Speaker Switch That Protects Tube Amplifier

Not sure about that, Marty (Viridian).

I agree that the load resistor could be made significantly higher than 8 ohms, which would allow a somewhat lower wattage resistor to be used. I recall reading somewhere that as a conservative rule of thumb guideline 32 ohms or thereabouts would be low enough to protect most or all tube amps from the possibility of damage, even if they are operated for considerable periods of time with signals going through them. (The possibility of damage from running with too light a load or no load is greatly reduced if no signal is being processed by the amp, although various scenarios can be envisioned in which a brief transient "signal" can be put into an amp's output stage even when no music is being played).

However I see several potential issues with using a make-before-break switch.

First, when the switch is being thrown there will be a brief instant in which each amp will be loaded by the parallel combination of the speaker impedance, the resistive load, and most significantly by the output impedance of the other amp (that being very close to zero ohms in the case of the solid state amp, and perhaps 1 or 2 ohms in the case of the tube amp). Obviously not a good idea, especially if signals are going through the amps. At the very least, it wouldn't surprise me if self-protection mechanisms in some amps would end up being triggered as a result.

Second, if the solid state amp has a small amount of DC offset in its output, say something like 20 millivolts, which I believe would not be uncommon in a properly functioning solid state amp, that would be applied to the secondary of the tube amp's output transformer for a brief instant when the switch is being thrown. That would cause a brief transient to appear on the primary side of the transformer, whose amplitude might very briefly be perhaps several hundred millivolts as a result of being stepped up by the turns ratio of the transformer.

If the tube amp is not powered up at the time, the energy of that transient would not have any place to go, since the output tubes would not be functioning. The result potentially being a large voltage spike due to inductive kickback, which is exactly the potential damage mechanism the dummy load is intended to prevent.

If the tube amp is powered up at the time that transient occurs, it would be introduced into the amp's feedback loop (if it has one), and offhand I'm not entirely sure that would be healthy under all circumstances either.

Mceljo's plan sounds good to me :-)

Best regards,
-- Al

Almarq - I was hopeful that you would provide input as I value your opinion. The revised plan is to use the dummy load resistor (a parts express item) on both inputs and also put 150 ohm resistors on both inputs to make it a fail safe design for a tube amplifier. I'd rather not have to keep track of the tube safe input. Each channel will have its own switch because it would take at least a 6P or 8P switch to do what we want.

Marty, note this statement in one of Mceljo's earlier posts:

To bridge the instant between the speakers and the dummy load resistor on the tube amplifier we will also add 150 ohm 5W or 10W resistors on each channel based on the recommendation from the Niles manual that was linked.

And in his most recent post:

The revised plan is to use the dummy load resistor (a parts express item) on both inputs and also put 150 ohm resistors on both inputs to make it a fail safe design for a tube amplifier.

So as I understand the plan neither amp will ever see a load that is lighter than 150 ohms, even for an instant. That seems reasonable to me, although I suppose it would be good practice to not throw the switch when music is playing (since the possibility of damage resulting from having a load that is too light is very small if no signal is present).

Best regards,
-- Al