ot - Firefly Transformer and earth questions (again!)

[sfr]

I ordered the transformers Doug suggested in the earlier thread about the Firefly started by Paul Marrossy.  ( here )

My question comes to the power transformer, the 186b120 that Doug recommended in that thread - does anyone know how to hook that up for use in the Firefly amp?  I'm a bit confused, maybe it's the number of pins I'm dealing with, I'm not very familiar with transformers with more than 4 wires . . .


Here's a quick scan since I couldn't find a tech sheet online.  the top of the image is the general info on the top of the transformer, and the bottom part is a scan of the info on the box.   I'm trying to find out how to hook this up in the Firefly Rev. 3 schematic here , I just don't see how to get the 207VAC I need out of this.  I'm sure I'll feel stupid about this later!



Also, I think I understand this - but someone just make sure I'm correct here - the earth connection coming off the third prong of my AC plug does or does not connect to my star grounding point?

[Peter Snowberg]

I quickly looked at that thread and didn't see any mention of the number 207, but in any case you want to use the transformer as a step-up, so you'll be using it in reverse so to speak.

Connect your AC inputs to pins 5 and 8, leave pin 7 unconnected, place a jumper between pins 2 and 3, and finally, take your new 230VAC output from pins 1 and 4.  

If you look at the diagram, each winding has a dot to indicate phase. If you were to jumper from pin 2 to 4 and then looked for output from pins 1 and 3, you will measure pretty close to zero volts because the windings will cancel each other out. If you connected pins 1 and 3 and then connected pins 2 and 4, you would see 115VAC across those pairs with twice the current you see in the 230VAC configuration. And finally, if you connected pins 1 & 4 and then 2 & 3, you would watch your transformer melt down. :shock:

Finally, yes, you want to connect your earth ground to the star point.

Best of luck on your build! It's a very cool circuit and I think you'll probably be very happy with it. Keep in mind that different tubes sound VERY different so try as many different makes and models as you can. Sonic nirvana is in there somewhere!

Take care,
-Peter

[sfr]

the 207VAC I mentioned came from the firefly schem., rather than the thread itself. (http://home.cfl.rr.com/dbhammond/Firefly_sch_R3.gif)

But yeah, thanks Peter!  Makes sense now.  I like to ask questions when I'm not a hundred percent sure, and dealing with line voltage.  




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[smoguzbenjamin]

I think when you're working with painful or even lethal voltages I think it's wise to ask questions before blowing something up, unless you really want to meet your creator :?

[Peter Snowberg]

Yes, it's always best to ask questions when it comes to line voltage. Getting shocked is no fun. Getting killed is even worse.

Take care,
-Peter

[sfr]

Quoting Peter from the earlier thread

For power, there's a lot of flexability. You could use a 115/230 step up/down transformer too. The higher the voltage, the cleaner the sound. The only thing you might have to adjust is the cathode resistor on the 12AU7.

Anyone know how I'd determine this, seeing as how I'd be using a slightly higher voltage?  Looking at the schematic I'd assume I want to measure out all the voltages marked in blue and make sure things work out right?  

Again, sorry for so many questions.  I should start taking the rest of these things to the Amp forums elsewhere.

[Peter Snowberg]

Questions are good. I think that's why we're all here. While this is an effects forum, Doug is a premiere effects designer and anything that helps more people get into tubes is more than worth it. You will find more at http://www.ax84.com

Tubes need bias just like transistors. With the grid at ground potential, you need to raise the voltage on the cathode to bias the tube into a region where it doesn't melt down.

The voltage at the cathode depends on the supply voltage, but if your supplies are the same it makes a good starting point and it's more than close enough for Rock & Roll in most applications if the tubes are the same between your build and the schematic. That means same make and model of tube.

The thing you have to watch out for is making sure that you don't pull too much current through the tube. For a 12AU7, that means don't pull more than 11mA.

I would replace the cathode resistor with a 330 ohm resistor and a 500 ohm pot. Use at least 2 Watt rated components. If you put a meter in-line with the cathode resistor, you can adjust the sound to taste as long as you don't pull more than 11mA. A much better way is to put a high quality 1 ohm resistor in the cathode and measure the voltage across it. The number of millivolts you see is the same as the number of milliamps running through the tubes. If you meter is less accurate, you could also use a 10 ohm or a 100 ohm and scale your readings accordingly.

See the excellent bias info at GEO in the Tube Amp FAQ section for more.

There is no "correct" value, just values that make your tubes last longer or shorter. If the bias voltage is high, the output stage will run in "class B" and you probably won't like the sound. As you lower the bias it will go through "class AB2", then into "class AB1" and finally into "class A". Most tube amps run in AB1 or AB2. If you go too low with the bias, you'll see the plates start to glow dull red. Try to avoid that.

Take care,
-Peter

[smoguzbenjamin]

What happens when the plates start glowing red?

[Peter Snowberg]

What happens when the plates start glowing red?

You can take that as an indication that your tube life is falling quickly. If things get too hot, the grid wires can melt and deform, and the cathode coating will be baked. The worst case is that the tube fails, shorts out, and kills your output transformer and or other components.

The flip side is that you may like the sound better. :? Something about a "swan song". :megreen:

Just be really careful to avoid a bright red glow whatever you do.

Take care,
-Peter

[Doug H]

If you see the plates glow red, or a red stripe form in the center of the plate, turn everything OFF before you do anything else. The tube will burn out and it may take the output transformer with it if you leave it on.

For all intents and purposes, this is a Class A cathode biased amp. If you measure the cathode voltage, you will see ~20v. It's all Ohm's law from there: V=I*R. Divide the cathode voltage by the value of your resistor. This gives you the current flowing through -both- triodes of the 12au7, since both triodes share the same cathode resistor.  Divide this by 2 for ea triode separately. Okay, now you have the current flowing across the plate of ea triode. Next you need the voltage across the plate. Measure the voltage from plate-to-ground. Now subtract the cathode voltage (divided by 2) to get the voltage across ea plate. Now multiply the voltage across the plate by the current you found earlier. This gives you the amt of power being dissipated by the plate. If you check the 12au7 data sheet, you will see the max plate dissipation is 2.75W. So you want to size your cathode resistor to stay below this value.

Check Randall Aiken's site in the "tech section" for some really good and easy to follow instructions for proper tube biasing.

Doug