A question about the primary side of the output transformer.

[christarak]

I am getting myself more wound up trying to understand output transformers. I’m hoping you can peel back a few winds to allow in some understanding.

In a push-pull output stage, the voltage between the B+ centre-tap and each of the plates should be the same, right? I checked Hammond’s specs and it supported this basic understanding with equal resistances. Also if I do a plate current measurement, I do a DC resistance between centre and the 2 plate windings and measure voltage drop. All good.

This morning I am working on a Joyo Tweed Tone 20 (I have been asked by the importer to not post the schematic, but its basically a Blues Jr). The OT DC primaries between centre and plate leads are 135R and 228R. Ha! Faulty transformer, me thinks. In comes the replacement from Joyo with the same winding resistances! What?

I then put 5.4VAC on the secondary of the OT and measured the 2 primaries referenced to the centre tap 80.1VAC and 80.0VAC.

Question 1: How is it possible that I have perfectly matched voltages but totally mismatched DC resistance (Fluke 87V)?
The only thing I can think of that one of those windings must travel twice the distance as the other winding for the same number of turns. Is this possible? Why don’t the Hammond transformer specs support that idea? I checked a Marshall JCM800 and the DC resistance on that was also perfectly matched.

Question 2: Does this possibility invalidate my alternative method of measuring Plate current = VDC across secondary/Rsec when I don’t use my plug-in bias checker?

Question 3: When Americans say “sodder”, is it spelt differently to the English/Australian “Solder”, or did they accidentally change the “L” into a “D” for some reason? I love America. I have done so many shows there in my day, that it feels like home when I visit, but I still find their take on English to be weird. For example, we call the path upon which we place our feet to walk a “foot path”, yet in America I am expecting pedestrians to be doing a crab-like “sidewalk”. I like to play a Fender, not having it covering the front wheel of my car. Odd. Vive la différence.

[The Dude]

1) It depends on how the OT is wound and whether the two halves are wound at the same time and are the same wire length or one winding is on top of the other. Either way, it's impedance that matters and not DCR. It's common for the two halves to measure slightly different DCR. It's nothing to worry about unless values are vastly different.

2) "Plate current = VDC across secondary/Rsec" is still a valid method.

3) Americans still spell it "solder", the "l" is silent. I won't defend the English language. It's quite a messy thing. For instance, why are daughter and laughter pronounced differently?

[Enzo]

Question 1: How is it possible that I have perfectly matched voltages but totally mismatched DC resistance (Fluke 87V)?
The only thing I can think of that one of those windings must travel twice the distance as the other winding for the same number of turns. Is this possible? Why don’t the Hammond transformer specs support that idea? I checked a Marshall JCM800 and the DC resistance on that was also perfectly matched.

You answered your own question. Transformers can be wound two ways. One is to use two equal length piece of wire for the two halves, and wind them at the same time. Result, equal resistance. OR you can wind half the turns with a wire, connect a center tap wire, then continue to wind the rest of it. Those outer windings will be longer and resistance higher for the reason you describe.

How can it work? Transformers work on turns ratios, not resistance. The extra few ohms on the uneven ones can alter the idle current slightly, but when the whole she-bang is say 6000 ohms for the tubes, having a 100 ohm difference in the wire won't matter much.

Question 2: Does this possibility invalidate my alternative method of measuring Plate current = VDC across secondary/Rsec when I don’t use my plug-in bias checker?

No, just be aware of it. You may find that regardless of the tubes, one side reads a few ma higher than the other. Just average the two. If you want 38ma as a target, then set it so the one tube at 36ma and the other at 40ma average your 38ma.

Americans say “sodder”,

It is always and has always been solder. We don't pronounce the L. Just like British people can't seem to pronounce the letter R at the end of a word. Took me forever to figure out what a "gaol" might be. I saw a book once, "How to speak Strine". Don't you pronounce "here" as "ih"?

To us, or at least to me, a "footpath is a dirt walkway. Like through the forest. Or on a school campus a trail across the lawn where constant foot traffic has worn the grass away. A sidewalk is a paved walkway on the SIDES of the road. We extend that sense to any paved walkway.

besides, I have no idea why your cars don't fall up off the road as it is.

[Enzo]

why are daughter and laughter pronounced differently?

because my daughter isn't funny.

[The Dude]

Oh, the Northeastern folks don't pronounce 'R's either- "Pak the Ca". "Dak roast" coffee is another one. I get a kick out of playing the East Coast whenever we go. It's a trip just listening to people talk.

[christarak]

1) It depends on how the OT is wound and whether the two halves are wound at the same time and are the same wire length or one winding is on top of the other. Either way, it's impedance that matters and not DCR. It's common for the two halves to measure slightly different DCR. It's nothing to worry about unless values are vastly different.

2) "Plate current = VDC across secondary/Rsec" is still a valid method.

3) Americans still spell it "solder", the "l" is silent. I won't defend the English language. It's quite a messy thing. For instance, why are daughter and laughter pronounced differently?

Thanks Dude. Its because the DC resistance were so vastly different, (almost 100%) at 135R and 228R, that I sought your advice.

A weird difference to me is how we pronounce "Les" in "Les Paul". I have to defend the American pronunciation to my mates vs our "LeZ Paul" explaining that its short for Lester. Its just odd to our ear. I get it, but I still can't say LeSS Paul.
Thanks mate.
Chris

[The Dude]

I guess "vastly" is a relative term and leaves some room for interpretation, so apologies for not being more descriptive/specific. FWIW: Those readings make perfect sense to me for a transformer that's wound one winding over top of the other. If one winding were say 10 times the DCR of the other, I might suspect a problem. What really matters is how the thing works in circuit.

[christarak]

Thanks mate

[Helmholtz]

The OT DC primaries between centre and plate leads are 135R and 228R.

That's around 70% difference and is actually more than I've seen up to now. As others explained some DCR mismatch is not unusual especially in smaller and cheaper OTs.
But as primary voltages are equal, turns numbers must be equal. Means that it's ok.
With correct secondary load the reflected primary impedance seen by each EL84 should be around 2k, the DCR adds to this.

Does this possibility invalidate my alternative method of measuring Plate current = VDC across secondary/Rsec

For this method the correct DCR for each side/tube must be used. As DMMs sometimes read wrong DCR numbers when measuring an OT primary, I would repeat the primary DCR measurement with shorted secondary to exclude meter errors caused by high inductance.

[Dave H]

Question 3:
Accents are funny things. I'm only 25 miles from Liverpool but can't understand a word they say.
I noticed Less Paul and soddering iron when I was in the US. What about laboratory? It sounds like lab-rat-tory in the US and interesting sounds like inner-resting.
I noticed when watching The Brokenwood Mysteries that in New Zealand they only use one vowel sound, the 'i' so if you want to fake a New Zealand accent all you have to do is sound all vowels as 'i'. It gives a whole new meaning to the phrase "All hands on deck"

[Chuck H]

Reminds me of a skit by a comedian named Izzy Eddard. Talking about differences in British english and American english:

"For example, you say (phonetic) 'erb'. And we pronounce it 'herb'... Because it's got a f#@&!ng H in it.

[Chuck H]

Word list of some silent L's that Brit's AND American's don't enunciate:

half
would
calf
salmon
could
would
should

Silent L's I'm not sure whether Brit's pronounce or not. Sometimes I think they 'sort of' do (considering different dialects):

walk
chalk
folk
palm

Note: Some Americans actually do pronounce the L in folk and palm. But never solder

[christarak]

If you want to fake a New Zealand accent all you have to do is sound all vowels as 'i'. It gives a whole new meaning to the phrase "All hands on deck"

Oh yes. From Sydney, I am closer to NZ (3hr flight) than I am to Perth (5hr flight) so both countries enjoy taking the piss out of each other. But we are ANZACs and stand by each other like brothers.

[christarak]

Reminds me of a skit by a comedian named Izzy Eddard. Talking about differences in British english and American english:

"For example, you say (phonetic) 'erb'. And we pronounce it 'herb'... Because it's got a f#@&!ng H in it.

Izzy is so funny. I suppose you've seen this classic "Death Star Canteen" skit. https://www.youtube.com/watch?v=Sv5iEK-IEzw

[Leo_Gnardo]

Word list of some silent L's that Brit's AND American's don't enunciate:

How about a silent "t" in 'Murica, and straightforward "say it like it's writ" in the UK: filet, sometimes spelled fillet. Feel-ay in the USA, fill it in the Queen's English. "I'll have the fill it of 'alibut please. Wif chips! And mushy peas." One of our tour personnel learned to be careful at the take-out when saying "please" - he got an order of peas he didn't expect. Maybe he looked like he needed his veggies...