Step 3: DC Resistance on the Output Transformer

 All we need is a small AC wall wart (anything between 4 and 10 volts AC would work), a multi-meter (we'll only need the AC voltage and the ohm-meter), a 1 Ohm / 1 Watt resistor, and that transformer of unknown quality.
 If the wires on the transformer lack any distinctive coloring, it would be handy to create labels.  This can be as simple as stripes of color using markers or folded tape with numbers.
 Using the ohm-meter, test all the possible combinations of wires to see what connects to what.  Usually the reading will be very low,  as a high DC resistance is rarely desirable in a transformer.  Draw out the connections and make note of the resistances. 
 On the output transformer I found four separate windings: two with two connecting wires each and two more that each had a center tap.
  For example: The yellow wire was 17.6 Ohms away from the red wire, but 8.8 Ohms away from the red/yellow.  I checked the resistance between the red/yellow wire and the red (8.8 Ohms) and confirmed that the red/yellow wire was directly in the middle of the red and yellow wires.
  Notice that the black-red-green winding has a slight imbalance.  This is fine.  The DC resistance is a good indicator but it only hints at the winding ratio; and the winding ratio is what we're interested in.
I wish there was a way to understand this more easily... I am not the smartest American in the bunch... ;0
<p>This is not the way i would entertain my desire to build a guitar tube amp , and no , there is no easier way to understand this load of twaddle !!! You dont have to be Einstein to build an amp or even to know not to bother starting with something you found at the local dump, that has several times been&quot;@#%#'ed by a D4 doser , and @#%#'ed on by the last seven hurricanes.. </p>
I've built a few tube amps myself from &quot;scrounged&quot; components... A couple comments, if you don't mind:<br> <br> -- For a tube power transformer (which it definitely is), that looks plenty large enough for a 6V6 PP amp. No problem. Probably big enough for 6L6's.<br> <br> -- Taking an educated guess, I'd say the AC mains primary is the Black and the Black/Yellow wires.<br> <br> Once you know the AC primary, it's simple to test the other wires for voltage. When &quot;recycling&quot; an old PT, note the tube compliment of the equipment it came from. That'll give you a ballpark as to it's current capabilities, for both output and heater.<br> <br> Somewhere in the 275-350V is the &quot;classic&quot; voltage range for a 6V6 push-pull amp.<br> <br> -- 99% of tube PTs had a center-tapped secondary, so the center tap is the GND, and only two diodes are necessary for full-wave rectification. Most rectifier tubes are dual diode, and it was cheaper (then) to double up the transformer secondary than to use two rectifier tubes (and power two different tube filaments).<br> <br> That's important--if you use a SS bridge like your PS schematic, the HV secondary will be 2X the original voltage, and can only supply half the original current.<br> <br> -- You can certainly substitute SS diodes for the original tube rectifier. Usually two 1N4007 diodes in series will replace a 5Y3 (or three 1N4007 diodes for larger rectifiers). Of course, that's two 1N4007s per diode of the 5y3, for a total of four. That would also leave you an unused 5V secondary you could use for switching relays or an indicator lamp, or whatever.<br>
Thanks for your input!<br> The power supply schematic at the bottom of the step one is more of a general PSU. Mostly I wanted to draw attention to how different B+ voltages are tapped before and after the choke. This (perhaps wordy) instructable was focusing on finding winding ratios, and PSU topographies will be discussed in the next. Yours would be the concise version :) Me, I like to ramble :P<br> <br> You said 99% of tube PTs had a center-tap-- are you familiar with the remaining 1%? The PT I have does not have a center-tap, and to compound things the heater supply is two taps in the middle balanced around the center of the HT winding. Thus:<br> <br> &nbsp; _HT<br> (<br> (<br> (_Heat<br> (<br> (_Heat<br> (<br> (<br> (_HT<br> <br> I haven't found any reference to a PSU like this-- can you offer any insight?
That's interesting...<br> <br> Could this be a later tube PT, after SS or selenium diodes were introduced? It certainly saves wire to have the heater wiring like that, if you use a bridge, and that makes sense...<br> <br> There are two ways to &quot;share&quot; the secondary windings: in the middle, and on the end. There's good reason, electronically, to use the middle--if the end of the coil were used instead, the low voltage portion would be an &quot;elevated&quot; voltage, referenced to ground. If the &quot;shared&quot; part is in the middle, that lower voltage isn't elevated.<br> <br> For the &quot;end&quot; coil winding, if the total secondary voltage was 0-300V, and the filament voltage a typical 6V, then that 6V would be offset from the maximum voltage. 294V to 300V, rather than 0 to 6V.<br> <br> That <em>does</em> actually work--except for HV there is a &quot;Maximum Heater-Cathode Voltage&quot; for tubes. A 12AX7, for instance, that maximum difference is 180V. This could easily be exceeded with a 300V (our chosen &quot;example&quot; voltage) power transformer...<br> <br> I have used a PT with a shared end coil successfully, and it did work. But that was a lower voltage (145V) secondary.<br>

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