[Steve]

Nice post Jake. Just a little more intimate detail to add.

Jarcher,

Steve can fill in with greater details but the primary concern is that the current spike will cause the insulation breakdown in a transformer.  The insulation is not like the stuff on your speaker cables or the Romex in your walls, it is a thin varnish type that allows the windings to be close together so that a magnetic field can occur.  Once the varnish melts somewhere in the long wires that are wound around the core it ruins this capability.  The transformer can not function as desired and must be replaced or repaired.

Jake, you bring up an interesting subject, wire separation. If I may elaborate. A variety of problems exist. As you indicate, if one were to take 100 turns and spread it out over two feet, 1" dia., little magnetic field would be created. As you also insinuate, if that 100 turns is within 1", same dia., then we have much greater field produced. For those who may be new, let's go on.

If we had copper wire turns perfectly together, no insulation at all, and no air gap, we would have maximum field, with zero (or near zero) leakage inductance. (We will see below that maximum inductance is important to low frequency response, and leakage inductance lowers the high frequency response.) Sounds great, but then we would have turns touching each other, a short, which of course won't work.

That 1 mm thin insulation, either varnish, enamel etc, which separates the wire turns, preventing shorts between turns, however, allows leakage inductance to occur. In fact, that 1 mm insulation is a major contributor to high frequency loss in the OPT. If one could use a super insulator, say 0,1mm thick, we could dramatically increase the high frequency response.

With that said, I believe, but don't quote me, that generally 2500 volts is the voltage used for testing OPTs these days. (Capacitors use 1.5 to 2 times their working voltage for testing as well.) This does not mean 2500 volts is the maximum working voltage, just test voltage near insulation breakdown. Maximum working peak audio signal voltage should probably be held to around 1500 volts. 1300 volts is a safer maximum though.

This means with 750 idle plate volts on the output tube(s), we could swing about 1300 peak audio signal voltage or so (before clipping). This is based upon the minimum plate voltage swing of 200 volts or so.

Capacitance between pri to pri and pri to secondary windings is also a major contributor to high frequency loss. That is why sectionalizing the winding is important as it lowers the capacitance across the winding.

A quote from the RCA Radiotron Designers Handbook, 1960, page 207.

Frequency response
At low frequencies the response falls off due to the finite value of primary inductance. At high frequencies, the winding capacitance and leakage inductance are responsible for the response limitations.

When I work on an amp on the bench I always connect a big 10W 8 ohm sand block resistor across the speaker terminal so the OPT has a load at all times.  Cheap insurance.

Nice advice and example for those new to tubes. Even though I have not used tetrode/pentode operation since I was a kid, always triode or ultralinear operation, as you recommend, a load on the output is always good insurance.

To expand a little more, Tetrode/Pentode operation is especially sensitive to voltage spikes as it presents a high resistance load, not low resistance load. I will give a few examples.

300b triode = 600 ohms plate resistance.
KT-88/6550 triode operation = 600 ohms (I believe I calculated such.)
6550/KT88 beam power operation = 20, 000 ohms (not much load)
KT66 beam power operation = 22,500 ohms
KT66 triode operation = 1,700 ohms

One other important point. What about power transformers? The same problem exists, especially on turn off. Without any load, such as testing old PTs on the bench, power turnoff could result in arcing and permanent destruction. However, within an amp/preamp circuit, one winding is almost always loaded upon turn off. It could be the input filter capacitor with SS or IDT rectifiers, either high voltage or low voltage. Turn on, however, only the filament winding would be loaded.

With choke input high voltage supplies, the filament winding (or dc rectified filament circuit) is the only load. I do not trust choke input filtering systems as a load since the choke isolates the input cap, which is what loads the power transformer when a spike occurs.   

Cheers

[FullRangeMan]

Seems to me with a gaped core these values are different, how much I unknow.
OPT are a compromisse between capacitance/inductance.

A long time ago Partridge, one of the best audio transformer designers of all time, say there a limit to the amount of inductance you can throw at a load.

[Steve]

Seems to me with a gaped core these values are different, how much I unknow.
OPT are a compromisse between capacitance/inductance.

A long time ago Partridge, one of the best audio transformer designers of all time, say there a limit to the amount of inductance you can throw at a load.

First part, would you clarify your point "these values are different", as it is confusing what you mean.   

Second part, Partridge is correct. I already explained in my above post in more detail. .

Cheers

[FullRangeMan]

Air gap cores are somewhat more resistant to magnetization than EI.
Your post are correct.

[Steve]

Air gap cores are somewhat more resistant to magnetization than EI.
Your post are correct.

Yes FMR, you are correct. Gap tends to limit changes in inductance, and saturation of the core.
If we refer to a choke; if there is no gap, we have a "swinging choke" as the inductance changes vs current change.
If we have a gap in the choke, the inductance of the choke tends to say constant vs current change.

Similar in a transformer. Nice point FMR.

Cheers

[FullRangeMan]

Thanks.
Do you know why toroidal cores are unused as OPTs??

[Guy 13]

Yes FMR, you are correct. Gap tends to limit changes in inductance, and saturation of the core.
If we refer to a choke; if there is no gap, we have a "swinging choke" as the inductance changes vs current change.
If we have a gap in the choke, the inductance of the choke tends to say constant vs current change.

Similar in a transformer. Nice point FMR.

Cheers

Hi Steve.
I hope I am not off topic, but my question is about toroidale transformer.
Why toroidale transformers are not more widely used for power transformer in amplifiers ?
Is it because they are more costly to manufacture,
they are bigger than standard transformer ???
Or maybe it's more difficult to do multiple taps on that type of winding?
It seems to me that as a power transformer they are better, more performing.

Guy 13

[Steve]

Good questions Gents. Price appears to be relatively high, over $300 each. And this is for small OPTs.

Size is another problem, relatively low height but large surface area doesn't leave a lot of area
for other parts.

Weight, core size is often small in favor of many turns. I have seen 100 watt OPT weigh only 5.5 lbs.
Using larger cores involves larger winding machines, which are quite expensive. As a result of small size core, it appears to saturate quickly
with just a slight current imbalance (PP). Potential is enormous as extremely high coefficient of coupling is possible, thus large inductance
is possible while at the same time very high frequency response is possible.

I almost had a nearby toroidal company on board to build large OPTs, about 15 years ago, but the owner died, and his sister took over instead of the son.
The project was scrapped.

Cheers

[FullRangeMan]

Thanks Steve.
I read somewhere the lamination core help to filter the dirty raw mains wall energy.
Toroids let it all pass through.

[Steve]

Thanks Steve.
I read somewhere the lamination core help to filter the dirty raw mains wall energy.
Toroids let it all pass through.

For power transformers, probably so since the high frequency response drops off earlier; but filtering should
take care of that easily If, If the filter section and layout is proper. If not, that is another story.

For power transformers, toroidals do offer the advantage of better magnetic field confinement, for less hum problems, but whether
that really helps that much is another matter since phono stages usually run a separate power supply chassis some distance away.

Line stages is another matter since many place the PT in the signal chassis and signals are weak, and the PT can be relatively small.

Cheers