bepone wrote: ↑Fri Aug 05, 2022 6:27 pm
R.G. wrote: ↑Thu Aug 04, 2022 9:17 pm
- what is the efficiency of tranformer (losses in primary+secondary) also meaning speaker volume (!), on 4 to 4 compared to 16 to 16 ohms?
If both transformers are well designed, no change in efficiency at all. Speaker volume depends on the speaker setup (individual speaker watts-to-SPL efficiency) and any series/parallel hookups. If the speaker setup is properly matched to the output impedance setting, almost no change for the speakers.
we dont have 2 tranformers, question is practically what is going on one standard single transformer, when using 4 ohms tap, and the whole secondary on 16 ohms?
The short answer is that the effects are too small to matter for the specialized case of guitar amplifier use.
A longer answer is as follows:
- For any given guitar amplifier OT, you have no idea what the windings are like inside it - how many turns of what gauge wire, and how hard that drives the magnetic field in the core.
- Not knowing wire gauges and turns per winding section and wire length means you can only measure the end-to-end resistance of the winding sections. It becomes very difficult to tell from external resistance whether the 4 ohm tap is the center tap of the 16 ohm winding or is a separate section connected only to the 16 ohm winding at the common terminal. Further, if you could tell that 4 ohms is a tap in the middle of the 16 ohm winding, you can only guess whether the wire gauge for the 4 ohm section of the 16 ohm winding is thicker as befits more current in the 4 ohm case. Ideally, it would be. But it would take some precision instrumentation and testing to make a good guess without tearing down the trannie.
- Not knowing the interleaving setup - or lack of interleaving, which is vastly more common in guitar amps - means it's hard to make any sense of the leakage inductances; about all you can do is measure them and/or the frequency response and make guesses. This is not impossible, but it takes some instrumentation.
- The power lost in the primary and in the magnetizing current is constant for a specific power level, as the primary is where everything involved with heating the primary wire and the iron happens. Any secondary at all that puts out the same power through the transformer allows the same primary and core losses.
- The secondary windings will be where all of the differences in secondary losses happen. There will be small differences in the losses, depending on how closely the wire gauges and window fill keep the winding-resistance-cause-heating. A properly-designed transformer would keep the total secondary losses down to (...crudely guessing based on experience) about a quarter of the total transformer losses. Again, guitar amp OTs are intended to be as cheaply made as possible while still keeping the customer base happy-ish. And remember that the PT and OT are the second and third most expensive parts in a guitar amp. The most expensive part is the cabinet and appearance items, btw. The OTs are simply not designed with best electrical/signal performance in mind.
- Guitar amplifier makers do not make their own transformers. They buy them from OEM transformer suppliers. The suppliers just have to make an OT meet the power handling and bandwidth specifications, so the amp makers may not even know what's inside the trannie, especially when they change suppliers over time.
About now you're getting frustrated again. You want me to simply say "4 ohms tap is better/more efficient/etc." or "16 ohms tap is better/more efficient/etc. because ...", and I'm telling you that I can't do that because the question is stated too simply. The complexities underlying transformer efficiencies and winding mean that without knowing the insides of the transformer, it is not possible to give an general answer to the question.
or if you have 2 speakers in the box, each 8 ohms, and you can connect them in 4 or 16 ohms total resistance. what is going on on taps 4 ohms and what is going on on 16 ohms? what is efficiency=?
Well, first of all, speaker impedance is not resistance. The DC resistance of an average 8 ohms speaker is often near 6 ohms. The rest of the "impedance" is from moving the speaker suspension and the air load, plus resonance effects of the cone mass and suspension stiffness. The graph of speaker impedance versus frequency is not a flat line, and it usually only crosses the nominal "impedance" a few times in the graph. This also means that you have to use identical speakers to do the tests, of course. Here's a graph to illustrate:
But yes, two "8 ohm" speakers that are identical and are each being fed the same power (volts*amps) will have twice the current and half the volts when paralleled as when hooked in series. So if the 4 ohm tap to common has half the resistance of the 16 ohm tap to common, the efficiencies in both hook ups will be the same, as the heating resistance losses will be the same in both.
Your question is the difference in efficiency. As I said earlier, the differences in transformer insides make this nearly impossible to answer generally. But let's look at a couple of cases. From the Hammond web site, the 1750U Marshall replacement, 100W.
Primary, 15.36R-0-15.36R
Secondary, 0, 0.10R(4 ohms), 0.10R(8ohms), 0.12R (16 ohms)
100W in 4 ohms is 20V, 5A; in 16 ohms, 40V, 2.5A. That means the peak current at 4 ohms is 5*1.414 = 7.2A, at 16 ohms 2.5 * 1.414 = 3.6A.
The loss in the 4 ohms section is 5*5*0.1 = 2.5W; in the 16 ohms, 2.5*2.5*(0.1+0.1+0.12) =6.25*0.32) = 2W.
At 100W, the losses in the primary; Ipk = 7.2/10.4 = 0.692. This happens alternately on each half of the primary, so we get the same power wasted by calculating a half-sine current in each half and adding the two. The RMS value of a half-sine of peak value 1 is 1/2. So the power in the primary (neglecting magnetizing current and core loss) is
(Ipk/2)*(ipk/s)*15.36*2 = Ipk/2 * 15.36 = 0.346*15.36 = 5.31W.
Then, putting 100W into 4 ohms, the losses in the windings are 5.31+2.5 =7.81W. Into 16 ohms, it's 5.31+2 = 7.31. The difference is 0.5W, out of 107 to 108W going into the primary. More, really, as sometime you have to consider magnetizing current and core losses. In this one case, the full 16 winding is 0.5W out of over 100w more efficient.
My experience is that (a) it's remarkably good that Hammond published this; good luck with other manufacturers getting the turns and resistance data (b) The difference in efficiency and even raw loss of power is very, very small, and that (c) even a tiny difference in the winding resistances and winding could easily make that come out different.
and then how much is parasite inductance Lp on 4 ohms tap, and how much is on 16 ohms? so simple question on one transformer. are the parameters the same?
Simple answer: No.
More accurate answer: it varies hugely depending on exactly how many sub-sections the 4 ohm and 16 ohm windings are sub-divided into, and where those are placed inside the core relative to one another. Leakage inductance goes down as the square of the number of interleaved sub-sections in general. The worst case for OTs is to wind a primary, then all the secondaries, or vice versa. Splitting the primary into two independent halves and sandwiching the secondaries between the two half-primaries cuts this but only in terms of leakage from half-primary to total secondary. There is leakage between the sections of the secondary. You could, for instance, make the secondaries from two lengths of wire wound side by side (i.e. bifilar winding) and get vanishing small secondary to secondary leakage, which is different from secondary to half-primary. But no non-hifi transformer will have that, as it costs more. Most guitar OTs have very few sections because it's cheaper. So no, it's not a simple question. Well, OK, it's a simple question that doesn't have a simple answer.
Which is the point.
easy to check all the answers and find conclusion.
Hmmm.
OK. I think YOU should check all the answers and find a >>single<< conclusion that I can't come up with a counter-example for.
"It's not what we don't know that gets us in trouble. It's what we know for sure that just ain't so"
Mark Twain
and you should know that there are a standard patterns to follow, interleaving Primary - Secondary in guitar transformers, so Fender does simplified (Cp smaller), Marshall does in another way (higher Cp) .. This measurement in picofarads + by looking in the tranformer winding from top you know what is the arrangement Primary-secondary. 
simply it is not the same. 
