Wiring different output jacks for different impedances

[theRagman]

I have an OT with 8 ohm and 16 ohm outputs. If I wire one output jack to the 8 ohm output of the OT and one to the 16 ohm output of the OT, are there any concerns about whether either jack or both are normally open or closed? What is the best practice there?

[passaloutre]

You don’t want a jack you’re not using to be closed. Use open jacks or just don’t wire up the tip shunt.

[theRagman]

You don’t want a jack you’re not using to be closed. Use open jacks or just don’t wire up the tip shunt.

Thank you! Easy enough.

Out of curiosity, what happens if a jack you’re not using is closed?

[sluckey]

Out of curiosity, what happens if a jack you’re not using is closed?

Then you will have either a partial or full short across the OT secondary. Just don't do it.

[HotBluePlates]

I have an OT with 8 ohm and 16 ohm outputs. If I wire one output jack to the 8 ohm output of the OT and one to the 16 ohm output of the OT, are there any concerns about whether either jack or both are normally open or closed? What is the best practice there?

Both jacks must be open-circuit; otherwise, the amp has no output.

Best practice to protect against no-load is to include a power resistor from Hot to Ground on one of the outputs.  However, it should be higher-resistance than "a normal load" so it doesn't steal power from the output (and generate excess heat).  Something like 100Ω across one of the outputs is sufficient (so approx 10x or slightly more than "a normal load"; much over 10x fails to provide sufficient loading to protect the OT).

[acheld]

Is there any reason not to use a single output jack controlled by an output impedance selector?  (Beside cost . .)

I've been doing that forever, with no issues.   I do use a shunt resistor to protect against folks not plugging in a speaker as noted by sluckey and HBP.

[JPK]

Any amp I've built has the impedance elector switch with one output jack. The shunt resistor is a good idea but I just make sure a cab is plugged when it's on. Also I don't change impedance while power is on.

[theRagman]

Is there any reason not to use a single output jack controlled by an output impedance selector?  (Beside cost . .)

I've been doing that forever, with no issues.   I do use a shunt resistor to protect against folks not plugging in a speaker as noted by sluckey and HBP.

I don't think there's any reason not to use a single output jack and impedance selector switch, in general. I was mostly constrained by what I had on hand while I was messing with something, and a hard limit on available work time that prevented me from ordering any new parts. And then I was just curious.

[pdf64]

Switches suitable for impedance selectors are expensive, bulky / difficult to mount, and seem more likely to fail than dedicated sockets for each OT secondary tap.

[acheld]

Switches suitable for impedance selectors are expensive, bulky / difficult to mount, and seem more likely to fail than dedicated sockets for each OT secondary tap.

Yup, they are expensive.   I think they are suited to heads, not so much combos, for obvious reasons.

I have not had one fail yet (knock on wood!) over the past 20 years, though I am admittedly a small volume builder.    I don't find them difficult to mount in any way.

They save some space, since I normally use Hammond 1650 series OT with 4-8-16 ohm taps.  Having three jacks would increase the chance of operator error IMO, though of course someone could forget to set the correct impedance.

But I'm not trying to say my way is the only way . . .

[theRagman]

Switches suitable for impedance selectors are expensive, bulky / difficult to mount, and seem more likely to fail than dedicated sockets for each OT secondary tap.

I will admit to being unpleasantly surprised at the price of switches I found for this purpose.

[tubeswell]

Is there any reason not to use a single output jack controlled by an output impedance selector?  (Beside cost . .)

I've been doing that forever, with no issues.   I do use a shunt resistor to protect against folks not plugging in a speaker as noted by sluckey and HBP.

Or two output jacks in parallel controlled by a DPDT (or 3PDT) switch - s'wat I do

[pdf64]

Sorry, the point I trying to make was that it's their bulk which could make them hard to mount, in regards of finding (appropriate) space.

The specs of switches marketed as impedance switches typically note a design life of a few hundred operations. Ok, that's a couple of decades for most of usage scenarios, but even so, they seem ridiculously expensive for a somewhat low spec product.

I suspect that dodgy impedance selectors have been a disaster for a lot of vintage Marshalls and other UK amps, resulting in many wrecked OTs etc.

Hence I'm prejudiced against them in principle