MAX Forty "Devil"

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Re: MAX Forty "Devil"

Postby Racing on Tue Jan 19, 2016 7:19 pm

Speaking of gridleaks then...
If you wish,go ahead and replace them 120k resistors with 150k or 220k if you wish. With that bias issue of yours settled,please try.
Then report back.
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Re: MAX Forty "Devil"

Postby Antti M. on Sat Jan 23, 2016 6:01 pm

Thank you again for your advice! I changed that bias system the way you explained. I used those 100k trimpots which I already had and made individual adjusts for tubes. And YEAH, adjust works like a charm! No problem anymore to adjust bias. I adjusted tubes to take about 21mA each at idle and anode voltage is about 380V now, so I believe it's now as recommended for these tubes.

But these sound modifications, could you recommend something? Sound is not bad, but could be much better...of course everyones taste is different and my cabinet(pretty cheap Laney 4x12 with some HH-elements) could be also better but anyway. Mostly I like 70's rock and heavy so sound should fit to that kind of music and texas boogie also. IMO as original there's too little bottom at second channel and maybe even too much at the first channel. Does that "James" modifications, which you spoke about, affect to that and sounds generally? And as I mentioned earlier I would like second channel to be as sensitive as first channel(or I don't know if first channel is too sensitive or is it just that too much bottom or what, but when I hit E-string hard, sound distorts wrong way, like too much volume). My friend has a Marshall head from -68( I don't remember model) and that kind of sound I would like to have.
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Re: MAX Forty "Devil"

Postby Racing on Sun Jan 24, 2016 12:13 am

Image

THERE ya go!!! Good for you! All to happy to help out :D

Yeah. As for modding. How shall i put this...?
Of course it´s a matter of taste. I honestly HATE being elitistic but the truth of the matter is that most that are into it are "copy&paste" kind of guys. In short they don´t really develop crap...
So.
What i guess i´m saying is that those of us that do..it comes down to beliefs and doctrine in a sense. I for one believe in letting tubes do what they´re good at. Amplify sound the "tube" way. Ie; when i put amps together i TRY to do this with a minimum of filters and what not. This is where the James network enters...
Most guitar amps work with what we call tonestacks. Stacks as the pots are "stacked" on each other. The James,as the Baxandall,does not. Both of them are indeed interactive networks and as i´ve come to settle on the James i´m going to take the time to tell you why.
My mentor in all of this,who´s one of them blokes that put together the REAL expensive stuff,told me to explore the active Baxandall. Which i did.

Most guitarist don´t really reflect on the FACT that most stacks are rather useless seing what´s REALLY available. Well. Me exploring the Bax i came to take to heart that there is such a thing as to MUCH control. In short the pots got insensitive,the way I fiddled around with component values.
So.
That made me think...see..Aleks is good at making you do that.. Due result was that i came to challenge the way less sensitive James,which is passive,instead and have come to sort of settle on a happy middle.
By setting the James up with the option of various mini switches that flip components in and out i get myself a tonecontrol that is as "wide" in what it does i could ever ask.
For some reason...the James is rarely used for guitar work. Guess due to it´s relative complexity. Some have tho...no argument. Like FBT...like DNS..like Orange and what not over the yrs.

That said the fact remains that actual TONE SHAPING i keep within the James. Basically as i see no reason to argue with results and this in turn lends me great leeway in how to set the actual gainstages up.

So. Long story. I´m aware of that.
The James i guess you recognize from the schematic of the MAX amp. Again. Do NOTHING else but replace the values for it with the ones that you find first thing as you fire the program tonestack calculator up.
Why?
Well,because it´s rather generic values that will hand you a baseline to work from. Getting experience with the James is half the job... But suffice it to say that already that will hand you a midfocused distorting guitaramp.
The thing is that you want to reposition that James to work off of the first stage plate. Ie;you install that James to work BETWEEN the two stages up front for that channel.
That´s...No1.
You finish that install off by installing the volume pot after the James has done its work.

Doing so will most likely make the amp "flobby" in the bottom end. In short you will have gainfactor out the ass seing the following two stages. Therefore..replace the cathode resistor decoupling caps of stock 25uF with 4,7uF ones. That...will help.

Now.
Out of "Can 2" you notice a 220k resistor. Increase that for 470k to 1M.

This is where the two volumes,of each channel,intersect. All good.
IMO though the designers made one half assed flaw after that because they made the master volume control part of the following stage...which is the concertina phaseinverter driverstage.
That brings that you want that stage to be as "neutral" as it can become. Therefore you do NOT want its bias to sway all over the place coupled to how the master control is dialed.
In short? Install a 22nF cap (which is overkill for the app) outbound of the mastercontrol. Before the signal hits the 220k gridstopper. This will make the driverstage completely stable no matter what is tossed at it and therefore way more predictable.

Concertinas in turn. Much can be said. Me i´ve grown tired of them. They do work though,no argument there. Word of advice is that you swap the stock 2.2k cathode resistor for a 3,3k one,which will add "warmth" to the sound.
What absolutely MUST be installed between the driver and the concertina though,for overdriven sounds to work,is a MASSIVE gridstopper. What you do is that you solder a 470k-1 M Ohm resistor in the signals way from the driver for the concertina. This will in essence completely erase any and all of that "fizzlin" sound once and for all that is inherent with concertinas as they´re driven up the proverbial wall.

But... Let´s take it from there.
Pull them mods off and report back. Tell me what you think. What you´d want more/less of.

Cheers! :rawk:
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Re: MAX Forty "Devil"

Postby Racing on Sun Jan 24, 2016 12:20 am

Further.
21mA@ approx 380VDC tells us something is amiss. At 21mA you should see inxs of 400VDC easy. This...of course with "european standard wall voltage". Ie; approx 230VAC at the socket.
If you DO see them 230VAC inbound..something IS amiss. Find out what is.

It may be a resistor that has drifted..a cap that is leaking..whatever...find out what it is.
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Re: MAX Forty "Devil"

Postby Racing on Sun Jan 24, 2016 11:57 am

In fact,if you want even more distortion out of the amp and in turn keep it "stock" still...just reposition the master control.
Let it do its thing between the concertina driver stage and the actual concertina phaseinverter.
Be aware that this entails installing yet another coupling capacitor. The pot needs to be completely decoupled...

That 470k to 1M resistor as a gridstopper STILL needs to be there too though..

Please also be adviced that the EL-36 powertubes are rather loud customers handed what they ask. A fresh MAX amp should deliver an EASY 50w clean.
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Re: MAX Forty "Devil"

Postby Antti M. on Sun Jan 24, 2016 5:40 pm

This was little difficult text for beginner such me... I tried to write/draw few changes to schematics which were little unclear to me...here is the pic.

Image

-First, this James thing, I believe it's where I marked, but where is R4 and R5 or should I add them? Or is that 100K before Master vol other of them? Do this James and ch1 vol pot. change places with each other, did I understood right? And what about 220k in the ch1 vol.pot where does it go(and 100k after James)? Picture sure would help a lot...

-Second, this ch2 resistor what I should change I guess I found in right place?

-Third, 22nF cap in master vol comes to there where I drawed? What type should it be or does it matter(or is there even different types of that size...)?

-Fourth, Cathode resistor in concertina, I guess I found it right?

-Fifth, Gridstopper between driver and concertina, did I draw it to right place?

-Sixth, I suppose 0,5W is enough for all these resistors?

-Seventh, What do you think, could I keep original 0,5M pots in James? I simulated it in tonestack calculator and it didn't made so significan't change...
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Re: MAX Forty "Devil"

Postby Racing on Mon Jan 25, 2016 11:12 am

You got it right apart from point 5.
That gridstopper goes the other side of the decoupling cap. Or put another way,make that resistor come as close as it can to the actual concertina grid. Physically.

That apart..yep. On the money i´d say.

The James then. Yes. You compliment the setup with a resistor. R4. The actual value of this resistor (aka separation resistor) isn´t super sensitive,therefore you can set it to 220k just as well as that is a way more common value.(I from time to time replace it with a pot of 500k to be able to adjust it).

Yes. 1/2w resistors will suffice throughout.

If you wish you can iterate the stock setup. Just set separation resistor value to 1 in TSC. Do so..as that´ll explain to you why the amp works/sounds as it does. (You click the values in TSC to change them. Capacitor reference works with both pF and nF)

From a practical point of view then. As you can gather... the components for each tonecontrol is installed to the rear of the pots involved. You should do the same,hence..neat and orderly fashion is the name of the game. Take your time.
Inside tip is that you make a mental note of which wire heads inbound...and which outbound. Mark these. Then pick the entire thing apart...including the pots.
Clean the rear surfaces of the pots with some emery cloth,spray them through an opening to clean them within (if you wish to you can dismantle them by prying their tabs back-in that case just use rubbing alcohol to clean it out as it is apart). Now install the pots "backwards". Ie; so that they protrude at the front facia of the amp. Ie;you can now install the components needed outside the amp but still with a solid reference for spacing as the pots are still in their respective location.

As for what leg of which pot to use...from the rear the leftmost leg is always the one that heads for ground in a James. In the case of the bass pot via a 10k resistor.
Just keep in mind which pot will be used for bass and which for treble. Point being that "building" the thing is WAY easier this way. When done you simply unbolt the thing and install it from within per normal and attach those two wires..in and out. What´s needed more is a third wire,and that´s a wire heading for ground. Do NOT ground at the rear/back of the pots.
Ground the James AND the following volume control to the tagboard where the actual gainstages ground.

R5 in turn isn´t needed as you replace R5 with a volume pot. Use the volume pot already in place between stage 1&2.

Between stage 1&2 there´s a gridstopper resistor of 220k outbound of the volume control. Replace that resistor with a 22k-56k one. Your choice.

If you look closely at the schem..inbound of that volume pot between stage 1&2 there´s YET another entry. The "Line" one. Well,it´s actually an output jack. As you most likely won´t need a line out..just delete that wire. Remove it and be done with it. The jack..might be needed later though so let that stay.
Should you ever want a line out jack this should be installed together with a voltage divider AND a resistor to lift ground somewhat WAY WAY WAY later in the gainstage chain. Just as would you an effects loop or similar.


As a general note. Making two stagers really rock means that you need to extract what signal is there to be found. As the amp uses a concertina and a concertina is always preceeded by a driver stage that leaves us with a shared THIRD stage..which from that respect CAN be a good thing. All depending what it is we seek.

For that very reason you might want to consider moving the master volume control to work between the driver stage and the actual concertina. This...is a matter of taste. Ie;try them both. As is ...and with it moved one stage down the line.
As a concertina on a whole have an amplification factor of 1 that brings that a volume control so installed works the same way from a practical point of view as does a PPIMV control. Almost at least...(the 3,3k resistor will add an ever so slight amplification factor amongst other things)
Ie;it might be an idea to try the amp out with an ADDED volume control to thus be able to adjust resolution of the driver stage. But...before doing that just try moving the master volume control between in and out of the driver stage.
(If you wish to run that way just scrap one of the speaker out jacks and install the pot in that hole)

As you can see in the schem the designers opted to use 220k plate resistors for the preamp throughout. This of course also impacts how each stage amplifies. What you can try is replacing them with more common value 100k ones. What suits you...again a matter of taste. The 100k ones will open the sound up quite a bit.

Now. I appreciate that the girth of this might make it a bit confusing to the novice. I appreciate that,it´s just that since this amp was designed (and many many others) our reference as far as sound and rocknroll has come to change quite a bit.
I guess what i´m trying to say is that there´s no shortcuts,and it does get even MORE involved really. Way more even....What sets the manner in which a gainstage amplifies is to a large extent a matter of two things as major factors.

1st... is the RAIL voltage. Ie; the voltage atop of the actual plate resistor used. Doesn´t the plate resistor set the voltage for the stage coupled with the cathode resistor? Yes it does,it´s just that the plate resistor we should regard for amplification FACTOR,as in AC voltage swing,and therefore we adjust the actual voltage via altering RAIL voltage as we set the plate resistor to a given value vs what swing we ask for out of the stage. Of course these two interact...

2nd.... is the cathode resistor value. This has the larger impact of the two. It will affect the working point and loadline of the stage is a rather profound manner and when adding gainstages...this is where it gets complex as the added various stages will amplify with a different bias if so setup. In other words this turns into an exponential function as we add stages,and hence..adds complexity as far as sound out of the amp.

Cool. That´s a lot of theoretical blabbering...then what´s it good for and how does one go about playing with this from a practical point of view? Well,first of all...way to few do. Play around with it that is,and seing the complexity of it as gainstages are added together..i can understand why.
Herein also lies the reason for why i recommended you to set that James up as neutral as they come. IF you opt to play around with primarily cathode resistor values that way you´ll be handed what the actual STAGES bring to the table. The "fine tuning" can always be done later by modifying the values of the James,and just take my word for it...the James network is as versatile as they come..
As you can see the amp uses 2.2k cathode resistors as a whole. This is rather common with "older" amps and wasn´t a MAX exclusive by any means.
If you compare that to how a Fender is setup with it´s 1.5k stages...how Marshall in the 80´s came to use a 10k stage for more distortion...and in turn the other end of the scale the Mike Soldano fabled 39k stage....and so on and so on..
The higher the actual value of the cathode resistor the less the actual gain/amplification out of that stage but..at the same time we alter loadlines and what not as well. In short..this is tricky business.
As you can probably gather...this is class 201... :mrgreen:

However...
Point still is that as you pull the mods off above,except this cathode resistor deal,you form what i opt call a baseline. Ie;them mods done there´s very little reason to go back a redo it as you keep modding,if that´s your wish. Ergo these modifications come first.
Evaluate what they bring...and be aware of the "news worth" of it. Ie;take your time evaluating.
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Re: MAX Forty "Devil"

Postby Racing on Mon Jan 25, 2016 11:21 am

Image

At the top of the picture a typical James installed to the rear of its pots.

Also mark the use of shielded wiring. I use regular braided coax wire for this.
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Re: MAX Forty "Devil"

Postby Antti M. on Mon Jan 25, 2016 7:30 pm

I tried to draw these ch1 changes and master vol repositioning... do they look about right or..?

Image

Image

And should I replace those cathode resistor decoupling caps to smaller in ch2 also? And my amp had caps in both cathodes in ch2, should I keep it that way?

I guess I can order some parts tomorrow then and start try modding this. 8)

BTW, I changed potentiometer knobs, cause few were broken, but few were ok condition. So I thought that I could send those which were ok to you as a small thank you if you want to have them as a spare parts?
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Re: MAX Forty "Devil"

Postby Racing on Tue Jan 26, 2016 12:37 am

Control knobs will be much appreciated!

Yes. You´re on the money apart from one thing.
Drop the cathode decoupling e-lyte of the concertina. Just delete it. It´s nor wanted or needed.

Yes. Keep the cathode resistor decoupling caps for the preamp. Both stages.
Just replace the ones for the "James" channel with 4,7µF ones.
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Re: MAX Forty "Devil"

Postby Racing on Tue Jan 26, 2016 12:44 pm

Oh!
Came to think of a small,but important,step.
This amp uses a full wave rectifier bridge. Ie;The bridge has a point that attaches to the chassis. Don´t.
With the amp upside down and facia facing you the e-lytes to the upper far left are the ones to focus on. This amp uses what´s known as a "half tap" to feed the screen grids of the powertubes. The full voltage is handled by two of in all 4 tabs of 2 e-lytes,then a third tab for the half voltage. For some reason they set the first e-lyte to take half of the smoothing capacity of the full voltage and the other half to take the halftap voltage. In turn the rest of the capacitance needed for full voltage is taken up by the NEXT e-lyte in order.
You should move these taps around so that the first electrolyte takes BOTH of the full voltage hits. Next up e-lyte thus serves the halftap and the PI.
Here´s the trick though. Being setup as it is the amp does NOT conform to what´s known as "the golden rule" and this should be taken care of. Now..first hit e-lyte takes the "first hit" on both its tabs. This e-lyte is either of the type that uses a separate washer for ground or one that uses a tab for grounding. No matter...
What you do is that you install a fresh wire from the bridge negative directly to that first hit electrolytes grounding point. From THERE in turn you run a wire to the central chassis grounding point.

This is known to "adhere to the golden rule" and is a surefire way of making sure the amp won´t hum in by appreciable amount. This is imperative.

If you happen to experience AC hum with the amp fired up anyway locate the heater winding artificial centertap. Cut those resistors out of the equation. Replace that with a 500 Ohm 1/4w trimpot that you install in the location where those resistors were. Then run a wire from the wiper of that trimpot to ground.
This is known as a "hum dinger" and what it does is let you set heater balance vs ground. By trimming away you will notice how any hum will differ in amplitude,if present.

On that topic...
The amps preamp and PI grounds all over the friggin place. It is ALWAYS a good idea to separate this and keep it apart. The CORRECT way of letting ground happen is to route each functions ground,as needed,to the grounding point of its stage. (Where the cathode resistors are soldered in).
Then run a separate wire from there to the grounding point to the grounding point of the electrolyte serving it. This practice falls under Kirchoffs laws. Google Kirchoffs laws if needed.

What´s more. Those i´ve had my head into have all suffered from weak entry jacks. The jacks used are regular Cliff ones and can be replaced directly,using the stock chromed nuts,with modern day counterparts from Cliff.
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Re: MAX Forty "Devil"

Postby Antti M. on Tue Jan 26, 2016 5:46 pm

Earlier you mentioned that if I just reposition mastervol pot it should be fully decoupled(how it's done?)...but not with these other mods( so my drawing was right except concertinas cathode decoupling cap)? Or did I understand something wrong?
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Re: MAX Forty "Devil"

Postby Racing on Tue Jan 26, 2016 9:03 pm

Fully decoupled in this case means that you shield the pot from DC.
That means,as both "sides" of the junction between the driver and concertina grid hold DC voltage...that you need a coupling capacitor at both ends.

If we do the R/C math on this we soon realize that we need less capacitance for those capacitors then we might first suspect...and still retain full bandwith.

Stock the mastercontrol is inbound for the drivers grid. There no voltage is present... However,if we don´t use a capacitor between the grid and the master volume the master will become a part of the gridleak,and hence this will vary with how the master is dialed and thus affect how the driverstage does its job.
If we place a capacitor between the wiperlug of the master and the grid we make sure that the only path to ground for the grid is via that...is it a 560k resistor.? (Don´t have the schematic in front of me ATM)
By making sure the driver only reacts to AC signal and nothing else is variable we set a stage up that will be way more predictable.
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Re: MAX Forty "Devil"

Postby Antti M. on Fri Jan 29, 2016 2:32 pm

So, if I understood correctly that's alright in the picture?
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Re: MAX Forty "Devil"

Postby Racing on Fri Jan 29, 2016 8:10 pm

WIth the add on in text form by me,yes.
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