OT Windings

[trem]

I see.....Thank You

[loudthud]

JM,

What wire sizes are available in metric sized wire? American wire gauges (AWG) are in sort of a log sequence. If you go up three sizes (subtract 3 from the AWG#) you double the current capacity. Odd numbered sizes are not widely available. Probably only special order in large quantites.

AWG......Diameter (.001 inch)(not including insulation)
20, 32.0
19, 35.9
18, 40.3
17, 45.3
16, 50.8
15, 57.1
14, 64.1
13, 72.0
12, 80.8

Note that when you double the diameter, current capacity goes up 4 times. Current capacity is based on temperature rise.

[trobbins]

I come across old PA output transformers a lot. They don't 'normally' present standard speaker level impedances for connecting to a 4, 8, 16 type speaker - and commonly have quite a few impedance tappings starting from say 24-40 ohm and going up to 300-1200 ohm depending on the power rating of the amp. However after some assessment, many of those output transformers do have winding segments that do align with 4, 8 or 16 ohm levels.

Often a suitable segment is only a small percentage of the total secondary winding turns available (eg. 5-20%), and often the secondary uses just one wire gauge, and often the primary winding is not even simply interleaved around the secondary.

In that situation, there could be some benefits in trying to use a 16 ohm speaker, compared to a lower impedance speaker on a smaller percentage of turns, if suitable winding segments were available.

One benefit could be a better coupling (lower leakage inductance of a secondary winding that incorporates more turns).

One benefit could be a slightly better thermal capability, as winding loss is spread over more turns, and thermal resistance to ambient or core would be marginally lower.

[PeanutNore]

So, in theory wouldn't the inductance of the OT secondary in combination with whatever parasitic capacitance is found in the speaker wiring and speaker form a low pass LC filter? So, the different inductances of different secondary windings would mean different cutoff frequencies for this LC filter? I can't think of any other way that (assuming the speaker load is matched to the secondary) the choice of secondary could affect the sound. Anyway, the capacitance involved is going to be so tiny that the cutoff frequency is probably way above the frequency response of any guitar speaker and probably out of the audio range.

[R.G.]

So, in theory wouldn't the inductance of the OT secondary in combination with whatever parasitic capacitance is found in the speaker wiring and speaker form a low pass LC filter?

The leakage inductance of the secondary, the magnetic flux from secondary currents that do not couple to the core or primary windings, would be a factor. The normal inductance of the core as seen from the secondary has been accounted for by its reflection into the primary and the drive side.

So, the different inductances of different secondary windings would mean different cutoff frequencies for this LC filter? I can't think of any other way that (assuming the speaker load is matched to the secondary) the choice of secondary could affect the sound. Anyway, the capacitance involved is going to be so tiny that the cutoff frequency is probably way above the frequency response of any guitar speaker and probably out of the audio range.

Transformers with multiple interleaved and cross-connected windings get very complex to analyze and more complex to predict the response of.

And while the capacitances may be small, they are not necessarily tiny compared to the leakage inductance impedances, and may, even if tiny, let enough current through to/from the high voltage primary swings to affect frequency response and phase response.

Good audio transformers are complicated, largely because of the 1000 to 1 range of audio frequencies to be handled.

[J M Fahey]

I come across old PA output transformers a lot. They don't 'normally' present standard speaker level impedances for connecting to a 4, 8, 16 type speaker - and commonly have quite a few impedance tappings starting from say 24-40 ohm and going up to 300-1200 ohm depending on the power rating of the amp. However after some assessment, many of those output transformers do have winding segments that do align with 4, 8 or 16 ohm levels.

Often a suitable segment is only a small percentage of the total secondary winding turns available (eg. 5-20%), and often the secondary uses just one wire gauge, and often the primary winding is not even simply interleaved around the secondary.

In that situation, there could be some benefits in trying to use a 16 ohm speaker, compared to a lower impedance speaker on a smaller percentage of turns, if suitable winding segments were available.

One benefit could be a better coupling (lower leakage inductance of a secondary winding that incorporates more turns).

One benefit could be a slightly better thermal capability, as winding loss is spread over more turns, and thermal resistance to ambient or core would be marginally lower.

PA transformers are VERY complex because they must match many different situations, from lots of very short distance speakers wired in parallel to 100 yards away (or more ) ones using high voltage and individual step down transformers.

So they often have multiple windings which are meant to be set in series/parallel as needed.

We discussed it in: http://music-electronics-forum.com/t38291/ , specially read post#10 with some practical examples, but here's a Geloso output wiring table, including series/parallel windings:



16 impedance settings, from 1.25 ohms to 500 !!!!
6 of them balanced !!!!

Maybe a very cheap PA used same wire for 4/8/16 , why not? ... but definitely not practical if same OT fed speakers and high voltage lines.

Similar winding technique applies in very high quality OPT , in this case not only for impedance flexibility but "because they can" , since they already split secondaries a lot, for interleaving:

for-sale-2-output-transformers

there they spec a transformer with 12 same turns, same wire secondaries ... obviously to get many different impedance options.
In that case they nominally have "only" 4/8/16 ohms secondaries ... but have primary impedances to suit "any" power tube on Earth:

Secondary layers x turns per layer x wire size, Cu dia, mm 12 x 63 x 1.0

Number of parallel Secondary sections x Secondary turns per section 12 x 63

If you are interested in high quality transformer design , read all of Mr Turner's site .... amazing.

[trobbins]

Yes I have some PA OT's that separate all their secondary winding sections, and provide a patch terminal panel for the user to configure. They are usually a dream for people wanting to restore the amp for guitar use, and the larger power types will likely allow all secondary sections to be placed in parallel and provide a standard speaker impedance match.

I also have some PA OT's that do not separate the many secondary winding sections. An old 12W AWA PA I have on the bench at the moment has six winding sections, with taps starting at 50 ohm for first winding tap, and increasing to 600 ohm at the furthest tap (end of winding). Luckily, the winding sections are varied to allow a wide variety of impedance matchings to be made between different taps, and there is a winding that is equivalent to about 12 ohm within that string, but of course it only allows a low % of secondary turns to be put to use, and the other windings remain 'dark', although I can use a sequential winding for feedback if wanted.

Some PA OTs have allowed access to the wiring going to the taps, and that can allow the winding sections to be separated and reconfigured - although some care is needed (or risk taken) as there may only be nominal wire varnish insulation between wires at each end of the 'output'.