[jarcher]

Something I've been wondering lately : I've seen it said many times that when on, tube amps should always be connected to speakers (load).  Is this a myth or a fact?  If true, what would happen if you leave it on without connecting it to speakers (or a load)?

I've usually always followed this advice, but recently had my tube amp on and warming up for maybe a few hours when I realized that the speakers cables weren't connected to the speakers.  I use one set of speakers for 2 channel + home theater and connect / disconnect them to different speaker cables / amps depending on which I'm doing.

In any case : nothing seemed to happen.

[DaveC113]

With no signal it should be no problem, but if there is a signal it can destroy the OPTs.

[jarcher]

Fortunately I didn't have any signal / music playing (or I would have had the speakers connected - duh).

Why are tube amps different than SS in needing to be hooked up to speakers / load with a signal to avoid damage?  Or is the same true of SS as well?

[FullRangeMan]

Fortunately I didn't have any signal / music playing (or I would have had the speakers connected - duh).

Why are tube amps different than SS in needing to be hooked up to speakers / load with a signal to avoid damage?  Or is the same true of SS as well?

Why are tube amps different than SS in needing to be hooked up to speakers / load with a signal to avoid damage?
As Dave said it is because of the OPT in the output, it is a device that heats up a lot and becomes short-circuit electrically if not there is a suited output to the signal(speakers).

Or is the same true of SS as well? There is many SS amps different cirduits, some are instable without a load also, so the manufacturer put a small capacitor before the output terminals.

Your amp is an OTL??
If so it dont have a output transformer to be damaged.

[jarcher]

Why are tube amps different than SS in needing to be hooked up to speakers / load with a signal to avoid damage?
As Dave said it is because of the OPT in the output, it is a device that heats up a lot and becomes short-circuit electrically if not there is a suited output to the signal(speakers).

Or is the same true of SS as well? There is many SS amps different cirduits, some are instable without a load also, so the manufacturer put a small capacitor before the output terminals.

Your amp is an OTL??
If so it dont have a output transformer to be damaged.

Thanks for the explanation.

None of my tube amps are OTL.  Will be even more careful in the future to make sure they are hooked up to speakers before turning them on though. 

[Steve]

Hi Jarcher,

Got a couple of seconds. For those new, and for simplicity's sake, the main reason is if no load is present, primary or secondary load, an inductive kick or huge voltage spike may occur, thus breaking down the insulation. This can be either primary to secondary winding, or primary to primary winding. Another possible problem is exceeding the maximum plate voltage of the tube.

The peak voltage produced is determined by the rate of change of current through the output transformer's (opt) primary winding. A load is present to "absorb" the spike current, thus reducing the spike voltage presented. Anything that produces a steep slope, high frequency, input signal may produce a spike voltage, or transient voltage spike.

With no load or virtually no load, such as using a tetrode or pentode with high plate resistance, and no speaker, a spike voltage may be produced and cause problems. A triode, or even ultralinear configuration, places a load on the transformer due to their relatively low plate resistance, regardless of speaker load. Negative feedback also tends to limit the spike voltage.

However, it is still good practice to present a load.

No such problems usually exist in SS equipment because usually, not always, an output transformer is not used.

Short circuits are more of a concern with SS amps due to what is called thermal runaway. This is due to excessive current, and self heating of the solid state devices. This should not be a problem in well designed amplifiers, but still, caution should be taken just in case.

Hope this helps in understanding Jarcher.

Steve

[JakeJ]

Thank you Steve Sammet!

Let's also always remember to turn on the preamp then the amp(s).  Turn off the amp(s) then the preamp when shutting down the system for the day/night.  This prevents any power surge from the preamp causing a transient and making your precious speaker drivers from doing the xmax boogie possibly causing damage.  This is true for SS as well.  Speaking from experience over nearly 40 years of audio involvement.

Of course a quality designed preamp that includes a mute switch will preclude a turn on thump too.

[jarcher]

Thanks Steve for the detailed explanation.

How likely are these spikes, what kind of damage do they cause under "no load" conditions, and what kind of tube amp designs are more or less sensitive to this kind of damage? The answer to some of these questions have already been touched on, it would be helpful to get further explanation.

[Speedskater]

A most excellent explanation Steve!
I had it happen at work in the R&D department. We had a big 750V AC test stand. When the secondary fuse opened, the voltage from the collapsing field shot through the roof. It took out a very nice Fluke DMM. 

[JakeJ]

Jarcher,

Steve can fill in with greater details but the primary concern is that the current spike will cause the insulation breakdown in a transformer.  The insulation is not like the stuff on your speaker cables or the Romex in your walls, it is a thin varnish type that allows the windings to be close together so that a magnetic field can occur.  Once the varnish melts somewhere in the long wires that are wound around the core it ruins this capability.  The transformer can not function as desired and must be replaced or repaired.

Causes of the spikes can be from a variety of things from poor design to inrush current at turn on.  Hopefully Steve will revisit this thread and add some detail and/or correct any blunders in my post.

When I work on an amp on the bench I always connect a big 10W 8 ohm sand block resistor across the speaker terminal so the OPT has a load at all times.  Cheap insurance.

[CSI]

Some tube amps are happy to run with no load. But they are the exceptions. I just bought a Decware Mini Tori II and Steve at Decware says it doesn't care if there is a load or not when on. But he cautions that it is an unusual amp in that regard. The best advise, as always, is check with the manufacturer of your amp for his recommendations. Then follow them.

[Steve]

Some tube amps are happy to run with no load. But they are the exceptions. But he cautions that it is an unusual amp in that regard. The best advise, as always, is check with the manufacturer of your amp for his recommendations. Then follow them.

Updated: There are several conditions that can allow an amp to be run without a speaker, as a low ohmage load is present.

1) Most any wired triode, such as 2A3, 300b, 845, SV83, SV84, and 6550/KT88/EL34 etc or even ultra linear mode will work. That is because the plate resistance of the triode, or UL mode, presents a nice load the primary of the OPT. The plate resistance is in the order of 600 ohms for 6550/Kt88s, 300bs, 2A3s, while the SV83, SV84s are nearer to 2k ohms or so. So the larger wired triodes actually dampen better, create a lower ohmage load, than the smaller wired triodes. Better damping means less chance of voltage spike.

2) Some amps have a resistor, or resistor series capacitor connected across the primary or secondary of the output transformer. Again a low ohmage/reactance load is presented to the OPT.

So practically any triode or UL amp will work with no load. With that said, it is still good practice, as Jake advised, to provide another load, just in case.

Cheers