Power Transformer testing? (kinda long)

[JJGross]

Hi all,
I posted this on another forum but have nothing after about a week so here I am asking here. I copied both posts in the other forum's thread to give a more complete picture of what's going on.

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Part 1

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I have a salvaged power transformer out of an old radio. It was originally powering a 5Y3GT rectifier, a single 6V6 and 10 octal tubes for everything from preamps to radio circuits.

My question is this:
I'd like to build a slightly hotrodded 5e7 clone like this:
http://www.kilback.net/homebrewtweak...hem-od3_r3.gif

Even though my original (salvaged radio) transformer was only running a single power tube, would it have enough to run another power tube since it was powering a dozen tubes in the original radio it came from? It has a single primary coil and a high voltage output with a center tap + what I'd assume are both 5v and 6.3v windings. Of course only powering it will tell me what they actually put out (edit :: see part 2 below).

The transformer itself actually has one more wire than the schematic shows. The schematic shows the 5v and 6.3v windings with no center taps, but there are the following wires coming out of the transformer:
2 black stranded (I'm assuming primary)
2 white solid wires
2 green solid wires + a green wire that is actually a loop returning to the core
3 red stranded wires

The schematic (of course) doesn't show me what colors are what. In addition, I have to guess at the colors because the read & white are all more dirty grey than anything else, even under the bells (which I have off).

Any idea if this transformer should be able to deliver adequate current for a 5y3 rec, a PAIR of 6v6 or 6l6 and 3 12ax7s?
It sure was running more tubes than that originally, 12 total including a 'Magic Eye'.
I'm sure the voltage is there, just not sure about current delivery in radios vs. guitar amplifiers.

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Part 2

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Rather than just hook up an unknown tranny to my house current, I decided to try powering it with a wall wart and doing a little math.

I hooked the transformer in question up to a 6vdc-300mA wall wart.
The power coming from my wall measured at 118.7vac.
With no load, the wart produces 7.29vdc.
Hooked up to the transformer, it's delivering 3.73v to the primaries.
I'm assuming the reason for the huge drop is because wall warts generally aren't designed to shove electricity through gigantic power transformers. The coils are probably offering a lot of resistance.

Green wires measure .026v and .011v C.T.
Red wires measure 2.64v and 1.32v C.T.
Yellow (or white or whatever they are) measure .019v (no C.T.)

Dividing my power coming into the wart by what's coming out connected to the transformer's primary, I get a factor of 31.823056300268096514745308310992 - lets call that 32 for simplicity's sake.

That means at 118vac mains voltage, multiplying by a factor of 32 ::
Green wires would deliver .832v
Yellow wires would deliver .928v
Red wires would deliver 84.48v

... da heck?!

According to the schematic for the radio this came from the first thing out of the rectifier was a 15uf/15uf/10uf cap can rated at 450v. A 5Y3 rectifier drops something like 50v as it does it's job IIRC. I can't imagine this thing ran on about 30something volts originally with a dozen tubes on it. I could post the schematic, but didn't because the power transformer is only shown as a part number and the schematic has no voltage values on it anywhere.

What am I likely doing wrong here? Any ideas? At first I thought maybe the primaries were really the red wires, but not with a center tap. Same thing with the green wires. Would it make more sense if the white (or yellow or whatever they are) are the primaries? I also thought maybe the transformer was no good, but if that were the case it shouldn't be passing any current through at least some of these windings - or would it?

I don't really want to just wire up the tranny to my house mains to measure it if I don't need to but if I have to I will. I'd much prefer to do this testing part at low-voltage if I can get away with it. I like my own internal electrical system just the way it is. I'd also like to not fry the transformer before I get a chance to do that running a guitar through it.

Any help is appreciated.

Cheers,
- JJG

[oc disorder]

um transformers need A.C. !
Electrons have to move to create magnetism which can collapse to induce in adjacent windings.

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Maniac is sitting by compère. He wears a top hat and an opera cloak.
Maniac Good evening.
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Maniac Yes, that is correct, I can...
Compère Entirely by hypnosis.
Maniac Yes ... I use no artificial means, whatsoever. (leans and picks matchbox off desk to light pipe, opens it and strikes match)
Voice (from matchbox) Aaagh!
Dove You've injured Mr Stools!
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We see a man in a white coat preparing to throw brick. He throws it gently. It lands on the desk in front of Keith. Keith stares at it as it falls.
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From Monty Python Episode 11
www.ibras.dk/montypython/episode11.htm

[Enzo]

You are assuming that 3v from the wall wart is making accurate assessments.

Standard color codes for PT wires:
yellow - 5v
green - 6v
red - HV
add a yellow stripe for center tap, except 5v where it wouldn't show, so add black stripe for CT on yellow , which you don't have

I suspect your white wires are really yellow. The heavy current heater windings would be the solid wire ones - more evidence.

I think I would have taken 6v from another transformer and applied it to the green wires. Then I could expect 5v from the 5v winding and whatever on the HV. CLose to the real voltages.

However, I have no problem connecting to the house current. You could:

1. wire a 100 watt bulb in a socket and in series with the transformer primary. if the tranny is shorted , then the light bulb will come on bright and nothign is hurt. If the tranny is OK, the bubl will come on dim or not at all.

2. wire a fuse holder in series with the tranny primary. A 1A fast ought to allow it to come up yet prevent a fire if it is shot.

3. do the same thing but with a little circuit breaker - like the cheap ones with the red reset push stick.

4. use a current meter in series with the thing set for AC amps, and bring it up slowly on a variac. The current will strat to rise if the thing is bad. If it is ok, it will come up fine.


And why not go over to RG's Geofex site and check out his transformer tester.

Too bad you didn't note the connections before removing the tranny from the unit.


Your tranny will likely be fine powering the heaters of the tubes you want - one added 6V6 taking the place of several octal signal tubes. The B+ is another story. Power tubes take current, the preamp and radio tubes are like 12AX7s, they take just a couple milliamps each. so one 6V6 might need as much B+ current as 10 preamp tubes. It isn't the number of tubes, it is what they are doing.

[JJGross]

You are assuming that 3v from the wall wart is making accurate assessments.

Standard color codes for PT wires:
yellow - 5v
green - 6v
red - HV
add a yellow stripe for center tap, except 5v where it wouldn't show, so add black stripe for CT on yellow , which you don't have

I suspect your white wires are really yellow. The heavy current heater windings would be the solid wire ones - more evidence.

I think I would have taken 6v from another transformer and applied it to the green wires. Then I could expect 5v from the 5v winding and whatever on the HV. CLose to the real voltages.

However, I have no problem connecting to the house current. You could:

1. wire a 100 watt bulb in a socket and in series with the transformer primary. if the tranny is shorted , then the light bulb will come on bright and nothign is hurt. If the tranny is OK, the bubl will come on dim or not at all.

2. wire a fuse holder in series with the tranny primary. A 1A fast ought to allow it to come up yet prevent a fire if it is shot.

3. do the same thing but with a little circuit breaker - like the cheap ones with the red reset push stick.

4. use a current meter in series with the thing set for AC amps, and bring it up slowly on a variac. The current will strat to rise if the thing is bad. If it is ok, it will come up fine.


And why not go over to RG's Geofex site and check out his transformer tester.

Too bad you didn't note the connections before removing the tranny from the unit.


Your tranny will likely be fine powering the heaters of the tubes you want - one added 6V6 taking the place of several octal signal tubes. The B+ is another story. Power tubes take current, the preamp and radio tubes are like 12AX7s, they take just a couple milliamps each. so one 6V6 might need as much B+ current as 10 preamp tubes. It isn't the number of tubes, it is what they are doing.

Thanks for the info, that's exactly the stuff I was looking for.
It's amazing how much info there is out there about building amps, but some of the little details like this are so hard to find. I searched for a couple weeks before asking.

Now I have some good starting points to hit it again.

Cheers,
- JJG