Yes, size does matter. Or not? (Transformer Question)

[dmeier]

Hi all,

I have built Hiwatt DR103 clone that worked fine at first for a few hours but then suddenly quit after a new startup. It did not die completely but had a very fuzzy sound and was lacking power. After hours of troubleshooting, I finally came to the conclusion it was the transformer and with the new one (from a different supplier) the amp is working and sounding again. But I still wonder, why the first one has gone.

The first supplier said the transformer was "Partridge Specification (same winding + spacing)". However, removing the covers and comparing his transformer to a real DR103-Partridge makes his quote sound like a joke! See the attached picture. This doesn't look like a 100Watter to me. At least, it would give me a hint, why the transformer failed: I run my Hiwatts always past 3 o'clock on the Master and this one probably couldn't take it.

Now, here's my question: What makes an OT take a certain number of Watts?

David.

[MWJB]

Watts audio is how much clean signal the OT will pass before distortion. This does not necessarilly relate to longevity.

Heat kills transformers, so the dc mA rating of the primary winding is the relevant aspect here, the hotter you idle the plate current & harder you drive the amp, the more heat you make.

[Chuck H]

That sure looks like a 50 watter to me. And I suppose it may be the correct spec for a 50 watt Partridge transformer. If it IS a 50 watt OT this would have caused a failure on two counts. One is that the smaller transformer couldn't dissapate the watts of a 100 watt amp and overheated (as mentioned) and the other is that there would have been an impedance mismatch for the speaker load causing an internal arc.

[Prairie Dawg]

And the way those transformers are set up and they're proximity they may be talking to each other in ways they shouldn't.

[R.G.]

The first supplier said the transformer was "Partridge Specification (same winding + spacing)". However, removing the covers and comparing his transformer to a real DR103-Partridge makes his quote sound like a joke! See the attached picture. This doesn't look like a 100Watter to me. At least, it would give me a hint, why the transformer failed: I run my Hiwatts always past 3 o'clock on the Master and this one probably couldn't take it.

Well, it looks like the same sized core stack, +/-, and the windings look about the same physical size in the attached photo. True, the Partridge mounting brackets make it look bigger, but the parts that count are at least arguably the same size; which leads to:

What makes an OT take a certain number of Watts?

The quick but not very informative answer is that it depends on the lowest frequency the transformer has to carry, the wire insulation materials, and the core materials.

The bottom line on how much power a transformer carries is always internal heating. The wires get hot from internal resistance losses, and the core material gets hot from eddy current losses. Ordinary iron core transformers can be made with special wire insulation material so they run and transfer power happily at temperatures which would much more than boil water. The core actually stays magnetic and therefore functional up to over 1000C - it will glow a dull red and still work as a core. The thing that fails is the wire insulation. Note that this is only for iron core transformers; ferrite cores become nonmagnetic at a much lower temperature.

The subtlety here is that the iron can saturate if the primary volt-time integral is too large, and that lets high currents flow in the primary, overheating it from wire heating. I know, your eyes just glazed when I said "volt-time integral". What that really means is that for a given core size, the frequency-voltage product on the primary is fixed. Higher voltage, the frequency has to go lower, and vice versa.

That means that if you have a transformer rated for 100W down to 40Hz (let's say it's a bass output) then it could successfully transform 200W at 80Hz, and 400W at 160Hz at least as far as the core is concerned. This is the fundamental reason we can have switching power supplies that produce hundreds of watts in a package the size of a pencil box - they run the transformers at upwards of a megahertz. The exact composition of the core materials also affects how much magnetic field it tales to saturate it. Power out is a function of how low you let the frequency go.

Notice that this is just the core's limitation on saturation. The wires still heat more at higher powers through the transformer, and the eddy currents in the core that produce heat will still heat more as frequency goes up, causing the wire insulation heating limit to come into play.

So - it's complicated. About the best you can do for this kind of comparison is to physically measure the core stack and eyeball how full the winding window it. If those are close, the transformers are roughly equal in power rating. That looks like the case from the photo, although yes, at first glance the sizes do look different. Measure the core stack. The vendor could have been telling the truth about same size and winding.

[dmeier]

Thanks guys for your feedback!

First of all, it really is (or should be) a 100W OT - I had purchased a 50W OT at the same time and that one is only half as big. As you can see from the picture, I assembled the amp similar to the original Hiwatts.

I am not 100% sure but I think when the amp failed, I had it noticed if the OT would have been very hot.

What I can tell from the pic with the transformers, is the fact that the windings are quite smaller or thinner, about 5-8mm.

It seems to me an odd issue, anyway: The general reasons for a failure did not occur (at least I wouldn't be aware of). But changing the transformer put the amp back to life.

[MWJB]

"I am not 100% sure but I think when the amp failed, I had it noticed if the OT would have been very hot." It may not be obvious, especially compared to the heat thrown out by other components like tubes & even the power transformer.

What kind of idle plate currents were/are you running on the power tubes?

[hasserl]

Well, it looks like the same sized core stack, +/-, and the windings look about the same physical size in the attached photo. True, the Partridge mounting brackets make it look bigger, but the parts that count are at least arguably the same size; which leads to:

I measured the width and thickness of the core and windings on my computer screen after expanding the pic to full size and the core of the Partridge is actually about 1/4" wider in the picture, and the widing on the Partridge is about 1/2" thicker, that's a lot of wire. I assume the picture, even at full size, is not accurate, with the actual dimensions larger than the picture, which would make the differences even larger than they appear to be in the picture.

What is the weight difference?

[dmeier]

Well, perspectives distort the effective dimensions. I hope this picture shows the difference better.

The core is exactly the same size/dimension. The difference lies in the windings. There is more than 10/12mm "missing windings" on left side of the windings and probably 6/7mm on the right.

The difference in weight - hm, can't really tell. The new transformer was not heavier or lighter when weighing the two each in one hand.

[dmeier]

First, I had not installed an adjustable bias and then the tubes were running on their limit (~50mA, 450V). With the voltage doubler in the bias circuit installed, I go down to ~42mA at 450V on the plates.

[guitarmike2107]

Insulation thickness may account the for different sized windings.. you have to measure the wires if you can ..

I may just have been a faulty trannie.. it happens

[MWJB]

52mA @ 450v (23W idle) is rather over a EL34s limit, I'd even be hesitant to run them over 40mA (18W idle).

There is a tangible difference in the heat generated by tubes idling at 32mA as compared to 42mA. I'm not saying that you need to run as low as 32mA, but you certainly shouldn't need to be 40mA+ to get a good tone.

[R.G.]

The core is exactly the same size/dimension. The difference lies in the windings. There is more than 10/12mm "missing windings" on left side of the windings and probably 6/7mm on the right.

The problem with measuring the overall wire protrusion is that, as guitarmike says, the internal layer insulation may make one winding bigger by being "fluffier" with thicker insulation, and perhaps even less copper filling the wire window and thicker layers of insulation. That brings us to:

The difference in weight - hm, can't really tell. The new transformer was not heavier or lighter when weighing the two each in one hand.

Iron and copper are both much heavier than insulation. Iron is pretty much the same between the two, so any difference in weight between them is almost entirely copper, insulation being almost zero density compared to the metals. If the core is the same size, and the weight is pretty close to the same, there is about the same copper inside, and so it's probably pretty close to the same power rating.

Insulation is good, insulation is what keeps the transformer working, but the power rating depends primarily on how much iron and copper you can use, and how hot the insulation can get before it quits insulating.