[Tube Pre-amp project] The JCM800

[QSQCaito]

Hi guys! This time I'll be building the preamp of the known Marshall's JCM800. 50w version, I believe 1989.. with high/low inputs, and master volume control.

Up to now I know (I believe) enough to start building this project, I've also searched a lot about it.

This is what I have to show:

http://www.wikiupload.com/download_page.php?id=181737

That's a project file, verified schematic and pcb layout.

And here we have the schematic from that same PDF:



I'm willing to do a completely new layout, as I've searched there are several things wrong, the power supply to begin with, it has to be on a seperate board.
So, lets start with this, which requires your help.

The preamp board will consist of:

The 2 transformers.(I believe they were 1amp, and 300 mili amp the other)
The 2 rectifying bridges
The 6 big 47uf caps
The 1 4700uf cap
The 12v voltage regulator( which regulator do you believe to be the best??)

The other pcb, I was planning to build it as it's in the pdf. Do you see any errors, or problems which could cause humm or noise, to be taken into account?


I would be adding 1 neon for the power switch.(when heaters activate)
and 1 neon for te standby switch(when you send the 220v to the circuit)


The V1b at the layout is to make the switching of the HIGH/LOW channels, which will be in a DPDT.


If I remember about other things, I'll tell you. AH! yes, I believe the master volume pot was way too high, needed change to around 25k. Also, do you believe a pair of 12ax7 would give better results than any other pair of triodes?


I think I have nothing more to say by the moment, I'm planning to do the layouts now. If you want to give any advice which will avoid future problems, they're more than welcome. I'm trying to keep this post tidy for if anyone wants to build this.

BTW, one question, heaters wires must be.. ehmm whichs the english wired. like turned, right?? Is it good to do that with all wires, the 3 pot wires, input, output, etc..

Also, star grounding would be the best option, right? And last, but not least, a 220 12v will fit in a box 4cm high??(it's not toroidal)


bye bye, thanks a lot.


I'll keep this up to date.

DAC

[Jaicen_solo]

Looks like an interesting circuit, and pretty simple too.
Only thing i'd reconsider is using the Hi and Lo inputs, just one with an input control would probably do just fine.

[bancika]

check out my tube reactor project, it's similar to this...(you can click on the logo below)

[AL]

50 watt JCM800 is the "1987" circuit. The 100 watt is the "1959". Then there were the master volumes -  2204 (50 watt) and 2203 (100 watt).

Looks like an interesting project. Keep us posted.   


AL

[snoof]

Also, do you believe a pair of 12ax7 would give better results than any other pair of triodes?


BTW, one question, heaters wires must be.. ehmm whichs the english wired. like turned, right?? Is it good to do that with all wires, the 3 pot wires, input, output, etc..

You won't really know if 12ax7's are the right choice until it is built, you can always sub in some 12ay's or 12au's if it has too much gain.

yes, def twist the heater wires.  one practice is to twist high voltage wires as well.  keep grid wires short, plate wires long.  when plate and grid wires cross, do so at 90 degrees.

you may want to add a switchable cathode cap (say 1-22uf) on V1a to switch between stock and an additional value for an easy tone shift.

as well as star grounding, another way to do it is to have a ground bus made of heavy gauge copper wire running alongside of the "preamp" PCB, and then another smaller bus for the "power" PCB.  I've used both with good results.

[QSQCaito]

no! I had a very long post, this thing just deleted. I'm going to be brief this time.

I've been occupied so I couldn't keep this to date, but I promise I will, I'm just trying to enter the university. I wanna be an EE

First things first, I'm not doing the layout until I've got the parts. Don't wanna miss sizes and all that stuff.

I was reading lots and lots, and came to the conclusion that I'm going to use an LM317 regulator for the heaters, set up at 6.3v. Here I encounter a problem, i've checked the data sheets but sill couldnt figure out how to set up this device. I believe this way would be the correct one, but still with that math can't choose resistor values.


Next issue, "Insulate any heater centertap, and make an artificial centertap across the heater supply from two 47 ohm 1/2W resistors in series. Ground the center of the two resistors. If you have more than one heater supply do this for each one."
Would you explain how this can be done? I believe putting to resistors from+ and from -, and putting common ends to ground. But, (watching the schem above[1st post]) I would have "two grounds" because the (-) wire would be to ground, the center tap too.)


Im completely dead with this "This is critical: Connect the rectifier side of the power supply to the negative side of the first filter cap with a single wire, preferably the centertap of the B+ winding itself. Then connect ONE wire from the negative side of that filter cap to your star ground point. Otherwise the current pulses from rectification will make intractible hum."

Talking about filtering, would it be good to put a choke after the first filter cap(now talking about the filtering of B+)


Last but not least, someone told me to use 1amp and 300 milliamp transformers. Isn't this too much? How can I figure out how much 2 heaters consume, and the rest of the circuit.


Thanks a lot, lots!


Bye bye

DAC

[the_random_hero]

Can't really help you with the other questions, but I do know heater current draw - for 12AX7 tubes, wiring the heaters in series will draw 150mA while in parallel it's 300mA.
I've also wondered about this whole pre-amp deal - can you just omit the rest of the schematic that finishes off the pre-amp section? There's no need for impedance matching for the output?

[QSQCaito]

I understand the concept of series and parallel, but how would i set the heaters of 2 12ax7 in parallel or series, besides, I'd wire them in series for less consumption, would this bring any disadvantage?

Bye bye

DAC

PS I someone has got any idea about the rest of the questions, it's much appreciated.

[the_random_hero]

Wiring pin 4 to +12.6V, leaving pin 9 open and wiring pin 5 to ground will have the heaters in series. Wiring pin 4 and 5 to +6.3V and pin 9 to ground will have the heaters in parallel.
You're not really wiring them for less consumption, you know, P will still equal VI.

[QSQCaito]

I see then, I'd wire it just as the schematic says I should.4 to v+, 5 to ground.

BTW, I'm probably going to buy my stuff tomorrow. I would reeeally appreciate If someone has an idea of which should be the ratings of the transformers. I would really appreciate this, thanks a lot guys!



Thanks!



Bye bye

DAC

[QSQCaito]

Sorry for the bump, I'm goign to buy components in a couple hours. Guess I'd have to live the transformers for another day.

Bye bye

thanks !


DAC

[bancika]

well, two tubes will draw 300mA@12V, plates don't draw much current, so with 600mA or more you're pretty safe.
If you live in the us you'll need different step up transformer (110->6) which will give you ~220VAC from 12V. If not, with two 220->12V you're all set.

[QSQCaito]

Bancika saves the day .

Ok, so a 600ma 220-12 transformer. And another 300ma 12-220 transformer.


:D


Thanks!

i'll keep this up to date as soon as I move on.



DAC


PS A 600ma fuse (or 500?) would be ok?

[bancika]

yeah, I used 1A but you'll be fine with 600mA (if you can find that value)
Keep us posted.
Cheers

[QSQCaito]

Thanks bancika

Due to problems( I lack medical certificate) I didn't go downtown, not to register university neither. That means I'll be going to buy my stuff, and see if I find a cheap uke on monday. Entering course starts august 6th


Bye bye!

DAC

[Johan]

50 watt JCM800 is the "1987" circuit. The 100 watt is the "1959". Then there were the master volumes -  2204 (50 watt) and 2203 (100 watt).

Looks like an interesting project. Keep us posted.   


AL

..not nececerily..."JCM800" referes to the WHOLE line of tubeamps made by Marshall in the 80's..including both mastervolume, nonmastervolume, channelswitching amps and even bass amps.
if you only go by what you find of the internet, it would seem only one ore two of all jcm800 models are jcm800...having worked ( as repairman ) for the marshall-agent in sweden, at the time, I say that is not true and typical most internet-generalisations...including and excluding whatever the latest net-guru says...the 1987 and 1959 were both introduced in the mid 60's and has lived through MKII, JMP, JCM as well as reissue versions
j

[MetalGuy]

I don't know what kind of sound you're after but the most you can get from this preamp is some crunch sound. Don't expect it to sound as an actual JCM800 amp.

[runmikeyrun]

i agree.  the real balls comes from their rock solid power amp sections, the NOS siemens el34s can slam!

[the_random_hero]

Did I read somewhere that there was diode clipping in one of the JCM amps? I thought it might have been the 900, but it's clearly not on this schematic...

[CGDARK]

I don't know what kind of sound you're after but the most you can get from this preamp is some crunch sound. Don't expect it to sound as an actual JCM800 amp.

I agree with MetalGuy and with runmikeyrun. A tube preamp depends entirely on a good tube power amp. Have you ever play a tube preamp with a solid state power amp?

CG