Using a speaker selector with 1 tube amp, 1 solid state amp, and 1 pair of speakers

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On the contrary. I've seen vacuum tube amplifiers output transformers short when the output is unloaded. If the (unloaded) tube amplifier is driven to clipping, the voltage on the primary of the transformer can become many times higher than the insulation is rated for. This can arc over, and carbonize a path and result in a short.


As a minimum, I'd put something like a 32 Ohm resistor, rated at 1/4th the 8 Ohm power rating of the amplifier across the vacuum tube's output to make sure it always has a load.


Another thing to be wary of is that some transistor amplifiers require a floating output on both the positive and negative outputs. Most switchers have a common ground, and using such a switch on this kind of amplifier (a bridge amplifer) will cause a short circuit between channels.


Hate to make this harder than it seems, but I'd hate for you to blow up an amplifier because you were told it was OK.


Bob Smith

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If you're also sure you never power it up and turn the volume control up on the tube amp while it is not connected to a load, that will be OK. If you turn up the volume because you THINK it's connected, and the amplifier goes into clipping, then you'll have problems. It's not guaranteed it will do damage, but the potential is there. If the transformers have enough insulation to take the overload, they may be OK. If, however, they don't, you will have an expensive transformer to replace, and the output transformer is about the most expensive thing in the amplifier.


The rule I always use is never leave a tube amplifier with an open output, and never place a short across a solid state amplifier. Tubes are very forgiving of a short on the output transformer, but not an open. Transistors (depending on the short circuit protection circuitry) are not.


I wouldn't take a chance.


Biasing the tubes with no output is probably OK, because usually when the bias position is selected, the input is disconnected ( I believe that's the way it works on my Marantz Model 9s).


With push-pull outputs, there isn't quite as much of a chance of doing damage either (though I've seen them burn out as well). This is because the conducting tube in the output stage will load the transformer when the other tube is cut-off.


A single-ended amplifier is much more vulnerable because when the output stage cuts off during clipping, the transformer (which is just a big inductor when not loaded) generates a back EMF to keep the current across it constant. UsingE= L(di/dt) , when the current instantly goes to zero (during clipping), the voltage can reach infinity. Usually this isn't the case, but it is possible for the voltage to go to several times the B+ of the amplifier. Most transformers are only rated to go to twice the amplitude.


All in all, as expensive as output transformers are, I wouldn't take a chance.


As I said, using a 32 Ohm resistor or so that never gets disconnected from the amplifier is probably the safest bet.

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Just put it across the speaker terminals in parallel with the speaker leads. It's going to rob some of your amplifier power from the speaker, in fact, for an 8 Ohm load, you're going to lose about 25% of your power (about 1.25 dB, just barely noticable) but you will be safe.


You should also make sure the switch is 'break before make' and that both positions can't be selected at the same time (many speaker switches allow both speakers to be connected at once) so that the solid state amp won't drive the transformer of the tube amp.


Even more important, if the tube amplifier is on while connected to the unpowered solid state amplifier, it could do some serious damage to the output stage of the solid state amplifier. With the solid state amplifier off, there is nothing to prevent the output transistors from being 'back biased' from emitter to base. If this is over 5 volts or so, the output transistors may be destroyed.


Sorry about all this paranoia, but I get queezy when the outputs of power amplifiers are switched while they are in operation, lots of things can go wrong when you're shoving so much voltage and current around and it gets 'misplaced'.



Bob