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Morgan Jones Power Supply Help.

post #1 of 21
Thread Starter 

Hello, I plan on building the optimized Morgan Jones headphone amplifier with feedback (found here http://gilmore2.chem.northwestern.edu/projects/showfile.php?file=cmoy5_prj.htm) for my electronics project in junior lab.  I have a good understanding of the amplifier cirucit; however, we have not discussed transformers or rectifiers.  Looking at the power supply circuit it looks like it was designed for a 220V AC source.  I need a power supply for a 110V AC source (I live in America).  I was hoping the fantastic head-fi community could help me design a circuit that will power the Morgan Jones amplifier from a 110V AC outlet.  I would appreciate specific part suggestions for the transformer and rectifiers.

 

I have one slightly off topic question as well.  Does anyone have suggestions for the brand (and type) of capacitors I should use for this project, and can any one suggest a brand of tube for this project?  There are so many options, my first instinct was to just buy moderately expensive caps and tubes, but I don't have any idea on how that would affect this specific amp. I am willing to spend a fair amount of money on this project (we get partially reimbursed for our project), and I would like to come away with a great sounding amp.  But if buying cheapish caps and tubes will suit this particular amp just as well as the more expensive components I would rather not waste money.

 

-Keabler

 

post #2 of 21

You should probably use figure 13.

 

The transformers will be rated 110 volts primary

and the values in the schematic for the secondary.

 

The main transformer steps the voltage up from 110 to 220 volts.

The filament transformer steps the voltage down from 110 to 6 volts.

 

I hope that answers part of your question...

post #3 of 21
Thread Starter 

 



Quote:
Originally Posted by Avro_Arrow View Post

You should probably use figure 13.

 

The transformers will be rated 110 volts primary

and the values in the schematic for the secondary.

 

The main transformer steps the voltage up from 110 to 220 volts.

The filament transformer steps the voltage down from 110 to 6 volts.

 

I hope that answers part of your question...


 

Hmmm. Maybe I'm stupid I could have sworn the schematics all said 220VAC @ 50 hz (I guess I was tired) that is why I assumed they were for British outlets .  So the 250VAC is just the secondary coils?   I suppose that makes sense.

 

I did some research.  Correct me if I"m wrong,  do I just need a solid state rectifier that can handle the voltage that comes from the transformer?

 

I guess it would be useful if some one could explain what the transformer ratings mean.  I get  that the schematic shows the Vout of the transformer, but what does " 250-0-250" means, and why do some of them have primary coils that look like "250-0-250?"

 

Thanks for your help.


Edited by keabler - 3/18/11 at 8:17am
post #4 of 21

Yes, diodes like the MUR840 would work well.

For the Filaments, 1N4001 would be all you need.

post #5 of 21

Take a look at the Allied 6K88VG

 

http://www.alliedelec.com/search/productdetail.aspx?SKU=2270113

 

Should do what you need. Search for, and download, PSUD2 to get started designing power supplies.
 

Quote:
Originally Posted by keabler View Post do I just need a solid state rectifier that can handle the voltage that comes from the transformer?

 

 


Yes.  1N4007 is fine for rectifiers. With the 500VCT secondary on that transformer, you'll need two diodes to do full wave rectification for a 200-250V supply.

 

For caps, cheap is also probably fine to get started.  Be sure they are rated at 350V or more. Panasonic TS-HA and TS-HB are as good as anything, but really anything is fine.  For tubes, the JJ 6DJ8 is hard to beat.

 

post #6 of 21

The primary voltage doesn't  matter, you buy the transformer that you need for your country... so 115V primary and around 250V secondary. The transformer can be rather small since you don't need much current (total consumption is less than 30mA so a transformer at 50mA should be just fine.

Better get a trans with two secondaries... so 115V pri. and 250V and ~6V sec. Doenload PSUD II (free) and start playing with it... its a great tool that is easy to understand and lets you design any psu. So 250V secondary isn't mandatory, 220V might do as well when you adjust the following components.

I've built the MJ a few years ago with a few mods... when you are interested I can mail you my schematic (adapted to your mains voltage).

 

When this is your first build don't waste your time with what capacitor will be best... don't buy chinese cheapos but Panasonic FC or similar for the psu and decent film caps for the small coupling caps.

post #7 of 21

Yeah, dsavitsk beat me to some of your questions...biggrin.gif

post #8 of 21

250-0-250 means 500 volts center tap.

 

Sorry, 1N4001 does not have enough current.

This is better.

 

Here is a little bridge rectifier that work for the B+.

post #9 of 21

 

Quote:
Originally Posted by Avro_Arrow View Post

The main transformer steps the voltage up from 110 to 220 volts.

The filament transformer steps the voltage down from 110 to 6 volts.

 


The original earmax and MJ clone used a (mains):18v transformer. This transformer ran the heaters with AC, AND ran the second transformer. 

The second transformer MUST be a ~22v:220v unit to get about 250v on the secondary. There was some discussion about this and selecting transformer voltages in the article on head-wize. 

This does buy some measure of safety* in idiot-resistance but from what I can gather results in B+ supplies that sag under load. 

 

You can do much better as a DIYer by just getting the part dsavitsk suggested, or something similar with a "real" high voltage secondary.

 

 

 

*Running the heaters in series like the original amp did is actually a good idea, its how they get the B+ voltages thats weird. If the gain tube is not installed in a 6.3v heater MJ type amp and the outputs are, this will almost certainly cause catastrophic failure of the amp on power on. If the heaters aren't hot: which they wont be if an element in a series string is missing, the tubes dont conduct! A built in safety mechanism.

 

nitpicking:

6dj8 dont have filaments they have heaters. 

Filaments are in DIRECTLY heated tubes. 

Indirectly heated tubes have heaters. 

Carry on. 

post #10 of 21

I would pick rectifier diodes that are rated well over 2 times the peak AC secondary voltage. The MUR840 is rated at only 400 volts peak repetitive reverse voltage, absolute maximum.


Edited by digger945 - 3/18/11 at 6:01pm
post #11 of 21
Thread Starter 

Ok so I need a transformer with a 250 VAC @ 50mA and 6VAC @ 2A secondary coils, and rectifiers that are ~ 2x the peak voltage of the AC signal they will be rectifying.

 

I was looking at the schematic and I have a few questions.  Is the 250V RMS voltage?  When I calculate the DC voltage from the rectifier the only way I can get something close to 320VDC out of the rectifier is if I use the full wave equation for a rectified VRMS signal (I just multiplied the ACV by sqrt(2)).  My second question is how does the voltage drop to 220V?  After the rectifier circuit the signal goes through an RC low pass filter, and a second inductor cap filter (what is going on in the LR part of the circuit? (is it just a band reject filter?)).  Does the voltage drop across the resistor?  Aren't RC filters suppose to not significantly affect low frequency waves?  Would some one be kind enough to walk me through the theory?  A qualitative analysis is fine, but a mathematical explanation is fine too.

 

Thanks for every ones help.

 

post #12 of 21

You want a transformer with a 250v OR 500vct secondary depending on how you want to set this up. and yea, a 6.3v winding too.

 

For the rest: 

Yes you should do the calculations on RMS volts.

 

Which Schematic did you get the power supply from?

 

There is current flowing, the resistor will cause voltage drop. 

 

I would suggest getting the books: Morgan jones Valve Amplifiers and another book by him Building Valve amplifiers. It sounds like you have some backround in electronics, they will be an easy read and you will learn so much more from them. 

post #13 of 21
Thread Starter 


 

Quote:
Originally Posted by nikongod View Post

You want a transformer with a 250v OR 500vct secondary depending on how you want to set this up. and yea, a 6.3v winding too.

 

For the rest: 

Yes you should do the calculations on RMS volts.

 

Which Schematic did you get the power supply from?

 

There is current flowing, the resistor will cause voltage drop. 

 

I would suggest getting the books: Morgan jones Valve Amplifiers and another book by him Building Valve amplifiers. It sounds like you have some backround in electronics, they will be an easy read and you will learn so much more from them. 

 

I have been using schematic 13, mostly because 250VAC transformers seem to be rather abundant.

 

Ok so here is what I calculate from a 250VAC transformer at 50mA.  

 

Rectifier: (full wave RMS equation) Sqrt(2)*250 ~ 350 VDC (not 320DC)

 

Voltage drop across the 3.3k resistor @ 50mA (assuming ideal current source):  .05A*3.3K ~ 165V

 

Vout DC: 350 -165 = 185V which is not 220V.  

 

 

 

I'm not sure what part of the calculation messed up.  I don't know if I should assume the transformer is an ideal current source or voltage source (or neither).  Can we not assume no power is dissipated by the caps and inductor?

 

I am physics math double major and am in Junior Electronics Lab.  In a lab we use pre-made power supplies so we didn't cover rectifiers or transformers (its more of a crash course than any thing).  I really don't think this calculation should be hard or hard to understand.  I would appreciate it if some one could offer some insight.
 

 


Edited by keabler - 3/20/11 at 9:20am
post #14 of 21

 

Quote:

 

Voltage drop across the 3.3k resistor @ 50mA (assuming ideal current source):  .05A*3.3K ~ 165V

 

Vout DC: 350 -165 = 185V which is not 220V.  

 

Nobody said that the current draw is 50mA... that is the spec of the transfromer. The MJ circuit consumes about 27mA when I recall correctly...

that equates to a 89V voltage drop across the first resistor.

 

I'd second nikongood in that it would be helpful to do some reading on the very basic stuff... then ask all the other questions here. Getting the

initial math right is one thing... but actually building a tube amplifier poses the next challenge (keyword: grounding, hum etc wink.gif).

 

While MJ's books are great but maybe a bit scientific for a beginner... you can also have a look at the pages on "valvewizard" 

 

-->http://www.freewebs.com/valvewizard/index.html

 

While it is written for building guitar amplifiers everything applies to the amp you are planning to build.

 

 

 

post #15 of 21
Thread Starter 
Quote:
Originally Posted by stixx View Post

 

Nobody said that the current draw is 50mA... that is the spec of the transfromer. The MJ circuit consumes about 27mA when I recall correctly...

that equates to a 89V voltage drop across the first resistor.

 

I'd second nikongood in that it would be helpful to do some reading on the very basic stuff... then ask all the other questions here. Getting the

initial math right is one thing... but actually building a tube amplifier poses the next challenge (keyword: grounding, hum etc wink.gif).

 

While MJ's books are great but maybe a bit scientific for a beginner... you can also have a look at the pages on "valvewizard" 

 

-->http://www.freewebs.com/valvewizard/index.html

 

While it is written for building guitar amplifiers everything applies to the amp you are planning to build.

 

 

 

 

I haven't had any experience with transformers or rectifiers (I have been reading and sadly the art of electronics books I have is missing the first chapter (the one with transformers in it)).  I was just hoping some one could walk me through what is going on in the circuit.  It shouldn't take very long right?  It looks to be pretty simple.  So do we assume the transformer is more or less an ideal voltage source?   And then calculate the subsequent current and voltage drops after?  


It's not that I have no idea what is going on I am just a bit confused as how to start.  

So is the VAC from the transformer the peak voltage (which makes VRMS ~ 177V)?  Is multiplying the VRMS signal from the transformer by sqrt(2) correct to calculate the DC voltage from the rectifier the correct calculation?  I don't think I'm looking at this circuit right because my numbers don't come out.

 

I do have a more-than-beginners- knowledge of electronics and I am in E&M right now so the theory isn't too heavy either.  I don't think I am starting the calculations right.

 

 

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