Thanks Bob. I guess Laurie only replied to me and not the list. Sorry to confuse anyone and drag this out.<br><br><div><span class="gmail_quote">On 2/27/06, <b class="gmail_sendername">Bob Weigel</b> <<a href="mailto:sounddoctorin@imt.net">
sounddoctorin@imt.net</a>> wrote:</span><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">Who is Laurie? THought that's what I said. Must have missed someone
<br>else saying the same thing... anyway there are several types of<br>'ground'. Please endure through this tutorial as it contains some hints<br>that will probably revolutionize your understanding on this matter.<br>This little journey contains many useful tidbits and is well worth the
<br>time to absorb for people who don't have a fairly high level<br>understanding of these things even.<br><br> Earth ground means there is a connection to earth potential.<br>"Common ground" could be earthed but not necessarily. In fact it could
<br>be totally isolated from earth and there may or may not be an earth<br>ground also that for intsance shields the casing for safety reasons or<br>for ES shielding.<br><br>For instance old fender amps and the like often have a ground switch.
<br>Yet..they had a two prong cord. ie.. not directly earth grounded at<br>all. The switch let you select which ac line coming in was touching a<br>coupling capacitor (usually .022 - .068uF with .047 the most common I<br>
believe) whose other leg was touching the case ground. Case ground was<br>used as a common tie point then for all lowest potential points in the<br>amp.... since THESE amps used only single sided voltages except for bias
<br>voltage I guess you could say.<br><br>In other words the power supply would have 440V- 490V to feed the high<br>voltage needs of the power tubes. This was often filtered again after a<br>choke to feed the screen grids of the power tubes.. This was dropped
<br>through resistance to attain another refiltered 350V ish supply for the<br>plate resistors of the inverter tube. Then dropped some more and<br>refiltered again for an even lower 200 some voltage supply for the<br>pre-amp tubes.
<br><br>Meanwhile a separate winding fed the heaters of the 6V tubes, and a<br>separate winding for trhe 5V rectifier in amps that used that method of<br>rectifying. This winding had HV insulation since the DC voltage would
<br>accumulate right on that heater! (as electrons rushed from the heater<br>to the plates powered by the HV windings of the power transformer; the<br>center tap of which is case grounded!)<br><br>Anyway these 6.3V windings were, in the old days just left to float
<br>often. (Eg. early Gibson EH-150 guitar amp). Today this is a source of<br>GREAT NOISE because we have so much more 60hz floating around than back<br>then. Quickly mfgs figured out that they need to put a low impedence
<br>from this winding to ground. However, in event of power tube failure<br>it's not a great idea to totally ground them as it can destroy the HV<br>windings of the transformer possibly especially if someone sticks the<br>
wrong fuse in. :-) So Fender put a 100ohm 1/2W resistor from each leg<br>of the heater and in case the HV shorts, they will actually burn up<br>before damage occurse to more expensive stuff. (100ma even going through<br>
100 ohms, which all power transformers can supply, =1W of power.)<br><br>Anyway, THE BIAS voltage in the bigger fender amps is designed to feed<br>a small amount of current BELOW CASE GROUND to the control grids of the<br>
power tubes. This allows operation with the cathodes grounded directly;<br>rather than running through a resistance to create a positive voltage<br>there (which limits the clean headroom of the amp even with a paralell<br>
capacitance to feed signal by the resistor; a common failing part always<br>also..). TUBES..work this way. If you allow the grid and cathode to<br>be at the same voltage, they run pretty much wide open and the tube<br>
glows and burns out quickly. (Not to mention it's already at saturation<br>so no signal dynamics are possible..) So the control grid must have a<br>voltage below that of the cathode. ~-14V for EL84 tubes... ~-40V for<br>
EL34 tubes and ~-50V for 6L6 tubes... etc.<br><br>To produce this voltage fender used an appropriate voltaged AC winding<br>and they fed it through a diode that points away from a filter<br>capacitance whose POSITIVE terminal..touches common case ground and
<br>whose negative terminal of course attaches to the diodes anode. The<br>other leg of the winding is grounded creating a charge pump that will<br>give a negative voltage buildup on that capacitance.<br><br>So anyway... I dont' know what the circuit looks like that you are
<br>working with. But if it's designed for a + and - voltage rail with a<br>common ground that is HALF WAY BETWEEN those voltages ...and you have<br>available something with +18, 9, and 0V...then yes treat the 9V leg as<br>
the common ground and attach the other two with 18V on the + and 0V on<br>the - and you should be ok. -Bob<br><br>amokan wrote:<br><br>> Thanks. Would I just tie all grounds on the circuit to a common point<br>> in the case?
<br>><br>> On 2/27/06, *Aleks Clark* <<a href="mailto:aleks.clark@gmail.com">aleks.clark@gmail.com</a><br>> <mailto:<a href="mailto:aleks.clark@gmail.com">aleks.clark@gmail.com</a>>> wrote:<br>><br>
> yea, just use an 18v wall wart. what would have been 0 would be<br>> -9, and what would have been +18 would be 9. Like Laurie said,<br>> it's a matter of relative potential.<br>><br>> On 2/27/06, * amokan* <
<a href="mailto:amokan@gmail.com">amokan@gmail.com</a> <mailto:<a href="mailto:amokan@gmail.com">amokan@gmail.com</a>>><br>> wrote:<br>><br>> So, are there wallwarts that will do +/- ? Am I better off
<br>> using an AC wallwart and then building one of the many power<br>> supply circuits available online?<br>><br>> I really just want something cheap and that only involves 1<br>> plug. I also want to avoid anything much more than a regulator
<br>> in my circuit to keep it small.<br>><br>> I've never really put much thought into this until now. All of<br>> my circuits are done on a breadboard with a bench supply and<br>> by the time I get them in a frac, they just plug into my main
<br>> supply that "magically" works :)<br>><br>> Thanks again everyone. Sorry to drag this thread on.<br>><br>><br>><br>> On 2/27/06, *Laurie Biddulph* < <a href="mailto:elby_designs@ozemail.com.au">
elby_designs@ozemail.com.au</a><br>> <mailto:<a href="mailto:elby_designs@ozemail.com.au">elby_designs@ozemail.com.au</a>>> wrote:<br>><br>> If the outputs are truly isolated from the mains power
<br>> (specifically that is not connected to the Earth pin -<br>> which they should be) then you can connect the positive of<br>> one wallwart to the 0V of the other. This (double) 0V
<br>> connection is connected to the 0V connection of your<br>> module and you then have +ve (from the free positive lead)<br>> and -V (from the free 0V lead).<br>><br>> What you won't get is tracking between the outputs so one
<br>> might go up while the other droops etc. If you are using<br>> these to power regulators in your circuit then this will<br>> be less of a concern. You will also need to power outlets
<br>> and if your wallwarts are large then it may not be<br>> possible to get 2 on to them (double) outlet.<br>><br>> The size of a wallwart is a possible indication of wether
<br>> it is a transformer or switching design but not always. A<br>> low power transformer unit will be quite small. Usually<br>> there is next to no weight in the switching designs. Above
<br>> should hol true for these as well as the outputs should be<br>> totally floating.<br>><br>> Best Regards<br>><br>> (Mr) Laurie Biddulph<br>> Mobile: 0404 846 943
<br>><br>> Elby Designs<br>> ABN: 70 022 727 605<br>> <a href="http://www.elby-designs.com">http://www.elby-designs.com</a><br>><br>> This e-mail and any files transmitted with it are
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<br>> liable for any losses as a result of any<br>> viruses being passed on.<br>><br>> ----- Original Message -----<br>> *From:* amokan <mailto:<a href="mailto:amokan@gmail.com">
amokan@gmail.com</a>><br>> *To:* synth-diy <mailto:<a href="mailto:synth-diy@dropmix.xs4all.nl">synth-diy@dropmix.xs4all.nl</a>><br>> *Sent:* Monday, February 27, 2006 5:20 PM
<br>> *Subject:* [sdiy] wall wart confusion<br>><br>> I didn't pay attention back in high school electronics<br>> when it came to power supplies so please forgive me if
<br>> this is a newbie question...<br>><br>> I've built bipolar supply kits for modular use but<br>> never really understood _how_ they work. Negative<br>> supplies are a mystery to me. I know there is a
<br>> difference between ground and the negative rail, but I<br>> don't know what the difference is. A 9v battery is<br>> in-fact bipolar, yes?<br>><br>> Long story short, I'm working on modules that will run
<br>> in a stomp box format and I'm trying to figure out the<br>> cheapest & easiest way to power them. I know I could<br>> use the dual 9v battery solution to power them
<br>> (assuming the circuits could run at 9v) but I'd really<br>> like to just use a standard 9v or 12v dc wallwart that<br>> I have laying around the house. How do I get a
<br>> negative voltage off of a standard 12v wall wart? Is<br>> it already there normally, or just a positive and a<br>> ground?<br>><br>> Sorry for rambling. If there is a web resource
<br>> explaining this, please let me know.<br>><br>> Thanks!<br>><br>><br>><br>><br>><br>> --<br>> <a href="http://www.nomesh.net">http://www.nomesh.net
</a> <<a href="http://www.nomesh.net">http://www.nomesh.net</a>><br>> <a href="http://www.radioresponse.org">http://www.radioresponse.org</a><br>> <a href="http://www.freebsd.org">http://www.freebsd.org
</a> <<a href="http://www.freebsd.org">http://www.freebsd.org</a>><br>><br>><br></blockquote></div><br>