transformer wiring

[dscottguitars]

Hi,  I have a power transformer but I don't have the wiring information anymore.  Is there any way to find out the specs based on the numbers?  I have not found anything through google. (I have a few transformers I could use info on)  Based on continuity tests I have determined the following:

Please let me know if my reasoning is correct...

2- black wires - primaries - measure 2.1 ohms
red wire and red/white stripe wire- secondaries - 19.3 ohms
red/yellow stripe wire - center tap - 9.8 ohms with each of the wires above
2- green wires - 6.3v filament - 0.6 ohms
yellow/green stripe wire - 6.3v center tap - 0.5 ohms with each of the green wires
blue and yellow/blue stripe wire - 5v - 10.6 ohms

Before I hook this up to power I wanted a second opinion about testing.

Thanks,

Daniel

[eleventeen]

You seem to be asking whether the ohms readings you have measured are "nominal" for what the various voltage outputs you've "designated" or "assumed".

Some are, some aren't. Depending.

2- black wires - primaries - measure 2.1 ohms < Fine

red wire and red/white stripe wire- secondaries - 19.3 ohms
red/yellow stripe wire - center tap - 9.8 ohms with each of the wires above < fine.


2- green wires - 6.3v filament - 0.6 ohms
yellow/green stripe wire - 6.3v center tap - 0.5 ohms with each of the green wires  < OK. Note how the low voltage winding here has such a low DC resistance...almost a short. Not much more than the resistance of your meter leads. I've seen 'em that low.

blue and yellow/blue stripe wire - 5v - 10.6 ohms <<< THIS seems VERY HIGH. Above, you have a 6.3 winding with resistance under 1 ohm. There's not a heck of a lot of difference between a 5 volt winding and a 6 volt winding....yet what you are claiming is the 5 volts seems awfully high resistance, wouldn't you say? And both windings need to have several-amp current capability, right? So they are not 50 ma windings or anything like that.

You can't directly compare ohms to ohms because the HV winding is comprised of thin, higher resistance wire, and the filament windings are wound with fat, low resistance wire.

Let's assume that your nominal take of color coding of the wires is completely wrong and you don't want to produce 1200 VAC or something outlandish like that (which would blow out most DVMs) out of a winding. In that situation, I would grab another transformer whose values you know and power up (what you believe to be) the primary of the "mystery" transformer with the 6.3 volt winding of your "known" transformer. 6.3 volts is about 1/20th of 120 VAC. Ergo, if you do this, the HV winding of the mystery tranny which might be  let's say 350-0-350. Go ahead and measure the voltage between one side and the center tap. The nominal 350 volts output should read 1/20 of 350 = 17.5 volts. Yes, I know things are unloaded and thus the voltages might be off, but you are trying to get a general idea of the lay of the land. Keep going. Your 6.3 volt winding * 1/20th = .315 volts. Your 5 v winding should read about a quarter of a volt. What you are interested in are the *proportions*, the input:output voltage ratios. If your readings are in these sort of ranges, you are good to go. Fire it up on a variac or regular 120 v line.....preferably with a lamp limiter in case you have a HV short....and make your real measurements.

That should do it as far as volts are concerned. Now as for amps...that's different. IF you have a 5 volt winding (powered from) 120 VAC then it really HAS TO be at least a 2A winding because that's the least current any typical rectifier tube draws, eg; 5Y3. Can you use a 5U4? Impossible to say. Throw one on, see if the voltage droops? For the 6.3 volt and the HV, these are more guesswork based on experience. What's the overall size? Big as a Twin? Or a Deluxe or a Princeton? The technology of winding non-toroidal transformers has not really changed in 50 years. But the thickness of the insulation between layers of windings has probably gotten better = thinner = smaller tranny for a given ampacity.

The problem arises in salvage transformers where you don't have a good idea of the tube complement of the original piece of gear, most specifically, even if you have a decent sized tranny (say, Bassman, Super) is it a 120-150 ma tranny (on the B+) which means it could handle 2 x 6L6 or is it a 50-75 ma secondary which points to a pair of 6V6. This, IMO, you cannot tell without performance tests. I have some surplus trannies which look great for amp use (based on size) but they only rate 75 ma (based on the printed diagram on the side) A Tektronix (or other) oscilloscope transformer might have 5 qty 6.3 volt windings, all of decent ampacity, and the thing is pretty big, but it may only put out 50 mils on the B+. And one of the B+ windings might be for only one section, meaning maybe only 20 ma. If your tranny (this one doesn't) has many filament windings, THOSE are the ones with fat wires and thus they build up the size of the thing and make it look like a high-power affair.

Hope this helps.

[dscottguitars]

Thanks, that helps greatly.  I understand what you are saying about the 5v wires I assumed, so maybe they are for bias voltage.  My main reasoning was attributing the continuity of the wires to know which paired with which.  Those last two was a guess because the secondary wires have a center tap and the green wires, which are usually the filament wire colors I've seen have a 'tap' although it's not half of the resistance, so I guessed that was a center tap for the filament.  I feel more confident about hooking it up and checking the voltages to verify what is what, it's just this is my first time doing something like this blind.  If I remember correctly, this came from an old Bogen 6L6 PP amp and may not have had a tube rectifier.

[eleventeen]

That makes complete sense. I was going to say that the resistance across your alleged B+ was maybe a skosh low for a (for example) 350-0-350 secondary...eg; it is more like a 200 volt winding...but that may very well speak to Bogen's frequent practice of using a SS voltage doubler as the main rectifier. If you need to do that: eg; use a doubler for your main rectifier AND pull bias from one of the HV windings...make sure you investigate and copy various Bogen schematics. It won't work unless you place a fattish capacitor between the winding and the backwards diode for the bias rectifier. Needs to be at least a .1 and maybe a .22. At big (600) volts, preferably.

[PRR]

The "5V" seems wrong, as was said.

Otherwise it seems reasonable. Black-Black is 120V. But don't do 120V yet. Get a 12VAC or 6VAC transformer and a 10 ohm 10 Watt resistor in series. Feed the "120V" with that. If the resistor smokes, there's a short. Otherwise the secondaries will show 1/10th or 1/20th of their "normal" voltages.

[dscottguitars]

I wasn't planning on using that tap for bias voltage, I'll just cap the ends and go with cathode bias.  I'm planning on using this (or going to try) for a two channel EL84 PP stereo amp with 3 preamp tubes in each channel.  Total of 10 tubes.  Otherwise I'll buy a different one if this won't work.  My thoughts are the voltage may be too high for the plates unless I can bring it down with a resistor, and using 10 tubes may help cut that B+ down too. 

If it were shorted, wouldn't I know that by the continuity testing?

[eleventeen]

"If it were shorted, wouldn't I know that by the continuity testing?"

Ehhhh, maybe, maybe not. You could have a carbonized spot where a spot short occurred...and it shorts to other windings, while the winding itself remains continuous end-to-end. IOW, it only shorts "dynamically", under power.

And if you had a short in your .5 ohm (6.3 volt) winding you'd never know it until powered up & under load. Besides, you probably did not test for continuity between those wires you measured and the frame of the tranny, and that of course would be curtains.

I realize these are pretty rare occurrences but should nevertheless be ruled out if possible if you are going to carve out a square metal cutout for a flat-laying tranny or drill holes for a standup one...and then have to go buy a new one anyway. You also want to check for that stinky burnt transformer smell.

[PRR]

> If it were shorted, wouldn't I know that by the continuity testing?

It could have a turn-to-turn short.

There's (say) 1,000 turns. Say 10 ohms total DC resistance. Say exactly 10.00 Ohms.

Short one turn to the next. The DC Ohm-Meter shows 9.99 Ohms. Except you never know the "right" DCR to 0.1% precision, nor is your ohm-meter that good, also 10 degrees temperature is a similar resistance change.

However for *AC* the shorted turn looks like a 999:1 transformer ratio, VERY high current in that one turn, and excessive current in the primary.

That's why I suggest testing with a lamp-limiter; or since you also want to confirm voltage-ratios and expect high voltage, a low-volt AC supply with a cheap series resistor.

[dscottguitars]

Okay, I finally got a small transformer, hooked it up and no smoke.  I then hooked it up directly and got the following voltages:

green wire to green wire - 6.7v
green wire to yellow/green stripe wire - 3.3v
That's a check for the heater voltages

red wire to red wire - 100v
red wire to red/white stripe wire - 50v
?? very low and confused now

blue wire to yellow/blue stripe wire - 240v
That's higher than I expected but would work for bias circuit

So I'm really confused as to why the red wires, which should be the secondaries for the rail to all the tubes would be so low.  Any thoughts on this??

I probably won't be using this but I'd like to know what's going on anyway.

Would any of you know of a transformer to use for a dual EL84 PP amp with 2 12AV7's and on 12AT7 on each channel?

[eleventeen]

There are a few possibilities, and it's not always possible to determine exactly what is what.

You could have a transformer for a solid state amp, designed to provide maybe 50-70 volts as the highest rail. Or, you could have a transformer with completely independent 120 / 240 VAC windings, instead of the usual method of "center-tapping" the 240 volt winding. Or.....both!  

Imagine a tranny with independent 120 volt and 240 volt windings. Fire up the 120 volt winding on 120 volts and lo and behold, 240 will come out of the optional other primary winding.  

That you have a CT'ed 6.3 volt winding is strong evidence that you have the right wires for the primary. So I can accept that conclusion.

Your 100 volt winding, red/red-white/red would make a fine bias winding, and as bias uses essentially no current, would work fine. You might have to whack down the voltage but that is no problem in a low-current situation. So I would be looking to the >>100 volt<< winding as a prospective bias winding.

As for the 240 volt winding, that could produce your B+ with a voltage doubler. And there is nothing wrong with that. I am building an amp now with a really nice looking potted tranny from some piece of surplus something. Windings: 6.3 V at about 3 amp. 140 volts, 100 ma. 22 volts * 2.5 amps, 22 volts @ 2.5 amps. Weird, huh? Well...I plan to place one of the 22 volt windings in series with the 140 volt and voltage-double that. The other 22 volt winding, through yet another V doubler (or perhaps a 4-diode bridge) will supply bias. Unloaded, this (140 v + 22 v in series) is producing over 400 volts at present but I suspect it will droop under load...perhaps a LOT. If that isn't enough under load, I'll put the other 22 volt in series and that should do it. (It's for 6V6's, a Princeton) Then, I will steal one side of my three-windings-in-series for a bias supply, thru a capacitor like Bogen. (MUST have that cap in there to steal one side of the HV winding that feeds a voltage doubler.)  

There are multiple ways to skin a cat. There are situations where you absolutely cannot get sufficient B+ if you insist on a tube rectifier. But generally, you can do almost anything with silicon diodes. Although, you can end up with not enough current and end up with 130 extra volts you really don't want to have which is a big pain in the butt. The real mystery is, for your situation, should you elect to use the 240 volt winding doubled, is there enough current capacity? That I cannot answer. I would ponder the relative DC resistances of the 100 volt versus the 240 volt winding for further evidence one way or another.

[dscottguitars]

Thanks, when I bought this it was a pair, one power and one output transformer from a Bogen, 6L6 PP amp.  I used the output tranny in a bassman clone power and my own preamp I made for my nephew.  I don't know anything about voltage doublers though and will probably just sell this on ebay.  I have only made two types of amps, the one above and a 6BQ5 PP, so my experience is limited.  I did find a transformer I forgot I had that I took off of a Motorola 3 channel amp that uses 4 6BQ5's so this should work perfectly.  Here is my schematic I'm using:

(the voltages are targets based on another amp I built)

[PRR]

> Would any of you know of a transformer to use for a dual EL84 PP amp with 2 12AV7's and on 12AT7 on each channel?

"18Watt".

That's a super-common two-EL84 amp, widely cloned and modded. The number of preamp tubes is not very important.

___________________________

> green wire to green wire - 6.7v
> red wire to red wire - 100v
> blue wire to yellow/blue stripe wire - 240v
> That's higher than I expected but would work for bias circuit

BIAS?? 240VAC will rectify-out at 330VDC. (Use a 4-diode Bridge Rectifier, *not* any 2-diode plan such as a bottle-rectifier.)

This *may* be good for your EL84 team.

The 100V AC CT can give you + and - 70VDC. If you really need fix-bias, you can divide this way down cheaply. OTOH if that winding is sub-one-Ohm, what you probably have is for a transistor power-amp, and may not have enuff current on the 240VAC winding to power a pair of EL84.