Jcm800 Downscale

[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.