Perplexing transformer question

[vigilante397]

I've been working on what is more or less an amp kit build and am having some trouble with the transformer. The gentleman that sold me the kit originally forgot the transformer and sent it to me later, but I'm worried he may have sent the wrong one.

The transformer is Chinese made and has no label indicating which lead is which so I guessed, and apparently guessed wrong. I've lost 6 fuses and fried my favorite multimeter trying to figure the dumb thing out, but here's what I've figured out:

White - White are 120V primaries (I'm assuming)
Green/Yellow seems to be chassis ground, as it doesn't connect to any other leads on primary or secondary side
White - Green - White I assumed were heaters with a center tap, but I'm getting 5VAC between each lead and the center tap for 10VAC between the two white leads with no load
Yellow - Yellow I assume is for tube rectifier heater, reading 7VAC with no load
Red - Orange - Red These are the ones that really bother me. I assumed these were my high voltage mains with a center tap, which should be around 275 - 0 - 275 for a simple EL84 push-pull amp like the one I'm building. However, I'm measuring 530 - 0 - 530 with no load. The very first time I fired up this amp (without measuring anything, of course) I literally melted the glass on one of my EL84's.

Is this a simple case of the wrong transformer? Is there some way to hook everything up that would still make everything happy?

Any help would, as always, be greatly appreciated.

[KarenColumbo]

May be a Photograph (or several from different angles) would help. Just shoot the bastard, upload the pix - this thing is gonna be found out soon enough. This trans sounds familiar somehow ... maybe I can compare it to some of my amps here

[R.G.]

White - White are 120V primaries (I'm assuming)

Probably. In modern electrical code use, white is the AC power line "neutral" wire, black is the live or "line" wire. It's most unusual to see two white wires for a primary, but as you're finding out, there is no real standard for what wire color is what on transformers.

Kudos for figuring out that a winding must be connected end to end and and can be detected with the ohmmeter setting.

Green/Yellow seems to be chassis ground, as it doesn't connect to any other leads on primary or secondary side

One wire, not connected to any others, green/yellow? Yep, a screen/ground winding to act as a shield between primary and secondary.

White - Green - White I assumed were heaters with a center tap, but I'm getting 5VAC between each lead and the center tap for 10VAC between the two white leads with no load

Yellow - Yellow I assume is for tube rectifier heater, reading 7VAC with no load

I'd make the same guess; but you're measuring about 29% high.

Red - Orange - Red These are the ones that really bother me. I assumed these were my high voltage mains with a center tap, which should be around 275 - 0 - 275 for a simple EL84 push-pull amp like the one I'm building. However, I'm measuring 530 - 0 - 530 with no load.

No question that this is the plate supply and that it's too high.

If those are the correct windings, verified by connections with an ohmmeter, I'd probably make the leap that you have the wrong power transformer too. The high voltage windings almost have to be correctly chosen. If red-orange-red was the primary, that would leave the only other high-ish voltage winding too low as well as making the heater windings be too low.

I would say that a quick call or email to the gentleman supplying the kit would be in order.

[vigilante397]

If those are the correct windings, verified by connections with an ohmmeter, I'd probably make the leap that you have the wrong power transformer too. The high voltage windings almost have to be correctly chosen. If red-orange-red was the primary, that would leave the only other high-ish voltage winding too low as well as making the heater windings be too low.

I would say that a quick call or email to the gentleman supplying the kit would be in order.

When the first fuse blew I took that opportunity to disconnect all the secondaries and check everything with an ohmmeter I emailed the gentleman last night (around 1AM ) and with it being Sunday I don't expect a response any sooner than tomorrow. I'll keep everyone posted on what happens, but frankly if I don't get a response within a couple days I'll just buy myself another transformer and have at it.

Also since it's Sunday and there's nothing else I can really do on this amp right now, here are some pictures of the transformer in question:

[Cozybuilder]

Are you sure thats not an output transformer rated at 18 Watts?

[vigilante397]

Are you sure thats not an output transformer rated at 18 Watts?

Pretty sure. That's a lot of leads for an output transformer the amp I'm building is a Marshall 18W clone, pretty sure that's why the transformer has been labelled like that.

[vigilante397]

UPDATE: the seller hasn't responded. I bought the thing from him on eBay about 6 months ago, and his eBay store now looks like he's in the business of medical supplies.

So I just hopped over (electronically) to Mojotone and got myself a PT from them, I figure it's an upgrade from the Chinese transforner anyway.

[thermionix]

So I just hopped over (electronically) to Mojotone and got myself a PT from them, I figure it's an upgrade from the Chinese transforner anyway.

Big time.  Mojotone carries mostly Heyboer-made transformers these days.  That's a good thing.

[blackieNYC]

How high do you think think it should measure with no load?

You've already found your answer, but amps have always been a little scary to me.  I like to set a tone generator for 1.20 VAC, instead of 120 VAC, and look for the corresponding output voltages - 300VAC will come out as 3V.  I'm sure some inaccuracies are imposed by drastically different current and voltage, different generator impedance, but it seems to work fairly well. How reliable does this made-up method seem to you guys?

[vigilante397]

How high do you think think it should measure with no load?

You've already found your answer, but amps have always been a little scary to me.  I like to set a tone generator for 1.20 VAC, instead of 120 VAC, and look for the corresponding output voltages - 300VAC will come out as 3V.  I'm sure some inaccuracies are imposed by drastically different current and voltage, different generator impedance, but it seems to work fairly well. How reliable does this made-up method seem to you guys?

I think I remember you mentioning this method on another thread and thinking "wow, that seems like an excellent method to not die." It seems like it would be perfectly accurate to me, transformer voltages aren't absolutes, they're ratios. Alas it was late and I didn't think of it at the time. I haven't been shocked by an amp in a while, but 1100VAC was enough to fry my DMM, so this method would have saved the poor thing.

As for what the voltages are supposed to be, I had actually bought two of these kits and assembled one for a friend (naturally that one worked perfectly ) so I was comparing my no-load voltages to his no-load voltages.

[tonight, we ride]

How high do you think think it should measure with no load?

You've already found your answer, but amps have always been a little scary to me.  I like to set a tone generator for 1.20 VAC, instead of 120 VAC, and look for the corresponding output voltages - 300VAC will come out as 3V.  I'm sure some inaccuracies are imposed by drastically different current and voltage, different generator impedance, but it seems to work fairly well. How reliable does this made-up method seem to you guys?

This sounds like a great method for getting transformer ratios at safer power levels, but I thought I should mention one note on doing things this way. Just like with other components, transformers can have issues that only present themselves at nominal operating power.

Transformer winding insulation can break down at higher voltage/power levels, while looking like everything is good at lower voltages/power levels. Meaning, you could test a transformer for the proper ratios at a lower voltage only to find that it is shorting, or partially shorting, some of the windings when at full operating voltage.

There are devices called Insulation Testers, often called "Meggers," that are made to test transformer windings safely at proper voltages, but I've found that the best way to test a transformer is to terminate all of the leads into insulated terminals on a terminal block then carefully check each pair of leads.

To OP, glad that you've got a new transformer on the way!! I always hate waiting on a single part but it is always worth the wait.

[blackieNYC]

Good point - Thanks twr, I always figured it wasn't a perfect method for transformer operation, outside nominal operation. Glad to hear it's still somewhat useful. Shorts that show up only when higher current is flowing seems a possibility to me.
I mentioned this before? Repeating myself again, looking for affirmation. Brain damage. Earned it.

[vigilante397]

I mentioned this before?

I thought you did, maybe it was someone else? Or maybe I'm imagining the whole thing?

[amptramp]

If you ever collect old test equipment, the EICO 950B capacitance bridge has a comparator function that allows you to determine turns ratio of a transformer (or ratios between the values of any parts).  This is one application that can make it worthwhile.  There is a full description of a number of tube-era bridges here:

http://www.ohio.edu/people/postr/bapix/capchkr2.htm

Collecting kinds of grows on you as shown by these four EICO 950B's:

[PRR]

> I like to set a tone generator for 1.20 VAC, instead of 120 VAC

"Tone generators" are often poor "power" drivers. I did modify my Heath to deliver some power, but that is a side-drift.

MY technique for mystery iron is to use a 6VAC heater transformer to push the assumed-120V leads. The heater transformer has an Amp or two of grunt, far more than most tone generators, and will comfortably drive about any "120V" winding you are liable to face. You now expect your 300V leads to show 15V, your 6V leads are 0.3V, etc. Unless there is an >500V winding on there (possible), you won't have any real fatal voltages. (I would still use Clip-Leads, NOT my fingers, to make connections.)

Note that if you do not know the PT's 6V from its 120V, you could feed 6V to 6V and get all the FULL voltages out. So first use resistance checks. A 120V winding will tend to show 4 to 40 Ohms. A 6V winding is 1 Ohm or less, usually the lowest-Ohm winding of all. 300V windings tend to be hundreds of Ohms.

[thermionix]

Variac.

[vigilante397]

Variac.

A while back I was working on a Jet City warranty repair at work so I was on the line with an authorized tech that was walking me through it. I mentioned that I was pretty sure the problem was the power transformer, so he was telling me how to go back debugging it.

Him "Try disconnecting the PT secondaries and leaving it unloaded and see if it still blows fuses. If so, that's your problem. If not, we can work from there."

Me "Okay, cool. I can definitely handle that."

Him "Great. You have a variac, right?"

Me "Of course." *complete lie*

Him "Great, that should get you taken care of. Well good luck!" *hangs up*

Me "Crap. Why would I say that? What benefit does it give me to lie about having a variac?"

So now anytime a variac is mentioned I'm reminded that I compulsively lie to impress other techs Needless to say I still do not own a variac.

[thermionix]

I compulsively lie to impress other techs

Make sure you include that on your security clearance form.

For testing for a shorted PT, hard to beat the light bulb current limiter.  I recently rebuilt mine after 20 years of service, I use the hell out of that thing.

[vigilante397]

For testing for a shorted PT, hard to beat the light bulb current limiter.  I recently rebuilt mine after 20 years of service, I use the hell out of that thing.

I've been meanin to start doing that but haven't had enough problem amps recently

So Mojotone took care of me. Between placing the order and having it on my doorstep was 3 days. Great documentation, hookup took about 5 minutes. Everything works great, EL84's no longer red plating, and killer sound