output transformer with double winding

[fredouille]

Hi all

a small double question

1) do you short to ground on the jack when nothing is connected ? I looks like dangerous if you forget to plug a speaking and bring HT on
2) how do you deal with 4ohm and 8 ohm double windings ? as I plan to use only 1 at the time , the unused winding shall be left open , I guess same than point 1 not grounded

[TUBEDUDE]

Tube amps would.rather see a dead short than an open circuit. With an open circuit the flyback spike can exceed the insulation of the windings and kill the O.T.
I have xconnected the shorting jack to a 27 Ohm 10.watt resistor in the past, but I just use the shorting jack on all but the high power builds now.
Does your transformer have 2 completely separate secondary windings, or are they taps on one coil?

[LOUDthud]

You need a switch to select which winding will be used. You can't just use two jacks and short the unused winding. If the unused winding is shorted, it will squish the output just like connecting a speaker to only the Extension Jack on a Fender amp. You can leave any feedback connected to one winding.

[martin manning]

2) how do you deal with 4ohm and 8 ohm double windings ? as I plan to use only 1 at the time , the unused winding shall be left open , I guess same than point 1 not grounded

You can protect the OT like this: https://ampgarage.com/forum/viewtopic.p ... 23#p267223

[fredouille]

hi all sorry I didn't see answers

the OT has a center tap and 2 outputs, provider told me to load 1 part of the winding or the other and leave unused one disconnected

I saw some comments in other post about grounding, I don't agree about shorting an OT, it is like putting a 4 ohm loan instead of 8ohm = double current = burning the OT

[martin manning]

hi all sorry I didn't see answers

the OT has a center tap and 2 outputs, provider told me to load 1 part of the winding or the other and leave unused one disconnected

I saw some comments in other post about grounding, I don't agree about shorting an OT, it is like putting a 4 ohm loan instead of 8ohm = double current = burning the OT

I think you mean a common and two outputs? The secondary is not a "double winding," it's a single coil with a tap for the lower impedance.

The traditional short at the speaker jack is only present while you strum the guitar a few times and realize that something is wrong. If the secondary is open-circuit when this occurs, it might be too late. The shorted secondary technique requires a single output, or an impedance selector switch if there is a multi-tap secondary. It has been in use for many decades. Some people will install a ~5-10x expected impedance resistor to ground in case a cable is plugged into the output, but open at the other end.
In your case, without an impedance selector, grounding one of the secondary taps through a resistor that is ~5-10x the expected load might save it. You also add another resistor connecting the output leads and use switching jacks to disconnect it from whichever tap is in use. See in the linked post.

[fredouille]

hi all sorry I didn't see answers

the OT has a center tap and 2 outputs, provider told me to load 1 part of the winding or the other and leave unused one disconnected

I saw some comments in other post about grounding, I don't agree about shorting an OT, it is like putting a 4 ohm loan instead of 8ohm = double current = burning the OT

I think you mean a common and two outputs? The secondary is not a "double winding," it's a single coil with a tap for the lower impedance.

yes sorry, so it is not needed to do anything I guess?

[martin manning]

If you are only going to use one of the taps, you can just install a shorting speaker jack, and add a resistor from tip to ground if you want to be extra safe.