Speaking of output transformer with no load . . .

[justonwo]

Would it make sense for tube amp heads to have the following:

Use a Switchcraft jack with a "switch" terminal that grounds the tip if no jack is present. That way, the HT fuse would blow rather than frying your OT, no?

Of course, that only solves the problem on the amp end. If you have a dangling cable that's not plugged into a cabinet, that's not so good.

[hasserl]

Yes, it would. Many amps are built this way, including most Fenders.

[Popoon]

Your post got me thinking,couldnt an alternative switching jack when unopened supply the OT with a resistor to save it from damage.
My next thought was to ignore the ohmage and consider the heat dissipation of the resistor.
If you are going to add a resistor it may not be practicable to fit a 50 watter but if it was a low power amp it may be fine,it is unknown to me if that is the case.
Another unknown occured to me,what if along with some added circuitry you could add a buzzer to indicate no load and yet still present the OT with a resistance to prevent meltdown.
Moving back to what i do know and i found that if i wanted an audble warning i could perhaps use a speaker,the amp head i will be constructing can have some room.
I feel it maybe worth considering,the real issues i guess would be the switching jacks available and the aesthetics of maybe having to use plastic ones.
Another option is a bit vague electronically but would involve a lamp of some kind as an indicator.

If you can flesh out some of the details then please do.

[Wakarusa]

It might easier to shut down the power section than to provide a dummy load. Perhaps a double pole relay on the input to the phase inverter -- one pole in the signal path, the other pole in a circuit to turn on a warning lamp/LED on the front panel. Have the relay coil controlled by the switch on the jack.

More interesting, but a lot more complex, would be a circuit that tests continuity on the output jack and shuts down the power section when the jack reads open. This would save you when the jack comes out of the cabinet but is still plugged into the amp.

Too, an old solution to the problem is to put reverse biased diodes in the output tube plate circuits. They'll zener or short to (hopefully) save the OT and power tubes.

[Popoon]

Use a locking socket in either jack or speakon form for the cab and the cable will pull from the amp output socket first.
Just another low tech approach.

[Popoon]

For a no load speaker mounted within the amp head i was considering one of these.
http://cpc.farnell.com/productimages/farnell/standard/LS0046006-40.jpg
A mylar 16 ohm 12 watt rms 100 mm fixing hole speaker.
It may be fine for my 6-12 watt amp head i am building but i see it's useable frequency is only to 200 hz.
Obviously it would never be used for extended periods at high output.
Maybe an eight ohm wih an eight ohm power resistor would be a better option.

[Wakarusa]

Use a locking socket in either jack or speakon form for the cab and the cable will pull from the amp output socket first.
Just another low tech approach.

Problem is that there are a heck of a lot of existing (and new) cabinets that still use the old 1/4" phone jack/plug. Speakons are pretty much standard for PA systems these days.. wondering when they'll finally be the standard for guitar amps/cabs.

[jca345]

Problem is that there are a heck of a lot of existing (and new) cabinets that still use the old 1/4" phone jack/plug. Speakons are pretty much standard for PA systems these days.. wondering when they'll finally be the standard for guitar amps/cabs.

Hey Todd-- actually, I personally am very glad they don't. Every Speakon experience I have had leads me to conclude that they resemble every other cheap plastic jack. The old 1/4 inch doesn't bend/deform if slightly jarred, is extremely reliable, has excellent contact integrity, is far easier to drill/mount/design for, and is far more durable overall with less "moving parts". It is also very easy to isolate a 3/8" long switchcraft #11 or #12A if you use a pair of fiber shoulder washers. Also, when using a 1/4 inch jack on the rear panel of either a head or combo, the advantage of a right angle 1/4 male plug becomes quite evident. I personally like G&H connectors mated to Switchcraft #11's and #12A's for my amplifiers. The old Switchcraft right angle 1/4" plugs are also bulletproof. I see no real advantage to Neutrik's Speakon at all. They look cool on a panel--that's about all. I will however design preamp's/DI's/power amp's using these for clientele if the Speakon is the only solution.

[Wakarusa]

Speakons aren't the be-all end-all for sure (though you can, for a price, get them with metal bodies instead of plastic in the STX line). I'm also a dinosaur in that, with very few exceptions, I use Switchcraft (11, 12A, 226, 280) or the heavier duty Cliff style (just cuz it's often easier than re-sizing a hole and/or finding insulating bushings for the SW11 & SW12A -- if you have a good source plz share). With very little care these parts will outlive the gear they're in.

I would like to see a locking connector with the durability of the Switchcraft parts though. Especially if it accepted both its mating locking part and a regular 1/4" phone. There used to be a company that made a little harness that would lock in a SW280, but I'm not sure where to find those any more (or if they're even still made).

Oh yeah... a tip for the SW226 right angle 1/4" phone... solder the internal ground tab to the body right up by the barrel threads. Makes a huge difference in mechanical reliability for this part. Just don't get the body so hot that the little press fit circle on the back of the body falls out.

[John Phillips]

It might easier to shut down the power section than to provide a dummy load.This is done on some Rivera models, including a stereo head I worked on recently. The cathode connection for each pair of power tubes goes via a normally-open switch in the speaker jack, so the tubes are only powered up when a plug is in the socket, and you can safely run the amp in mono with only one side connected to a cab.

It still doesn't save you from the cable being disconnected at the cab end though. The way that can be done is a little more complex but was used in the Ampeg SVT - the special speaker cable to the cab was four-conductor, two of which just went to the cab and back (with a shorting link inside the cab) to turn on the power section... I can't remember exactly how.