newbie question

[Downtownfish]

Can you run a speaker switching box backwards? I want to run two amps to one set of speakers without switching cables all the time.

[willland]

I don't think so but you can get one of these:

http://forums.audioreview.com/showthread.php?t=25538

Bill

[Downtownfish]

That thing is odd with the 12 volt firing switch. I am afaid that would cause to much distortion to the sound, as stated in that thread. Thank you very much for the info though.

[DrWho]

The 12V is for driving the relay...there is no way in the world a relay is going to add any distortion to the sound. The nice thing about using a relay is that it opens up options for having it automatically trigger when the receiver turns on...otherwise there is absolutely no difference between it and a normal switch.

Usually you can use a speaker switching box in reverse, but you need to make sure it's using a "break before make" switch, that way you don't intermittently short the two amplifiers into each other. I would also make sure it's switching both the positive and negative terminals at the same, and not just sharing a common negative terminal. Not all amplifiers have the same reference on the negative terminal so sometimes you can create a short by connecting two negative amplifier terminals together.

One other comment...I've read that some tube amplifiers can go unstable with no speaker load attached. I don't quite understand why that would ever be the case, but it could potentially create some issues if you don't have a speaker load hooked up when the tube amp is turned on. In my mind, any amplifier should be stable with no load present, but that's a totally different issue. I am yet to find a true technical reason for this, however.

[Groomlakearea51]

You can also make a simple "break before make" relay with "either, or" switches. For added safety, you can add a center switch that simply will not allow anything, regardless of what the other switch is set for to work.

Dr. Who, I've heard the same thing about tube amps. Sansui AU-111 has those instructions.... Don't know why, but I always have something hooked up anyways to prevent Murphy's Law from taking effect.....

[djk]

"I don't quite understand why that would ever be the case,"

What happens when you interrupt the current charging an inductor?

The voltage rises to infinity (in this case the insulation breakdown point of the transformer).

To those foolish enough to not understand this, take a smallish inductor of say a couple of henries, give it to someone with a D cell, and have them touch the battery to the leads of the inductor.

MAKE SURE THEY DO NOT HAVE A HEART PROBLEM BEFORE YOU START !

When they remove the inductor leads from the battery they will get a big shock.

I can't remember what value we used for the choke (inductor) in high school, but it wasn't very large. This can be a very dangerous experiment, don't earn the Darwin Award.

A 100R/2W resistor on the output of most tube amplifiers is enough to prevent damage, and not load the amplifier excessively.

[DrWho]

"I don't quite understand why that would ever be the case,"

What happens when you interrupt the current charging an inductor?

The voltage rises to infinity (in this case the insulation breakdown point of the transformer).

While
that may be true for an inductor, in this case you've got the primary
side of the transformer being actively driven by the output stage. I
would expect a good design to be capable of dissipating the flyback
energy from such an event. Heck, you don't see transformer coupled
line-level equipment blowing up when cables are pulled out...

[vondy]

I just got one of these and it works great

http://www.russound.com/ab2.htm

Mine is from a brother-in-law who owns a HT business so I don't really know where to get one or how much.

Here's the Niles.

http://www.jr.com/niles-audio/pe/NIL_DPS1/

[Marvel]

While
that may be true for an inductor, in this case you've got the primary
side of the transformer being actively driven by the output stage. I
would expect a good design to be capable of dissipating the flyback
energy from such an event. Heck, you don't see transformer coupled
line-level equipment blowing up when cables are pulled out...

You also don't have 300Volts going through the primary, either. It is the rising and collapsing magnetic field in the primary that generates the voltage on the secondary. It needs a load on it.

[USNRET]

I personally used the Niles SPK-1 for quite some time in my hybrid HT/2 channel set up. To ensure there was no interconnectivity of the associated amps I used a Niles IPC-6 controller to turn control the timing delay between amps and switch. Using my Harmony 880 I set an activity to mute the HT receiver first then sent the trigger to start things switching.

If anyone is interested, I have a few of the SPK-1s, one IPC-6 and a couple of the Niles AC-3 voltage triggered power stips laying around.

[djk]

Tube designer David Berning:

"Audio-output transformer-coupled amplifiers should not be operated without a load connected because the inductance of the transformer’s primary can cause the plate voltage to reach a sufficiently high voltage to break down insulating materials, resulting in amplifier failure and even fire."

Cary Audio:

"NOTE: Some other tube amplifier designs will oscillate or go into overload if they are used without a speaker or resistive load attached to the speaker output terminals ."

Manley Labs:

"...open circuit are to be avoided. The result could be damage to transformers, tubes."

Marshall (guitar amplifiers):

"WARNING! Never use the combo without a load attached!"

"As is the case with any Marshall all-valve amplifier it is imperative that: a) the amplifier is connected to a load whilst in operation and the impedance selected on the amplifier matches the total impedance of the internal speaker and/or extension speaker cabinets."

Mesa Tigris (their class A integrated amplifier):

"Turning on a tube amplifier without a load connected to the output terminals can cause serious damage to the amplifier, especially if you accidentaly run signal into it. Such damages are NOT covered by the warranty."

Opera-Consonance:

"Note: Never turn the amplifier on unless there are speakers connected to both output terminals. Operation of the amplifier without a load can damage the output transformers."

Peavey:

If (for whatever reason) the SECONDARY of the transformer has no “load,” the magnetic

energy created by the primary (and “stored” in the core) has NO place to GO! As the alternating

signal in the primary goes down, the magnetic field built up in the core “collapses,” thus inducing

an extremely high voltage “spike” back into the transformer’s primary windings. Because this

energy is substantial, and because the primary coils have many turns of wire, an extremely high

voltage “transient” (often 3000 volts or more) can be developed. The primary of the output

transformer is directly connected to the tubes and tube sockets of the amp, and neither the

tubes nor the sockets (and associated wiring) are designed for that kind of voltage. These

electrical “spikes” can (and often do) cause “arcing” inside the transformer and/or the tube

elements, especially between the terminals on the tube sockets and the associated wiring.

Remember that we learned in high school physics about the “law of conservation of

energy.” This taught us that energy can’t simply “disappear,” it has to “go somewhere.” When a

tube output transformer doesn’t have a place for the energy to go (i.e. a speaker or its

equivalent) the energy developed in the magnetic core “flashes back” into the amplifier, often

destroying the output transformer, the tubes, the tube sockets, and/or the associated wiring.

This is WHY you should NOT operate a tube amp without a “load!”

An “UNLOADED” tube output transformer produces high voltage “spikes” in EXACTLY

the same manner as an “ignition coil” does. The difference being that a tube amp output

transformer is NOT designed to operate that way, but an ignition coil IS! Operating a tube amp

with its transformer unloaded can easily damage or destroy a perfectly good tube amplifier by

the “flyback” action described above. Hopefully, the above will provide a little background as to

exactly WHY it’s important NOT to operate an amplifier WITHOUT a proper “load” for the output

transformer’s secondary output terminals.

Peavey (recognizing that accidental operation of its tube amps without a proper load is a

possibility) has endeavored to try to protect the output tubes, sockets, wiring, and the output

transformer against damaging voltage spikes as described above. Although there is no 100%

“fool proof” method to protect an amplifier against the spikes generated by an unloaded output

transformer, the most effective protection is by use of so-called “flyback diodes.” “Flyback” is an

old term applied to high voltage systems in tube type TV sets, but it has come to be applied to

any high voltage transient (spike) in electronics. Silicon diodes have certain characteristics that

engineers find very useful. At a certain voltages, silicon diodes encounter a “threshold” and

conduct energy in the reverse direction to their normal operation. This “avalanche effect” occurs

at very high voltages and thus inclusion of these into the output circuit allows some measure of

protection for the tubes, output transformers, sockets, and wiring. If voltages occur above their

“avalanche” level, the diodes conduct in the “reverse direction,” thus the destructive voltage

transient generated by the unloaded output transformer is shorted to ground. Although this

method has proven very effective, it should be noted that super high voltage transients can

cause arcing either inside the output transformer, the output tubes themselves, and especially

between the terminals of the tube sockets. Cheaper plastic or phenolic output tube sockets are

especially prone to this and can be easily identified because they are usually BLACK or

BROWN. If a voltage transient “arcs” between the two socket terminals, it usually leaves a

“carbonized path” between the terminals that REMAIN CONDUCTIVE which can (and often

does) render the amp unserviceable. This is WHY Peavey now utilizes white GLAZED

CERAMIC sockets for our output tubes, since these white ceramic sockets are MUCH more

resistant to arcing between terminals and the “carbonized path” problem is totally eliminated.

Hopefully, the above will give players a better idea why tube amps should never be

played or operated without a speaker load, or a proper termination by a load resistance of some

kind.