Jcm800 Downscale

[Austin]

I used two triodes in cascode for a power section once, cascodes are supposed to have more of a pentode sound. I used a 6080 driving a triode strapped el84 on top, I thought it sounded very nice. With 40 volts you could make a cascode with a single dual triode 6080 or two of them with both halves parallel and drive them class a2 with a mosfet or chip amp or even better make a push pull amp with two cascodes that might sound pretty cool and still be on 40 volts. Kinda far out I know but it would be very simple and especially with a chip amp dc coupled to the 6080 driving it into REAL saturation and might even sound pretty rich in harmonics. I imagine it would be good for a couple watts at least..

[printer2]

Well if you follow some basic concepts like not working on low voltage circuitry with sweaty or bleeding hands I doubt a brush against a voltage rail will do more than wake you up.

[Austin]

I have been shocked a few times with high voltage.. Those dang caps.. My arm was sore and tingled for an hour afterwards once.

[Chuck H]

When I was really green I shoked myself BADLY at the junction of the rectifier diodes while holding my guitar with the other hand!!! I thought someone had grabbed my shoulders from behind. Then my vision started to pinhole. Like when you turn off an old black and white TV. That's when I realized I was in trouble. My will overcame the paralysis and when I pulled my finger free I was literally blasted over the back of my chair!!! Landing on my back behind where I'd been sitting. My left hand clamped down so hard on the fingerboard of my guitar that it pinched two strings broken and crushed three frets down into the wood of the fingerboard!!! No lie. I had a charred flesh nugget the size of a pea in the index finger of my right hand for several weeks.

I told the tale at the local music store and the guys started calling me "Sizzlin' Chuck". Anyone not in the know thought it was for my playing. Tee hee.

Not trying to rob thunder here, but it was a big event in my life. I'm very careful now, of course. And I hope you all are too. I've known two experienced electricians that died from electrocution. Please, everyone, never become cavalier and be careful. The helplessness of electrocution is like drowning, or burning. You know it's your ass, and it's terrifying. No one I know here should go like that.

[truckfighter]

Hello Everybody,

Thank you so much for all the replies, and I am sorry I only answer now, but I have been busy for the weekend.

The 40 Volt limitation is an safty issue and a limit set by my university. It is states that no two points on the circuit can exeed this limit, so a +40/-40 design is out of limits as this clearly shows a maximum Voltage differential of 80 Volts. So for now 40V is the absolute limit. I could politly ask my proffessor for maybe 50 Volts, but I don't think that that would make a big difference therefore I will keep it to 40 for now.

About my education. Everything between my degree and me is this thesis right now, so you can basically consider me an engineer. Someone said that they don't teach you anything about tubes at collage and this is absolutely right. Beeing a musicion I was always fascinated with tubes and I decided to ask my professor if I could write my thesis about them. All I know about tubes is what I have read on the Internet and books, I have zero practical experienc.

So when I was going about this project I needed to state what I want to achive. With a HiFi amp the recurements are easyly defined, good linearity, and the same amplification over the band of frequencies. With a guitaramp it is kind of hard to define the requirements, so my aproach was to copy the tone of a JCM800, just because its one of my favorite amps.
I knew, that I would get problems with the low Voltages I am limited to, so I started to read some datasheets to find out what tubes would be suitable. I ran across the ECC88 and ECC86 as preamp tubes and they seemed a good chice to me. Especially the ECC86 since its operation Value is rated at 12 Volts. It was my understanding, that i could design a cuircuit with them that would give me the same distortions in each of the 4 preamp stages, only limiting the outputpower which I don't greatly care for. If it comes out to be a amp with 1 Watt of outputpur, that would be fine with me.

Someone droped the word "starved plate". Am I understanding it right that this means a tube operating at a lower Voltage than it is designed for? If so I think the tubes I have been looking for wouldn't be considered "starved" in a 40 Volts circuit.
Let me add some links on them:

ECC86 /6GM8

http://www.shinjo.info/frank/sheets/035/6/6GM8.pdf


Ecc88 /6DJ8

6DJ8 - Wikipedia, the free encyclopedia

http://www.hebertech.com/view/1366-sylvania-6dj8.pdf


Anoter question was about the stages I wanted to use. I was thinking of keeping the basic circuit therfore 2 tubes which eqauls 4 stages in the preamp.

I really havent looked into the poweramp section too much yet, but ran across the EF98 which might be a good bet.


I hope I have answerd all the questions that accured if not, pleas ask again.

Thank you so much for your input again, this is more feedback than I hoped for!!


greetings Florian

[J M Fahey]

When I was really green I shoked myself BADLY at the junction of the rectifier diodes while holding my guitar with the other hand!!! I thought someone had grabbed my shoulders from behind. Then my vision started to pinhole. Like when you turn off an old black and white TV. That's when I realized I was in trouble. My will overcame the paralysis and when I pulled my finger free I was literally blasted over the back of my chair!!! Landing on my back behind where I'd been sitting. My left hand clamped down so hard on the fingerboard of my guitar that it pinched two strings broken and crushed three frets down into the wood of the fingerboard!!! No lie. I had a charred flesh nugget the size of a pea in the index finger of my right hand for several weeks.

I told the tale at the local music store and the guys started calling me "Sizzlin' Chuck". Anyone not in the know thought it was for my playing. Tee hee.

Not trying to rob thunder here, but it was a big event in my life. I'm very careful now, of course. And I hope you all are too. I've known two experienced electricians that died from electrocution. Please, everyone, never become cavalier and be careful. The helplessness of electrocution is like drowning, or burning. You know it's your ass, and it's terrifying. No one I know here should go like that.

Chuck H, I know what you are talking about, it's TERRIFYING because YOU HAVE NO CONTROL OVER YOUR MUSCLES.
You were lucky because one of your contacts was through the tip of a finger and your (involuntary) arm contraction pulled it away.
If you had been grabbing both points YOU WOULD BE DEAD.
This post should be a sticky and obligatory reading for everybody trying to become a member.
No kidding.

[J M Fahey]

OK Florian, then build it within your limitations and keep us informed day to day.
It will probably not sound like a "full" JCM800 but will be interesting anyway.
You will be able to keep EQ and such, but gain and distortion will be less; you will also need to redefine cathode and plate resistors according to new tubes used and available voltage.
In most gain stages you'll be around 20/25 V on plates so load and bias to gety around that value.
Good luck .... and post the step by step.
PS: I'd start by developing a working preamp first and drive a chip amp with it.
A functional power amp is on itself an entirely different project.
Save it for when you want to become a Dipl. Ingenieur Ingenieur

PS: Now that I think of it, you already *are* an Engineer, this project is simply the cherry on the cake.
I understand the 40V limit for School Kids at Science Labs and such, doubly so because Kids can be irresponsible, but for an Engineer !!!!!
C'mon , let's be serious !!!!
I already mentioned our working with tubes and *very* high voltage stuff (basically open chassis scopes) just on the 1st year at University , scary powerful 3 x 380V motors by 3rd year and our *actual* "Diseño y proyecto Final" on 6th (and last) which would be the equivalent to your Thesis and then some.
It was the course which defined whether you were given a Degree or not, and our 6 student team was to design (and build a working prototype) for a controller and synchronizer Power control system for the big motors used in a big Newspaper printing press.
They do not work with paper sheets but a mile long continuous roll of paper , which runs through a half-block long continuous printing machine at *very* high speed (you only see blurred images, go figure) and all transport cylinders must work at *perfect* syncronization or the paper either tears or slacks touching the floor.
Both are disasters and can cause losses of thousands of dollars.
We used then new optical encoders , TTL logic, but then it was fed into huge thyristor controlled power packs.
Nobody would ever *dream* of limiting us to 40 volts !!!!!!!!
Our stuff *had* to work on an actual Factory.
It's like limiting last year Doctors to applying band aids and not getting close to sick people, for fear of contagion !!!
Sorry but this Politically correct, Liability run World is going down the drain.
I'm sure Engineers in China or India do not work under those limitations.

PS2: can KM6 shed some light on how things are run in Russia today?

[Steve Conner]

I think a good plan would be to study the Marshall Lead 12. Marshall themselves intended it as a solid-state soundalike of their tube amps. And it will run fine off less than 40V.

Try to understand what they were thinking in the design process, and modify it to sound more like a JCM800. That would make a good thesis IMO.

[Steve Conner]

I think a good plan would be to study the Marshall Lead 12. Marshall themselves intended it as a solid-state soundalike of their tube amps. And it will run fine off less than 40V.

Try to understand what they were thinking in the design process, and modify it to sound more like a JCM800. That would make a good thesis IMO.

[soundguruman]

The correct way to determine if the amp is working right...
5 millivolts sine wave RMS 1 KHz at the input is supposed to drive the output stage clean and all the way to (full output) clipping.
If that's not happening, the output bias is too cold, etc...or something else is goofy.
or it could be over- sensitive. Too hot in the bias, etc...
The whole thing about the Marshall is the input sensitivity. That's how you measure it.

The basic idea is a relatively clean (capable of cleanliness) preamp that can overdrive the output tubes into full output wattage clipping.
The hotter the bias, the easier it can be over-driven. The hotter the bias, the more sensitive the amp becomes. The hotter, the less crossover distortion, etc...
And so, the idea becomes making it as hot as possible, without exceeding the rated limits (too much), and over-driving the output, until the output tubes scream for holy mercy.
At that point you have some real Marshall sound, overdrive, etc...

And then you can use your Power Brake, Hot Plate, Weber Attenuator, etc...as a master volume control.
The preamp master volume, PI master volume, really spoils it.

You need to overdrive the output tubes, and heat 'um up good, to get the real sound.

[Austin]

I believe that less volts would actually give you more gain not less, even though you would be making less power. Can anybody verify this? The input signal stays the same size and the supply volts gets smaller, the input voltage is now bigger in proportion. Lets say a 2 volt humbucker input compared to 200 volts on the plate of a 12ax7, 1/100 of the plate voltage vs. 20 volts on the plate and 2 volt signal is 1/10th. So 1/10th of 200 volts would be like swinging 20 volts at the input. Although 12ax7 is maybe a bad example because it wouldn't be linear down that low volt area does it still make sense?

[loudthud]

What you have to look at is how the output clips with reference to the input stage by stage. Example: If the input drives the output of the first stage to clipping at 1V in the JCM800, you have to adjust the gain so that the an input of 1V drives the output to clipping with your reduced B+. The input stage I'm working on to emulate the 12AX7 runs on a 30V rail and has a gain of about 5. The second stage gets a little more difficult. What voltage at the input of the first stage of the JCM800 will drive the output of the second stage to clipping if the volume is all the way up? It's something around 100mV or less.

Edited for clarity

[soundguruman]

I believe that less volts would actually give you more gain not less, even though you would be making less power. Can anybody verify this? The input signal stays the same size and the supply volts gets smaller, the input voltage is now bigger in proportion. Lets say a 2 volt humbucker input compared to 200 volts on the plate of a 12ax7, 1/100 of the plate voltage vs. 20 volts on the plate and 2 volt signal is 1/10th. So 1/10th of 200 volts would be like swinging 20 volts at the input. Although 12ax7 is maybe a bad example because it wouldn't be linear down that low volt area does it still make sense?

With less voltage the preamp clips sooner, that's not what you want.
You want more voltage and lots of headroom in the preamp.
Otherwise it will sound muddy.

See, all that tasty clean signal will now torture the grid of the output tube.
That's where the crunch originates, but not muddy. No, you don't want mud, you want crunch.
yum.

[truckfighter]

Hi Juan Manual Fahey,

Those are rules not made up by me but my university. I also have to say that I feel quit comfortable with the limitations since this is my first tube project and I think if I get it to work its going to be an interesting amp other beginners will feel comfortable working with.

best regards, Florian

[truckfighter]

Hello loudthud,

this is exactly how I planed to go about this project! The big problem is that I don't have a JCM800 to figure out what the several stages are doing at a given input. So my aproach was to run a simulation of the JCM800 to figure this out. I also know that there are severel problems with the spice models for tubes so I don't trust my simulation too much without it beeing confirmed.
So again, if anybody hase some oscilloscope pictures of the stages with a given input. I can of course post the pictures of my simulation again, however I need to know what Input signal anyone would want to see me use.
Right now I feel like I know what to do, but have no clue what to expect ;-).

best regards,

Florian