Attenuator shortening tube amp output?

[ComfortablyNoob]

Greetings folks. Long time I did not log in but I feel the itch to build something.

I want to build a resistive L-pad type attenuator for my home built 5E3 guitar amp. I found this one and I can build it now because I have all the components at home, although it's not the best design, I know.

Now... Is it just me or there is a possibility if the switches are both in a certain position and no speaker is plugged in the output of the attenuator that my amp will actually be shorted??? I already smell the smoke...

Thanks a lot for your help!

 

[jeff5may]

Not if you plug it in correctly.... the output (speaker) jack has a shorting terminal built into it, so if you unplugged the speaker while the amp is making noise, the amp would still see a load to dump its power into. Many more output tubes die from being underloaded than overloaded. Now if you connect the attenuation box up backwards while the amp is running, the amp would see a short across the terminals when the tip of the plug touched the tip terminal of the jack. Once the plug is inserted enough to push the terminals apart, the short is opened.

To make the box stupid proof, you could put a power resistor in with one leg attached to the shorting terminal and the other leg attached to the common connection. That way, even if the box was connected backwards, the amp would still see a load when the above condition happened. Power resistors are cheaper than amp repairs.

 

[ComfortablyNoob]

Thanks for your reply Jeff. Much appreciated. Your explanation makes sense and I'll follow your suggestion to add a power resistor to the output jack.

Thanks very much

 

[Enzo]

The worst thing you can do to a tube amp is play it with no speaker connected. It can send voltage spikes back into the primary and arc your output transformer, destroying it. Far better to have a shorting jack - like most Fender amps - so the transformer is shunted when the jack is empty. It won't blow the amp up.