dechiphering an output transformer

[pjd3]

The polarity of the negative feedback for one thing (if there is any) is something rather critical that would come into play there.

PJD3

[thetragichero]

it's a good idea to tack solder the plate leadsv with an unknown output transformer until you confirm if you've got it hooked up right for that exact reason

[trobbins]

Testing OPTs by applying a significant voltage to the secondary can cause a large voltage on the primary, and I would not recommend that technique for the following reasons:
- the voltage on the primary would typically exceed a safe level, and that introduces risk, especially to those that are watching a youtube video in order to find out how to do the measurement.
- the secondary winding being driven by a voltage, may end up saturating the core, especially when testing transformers with unknown windings/impedances, and that may not be obvious to those new to testing.
- most cheap DMM's nowadays are very proficient and accurate at measuring ACV down at the mV end, so applying even 5Vrms across a half-primary winding can achieve an accurate and measureable result for secondaries.
- as most OPT's are for push-pull, it is very easy to apply 5-12Vrms to one half primary winding, and measure the voltage on the other half-primary winding (and use that measurement for calculations). And by swapping over the half-primary windings, the symmetry and confidence in the measurements is improved (especially if there is a shorted turn or other quirk inside the transformer - which is not too uncommon for vintage transformers that have had a 'life').
- Driving a secondary winding and taking its voltage as a measurement for calculations introduces an error due to the driving signal current. The level of error can be indicated by driving a half-primary winding, and measuring the voltage on that winding as well as the other half-primary winding - they will be a bit different.

As such I disagree with the video's comments about 'taking more measurements to get more accuracy', nor would I recommend making flylead connections to mains AC, or provide a very minimal comment about the safety of the high voltage on the primary.

[dorrisant]

the voltage on the primary would typically exceed a safe level, and that introduces risk, especially to those that are watching a youtube video in order to find out how to do the measurement.

+1



Some of you are overthinking this. The advice given here certainly exceeds most YT "experts" by miles. Seriously guys, I've built so many amps that I've lost track. Most of those were built with donor iron and chassis. So I've used the outlined procedure enough, I'm familiar. If you notice, the specs for OT primary impedance are all basically rounded off to the nearest hundred: 4.2k, not 4.247k. So a ballpark number is all you need to shoot for. If you want to understand the magical tone that different OTs are responsible for then you should really be considering what the laminations are made of and how they affect things. Now there's a topic worth staying subscribed to.

[nworbetan]

trobbins is very good at overthinking things. I've read a lot of his papers and greatly appreciate his overthinking.

[pjd3]

Watching a youtube video to confirm a method doesn't equate to an admission of not being familiar with the ramification of power and what it can do to humans and equipment. After testing fuses and equipment in a high power test lab (1500 volts - >100KA of current) for nearly 2 years was a great learning experience in this. We were at any time, one or two button presses away from blowing up an entire huge room, or killing somebody. The days were full of calculating for time constants, power factor, phase angle, current, introducing power at the proper angle of the 60Hz voltage, etc. To not do any of those things accurately could lead to horrendous outcomes a great deal of the time. What it mostly taught though, was to stay humble and far away from being too much, "in the know". So, I stay humble and watch videos made by those who have been at particular things decades longer than I have. It only cost a little bit of time, and hell, we might even get something out of it.
Best,
PJD3

[pjd3]

The specs for OT primary are rounded off but, I wouldn't expect them to be rounded as far as doubling or halving the typical range of value that is customarily found in guitar tube amps of this topology/power. That's why it seemed at least reasonable feasible to at least try a 4K or 4.3K ohm primary in the calculations without expecting crazy off tap impedance's.

My initial approach was not to assume too much of anything. I went at this assuming I knew nothing about the transformer. What I did know, is that the OT was implemented in an amp that used a pair of 6L6GC's with a fairly common range plate voltage. Yup. The Bogner schematic tells me its a 4, 8 and 16 ohm tap. It is still a mystery to me how I got startlingly close to 2, 4, and 8 ohm taps from measurement and calculation. That's OK though. I'm sure worst things will happen even over the next hour, the sink will clog or something.

Thanks everyone, I do appreciate the effort you're all put in to my growing pains!
Best,
PJD3

[sluckey]

I use a variac and filament transformer to apply exactly 1VAC to the secondary winding of an unknown OT. This makes the math easy. The voltage is usually approx 30VAC on the primary. Very safe IMO.

[pjd3]

And this is how I would do it if I had better access to a variac. Of all the darn things I have hanging around my house I can't find a little ac transformer to save my life. Theres got to be one in here somewhere. My work would normally be happy to let me bring the chassis in and use the equipment but, they have many of us working at home till Covid pans out.

I might see if a guy can sign one out for me. I did bring a function generator home. I figured I could use that but, but what? I'd rather use a variac.

Thanks Sluckey,

Hey, I just thought of something! Why don't I go buy a real transformer that has specs N' everything? naw, thats too easy and reasonable.