dechiphering an output transformer

[pjd3]

Hello there folks.

So, did research, watched some vids, signed out a Hewlett-packard digital function generator and winding up to test this 3 tap output transformer that came in the Bogner alchemist.

I spent quite a bit of time looking for specs for this unpopular output tran so decided to skip over things and figure it out myself. Plus, I just thought this to be a good practical electronic excersise.

As I was leaving work with Hewlett-packard 15 MHz Function/waveform generator in hand, one of the techs that has also repaired amps in the past warned me about hooking the waveform generators output to the very low impedance coils of a guitar amp output transformer. I have used these numerous times before for testing in work but, they are usually outputed to a high impedance or resistance input, well, certainly higher than a handful of ohms. He told me I stand the chance of destroying the waveform generator should something happen. I was assuming that he meant loading the output badly causing a high current draw.

So, I was all ready to go with this output transformer tap de-ciphering test, and I'm afraid of blowing an expensive function generator that I signed out from work. I'm working at home for the while due to covid but, can be authorized to stop in for components and necessary stock.

Any thoughts on this would be great. I know that there are settings for the output in this thing, 50 ohms, etc... It happens to be the Hewlett-Packard 33120A if anyone is familiar with the model

Thank you1
Best,
PJD3

[TUBEDUDE]

You could run it through a 1k load first (in series), then make all your measurements after the reisitor.

[sluckey]

Your techy friend doesn't know what he's talking about. Or maybe you didn't understand what he said.

EDIT... I apologize for that hasty and harsh statement. I certainly understand your friend's concerns. I've done exactly what you propose to do using my HP 3310B Function Generator, but I connect the FG output to the primary of the OT. Never had any problems.

[dorrisant]

If your looking for the impedance specs, you could just test it with a cheap wall-wart supply... something around 10 vac. 10 volts exactly makes the math a bit easier. I use a 13 vac and run it down to 10 on a variac. Not necessary though, so do what you have to.

Apply 10 vac to the primary leads. Leave the secondaries disconnected from any load but connected to ac voltmeter. 2-10 volt range works. If you have identified your secondary common, note the voltages between the common and the other secondary leads.

Here's the formula I use...

1 ÷ (one of the secondary voltages derived from above) × primary input voltage (close to 10 vac)

That will give you the turns ratio of that particular secondary winding. Do the same for the other secondary windings.

Now if you take that result and square it, you have the amount of impedance that will be reflected across the primary winding with a 1 Ω load across the secondary.

Example:
10.8 vac supply applied to the primary
0.385 vac across the secondary leads
(1 ÷ 0.385) × 10.8
2.5974 × 10.8
28.051

Just round that off to 28. Then your impedance ratio is 28:1

Square the 28 = 786

So for every 1 Ω of speaker load on the secondary, the primary will see 786Ω.

4 Ω on this tap would reflect (4 × 786) 3147 Ω to the primary.
8 Ω in this same tap would reflect (8 × 786) 6295 Ω to the primary.

If you know the power tube type that the OT was designed for, you will be able to determine which tap was intended for each impedance.

... I think.

[pjd3]

Thank you all,
After I did this post, it did occur to me that I could apply the signal to the primary side. If there was in fact anything to worry about at all, the primary side would offer much less of a load. I have a pretty good fluke meter with me that should take some accurate voltages on the secondary side once applied on the primary. It seemed all the articles and vids I looked at were applying the voltage on the secondary taps so, I just erroneously got that stuck in my head. But, this ought to be able to happen both ways.
The electrons and magnetic fields dont care what side of the OT the speakers are attached to right?

And sure, adding a 1K ohm resistor in series with the signal output appears to be reasonable approach. I just take my input measurement after the resistor because thats what the transformer will be seeing.

Thanks everyone, you've given me some good ideas. Maybe I'll try a few approaches just to prove to myself that they all work!

Best,
PJD3

[pjd3]

And to mention, the amp came with 2 6L6GC's. That probably tells me alot about what to expect for impedance ratio's and expected primary/secondary impedances.

what is a tad confusing to me is that the speaker outputs were originally (and labelled) a single 8 ohm plus, two jacks for an 8ohm pair or single 4 ohm, the latter obviously a 4 ohm tap paralleled to two jacks. BUT, there are 4 wires coming from the secondary (Black, Yellow, Orange, Green).
Although I've seen differing wiring color schemes with similar colors, I have to conclude that one the wires is a common and the remaining 3 are typical taps you would find on many guitar tube amp OT's. So did they leave one tap unused? I just would not have expected to see a wasted tap.

A couple of videos I watched which I though were done very well, just used a simple 120/12 vac transformer and fed that to the secondaries of the OT. If I run into anything odd, I can just bite the bullet and go get one, do what I know works because I've actually seen it done that way..

Thanks,
PJD3

[pjd3]

And Dorisant,

Thank you for taking the time to spell it out so clearly. I will take a good look at your example and see how it fits in with what I have come to understand.

What I did the other night was watch Uncle Dougs vid on how to decipher output transformers. I watched it until I understood it, wrote down the steps and accompanying math, and even did out the chart to place all the readings and calculations. I figured it would be no problem outputting 1, 5, 10 volts into the taps and just record the readings on the primary, get the winding ratio's from the voltage ratio's, square those results, and divide the expected primary impedance of the tubes by each impedance ratio for the individual taps impedance. Ha, then the tech scared the shit out of me. Sluckey says he doesn't know what he's talking about and that could very well be the case in this instance. But, the guy has been working there for over 5 years and is pretty deep into the workings of the companies defribulators. I had to wonder if he had found out anything the hard way in his time there.

Thank you once more!
Best,
PJD3

[nworbetan]

Here's the formula I use...

1 ÷ (one of the secondary voltages derived from above) × primary input voltage (close to 10 vac)

That will give you the turns ratio of that particular secondary winding. Do the same for the other secondary windings.

Now if you take that result and square it, you have the amount of impedance that will be reflected across the primary winding with a 1 Ω load across the secondary.

Example:
10.8 vac supply applied to the primary
0.385 vac across the secondary leads
(1 ÷ 0.385) × 10.8
2.5974 × 10.8
28.051

Just round that off to 28. Then your impedance ratio is 28:1

Square the 28 = 786

So for every 1 Ω of speaker load on the secondary, the primary will see 786Ω.

4 Ω on this tap would reflect (4 × 786) 3147 Ω to the primary.
8 Ω in this same tap would reflect (8 × 786) 6295 Ω to the primary.

A much simpler formula is that the turns ration is equal to the voltage ratio, or: primary voltage divided by secondary voltage = turns ratio

10.8 / 0.385 = 28.051

[dorrisant]

A much simpler formula is that the turns ration is equal to the voltage ratio, or: primary voltage divided by secondary voltage = turns ratio

10.8 / 0.385 = 28.051

True.

I would bet that the black wire is your common.

Also, it seems that the circuit didn't use the 16 Ω Tap at all...

[pjd3]

Thanks guys,

Thats what I thought was going on, a single 8 ohm tap, with the 4 ohm tap paralleled out to 2 other jacks, and an unused tap of some unknown impedance (until now!). Now I know what the toted impedances of the taps are. That would make it even easier to figure out - inject really any voltage across the primary and the 3 taps from lower to higher voltage readings would reveal the 4, 8 and 16 ohm tap. (And after looking at various classic OT color codes, I had to assume that the Black wire was probably common).

Thanks for posting the Alchemist schematic. I'm scratching my head wondering how I missed that before.

Should be good to go now, thank you for your time and efforts here! Its good to be sure, especially at my stage of the game.

Best,
PJD3

[pjd3]

Hello again. I was hoping some of you could comment on the results that came of my deciphering (if thats what actually happened)!!

The Bogner Alchemist schematic calls out 3 taps of 4, 8 and 16 ohms on the OT secondary. Thats what I expected to see.
However, after applying a low voltage to each tap and measering across the brown and blue wire of the primary, and of course following through with the math, this is what I got:
Orange to Blk = 8.4
Yellow to Blk = 4.1
Green to Blk = 2.08

I did use a 1K resistor in series with the function generator output and measured the tap(s) input voltage at the taps, after the resistor. (I set it for a 1kH sine wave).
So this seemed to bring me to classic values except, the schematic shows a 4, 8 and an unused 16 ohm tap.
Ya think they just used a different OT with 2, 4 and 8 ohm taps? And I did out the math according to a pair of 6L6GC's with a plate voltage around 425 - primary impedance of 4.3K ohms
At first, I thought I thought the Fluke meter was maybe reading Peak ac voltage instead of P-P which the function generator clearly showed (P-P in big letters). But, I hooked the meter directly to the func-generator and set for one volt P-P, the meter read .708 volts which I had to take as an RMS p-p reading.
Again, this is the only amp I've ever seen using transformers that came up with zero specs on. I literally spent 3 hours one day and came up with nothing on these Shaungue transformers. Line six must have had some deal to not disclose certain things, I don't know what else it could be.
Of course, I keep looking for "operator error" in all of this but, I seemed to have followed the correct process for determining secondary tap impedances and came up with very common speaker tap impedance's.

Comments welcome!
Thanks so much for coming around,
Best, PJD3

[nworbetan]

And I did out the math according to a pair of 6L6GC's with a plate voltage around 425 - primary impedance of 4.3K ohms

Yep, probably operator error. Without seeing your math it looks like you got the turns ratio right, but then to find the reflected impedance you need to square the turns ratio and multiply it by the speaker load, like dorrisant explained.

[dorrisant]

Not to be rude, but spending 3 hours searching for specs on an unknown OT is a bit inefficient. There are plenty of sources of good info on how to test for specs. Use those tips and you'd spend less than 3 hours to know more than your 3 hour search. The tests can be done to find what you need with simple tools or items (wall warts) that you don't have to worry about frying.
On the flip side of that, you could order a new or used set of TXs with the specs you are looking for for not too much coin. Compare the time your labor is worth multiplied by the number of hours you're gonna spend researching to the ease of knowing what you have in hand. I totally get that you want to know, or more likely want to know the method to test for these specs. I get it. Test every OT you can lay your hands on till you get a method down. That will teach you.

[pjd3]

OK, lets see if I can organize this without destroying anyone's eyes"

• Secondary input V to tap Vout (brown/blue) Winding ratio impedance ratio plate-plate load/winding ratio squared
  6L6GC
  Orange .5 11.26 22.52 507.1 8.479
  Yellow .250 8.01 32.04 1026.5 4.18
  Green .150 6.81 45.4 2061.16 2.08

So, pretty straight forward it was - divide Vout by Vin, get winding ratio, square each winding ratio gets you the impedance ratio, then divide expected plate-plate impedance by the impedance ratio for that particular tap impedance. Ha. Sure seemed to work for Uncle Doug! And maybe it did for me!

And its OK to be rude to me, I can take it. The reason I spent 3 hours searching for OT info is because I usually find what I'm looking for 97% or the time. Gets me adamant a little. But, thats alright. All thats left to do is wire up the taps to the speaker output jacks and connect the NFB to one of the taps. Then I'll drop it off to the local amp tech who knows Marshall type circuits a hell of alot better than I do, and let him give it a good health check and a bias. It will be nice to own a well functioning 6V6 Plexi plus. Thank you everyone for your comments and guidance. This is a pretty good trip, this amp building stuff.

Best,
PJD3

[pjd3]

Oh darnit, all the spacing went away when the last post was submitted. It looks like an unreadable nightmare now.

Sorry!

{PJD3