How to figure out Primary wiring without a color code guide

[DrGonz78]

I have a couple of these amps that I bought as starter kits to build 18 watt EL84 amps. The PT's are from China and have universal Primary taps. On the transformer it says Input: 100v | 120v | 220v | 230v | 240v, but gives no color code info. I have tried to contact the seller of the amp but he did not have any info. The maker of the transformer is Ningbo Chaobo Electric Co. by way of Axl Musical Instrument co. and it seems like a dead end finding info from those sources.

The primary wires are in a row: Black | Wht\Blk | White | Brown | Blue | Purple. The first logical idea for me was that: Black & Wht/Blk = 100v | Black & White = 120v | Black & Brown = 220v | Black & Blue = 230v | Black & Purple = 240v. Now that is just an idea of what it could be and just putting it out there as a thought. The DC resistance is: Black & Wht/Blk = 2.8ohms | Black & White = 3.4ohms | Black & Brown = 15.6ohms | Black & Blue = 16.7ohms | Black & Purple = 18.2ohms. Thankfully the secondary side will be much easier to sort out and I could always inject AC (low voltages) from secondary and measure voltages on the primary too. Just looking for some guidance on the matter as the best way to proceed from here. Thanks in advance for any help.

[DrGonz78]

One more thing to note... It is a stripped down hand wired type VHT amp EL18 is what is written on the transformer. If you look at the Special 12/20 VHT schematic it shows switch to be able to set Primary input to 100v | 120v | 230v. So I would be able to detect if there are two separate primary windings correct? Also, since this PT has 5 separate available inputs it reminds me of this type of primary winding.

[eschertron]

I would expect the primary resistance from the zero reference to be grouped (the 100's to be about half of the 200's), so I find it hard to believe that 120vac = 3.4 Ohms while 240vac = 18.2 Ohms. I think the largest resistance between any two wires would be the zero and 240vac lines, and then the others would fall into place linearly between them. Did you try Ohm-ing out any pairs other than black-and-some other color?

[g1]

Your resistance readings imply that there are not 2 separate primary windings, as you have continuity through all the taps.

[DrGonz78]

I would expect the primary resistance from the zero reference to be grouped (the 100's to be about half of the 200's), so I find it hard to believe that 120vac = 3.4 Ohms while 240vac = 18.2 Ohms. I think the largest resistance between any two wires would be the zero and 240vac lines, and then the others would fall into place linearly between them. Did you try Ohm-ing out any pairs other than black-and-some other color?

Okay I will try to find those two wires and then see where it goes from there, thanks much!!

Your resistance readings imply that there are not 2 separate primary windings, as you have continuity through all the taps.

Yeah that is what I figured and honestly that makes it a little easier, I think.

[olddawg]

Do you have a signal generator or a cheapo AC output low voltage/current wall wort? You could try different combinations and measure the ratio at the secondaries. I would try black and white first.

[DrGonz78]

Well if the largest DC resistance between two wires would be the 240v tap then Black & Purple is the greatest resistance at 18.2ohms. So to me it seems that I might have got lucky on first draw? I have two of these transformers and they both measure the same DC resistances.

I do have signal generator and a variac so I can go about measuring the AC volts secondary ratios. I will start that direction tomorrow and see what I get, thanks.

[eschertron]

Well if the largest DC resistance between two wires would be the 240v tap then Black & Purple is the greatest resistance at 18.2ohms. So to me it seems that I might have got lucky on first draw? I have two of these transformers and they both measure the same DC resistances.

Is that after testing every possible combination of two wires?

I do have signal generator and a variac so I can go about measuring the AC volts secondary ratios. I will start that direction tomorrow and see what I get, thanks.

To be cautious, I'd put my supply voltage across the 240v winding first (knowing that every reading should be low, but in a perfect ratio) just to be sure. I know the high resistance winding will be safe, not so sure about a winding with only a small fraction of that (i.e. black and white, 3.4R)

[Enzo]

Since it is one primary winding, perhaps the 220/240v portion of the primary is a smaller gauge wire than the 120v part, thus explaining the higher resistance?


it is a transformer. Put it on your bulb just in case, but I doubt it matters. APply 120v to what you think is the 120v winding taps. Now do you get about 240 out the 240 tap? Do the secondary voltages come out making sense? Roughly 5v and 6v and several hundred volts? Or bring it up on a variac if you are timid.

[DrGonz78]

Since it is one primary winding, perhaps the 220/240v portion of the primary is a smaller gauge wire than the 120v part, thus explaining the higher resistance?

I had that thought in my head last night but started to doubt my thinking. Is that also similar to why you will see a lesser amperage fuse in 220-240v primary application than the USA type primary fuses? Maybe because higher resistance winding has less in rush current upon power ups?

[Enzo]

No. The circuit is the same on the secondary side, regardless of mains. So if the amp draws 50 watts, then 50 watts is what it needs from the wall. The fuse is smaller simply because 50 watts from 120v needs twice the current as 50 watts from 240v.

The last thing a tiny amp like a Deluxe is worried about is inrush current on the mains.

[DrGonz78]

Oh I forgot to update a conclusion to this thread... I connected 120vac to the black & white primary wires. I measured 5.4vac for the heater voltage to the tube rectifier and 6.6vac for the general heater supply. I figure that under load it will drop down to 5vac and 6.3vac. High voltage side came to 640vac as I recall and that is a 290vac CT secondary. So it seems to me that the black and white wires are correct 120v primary connects.

[52 Bill]

Oh I forgot to update a conclusion to this thread... I connected 120vac to the black & white primary wires. I measured 5.4vac for the heater voltage to the tube rectifier and 6.6vac for the general heater supply. I figure that under load it will drop down to 5vac and 6.3vac. High voltage side came to 640vac as I recall and that is a 290vac CT secondary. So it seems to me that the black and white wires are correct 120v primary connects.

Glad to hear you got it figured out.

As a point of info here, I think that what Enzo was suggesting earlier was to apply 120 to the suspected 120 volt terminals and then measure the ac voltage on the other primary taps to see what they read. In a single winding primary, if the correct voltage is applied to the two taps that are in fact designed for that amount of voltage, the voltages on the other taps will read what they should be.

A better explanation would be with 120 volts connected to the black and the white taps, you should read 100 volts between the black and the white/black terminals, 220 volts between the black and the brown terminals, etc.

[DrGonz78]

As a point of info here, I think that what Enzo was suggesting earlier was to apply 120 to the suspected 120 volt terminals and then measure the ac voltage on the other primary taps to see what they read. In a single winding primary, if the correct voltage is applied to the two taps that are in fact designed for that amount of voltage, the voltages on the other taps will read what they should be.

I will check this out tonight after work. I see where Enzo said that now and thank you for pointing it out.

[DrGonz78]

Well I confirmed that wht/black is 100vac, brown is 220vac, blue is 230vac and purple is 240vac. Thanks to everyone for helping me understand this better, much appreciated!

One last question... Since Black is the common wire then would I want that wire connected to neutral or does it even matter? I currently have it hooked up black to hot, but I see sometimes see "Common" or "0v" connection side of primary. Just have to ask that much to see if there are any other suggestions, thanks.