OT, the bigger the better? And which one to choose?

[Chuck H]

When comparing the amp's characteristics when suitably loading the 4 ohm output (compared to the 16 ohm output), might poorer coupling increase the magnitude of the back emf spikes resulting from heavy overdrive?

Makes sense to me.?. If the load suffers a very high impedance at HF where can the power go? I've used small cap value shunt filters across the OT primary to squelch spikes with very little tonal change.

[Enzo]

Just catching up, but something a way back caught my eye.

if you have two same number of turns secondaries and you don't want to rewire them (in parallel) then you can still use one of the halves to drive a 4 Ohm speaker however you won't be able to get full power out of the OT because it won't be able to handle the current. You can use it at low volume levels though

.

Unless I missed that we were discussing a particular transformer, this does not hold. The fact that there is an unused parallel winding has no impact on the in-use winding. It would handle the current if the wire were of proper size. We cannot assume the individual windings have just barely enough current capacity to drive one speaker but not two. It might be the case, but is not a given.

[Gregg]

What I meant was if you have an OT with only one 16 Ohm winding (tap) consisting of two halves in series with appropriate wire thickness for 16 Ohm operation it would be usually thinner than the wire that would have been used for only 4 Ohm operation. In that case you either 1/ could rewire the two halves in parallel (assuming they are externally to the coil/bobbin connected) or 2/ run a wire from the connection between those two halves and use only one of them for the 4 Ohm tap. In case of 2/ due to the thinner 16 Ohm wire size that winding will have less current capability compared to if the two halves were in parallel.
As a result you can still achieve full power but not with the same low frequency response you would have had with the two halves in parallel or if thicker 4 Ohm wire was used OR you could achieve the same low frequency response at lower power.

[Mike Sulzer]

What I meant was if you have an OT with only one 16 Ohm winding (tap) consisting of two halves in series with appropriate wire thickness for 16 Ohm operation it would be usually thinner than the wire that would have been used for only 4 Ohm operation. In that case you either 1/ could rewire the two halves in parallel (assuming they are externally to the coil/bobbin connected) or 2/ run a wire from the connection between those two halves and use only one of them for the 4 Ohm tap. In case of 2/ due to the thinner 16 Ohm wire size that winding will have less current capability compared to if the two halves were in parallel.
As a result you can still achieve full power but not with the same low frequency response you would have had with the two halves in parallel or if thicker 4 Ohm wire was used OR you could achieve the same low frequency response at lower power.

Yes, I see what you are saying. This is like the primary of a power transformer that can be wired wither for 120 or 240. You have two primary windings: series for 240, parallel for 120. That way you make optimum use of the copper for both cases and get the appropriate primary inductance also.

The problem with an OT secondary is that you want 8 ohms as well as 4 and 16, and providing all three with optimum copper use and a simple selection scheme is not going to happen. So it might make sense when using a single winding with taps to use heavier wire from 0 to 4, medium wire from 4 to 8 and smaller wire wire from 8 to 16. But this might make winding for low leakage inductance harder, and so I think in practice you just accept somewhat higher copper loss when using 4 ohms.

[Gregg]

In theory from calculations the secondaries' wires should have different thickness. In practice if you have 3 taps for 4, 8 and 16 Ohms and the corresponding sections are in series and if the coil space would allow it they usually would use the same size wire for all of them and it's the thickest one that can handle the 4 Ohm current. So it depends on the particular transformer we're looking at.

[Mike Sulzer]

In theory from calculations the secondaries' wires should have different thickness. In practice if you have 3 taps for 4, 8 and 16 Ohms and the corresponding sections are in series and if the coil space would allow it they usually would use the same size wire for all of them and it's the thickest one that can handle the 4 Ohm current. So it depends on the particular transformer we're looking at.

Yes "handle the current" has a least these two specific attributes: 1. Keep the transformer from getting too hot. 2. Keep the fractional power loss in the transformer versus the load to an acceptable level.

[R.G.]

In practice, #1 is the primary concern for all but very small transformers. For transformers above a few watts, you generally get #2 handled by default if #1 is satisfied.

In designing a transformer, a prudent designer tries for equal current density in all the wires under all conditions. That semi-automatically means that there are no hot spots caused by current distributions. However, as noted, it's not possible to get equal current densities in all windings in a multi-tapped transformer without making sure that all wires are used in all configurations by rewiring sections in series/parallel. For an interesting look at how this might be done, look at the article on the Williamson amplifier with the transformer design/winding section in it. Many secondary sections are interleaved with primary sections, and these .. um? 12? ... sections are then hooked up in various combinations for different speaker loads.

It's not the kind of thing you'd do with a tap changing switch or plug though.

Frankly, the issue of how to get current distribution and the smallest lamination and least copper are only trivially related to modern amps. Transformers are relatively much cheaper today, and it's simpler to use a bigger lamination and more copper (in a design setting) than to design to within a gnat's eyelash of perfection.

[tedmich]

design to within a gnat's eyelash of perfection.

such designs are not cheap!

http://www.lundahl.se/wp-content/upl...0_3_7_9202.pdf

[SoulFetish]

It's not the kind of thing you'd do with a tap changing switch or plug though.
.

It would certainly be a challenge to configure. But selector switches are just so convenient, wouldn't it be too tempting not to try? Schurter makes some nice panel selector switches for mains and impedance. I wonder if the distribution RG was talking about was what they had in mind for the mains voltage switch here:
69e069a33e7db3bbec4b2c39b7b1064c.pdf

[trobbins]

Shocki, the thread certainly illustrates how many influences there are when subjectively comparing even the same amp/combo, both from within the design of the OT and from external influences.

Having the same amp or combo, but a different speaker or speaker enclosure can make a very subjective influence. The low frequency roll off of the amps output (as determined by the OT's primary inductance) adds to that influence, but could also depend on the inter-stage coupling network corner frequencies if they are high enough. The amount of low frequency feedback that occurs in the bass end effects the damping and subjective performance of a speaker in a given enclosure, and that feedback level is influenced by the low frequency roll-off imparted by the OT. The DC imbalance in the OT can noticeably increase the output stage RL roll-off corner frequency, and the DC imbalance is not just static bias current imbalance, but is additionally influenced by tube dynamic mismatch.

The configuration of OT winding sections certainly influences many of the leakage inductance levels that impact on higher frequency performance. The simpler to comprehend leakage inductance affects high frequency roll-off, which may only subtly be heard given the natural roll-off of the speaker response - but subtle sibilance can be an influence. The harder to comprehend leakage inductance effect is when the two half-windings change from both conducting current (class A) to when only one half-winding conducts (the other valve is in cutoff). When the signal passes through that class A to B transition, a different leakage inductance effect causes distortion. Those different leakage inductance levels are influenced by the level of simple layer interleaving, and just using the '4 ohm' section of winding, and not the full set of winding sections that make up the '16 ohm', would have some form of influence.