. Will Use the chassis and drivers in this hybrid project.

J M Fahey

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Re: Rebuilding Marshal Amp
« Reply #4 on: March 31, 2015, 08:13:32 AM »
That instructables preamp won't do it, it's almost a toy, just a classroom demo on how a tube works.

Pity that Valvestate died, because it's the best of them all.

In your case, I would use just the cabinet and speakers , buy a used guitar head , any kind you like and can afford, and use it to drive that cabinet, which has excellent speakers.

The original ones are 4 ohms each, so you should connect them in series to make an 8 ohms cabinet.

DrGonz78

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Re: Rebuilding Marshal Amp
« Reply #5 on: March 31, 2015, 08:26:58 AM »
How bad is the damage? Got some pics?
“A person who never made a mistake never tried anything new.” -Albert Einstein

Roly

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Re: Rebuilding Marshal Amp
« Reply #6 on: March 31, 2015, 08:51:54 AM »
Quote from: CraftyZA
then some resistors blew up, and some tracks melted.

It would seem that you need to read this thread;
http://www.ssguitar.com/index.php?topic=2093.0

A limiting lamp really is an essential tool when working on solid-state amplifiers.




On the input socket pins 4 and 5 should be connected together to silence the amp when nothing is plugged in.


Quote from: CraftyZA
know what it will sound like?

Running on only 60 volts it will have little or no headroom to clipping. 

There are good reasons why the lowest supply voltage RCA characterised the 12AX7 for was 90 volts, and why all of the classic era valve guitar amps invariably used around 200-300 volt supply.

There are two basic requirements when setting the idle or DC conditions of a triode stage (selecting the anode and cathode resistor values);

- the anode voltage should idle about half the supply,

- the cathode voltage should be higher than the largest expected signal peak.

As the supply voltage is lowered it becomes harder to satisfy both of these requirements, and in fact the anode voltage in the Instructables design will idle around 50 volts meaning an output headroom of only 10 volts to clipping.

We normally assume that a guitar has an output of around 500mV average with peaks to 1 volt.

In the classic application the cathode voltage is between one and two volts, so it is quite hard to clip the first stage even if you belt the guitar.

On a 60V supply the cathode voltage drops to around 450mV so even average playing is getting you into clip territory.



from RCA databook

One of the things about these triodes that guitarists like is the fact that they are not actually linear, that positive signal excursions tend to get stretched while negative excursions tend to get compressed, in other words they produce some harmonic distortion.  The larger the signal swing on the grid the more this occurs, conversely with a limited grid swing there is less of this effect, and a low supply voltage seriously limits the available swing until clipping occurs.

This classic load line from valvewizard illustrates the point;

Vsupply = 320 volts

The distance along the blue 100k load line between Vg=0 and Vg=-0.5 is much greater than the distance between Vg=-2.5 and Vg=-3.  The spacing between intersections gets continuously shorter between Vg=0 and Vg=-4, so as the signal level goes up so does the harmonic distortion.  In a preamp this is what "tube tone" is all about.

If we move the 100k load line to the left so it runs from 60 volts to 0.6mA we can see that there is much less swing available to clipping, but also that this compression effect is less, so we will get less harmonic distortion before the onset of clipping, less "tube tone", then grunge.

I understand that people are afraid of high voltages, but running a preamp on voltages less than about 90 is a bit like running your car on kerosine because you are afraid of petrol.

Getting a 200-300V supply at a couple of milliamps is trivial, certainly more trivial than that switch mode converter, and a couple of small back-to-back transformers don't have to be sealed up in a metal box and carefully filtered lest (albeit supersonic) switch mode artifacts turn up in your signal chain.

{BTW the 100k anode load resistors R1 and R2 don't need to be 1 watt, half watt is sufficient even on a 300 volt supply}

HTH
If you say theory and practice don't agree you haven't applied enough theory.

CraftyZA

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Re: Rebuilding Marshal Amp
« Reply #7 on: March 31, 2015, 09:31:28 AM »
Sorry, did not take pics. I had the amp with somebody I really trust. Even he said to not spend more money. Everything is fried inside the power stages. It has 2 power stages. I think the one is used for chorus. All circuits in the line of the first stage was blown.  So not the chorus channel.
After salvaging some parts I discarded the board. Got enough 30% - 70% projects floating around. Time to clean up.
Towards the end of the year, I'm going to build a Twead like clapton used. But that is a project for later.

Roly, Thanks for that explanation. Strange to think that there are even 12v circuits on the net. Valve Caster 2.0 runs on 9 volt batteries  :duh

I will do some more searching. I've got 2 x 12x7 tubes (not matched), and have not bought components yet. They are all in an online basket. Still researching.
However, I have ordered a 60v transformer vir 6v seconadry. Can use voltage doubler on that 6v and try to regulate it to 12.6 for the filaments. However, seems like that 60v will not be enough.
I'm sure I can use it in another project somewhere.





Roly

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Re: Rebuilding Marshal Amp
« Reply #8 on: March 31, 2015, 10:18:29 AM »
Year, sure, they will still work down to stupid voltages, but you did ask what it would sound like, and from a techo-guitarist point-of-view, better on higher supply voltages.  I'd guess the people at RCA (12AX7) and Mullard (ECC83) would raise an eyebrow at supply voltages less than 90.  "But why" I can hear them asking, "when it is easy to run them at their design voltages?"

Quote from: CraftyZA
I've got 2 x 12x7 tubes (not matched)

There is no point at all in paying for matched preamp valves, they aren't operating in push-pull like an output stage.  Matching output valves is a Hi-Fi imperative and implies a reasonable quality output transformer, but again some mismatch in the output valves and a mid-fi output transformer gives rise to distortions that guitarists like.  One tech here goes as far as saying that "what you do for Hi-Fi, try doing the opposite for guitar" - a bit extreme perhaps, but guitar amps ain't Hi-Fi amps and we need to constantly question the "truths" that sneak in from the Hi-Fi world because they often don't apply.

Given the very low current draw of preamp stages a voltage multiplier such as a doubler;


... tripler;


... or quadrupler;


... is quite practical.
If you say theory and practice don't agree you haven't applied enough theory.

CraftyZA

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Re: Rebuilding Marshal Amp
« Reply #9 on: April 02, 2015, 08:19:05 AM »





Am I right in saying that if I increase the gain on the left I can create a stronger distortion, and the output from the valve on the right will stay relative static. Thus I can have a 2v output signal, and by really overdriving it, that signal should still stay close to 2v? I'm going to order parts for this now, and pick it up on saturday. Should make for a nice weekend project :-)

g1

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Re: Rebuilding Marshal Amp
« Reply #10 on: April 02, 2015, 12:26:03 PM »
  I doubt that having those 2 pots ganged together will track the way you expect.
Also, you are vastly limiting the tonal possibilities you would have if you use 2 separate pots.

J M Fahey

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Re: Rebuilding Marshal Amp
« Reply #11 on: April 02, 2015, 02:44:04 PM »
The big problem with that preamp is that it's too crude, too raw in a bad way, has no EQ at all and the guitar will sound dull and boomy.

I suggest you build *at least*  a simple "Fender type" preamp which will be vastly superior.

One such preamp is the Alembic, ripped straight from a Blackface Twin/Super/you-name-it-Reverb.



why build a crude classroom example when with the same effort and cost you can build the real thing?

They built it stereo but you can build a single channel, of course.

And they also double voltages to a sensible value, basically to make winding the small power transformer simpler.

By the way, inside the Stereo Chorus Valvestate you *do*  have a step up transformer, fed from the 30 or 30+30V secondary giving you some 250V +B ... USE it  ;)

Alembic used that tube preamp to drive monster racks chock full of big PA amps and mountains of speakers, here's a somewhat famous band using  that:



No need to to that extreme, of course, but you can build that preamp inside your VS chassis, and use existing transformers to power the preamp and 2 x  LM3886/TDA7294 chipamps and driving the original speakers.

You'll have a "hybrid Twin" of sorts, very close sounding to the real thing, for peanuts.

Of course, use pedals for dirt and reverb.
« Last Edit: April 02, 2015, 07:26:58 PM by J M Fahey »

Roly

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Re: Rebuilding Marshal Amp
« Reply #12 on: April 02, 2015, 07:22:53 PM »
{Now that's what I call a stack - dig all that cone area  :lmao: }


JMF provides some excellent advice.   :dbtu:

Assuming that the Alembic is running on a supply of around 250 volts you have a very classic preamp designed by Leo Fender (who had just the odd clue or two), a very well worn path for very good reason - it works well with guitar and has been a proven winner for over 50 years.  You will note that this circuit comes via R.G.Keen's site, another gentleman with the odd clue.   8|

{mind you, in my opinion most of the classic sound of a valve guitar amp comes from a valve output stage driving an output transformer, but that's a whole other discussion}


Quote from: CraftyZA
if I increase the gain on the left I can create a stronger distortion

How?  A triode is not an op-amp with a huge hidden reserve of gain available for the tweeking of a resistor value.  The first stage is already operating at or close to flat tack (see the "V.G.", Voltage Gain, column in the RCA table above) and you are already trying to thread your guitar signal through the eye of a headroom needle.  Increasing the gain would only make things worse.

Why?  Your question suggests that you have missed the point of the second part of my post above.

The characteristic "tonality" of a triode (or JFET) comes from using it in its linear range between cutoff and saturation.  If you don't want the characteristic tonality or voicing of the valve and only want square waves then you can simply use a 50-cent op-amp and diode clipper and save yourself an awful lot of bother; the fact that it's a valve just becomes purely decorative.

A problem with the low supply voltage is that the linear operating window is seriously restricted and the gain stage prone to very easy, far too easy, overload (clipping).  Using a higher supply means that the triodes have a larger linear operating range where you will get the classic characteristic preamp distortion - "clean" but harmonically rich.

What you are suggesting would result in wall-to-wall clipping under just about all playing conditions and makes the choice of the gain device quite immaterial.

Do you want an amp that has some character, a range of voicing, or do you just want a grunge engine?

Perhaps you should build what JMF rightly calls "a toy, just a classroom demo" so that you can experience what we are trying to explain. 
If you say theory and practice don't agree you haven't applied enough theory.

CraftyZA

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Re: Rebuilding Marshal Amp
« Reply #13 on: April 03, 2015, 05:34:33 PM »
This seems fantastic!!
http://www.alembic.com/prod/f2b.html
Thanks for the input. I think this alembic is the one :)

CraftyZA

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Rebuilding Marshal Amp
« Reply #14 on: April 04, 2015, 01:46:35 AM »
This looks so simple





Do you want an amp that has some character, a range of voicing, or do you just want a grunge engine?

A bit of both. You can have both, if you don't overdo the overdrive bits.

Jack white
Pixies
The who
Clapton

Otherwise one can simply hook up a boss mt2 and crank it all the open, but that sounds crappy.. That MT2 must be worst peddle ever. I like warm tones, with a touch of crunch.
But as you said in one of your earlier replies,  one can achieve that with a peddle.
« Last Edit: April 04, 2015, 02:10:09 AM by CraftyZA »