Dual impedance question

[MacGwyn]

Working on a Fender clone w/ 2 6L6 output.  Decided to get a dual impedance OT, 4 and 8.  Instead of using a switch (like most Marshalls) I decided to wire 4 ohm to one jack and 8 ohm to the other (labeled of course).   It occurs to me that there might be trouble with the OT load if only one of those jacks is used, while the other is left open (which would be normal operation).   Any opinions on whether this option is safe... as long as one of the two jacks is plugged into a speaker?

[sluckey]

I would recommend only using one jack at a time. No problems doing this.

[jeff]

They can't both be shorting jacks like you'd normally use.
Either use regular jacks or wire the switching jacks in such a way that plugging into either jack breaks the connection to ground.
What you don't want is to plug an 8 ohm speaker to the 8 ohm jack and have the 4 ohm jack short the 4 ohm tap to ground because there is nothing plugged into it.

Normal jacks will work but won't protect in the unlikley event you turn on the amp with no speaker load

Switching jacks will work but must be wired so that the switches of both jacks are in series so plugging into either one breaks the connection to ground.

But two shorting jacks will not work.

[sluckey]

Switching jacks will work but must be wired so that the switches of both jacks are in series so plugging into either one breaks the connection to ground.

How would you do that?

[PRR]

> trouble with the OT load if only one of those jacks is used, while the other is left open

The music will leak out, make a mess on the floor.

Seriously-- not a problem.

I have 120V outlets for lamps and 240V outlets for dryer. If I don't plug-in any lamps (or leave them all switched off), the 240V outlet still works fine; and vise versa (light with no dryer).

That assumes NON-shorting jacks. In small cathode-bias amps, shorting jacks are a nice touch because the amp won't easily get un-loaded. (However a speaker cable not plugged to speaker will do it.) Cathode-bias amps can usually be shorted forever, at any signal level. Fix-bias amps (and most all transistor amps) will pull more power into a short and bust a gut, they should not be shorted.

A series-switch affair to un-short if 4 *or* 8 is used is possible but needs a different sort of jack than the usual 3-tab. You can find examples. It is not commonly done. It does invite a screw-up *when* the jack goes bad and a later repair-tech (maybe you, after many years) does not understand (remember) the scheme.

[jeff]

Cathode-bias amps can usually be shorted forever, at any signal level. Fix-bias amps (and most all transistor amps) will pull more power into a short and bust a gut, they should not be shorted.

I'm so glad you mentioned this. I was under the impression ALL tube amps should have a shorting output jack for protection. Better double check my amps.

Switching jacks will work but must be wired so that the switches of both jacks are in series so plugging into either one breaks the connection to ground.

How would you do that?

Certian jacks have 4 tabs. tip, ground, and two tabs that make a connection when nothing is plugged in but break connection when something is plugged in. so basicaly a jack with a seperate SPSTswitch. It works kinda like a shorting jack but instead of one finger connecting to the tip there's two fingers that touch each other. There is a plastic tip connected to one of the fingers so when the cord is plugged in it pushes on that to break the SPST connection without actually making contact with the tip. First time I saw one was the input of a fender PA100. They had it wired so one tab of the SPST section went to tip one to ground making it a shorting jack.

I think you could also do this with a Cliffs jack's ring section, provided the finger that touches the plug goes to ground and the other side(the side the finger hits when nothing is plugged in) goes to the tap. Put two in series and each tap to its own jack's tip.

I've done something similar to this series break when I built an amp with a single 8 ohm tap and wanted two output jacks but didn't want to have a Main Output and an Ext. Both jack's tips are connected to the 8 ohm tap and short it when nothing's plugged in, but plugging into EITHER jack breaks the connection. Guess it was my way of idiot-proofing it???

If you wired from the 8 ohm tap to one switch then to the other then to ground plugging into either would break the connection.
 
So it seems like the best way would just be two regular two tab(tip/ground) jacks one to 4 one to 8