Jcm800 Downscale

[truckfighter]

Hello community,


I'm trying to built a low power version of a Jcm800 for my bachelors thesis.
As my university limits me to working with voltages below 40 volts I can't take the meassurements on the JCM800 myself. I would need them to verify the simulation of the JCM800 I did. While I found a lot of information on the DC Voltages on the internet, i didn't finde any on the AC Voltages for a given input Voltage. So if anybody captured the AC Voltages after the individual stages with an oszilloskope I would very much apreciate those pictures so I can compare them with my simulation. Obviously I would also need to know what input signal was used. I would prefer a sine signal on the input.

Right now my simulation only consists of the preamp stage, so those meassurements would do for now.

I will attach the schematics of my Simulation as well as the AC voltages I meassured.


This is the schematic my simulation is based on
Attachment 19752


This is my Simulation using a 800 mV RMS sine signal at 1Khz.
Preamp and Mastervolume are set to 100%
The Potentiometers of the Eq are all set to 50%, assuming that they are linear Potentiometers. I dont have this confirmed yet.
I assumed a high resistanz at the output.

As it is very easy to change the settings in the simulation, any meassurements you did would be welcome since I can adapt the Simulation easily.

This is the Simulation running:




The cable color matches the signal color on the scope

Settings are:

Green 20 Volts/Division
Yellow 50 Volts/Division
Orange 1K Volts/Division This seams very suspicious!!
Red 50 Volts/Division


Like I said, I don't have the possibility to hook up a scope myselfe and therefor I don't know what to expec. I really don't trust the Tubemodels I am using so I hope for some Input.


Best regards from Germany,

Florian

[J M Fahey]

1) Is a simulation enough or you will actually have to build something?
Because in that case you will be well beyond your 40V limit.
2) Treble pot: 250K linear
Bass pot= 1 M log
Mid pot: 25K lin.
Volume pot: 1M log.

[olddawg]

Realistically you will never get a JCM800 with lower power. Sound pressure, cone movement, and a pinch of subtleties related to volume are in your way. In practice though, a modified 18 watt Marshall clone with a Rat pedal and an OD pedal through a 1960a cab will get you close. I have seen people actually graft a JCM800 preamp on an EL 84 pp output. Using an oversized OT also helps a lot. I haven't modeled this of course, but it certainly works in practice.

[Steve Conner]

This might be relevant: Fet version of the JCM800

Best effort I've seen in the public domain, but it needs 180V. I don't think life would be worth living if I could only use voltages below 40V.

Come to think of it, even a 40kV limit would be harsh.

[truckfighter]

Hello,

thank you so much for your quick replies. I have to actually build it. My aproach will be using a similar schematic with different, so called batterytubes, namely the ECC86. I was thinking of properly researching the the JCM800 stages for the distortion then try to achive the same distortions with the little ecc86 tubes. I know that i will not get the output power, but i don't really care for that too much. I think it would be a neat practice amp that i can play ad a moderate volume.
The problem is that i cant get my hands on a JCM800, and am also not allowed to do meassurments on them. So my best bett is to get a simulation going and work from ther on on.
I hope I made my case understandable, forgive me for english is not my mothertongue ;-)

If there are any more questions about the project pleas ask.
For now i need to confirm my simulation is working right.

best regards

Florian

[printer2]

Save your money in the ECC86 as they have no more amplification factor than the 12AU7. The 12AU7 has been used in low voltage guitar pedals/amps and while they are a far cry from being a 12AX7 they will work at the low voltages. For the output tube use a 6CW5/EL86. It is a lower voltage-higher current tube and you could probably get a couple of watts out of a pair at 40V.

[Austin]

I don't know if this helps but 6080 or 6as7 as a power tube can run on only 40 volts but they are not pentodes they are triodes and won't sound the same. They don't cost very much though.

[J M Fahey]

Sorry but trying to build a JCM800 or anything even remotely close while restricted to 40V tops is an exercise in futility.
There are *killer* SS amps within your restrictions.
The Marshall 5005/Lead 12 even sounds like a mini JCM800, has been used by distinguished musicians (ZZTop rings a bell?) and sounds 1000 times better than anything built around either starved 12AX7 or poor performance designed-for-low-voltage tubes.
All it needs is a 4x12"
Which is not off limits for you.
If budget is a limit, a MOD 1250 in 4 ohms is a cheap and very good (for the price) speaker.
jm2c

[printer2]

Maybe an exercise in futility but definitely an academic exercise, which I think is the point (Although I disagree with the 40V limit, that is the way academia is going). Actually I think it would be interesting to see what can be done at the 40V limit (not running starved plate). Can you trade off headroom for gain, how much gain do you actually need, given the low voltage how will the component values have to be compensated to get even close to the desired result? Would you run two triodes in series to make up for the stage's gain? All kinds of questions that running a simulation could possibly point you in the right direction.

[loudthud]

Is that 40V as in +/- 20V or +/-40V absolute max to ground? The clowns that make up these rules never seem to understand the difference. A 50W at 2 ohm power amp can be made from a pair of 19V laptop power supplys. JM.022

[printer2]

Is that 40V as in +/- 20V or +/-40V absolute max to ground? The clowns that make up these rules never seem to understand the difference. A 50W at 2 ohm power amp can be made from a pair of 19V laptop power supplys. JM.022

Probably 40V total from my past experience. It is all about 'safety'. You can shock yourself with greater than 50V. They really did not like the fact we used 240V in some of the labs we ran at the college I used to work for. They actually wanted the labs to be simulated on a computer to get away from any liability.

[J M Fahey]

Is that 40V as in +/- 20V or +/-40V absolute max to ground? The clowns that make up these rules never seem to understand the difference. A 50W at 2 ohm power amp can be made from a pair of 19V laptop power supplys. JM.022

I'm sure the rule was written by the school lawyers, who want to avoid being sued by angry parents for endangering their kids ... which is ok with me.
Remember that the "grade 7 (C7) contractor's certification" shown by soundguruman, who is an adult with 40 years experience, certified to work as an electrician, allows him to install under 91V equipment only, so it looks like the School rule which is made for uncertified kids tries to play it safe.
Also remember that truckfighter is not even allowed to measure one, probably meaning he can't get even close to an open, live one.

Sorry but I think those rules were *designed* to make students work with SS stuff, and not that high powered either.
(No LM3886/TDA7294 amp either).
Although a *bridge* amp fed +40V single supply can reach *very* loud 100W into 4 ohms.

[Enzo]

Doesn't seem like a college today would be teaching tubes anyway.

SO just exactly what is the assignment? Are they expecting you to build a tube circuit but not test it in operation, so your sim has to work, and you build a non-powered circuit? Or is the tube part your idea and they are really just expecting a solid state design? I can't imagine a entry level circuit class expecting you to engineer a working tube circuit for 40v that would emulate a full voltage circuit.

I'd agree the 40v rules are for safety, but I doubt they intend them to force students to solid state instead of tube. I suspect the solid state part was already a given. In a solid state amp, any principal that can be expressed in say an 80v circuit can also be expressed in a lower voltage circuit.

[Austin]

Doesn't seem like a college today would be teaching tubes anyway.

SO just exactly what is the assignment? Are they expecting you to build a tube circuit but not test it in operation, so your sim has to work, and you build a non-powered circuit? Or is the tube part your idea and they are really just expecting a solid state design? I can't imagine a entry level circuit class expecting you to engineer a working tube circuit for 40v that would emulate a full voltage circuit.

I'd agree the 40v rules are for safety, but I doubt they intend them to force students to solid state instead of tube. I suspect the solid state part was already a given. In a solid state amp, any principal that can be expressed in say an 80v circuit can also be expressed in a lower voltage circuit.

Could you get graded on coolness if you were able to make a high performance tube amp on only 40 volts? Besides if they think it is solid state thten kids are going to be doing positive and negative 40 volt rails which is really 80 volts right?

[Merlinb]

Don't forget about the sub-mini valves like 6111. Some of those work quite effectively at low voltages.

It is also possible to augment valves with some solid state circuitry to make them work better at low voltages. OK, that's departing from the traditional JCM by a long way, but you might still be able to emulate it.

Perhaps you could build a low voltage valve preamp that emulates the JCM, and then feed that (output transformer and all) into a solid state chip amp that will give you the pure volume while only running on 40V...

[Merlinb]

Doesn't seem like a college today would be teaching tubes anyway.

They don't, but I suspect they're just glad to find a student who actually engages with the subject. They're usually pretty open to anyone who has project ideas of his own, as most students are pretty apathetic (I know plenty who get through their degree without even learning to solder, and probably won't after they leave! They go on to be software monkeys.)

At my uni the safety limit is officially 50V, but they will go up to 60V if you're polite. Beyond that and you need to do a health and safety appraisal and build in various safety measures that make it nearly (but not quite) impossible to proceed.

Do your tube models actually include grid current? Most of them don't...

[Austin]

Now that is a thought. Using sub miniatures to make a miniature jcm800. You could maybe use the same tone stack and layout and have an actual pentode amp. There are plenty of subminiature pentodes right? Just use like 20 of them in parallel for the output section.. Muha

Actually 6y6 tubes can run on lower volts and still are pentodes and might sound okay on the 40 volts. 40 volts negative and 40 volts positive so really 80 volts?.

[J M Fahey]

Thanks God when I studied Engineering (1969 to 1975) we used tubes, both in the Lab instruments and in our own projects. I remember dropping an EL84 on the floor (it was part of some Atomic gizmo, similar to a sophisticated Geiger counter) and running to a friend's house a few blocks away to pull one from his guitar amp, so we could finish the experiment before the deadline.
Also remember an Earth Magnetic Field experiment, where we used what was basically an *open* chassis oscilloscope (yes, couple thousand volts available) , no sweep so the dot was static in the center, and moving the chassis first pointing North/South and then East/West and measuring the dot displacement and calculating Earth Magnetic Field with precision.
And not only high *voltage* but *current/Power*: the Electrical Machines Lab was a 30's Science Fiction movie set of complex *thick* wiring, exposed "knife" switches, and lots of electric motors, transformers, resistive load boxes, generators, electromagnets, starting motor relays, capacitor banks, etc. , everything powered with 3-phase 380V.
Was there any accident (or death?).
None, zilch, we were University Students and were supposed to keep ourselves safe, period.
Any stupid misbehaving moron was thrown out of the Labs (and soon out of the University) for good.
We even had a few Military Engineers as teachers, no "desktop generals" but guys with *years* of practice running Military Factories , which besides weapons manufactured Steel, Copper, Heavy Acids (Sulphuric, Nitric, etc.), petrochemicals, cars, copper wire, sheet metal, etc.
Real no nonsense, feet on the ground guys.
Simulation?
Yes, a little, a computer was room size, (it had to be air conditioned), we wrote our own FORTRAN programs and data output was a 16" wide, as-long-as-you-need sheet of paper, printed on a similar to teletype *noisy* printer.
But we learnt our trade well.

[Enzo]

Yes, we sure CAN come up with low voltage tubes and fancy circuits, but considering the context of the original poster, it sure sounded like a real basic level project.

[J M Fahey]

Just quoting the project definition:

I'm trying to built a low power version of a Jcm800

coupled to:

my university limits me to working with voltages below 40 volts

Has only one answer: sorry but no.
Yes, I know that:
1) there are tubes made to work with 40V .... but they are not 12AX7 !!!!!
2) 12AX7 *may* work with 40V ... *very* poorly , with seriously compromised gain, wildly less than what's needed for JCM800.
3) even so, they will need very different cathode and plate resistors ... that alone means the circuit will be very different, so no more JCM800
4) you can add more stages to make up for lost gain ... but it will drift even further from what JCM800 does.
5) since it's not a linear amp but a heavily overdriven one, tubes behave different both from what's on their datasheets and what's been incorporated into their simulation models.
All kinds of undocumented quirks come into play.
No datasheet shows what happens with 60/90V RMS (180V PP) applied to a grid, yet that happens all day long .
OK, maybe a voltage divider lowers that to 30V RMS. Still way off the playing field.

So, in a nutshell, some tube amp (or preamp) can be built to work with 40V. Yes.
Will it work or sound or clip like a JCM800? Sorry but no.

[printer2]

Just quoting the project definition:

coupled to:

Has only one answer: sorry but no.
Yes, I know that:
1) there are tubes made to work with 40V .... but they are not 12AX7 !!!!!
2) 12AX7 *may* work with 40V ... *very* poorly , with seriously compromised gain, wildly less than what's needed for JCM800.
3) even so, they will need very different cathode and plate resistors ... that alone means the circuit will be very different, so no more JCM800
4) you can add more stages to make up for lost gain ... but it will drift even further from what JCM800 does.
5) since it's not a linear amp but a heavily overdriven one, tubes behave different both from what's on their datasheets and what's been incorporated into their simulation models.
All kinds of undocumented quirks come into play.
No datasheet shows what happens with 60/90V RMS (180V PP) applied to a grid, yet that happens all day long .
OK, maybe a voltage divider lowers that to 30V RMS. Still way off the playing field.

So, in a nutshell, some tube amp (or preamp) can be built to work with 40V. Yes.
Will it work or sound or clip like a JCM800? Sorry but no.

Might be fun trying though.

[Chuck H]

Wait a moment... Is the project required to operate on 40V as in input voltage? or are no voltages inside to exceed 40V? Because there are a bunch of ways to make 40V into a lot more volts.

[J M Fahey]

As I see it, there can be no more than 40V across any 2 points in the circuit.
Not even +/-40V rails which means there's never more than 40V differential to ground, but you could conceivably touch one rail with the right hand, and the other with le left one.
So no smart little SMPS inside either, such as used to power Nixie tubes.
Nor "Real Mc Tube" or "V-Twin", powered by 12VAC but having internal reversed 12V/120V transformers.
Don't think you can make a credible JCM 800 with 40V tops, *WAY* too restricted, but there are other options if you insist on tubes.
Check my post covering *commercial* low voltage tube preamps/distortion.
Direct Coupling with Zener Diodes - Page 2

[J M Fahey]

As I see it, there can be no more than 40V across any 2 points in the circuit.
Not even +/-40V rails which means there's never more than 40V differential to ground, but you could conceivably touch one rail with the right hand, and the other with le left one.
So no smart little SMPS inside either, such as used to power Nixie tubes.
Nor "Real Mc Tube" or "V-Twin", powered by 12VAC but having internal reversed 12V/120V transformers.
Don't think you can make a credible JCM 800 with 40V tops, *WAY* too restricted, but there are other options if you insist on tubes.
Check my post covering *commercial* low voltage tube preamps/distortion.
Direct Coupling with Zener Diodes - Page 2

[Austin]

As I see it, there can be no more than 40V across any 2 points in the circuit.
Not even +/-40V rails which means there's never more than 40V differential to ground, but you could conceivably touch one rail with the right hand, and the other with le left one.
So no smart little SMPS inside either, such as used to power Nixie tubes.
Nor "Real Mc Tube" or "V-Twin", powered by 12VAC but having internal reversed 12V/120V transformers.
Don't think you can make a credible JCM 800 with 40V tops, *WAY* too restricted, but there are other options if you insist on tubes.
Check my post covering *commercial* low voltage tube preamps/distortion.
Direct Coupling with Zener Diodes - Page 2

Dear JM no disrespect intended sir,
Just remember that there are people that have died from picking up a car 12v car battery with both hands by the frayed copper cables... Your blood and even your sweat can make a low enough impedance path to allow enough current across your heart to hurt you. It doesn't take very many millamps at all across your heart to kill a perfectly healthy person. I don't know how much current, different people say different amounts.. So not even with perfectly dry hands would I try it left on one right on the other.. So please I would urge you to watch out what advice you give to people on a public forum like this and remember that lots of different people may read for years what you write.

"but you could conceivably touch one rail with the right hand, and the other with le left one" Is in my opinion very bad advice and though it might not hurt you if your hands are very dry, please don't try it to find out.

How much current can kill a human

http://wiki.answers.com/Q/How_much_a...l_kill_a_human

That last one says the human body can have a resistance as low as 300 ohms. Scary. 130 ma at 40 volts.

[Chuck H]

Austin, keep in mind that a battery has terminals exposed with no additional loads, resistances in the current path or limitations of avilable current (400CCA to 1500CCA for an automotive battery!!!). Your advice is spot on though. And I agree. But WRT the subject at hand I think it's less of an issue.

[J M Fahey]

Dear Austin, please re-read my post, what I am telling everybody is ****NOT**** to touch such rails , because it's ****DANGEROUS****
Don't know about 12 Volts, but I have felt tingling in the back of my hands (which are somewhat more sensitive than, say, fingertips) with 24V DC, working in a very hot and humid cellar. (Boiler Room).

[Austin]

Whats funny is a cheap disposable camera flash can have 300 volts inside of it.... 40 volts for a limit seems kinda lame. Of course you could weld a bumper onto a truck or weld a broken plow blade back together with less than 40 volts.. There is no limit to the amperage at 40 volts right? You could get a bunch of super capacitors and charge them all up with the 40 volts and blow some stuff up.. Or build a rail/coil gun with them... Since it is far too dangerous to build a little tube amp. Show them you can still find ways to be dangerous er- I mean "do cool stuff" with there lawyer imposed voltage limit. What do lawyers know about voltage and current anyways and why do they get to make the rules? Hehe just kidding around.

[loudthud]

Those disposable camera flash units make a great B+ supply for a tube preamp or fuzz pedal.

[printer2]

Just don't go dripping blood into your amp, dry hands are a good thing also, and under 50V should be no problem.

[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