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tube amp with no load

Eddy

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I have a transistor amplifier and a tube amplifier (Oldchen EL34-B), both of which I want to use on my speakers (not at the same time, of course). To switch between these two amps I have an amp switcher (Dynavox AMP-S MKII).

When I switch the amplifier switch from tube amp to transistor amp, there is no load on the tube amp (since it is no longer connected to the speaker). I've heard that a tube amp can be damaged under no load. I have a few questions about this:

a) Can a tube amp be damaged even if there is no input signal to the tube amp?

b) Can a tube amp be damaged even if the volume control of the tube amp is set to zero?


(Maybe I can switch that way without having to turn off the tube amp right away.)
 

DonH56

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The problem with an unloaded tube amplifier is that the output transformer is subject to very high voltage spikes with no load, thousands of volts on the primary (tube) side, and that can cause tubes and transformers (among other components) to arc and fail. The input signal (or volume control) does not really matter (can only make it worse, but it is already a Bad Thing).

The usual solution is to add a power resistor of 50~100 ohms *** across the tube amplifier's output terminals. That is usually enough to prevent damage without affecting how it drives your speakers. Some amps may include an internal snubber circuit (ask your amp's manufacturer).

HTH - Don

*** With more time to look, I checked out the article @Blumlein 88 linked and a couple of others, along with one of my old tube design books. For audio, I am seeing recommendations in roughly the 100 ohm to 1000 ohm range, with some dependency upon the amplifier design. My vague memory is using 220 or 470 ohms to protect the amps I was using back then (ARC D-79, plus Eico and Dynaco models).
 
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mhardy6647

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As above -- running a vacuum tube (power) amplifier is, at best, risky -- and, yes, even at zero volume.
Best advice: don't do it.
Buy or rig up a (break-before-make, i.e., nonshorting*) switch box that connects the vacuum tube amplifier to some high-resistance (maybe a hundred or a couple of hundred ohms) power resistors if it's disconnected from loudspeakers.

________________
* out of deference to the ss amp -- a transformer-coupled vacuum amp shouldn't much care about operating into a short.
 
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