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



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...