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Can a tube amp be damaged by a speaker switch?

Help please from the tech crowd.

I had a room with two systems....one for audio and one for TV. The audio is a Leben tube amp into Altec Valencia speakers. The TV is a Marantz receiver into a group of five budget small speakers. My goal was to use just the Valencia's for both amps because the big Altec's are so much easier to hear dialogue on the tv. I also want to clear up wire clutter in a cramped cabinet. 

My solution was a Luxman AS-55 speaker switch which a very high quality unit designed to allow 3 speakers and one amp, or in reverse setup, three amps and one set of speakers. During set up, I had a question and called the Luxman US manager who picked up the call and was so gracious with his expertise. Great service. The problem is after hearing my questions he said..." we do not recommend using a switch with a tube amp as running tubes without a load can damage the amp...even break it. 

I can use the switch if the the tube amp is turned on only when running into the speakers. The problem with that is the Leben really needs a good half hour to warm up and I would like to do that while watching tv. It will make it cumbersome to switch back and forth between the two. 

Eliminating the Luxman AS-55 switch and just using two sets of speaker wires is an option, but I really do not want to have to plug and unplug the bananas on my Audience cables as that can only hurt them and its a hassle. Im also worried about potential damage to the Altec's with two amps hooked up. 

My question is for the experienced tube users. How much risk am I running if I use the Luxman switch as I intended? Can it really damage the amp or is the Luxman rep exageratting the risk for laibility reasons. What is the potential damage?

 

128x128tamarack4
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cleeds
5,957 posts
 

tamarack4

... we do not recommend using a switch with a tube amp as running tubes without a load can damage the amp...even break it ...

Absolutely true.

larryi
4,029 posts
 

Running a tube amp with no signal into the amp, and hence, no output, should not be a problem.  But, it is too easy to accidentally have the amp amplifying without a load.  One thing you could do is install a dummy resistor to act as the load when the box is switched to play a non-existent speaker.  just google 8 ohm dummy load and you should be able to buy the right set up (dummy resistor plus a suitable heatsink).

imhififan's avatar
imhififan
2,204 posts
 

What is the potential damage?

If the load accidentally disconnected from the tube amp while playing, The inductance of the output transformer will produce a large fly-back voltage, this is lethal for components in the amp, it can damage the output tube or will arc across the output transformer primary winding and burn it up.

One solution is to install a 100Ω 5-10W wire wound resistor on each channel of the tube amplifier output terminals. This will always load the tube amplifier, but it will also take away some of the output power since it’s in parallel with the speaker.
 
The problem with that is the Leben really needs a good half hour to warm up and I would like to do that while watching tv.
My solution was to use a high quality 4P3T MBB rotary switch and only use a dummy load on the amp with no speakers connected. This circuit design allows both amplifiers to be powered ON at all times.
 
 
 
tamarack4's avatar
tamarack4 OP
1 posts
 

What a wealth of knowledge this group is. It sounds like there is no quick and easy solution, but there are options. I'll try to digest this and run your suggestion by my audio repair friend.

Thanks so much.

erik_squires
16,118 posts
 

This is amplifier dependent. Almost all solid state amplifiers are stable into no load, but not all tube amps were.

Check with the vendor.

In _all_ cases you must ensure that the switches used are "break before make." That is, they open completely before making the next connection. The alternative, make before break, can be disastrous for any amplifier.

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