secondary in parallel/series with primary

[zigzagflux]

Originally Posted by DF96

That winding will have been built for high voltage operaton, because the transformer designer will know that a 5V heater winding is intended for a rectifier. Not all secondaries are.

That is quite a large assumption that the manufacturer knows I will be elevating the 5V winding, and designs that specific winding with additional insulation. Along those same lines, what if I want to build a PI or cascode, and intend to elevate one (and only one) of the 6.3V windings to 100, 200, or even 300V. Again, no one panics. Can I assume the mfr knows this, and builds in additional insulation? Of course not. The only real assumptions we can make without getting specifics from the manufacturer is one of two methods:

1. NO secondary winding is built with additional insulation. We should then only use individual xfmrs for DH rectifiers and cascodes (my ideal preference).
2. ALL secondary windings are sufficiently rated to handle a few hundred volts, to accommodate DH rectifiers and cascodes.

We cannot know which is true, but we can minimize the unknowns by connecting the secondary winding in the neutral end.

[AndrewT]

I would expect that a bi-fillar secondary winding to have low isolation capability.
I would further expect that a transformer for a valve amplifier will always have more isolation capability between the separate secondary windings.
I would not expect the isolation test voltage between primary and secondary to be applied between secondary windings, unless the specification tells the user so.

[RJM1]

If you had a megger you could test the insulation break down voltage non destructively.
I was lucky enough to work for one company who were selling a product to the US navy and they would not accept the results done with the 1933 hand cranked megger that the company had. They had to buy a new digital version for over $1000.00 and I bought the old hand cranked one for $5.00. It's also great for bringing up worms for fishing if you put the two leads into the ground about 10 feet apart

[DF96]

Originally Posted by zigzagflux

That is quite a large assumption that the manufacturer knows I will be elevating the 5V winding, and designs that specific winding with additional insulation.

No. The transformer manufacturer knows that octal rectifiers have 5V heaters, which are either directly heated or at least connected to one side of the cathode. Other valves tend to have 6.3V heaters. So on a valve mains transformer it is quite possible that the 5V secondary has better insulation than the other heater windings.

what if I want to build a PI or cascode, and intend to elevate one (and only one) of the 6.3V windings to 100, 200, or even 300V. Again, no one panics.

There is a world of difference between a relatively high impedance HT/B+ DC supply and the very low impedance of an AC mains supply, so there are very different safety implications of insulation breakdown.

we can minimize the unknowns by connecting the secondary winding in the neutral end.

True, provided that you can always be certain which side is neutral. You can't trust building wiring.

Originally Posted by AndrewT

I would further expect that a transformer for a valve amplifier will always have more isolation capability between the separate secondary windings.

Yes, I would expect that too. It would probably be safer to do this trick with a spare heater winding on an HT transformer, than with a general-purpose low voltage transformer.

[Tony]

Originally Posted by AndrewT

I would expect that a bi-fillar secondary winding to have low isolation capability.
I would further expect that a transformer for a valve amplifier will always have more isolation capability between the separate secondary windings.
I would not expect the isolation test voltage between primary and secondary to be applied between secondary windings, unless the specification tells the user so.

really depends on insulation properties of the magnet wires used.....

the McIntosh MC270 OPT used bifillar windings with no other insulation other than the enameling on the wires themselves......

[Tony]

Originally Posted by DF96

No. The transformer manufacturer knows that octal rectifiers have 5V heaters, which are either directly heated or at least connected to one side of the cathode. Other valves tend to have 6.3V heaters. So on a valve mains transformer it is quite possible that the 5V secondary has better insulation than the other heater windings.

i also use the 3 volt 3DG4 octal rectifier......can be had for a dollar each at Rogalski's....

[Elvee]

Originally Posted by Tony

the McIntosh MC270 OPT used bifillar windings with no other insulation other than the enameling on the wires themselves......

Yeah, but then there is no risk to human life.
Quite different when you mix the primaries and secondaries of a mains transformer.

[Tony]

Originally Posted by Tony

such pratice is not encouraged here unless you are using isolation transformers, electricution hazards are real.......

i wonder why we are still having discussions.....

[DF96]

Originally Posted by Tony

i also use the 3 volt 3DG4 octal rectifier......can be had for a dollar each at Rogalski's....

That is part of the fun of DIYaudio forum: make almost any 99.9% true statement and someone will report an exception!

Nevertheless, a 5V winding is almost certainly intended for an octal rectifier as most of the commonly used rectifiers had 5V heaters even when the partnering signal valves used 6.3V.

[AndrewT]

Originally Posted by Tony

such pratice is not encouraged here unless you are using isolation transformers, electricution hazards are real.......

Originally Posted by Tony

really depends on insulation properties of the magnet wires used.....

the McIntosh MC270 OPT used bifillar windings with no other insulation other than the enameling on the wires themselves......

Originally Posted by Tony

i wonder why we are still having discussions.....

In the first instance, you tell us clearly that there is a Life Hazard.
In the second instance you tell us that the capability of wire to wire enamel insulation is up for debate.
The third instance asks why "you" are still discussing the topic?

Rules for some and not for others?