Output Transformer Saturation Surpise

[Gregg]

I do not know if using a gap is the most efficient way to make a transformer for a single ended amplifier, but the gap is not essential for a good transformer.

A SE OT must have air gap per definition. Whether the Champ can get away with a non-gapped OT and/or it's tone (complete crap to my ears) is due to this "mistake" is an entirely different subject.

[J M Fahey]

There are some complicated questions regarding the transformer. I do not know if using a gap is the most efficient way to make a transformer for a single ended amplifier,

There is no other way.

but the gap is not essential for a good transformer.

Oh yes it is for any transformer carrying significant DC, as in *any* class A design.
Unless you speak of paraphase, which splits the DC inductor and the AC transformer into 2 separate cores.
And you still need a gap in the DC carrying one, so you havenīt really avoided anything, just split the job.

To see this, consider designing a transformer, same specifications, except specify a minimum operating frequency half of what we need and ignore the dc bias current through its primary.

But you canīt ignore it, no way.

In the design of such a transformer the B field must be kept below some value that would cause the core to saturate. The relationship between the maximum field that is produced and the voltage across the transformer is given by Faraday's law of magnetic induction, which takes the following form here (eq. 22-4b, Radio Frequency Electronics, Jon Hagen):

Bmax = Vmax/( N*omega*(core cross section) ) < (values that causes core saturation)

Now we look at how Bmax changes when the dc bias current is introduced for SE operation. Since the flux no longer changes between positive and negative values, but rather varies between zero and twice that value, Bmax is twice as big as shown above. But we can make it stay at the original value by making the lowest frequency (omega) twice as high, that is, the value we originally wanted. This is one way to make an SE transformer without a gap.

You didnīt understand it, you are only considering the AC component.
The DC component still has ZERO Hertz frequency and is by far the HUGE saturator there.

Not necessarily the best.

Callit "terrible" and weīre talking.

But what results in the cheapest good (or bad) transformer? The gap effectively lowers the permeability of the core, allowing more voltage before the maximum B field is reached, but it also lowers the inductance, which raises the minimum operating frequency.

Thatīs an unavoidable compromise.
You canīt beat Physics. [tm]

I do not think you can answer the question without going through the whole design process, which requires finding the best compromise in a coupled set of inequalities for Bmax, the losses, and the minimum inductance.

And donīt ignore the DC component.

ALL of Champ (and any other SE output transformers, good or bad), were gapped, even if using the minimum type of gapping achieved by stacking all E one way and all I closing the circuit, relying of cutting imperfections to provide a minimum "de facto" gap.

And even so, it costs nothing adding a single sheet of brown paper between them.

Cheapest audio appliance found at homes in those days was an American 5 or similar AC/DC radio and even so OTs were gapped, let alone a Champ which was way more expensive.

Translate those prices to modern ones and Champs were expensive for what they were.

EDIT:

I'm not sure that I'd agree that you want an air gapped SE transformer with a champ.

Not sure about what MM made, because in some model they *might* have followed "Internet Forum lore" but am quite certain no SE Fender OT was ungapped, ever.

It was "common knowledge" way back then, "the way things were made" and he wouldnīt have ignored that.

Even more, he didnīt actually design the transformers himself, what for?, (same with speakers of course), he just ordered his supplier-of-the-day "please quote 500 single 6V6 OTs. primary impedance xxx , secondary yy ohms, vertical mount, 8" leads, primary brown and blue, secondary white and green" and then argue price, conditions and delivery time.
Like any Factory owner/manager would.

[Mike Sulzer]

All the gap does is lower the effective permeability of the core, and so the voltage at the lowest frequency can be larger. (But the inductance is also lower and so the magnetizing current rises, and so the actual useful lowest frequency rises somewhat.)

Maybe the solution that I have suggested is wrong, but it is certainly wrong to say that a gap is essential. Any core can support some level of dc current without saturating. That is, the B field produced by a dc current must be less than the B field that causes saturation, and for the transformer to work, it must be less by enough to allow the sum of the dc and ac components to stay below that limit.

So if you guys want to show my solution is wrong, you have a lot more physics to do.

I can suggest a hint as to the misunderstanding here. It is true that the magnetizing current would have to rise to infinity to support a "zero frequency" transformer. But that is not what is happening here. The dc current is fixed to a finite value by the biasing in the circuit. It does not go to infinity as would be required for some kind of "runaway" saturation.

[nickb]

I did some very similar tests, reported and analysed in Chapter 8 of ‘Guitar Amplifier Overdrive’, Neumann and Irving, 2015.

I don’t think the transformer is actually saturating. At low frequencies the reactance of the primary inductance (also known as the magnetising inductance) drops below the proper load impedance (reflected through the OT from the secondary). (The proper load and the magnetising reactance are in parallel.) This means that the output tube is trying to drive into a very low impedance. What is nominally ‘a 5 watt amplifier’ then becomes (say) ‘a 0.5 watt amplifier’ and it is the tube itself which is causing the distortion, when you try to drive it a bit harder.

Cowardly taking the middle ground, my final conclusion of this is that it's a bit of both.

Based on the AC 50Hz test we can see the core is well up the BH loop and the mu is falling. This means the inductance when running with DC 45mA or so is much lower than the manufacturer specifies. Also, they measure with DC current=0. I think that are calculating the inductance as Vrms/Irms/2/pi/f. Because of the current waveform distortion I suspect the value derived by this method is quite inaccurate and oversetimates the inductance

Thus my "conservative" use of 40H for the working inductance was probably far too high.

Going back to the OP, I note the plate current had dropped to almost zero, I missed this before. That is a pretty strong indication that the swing is large and so the effective impedance has dropped due to the much lower magnetising inductance.

One consistent theme that screams out is... no-one likes this amp.

[Malcolm Irving]

.... It is true that the magnetizing current would have to rise to infinity to support a "zero frequency" transformer. But that is not what is happening here. The dc current is fixed to a finite value by the biasing in the circuit. It does not go to infinity as would be required for some kind of "runaway" saturation.

Also, the primary winding has some resistance, which will limit the magnetising current.

[Mike Sulzer]

Here is a link to some material on air gaps in magnetic circuits from a more general view point than just transformers: Why Have an Air Gap? | Power Electronics

[benito_red]

... Maybe the solution that I have suggested is wrong, but it is certainly wrong to say that a gap is essential. Any core can support some level of dc current without saturating. That is, the B field produced by a dc current must be less than the B field that causes saturation, and for the transformer to work, it must be less by enough to allow the sum of the dc and ac components to stay below that limit. ...

yes your solution is completely wrong. You should explain why worry so much about airgap. The idle current for a SE 6V6 is arount 40 or 50 mA and without air gap to avoid saturation you should make a transformer as big as a 200/300watt PushPull (or even more), and that makes no sense and no added value but much more cost.
Anyway don't beleve to me just ask anybody that builds transformers.

[benito_red]

Here is a link to some material on air gaps in magnetic circuits from a more general view point than just transformers: Why Have an Air Gap? | Power Electronics

the text you have provided refers to ferrite cores for high frequencies, that is not suitable for audio frequencies:
In fact this is what it states:

" Fig. 1 shows two idealized B/H loops for a typical Ferrite core. The steep slope (high permeability) is for a core without an air gap and the more gradual slope for the same core with a small air gap. To start, we will assume the core is to be used for a high frequency transformer or an inductor. (A transformer or inductor, is only polarized with ac, while a choke or fly-back transformer has both dc and ac current components). "

audio transformers are different beasts and do not use ferrite but silicon iron or grain oriented (usually M6 type)

[Mike Sulzer]

the text you have provided refers to ferrite cores for high frequencies, that is not suitable for audio frequencies:
In fact this is what it states:

" Fig. 1 shows two idealized B/H loops for a typical Ferrite core. The steep slope (high permeability) is for a core without an air gap and the more gradual slope for the same core with a small air gap. To start, we will assume the core is to be used for a high frequency transformer or an inductor. (A transformer or inductor, is only polarized with ac, while a choke or fly-back transformer has both dc and ac current components). "

audio transformers are different beasts and do not use ferrite but silicon iron or grain oriented (usually M6 type)

The same principles apply; so look for the common ground. If you read the whole article carefully you will see that he has some material that applies to inductors and SE transformers near the end. Figure one shows a change in slope with/without a gap. Where dos it say it that what applies for a typical ferret core does not apply, with some possible differences, to transformer steel? Both materials saturate, and more generally they both have hysteresis curves that differ in details.

There is a lot of material on SE transformers on the web that is mostly hysterical garbage; it appears to be some sort of cult.

This is not an easy topic, and I doubt that most people without a good engineering or physics background can understand it. But everyone should be able to see what is based on facts, and what is just blustering.

[Mike Sulzer]

yes your solution is completely wrong. You should explain why worry so much about airgap. The idle current for a SE 6V6 is arount 40 or 50 mA and without air gap to avoid saturation you should make a transformer as big as a 200/300watt PushPull (or even more), and that makes no sense and no added value but much more cost.
Anyway don't beleve to me just ask anybody that builds transformers.

OK, this, which I just saw is an example of such nonsense. How have you shown that you would need such a large core? You have not.

People who build transformers are seeking the least expensive solution. You have no idea how that is done.

[bob p]

Possible solutions?
1) a bigger OPT
2) lower the idle current so you get a better LF response at the expense of less 1KHz output power
3) increase the corner freq of the drive to the output tube grid to make it harder to get the nastiness on lows during real world playing
4) removing the feedback should make things drop off in a more progressive way.

Getting back on topic, I think that removing the NFB circuit improves the tone of a Champ, if you're interested in that cranked tweed champ tone that Clampton made famous. Personally I find the more linear transition into distortion and saturation to be more musically useful. I also think that LF around 140 Hz and below isn't part of that tone, and that the 1k and up zone is what matters most. As I mentioned in an earlier post, fussing about HiFi transformer concerns isn't part of this formula. The great sounding champs of the 50s all have inexpensive shitty little transformers. A big iron air gapped transformer isn't the answer here. Instead of worrying about making the amp better, I just enjoy it for what it is. Just my $0.02.

[benito_red]

The same principles apply; so look for the common ground. If you read the whole article carefully you will see that he has some material that applies to inductors and SE transformers near the end. Figure one shows a change in slope with/without a gap. Where dos it say it that what applies for a typical ferret core does not apply, with some possible differences, to transformer steel? Both materials saturate, and more generally they both have hysteresis curves that differ in details.

There is a lot of material on SE transformers on the web that is mostly hysterical garbage; it appears to be some sort of cult.

This is not an easy topic, and I doubt that most people without a good engineering or physics background can understand it. But everyone should be able to see what is based on facts, and what is just blustering.

ok beleve wath you want

[Gregg]

Honestly I can't understand why some of you are trying to reinvent and/or rewrite more than 100 years of transformers theory and practice. Yes, in some applications you can get away without air gap or you can run a PP amp with one tube only, so what?

[nickb]

Getting back on topic, I think that removing the NFB circuit improves the tone of a Champ, if you're interested in that cranked tweed champ tone that Clampton made famous. Personally I find the more linear transition into distortion and saturation to be more musically useful. I also think that LF around 140 Hz and below isn't part of that tone, and that the 1k and up zone is what matters most. As I mentioned in an earlier post, fussing about HiFi transformer concerns isn't part of this formula. The great sounding champs of the 50s all have inexpensive shitty little transformers. A big iron air gapped transformer isn't the answer here. Instead of worrying about making the amp better, I just enjoy it for what it is. Just my $0.02.

...and that's exactly what I did i.e. left it alone. I just wanted to be 110% sure that I hadn't overlooked something. The suggested solutions were just points for discussion, that never got discussed after all as it turned out, since this is the theory/design forum.

I still can't get over the list price for this.

[bob p]

It makes as much sense as the $1500 used EC tremolux. One of those popped up at my local GC last week and it was gone in 2 days.