Just when I thought I had come to understand OT secondaries,,,

[R.G.]

It’s a lot more revealing to put 100-120 volts on the primary and measure the secondary than such a low voltage like 12 volts.

YIKES!!! That could easily lead to death by revealing your susceptibility to heart stoppage by the AC mains.

The vast majority of readers on this forum have no training and little if any experience in dealing with AC mains voltages, other than soldering the AC mains wires to fuse, switch and power transformer. And an even vaster majority do not have any source of 100-120VAC other than cobbling up a connection to the AC wall socket. A direct use of the AC wall voltage makes the entire planet the other side of deadly voltage connection.

The likeliest scenario for an inexperienced someone trying to use 120Vac mains to test an OT is to hack-wire a line cord directly into the primary of the OT or to bridge the AC wires on an amp chassis to the OT primary - possibly on something like clip leads or lengths of hookup wire.

At a minimum, an isolation transformer is needed to make using mains voltages less deadly. I have an isolation transformer but I would be mildly astounded if there were more than a dozen forum readers (including especially guests) that have access to one.

I think that we need to point out to the unseen beginners and guests where there are likely to be unexpected deadly traps.

[maxkracht]

The likeliest scenario for an inexperienced someone trying to use 120Vac mains to test an OT is to hack-wire a line cord directly into the primary of the OT or to bridge the AC wires on an amp chassis to the OT primary - possibly on something like clip leads or lengths of hookup wire.

At a minimum, an isolation transformer is needed to make using mains voltages less deadly. I have an isolation transformer but I would be mildly astounded if there were more than a dozen forum readers (including especially guests) that have access to one.

I thought an iso transformer was a standard piece of kit, but maybe I'm one of your dozen. Aside from the safety hazard, putting more voltage than necessary into an unknown transformer probably isn't a great idea for the transformer's sake incase you misjudge something.

If someone insists on testing transformers with clip leads or bits of hookup wire, I would recommend getting some Wago style connectors. Not as good as a proper test setup, but at least there is less potential for exposed metal.

[martin manning]

Most people who have been at this game for a little while have an autotransformer (Variac), but isolation transformers are pretty rare.

[R.G.]

+1 - what Martin said.

Variacs and other adjustable voltage transformers are not in general isolators, although a few of those exist.

You could be one of the hypothetical dozen; I read your post carefully and you did say indirectly that you have an isolation trannie. If so, GREAT!!
What set Martin and me off was the slightly unclear association of not putting too much voltage on a part with isolation. They're two different things.

I know the difference intimately, as I melted the insulation off an oscilloscope ground lead by presuming that a variac isolated. It was a long time ago, but the smell of burning insulation stuck with me.

[maxkracht]

I almost always have iso to variac to dim bulb tester with a bright led that tells me when power is on and when it's limited. Guess I took for granted that the average person would make 12vac with a transformer but 120vac without... Plenty of insulation melting experiences and the like in my past. An abundance of caution is good.

[pjd3]

Yeah, intuitively, applying that level of voltage to an output transformer is something I wouldn't have done, and likely would never do unless there was something in using 120vac that was going to tell me something relevant that 12vac would not.

I've even wondered if applying the 12vac to the taps as opposed to primary is even a good idea, seeing there there is a notable winding ratio going on in an OT and pretty large voltages would be appearing on the primary, and that voltage would increase depending on what tap I was applying the voltage to.

Fortunately, I've worked in places like high voltage, high current test labs where making a mistake was not an option, without killing someone, yourself, or taking out an entire wall of massive resistors and inductors. (Mersen fuse test lab - two 1500vac generators capable of up to 100,000 amps at short duration). That place gave me nightmares but earned me a huge respect for electricity.

thank you,

Phil Dl

[sluckey]

I have a dedicated small 12V CT transformer with line cord connected to the primary and gator clips on the 3 secondary leads. I use this to check voltage/turns ratios in order to calculate impedance ratios. I plug the line cord into my variac and adjust for 1VAC on the secondary side. Then connect this 1VAC to the OT secondary and measure the primary voltage. This makes the voltage/turns ratio simple and straightforward. No math required. Then simply square the measured primary voltage to calculate the impedance ratio.

[pjd3]

I like that Sluckey.

I Volt - removes a step towards the finding.

Thats what I need at home. A variac. It's always a bit stressful when sparking up a new amp build with the "hope" that you did it all right. Of course, you can make up for that by going over the wiring 3 times (which I do) and testing every possible place for shorts (which I try to do). And looking at all voltages before committing to tubes.

so far, so good. Even my "Sluckey plexi 6V6) didn't smoke on me.

Thanks all,
Best,
Phil D

[R.G.]

If you don't have a light bulb limiter, get/make that first.

[wpaulvogel]

It’s a lot more revealing to put 100-120 volts on the primary and measure the secondary than such a low voltage like 12 volts.

YIKES!!! That could easily lead to death by revealing your susceptibility to heart stoppage by the AC mains.

The vast majority of readers on this forum have no training and little if any experience in dealing with AC mains voltages, other than soldering the AC mains wires to fuse, switch and power transformer. And an even vaster majority do not have any source of 100-120VAC other than cobbling up a connection to the AC wall socket. A direct use of the AC wall voltage makes the entire planet the other side of deadly voltage connection.

The likeliest scenario for an inexperienced someone trying to use 120Vac mains to test an OT is to hack-wire a line cord directly into the primary of the OT or to bridge the AC wires on an amp chassis to the OT primary - possibly on something like clip leads or lengths of hookup wire.

At a minimum, an isolation transformer is needed to make using mains voltages less deadly. I have an isolation transformer but I would be mildly astounded if there were more than a dozen forum readers (including especially guests) that have access to one.

I think that we need to point out to the unseen beginners and guests where there are likely to be unexpected deadly traps.

What I meant was with a fused variac. We put several hundred volts DC and AC on our output transformers every switch on and play. We also always put mains voltage on our power transformers. I see it no differently.

[SoulFetish]

like the others, I don't make this measurement using resistance. I use my signal generator and my oscilloscope. It's very safe and very accurate, even applying lower voltages to the primary. My signal generator can output an undistorted 10V p-p sine wave, and my o-scope can list and measure RMS, Peak, mean etc. But you shouldn't necessarily even need all that.

In cases when an OT uses unfamiliar color codes, you can test all the leads for continuity to determine which leads belong to a given winding, and apply a 1k/100mV (or some other low voltage) to one of the windings to determine which is the pri, and which is the secondary. This is a quick measure, don't need precision; Also, often the secondaries of the OT will have a much heavier gauge wire, so you can use that to make an educated guess.
Once you've determined which is which, u can use a 1V-5V across the primary and read the voltages across different secondary leads to determine which is which. One helpful tip is that if you suspect secondaries for com-4-8-16ohms, the 4 ohm tap is the center tap of the 16 ohm tap. Anyways, I had to write this quick while I was on break, so hopefully this is helpful. Feel free to add something I may have omitted or correct something I had wrong. Cheers.

Back to fixing amps.....