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Tube amp with no load

B

Blue Star

Active Member
Hi guys,

I am a bit confused on tube amp startup or run without any load i.e. speakers connected. All the literature on net says that tube amp's should never be started up without speakers connected. I have been following the practice. To be honest I purchased two set of speakers such that I dont start up anything by mistake and fry it.

Last week I was in HK at a Chinese tube amp dealer who is really knows a lot. He mentioned there is absolutely no issue running a tube amp without speakers connected. He mentioned the power transformers can absorb anything. To demo he took out the speaker cables while the amp was powered and running music.

So which one is true ? or its just that newer tube amps have some protection built in ?

Thanks
BS
 
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Sure you might get away with it the majority of times , but I wouldn't want to risk the Output Transformers from a voltage spike. He probably figured it was an old wives tale as he was getting away with it.

Randy
 
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You can do it, but it's not recommended. Voltages can get up high enough to arc the output tubes or break down the output transformer insulation. Can you drive your car 55 mph in 1st gear, sure but....

That being said, starting up the amp unloaded with no or very small signal is not likely to hurt anything, but why would you?

As far as any transformers taking "anything" - That's total fertilizer. Any device can be damaged/destroyed. I was a service tech for years and have seen speakers that came in for re-coning where the cones had actually caught fire. A well known '80's group playing the Syracuse Dome. Wanted them tommorrow- before the glue was dry.
Never say something like that, there's people out there that laugh at what's unbreakable to the rest of us.

I don't understand the 2 sets of speakers thing - Don't you leave them connected when you're not using the system? And if you can remember to connect one set before you power it up, where's the 2nd set come into play?
 
I would not start the main amps without the speakers connected. The reason I ask is this. I have just bought a tube amp to run my headphones in office. It allows for a speaker/headphone selector. The dealer told me its perfectly OK to run this amp without any speakers connected which is contrary to whatever I had heard earlier.

http://www.toneimports.com/lmaudio/mini218.html

On the two set of speaker thing. I have two tube amps at home and I keep each connected to one pair of speakers all the time. The KEF and JBLs have different signatures which I like as a variation plus keeping both conencted gives me peace of mind that I wont fry one accidentally.
 
I wouldn't loose any sleep over it .Most well built tube amps are stable with out a load , If you don't have them driven , anything that was originaly in a console or has been modified , or put together as as a kit i would buy or make dummy load for the amp . Two 20 ohm 10 watt resistor would be fine across the output terminal shoud be fine as a dummy load for your amp
 
The amp with the headphone out has a internal load resistor which keeps the transformer balanced when using the phones. Usually it's just a 8 ohm load with a voltage divider to drive the headphones. It's switched in by the headphone jack usually, but sometimes there's a selector switch.

With this arrangement, your headphones count as the load when you are using them, so you're not running the amplifier unloaded. The speakers are silenced (internally disconnected) and not providing a load when running the phones anyway - they're out of circuit even if you have them hooked to the speaker terminals.

If you are using it as a headphone amp only, you don't have to connect speakers, just make sure the phones are plugged in before power up.
 
If you amplifier has global negative feedback, it is usually safe to disconnect the load.I understand the requirement to switch speakers between two amps. I have similar setup for listening tests. So far I didn't see any ill effects. I just to not run tube amp for long time without load. But few minutes in that mode didn't damage it yet. If there is no global feedback, then yes, there is a problem.
 
You don't want to drive a tube amp into clipping with no load. You can drive it into clipping. You can leave it unloaded. Do both, and the only question is WHERE it will arc over - if you're lucky, you'll only destroy a tube socket, but output transformer is more likely.
 
I've always followed that theory also, but watching Dennis Had work and play with his amps is pretty interesting. he has no qualms about having them running without a load for at least short periods of time. He tends to treat them in a manner which I would have called rough, but I suppose if you've been building them for 40-50 years, and you know your circuit's stable, you'll know what they can take.

Of course, I've also seen him set one of mine on fire....:yikes:
 
Blue:
Last week I was in HK at a Chinese tube amp dealer who is really knows a lot. He mentioned there is absolutely no issue running a tube amp without speakers connected. He mentioned the power transformers can absorb anything. To demo he took out the speaker cables while the amp was powered and running music.
Click to expand...

Whenever a dealer mentions there is absolutely no issue... :thmbsp: :sigh: :scratch2: :no: Anyway, that so-called dealer might seem to know a "lot" but he is a far cry from knowing and understanding the actual basics and the crucial theory behind the issue. Not sure if Blue got confused there where the 'dealer' seems to have made that claim about the power transformer, but it's the output transformer(s) and output tubes that are at risk of damage. The power transformer is relatively unlikely to be affected from that in this case, due to the configuration of the power supply's related components, so maybe that dealer is ever so slightly 'right' (not). If half the story is only half or less right, it's still half -or more- wrong.

Everything that has been mentioned so far doesn't include one of the 'primary' (transformer humor) causes for the risk of damage. This is where understanding actual theory that applies to the real world might help with grasping the concept.

Yes, you might be able to get away with disconnecting the spkr load from some tube amps. Just take a look at a whole slew of tube-based guitar amps that have a 1/4 inch spkr plug you can pull (most do, some don't), but this example of mine is not the entire scenario and here is why: What you might not know is virtually all the tube-guitar amps with removable spkr plugs have shorting jacks to protect the output transformers from the no-load condition. This widely accepted manufacturing routine should serve as a major clue.

Feedback might help but it certainly has limits, so it can only do so much, which in most cases is nowhere nearly enough to prevent an arc over condition. I can personally guarantee from both firsthand knowledge of the theory and real-world experience that feedback is no 'guarantee' at all. In other words, feeling all warm and fuzzy about the presence of feedback could see you heading straight into damage-land, so feedback is definitely a moot point.

The most critical issue is the buildup of spikes that occur during power-up with no spkr load on the secondary. This is one of the major factors that are the root cause for the excessive and damaging spikes of voltage that feed backwards thru the output transformers and into those output tubes that are connected to the primary windings. This and other conditions may apply.
 
Completely agree with Tom. Of course, there's always the poorly designed NFB amplifier too, that goes unstable without a load connected. The original Heath W-3 was a good example of this. It didn't hurt the OPT, but any tweeters connected were always in immanent danger! It took the W3-AM to resolve the stability issues, and included the first application of Heath's much ballyhooed "tweeter saver" circuit -- a Zobel network across the OPT's secondary to always maintain a minimum load at supersonic frequencies.

Dave
 
It doesn't take much of a load to be safe for a stable amp. For example, my Scott LK-72B has a speaker on/off switch. When the speakers are off, it connects a 60 ohm resistor to each channel's 16 ohm tap. This is enough to prevent thousands of volts from arcing somewhere.
 
Hiya,

Yet another thread that we need an expert consensus on and a Sticky.

My doofus opinion is the same as it always has been. Adding a resistive load is too easy to not do unless someone is impatient or a cowboy or something.

I find even the best cowboy will get bucked from the bronc eventually. So its just a matter of time before one encounters a situation where they wished they had a load on the amp. I don't think the reverse is ever true. When has a load on a amp ever caused a problem assuming the amp does not already have issues that in fact would be an issue no matter what. (Yes I run them sentences long and longer to the point of absurd. Consider it a mental exercise)

Frannie
 
I don't think the issue is stability.

I think the issue is leaving an inductor (output transformer secondary) open while current is flowing through the primary. Inductors like to maintain a current flow. With no load, the secondary will want to maintain a current flow. With no current, the voltage will rise. and rise, and rise, until it is large enough to arc through varnish, or arc over an air gap.

Now, I've worked on many tube amps and have never lost an output transformer or any of that. When your on a test bench, you don't really have the time to shut everything down before connecting and disconnecting stuff.

Was it because negative feedback was used? Was it because it's a non issue? Was it because there's usually some non apparent leakage path that prevents the extreme voltage rise? I don't know exactly, but I have never confirmed the dangers in all the stories I've heard about it.

On the other hand, why tempt fate?
 
I also tend to err on the side of caution. I have accidentally (and a couple of times on purpose before I knew better) run things with no speakers. I recall being amazed that the output transformers will sing with no speaker hooked up. I got lucky and never blew anything up, but now I hang a load resistor on there just to be safe. Mine are actually 20 ohms, which isn't absolutely ideal but they're on hand.
 
Jon -- I agree that stability (or the lack of it) is not the basic issue. What it is though is potentially another "signal" source that unsuspecting users might not be aware of. With no input connected, and otherwise stable amplifier will care little if a load is connected or not. But if the amplifier in question is unstable and the load is removed, the instability can be all the signal that is needed to damage the OPT, as surely as if a signal had actually been applied. The end results are the same.

Dave
 
Would anybody happen to know about a Knight KN928... It has a Phones jack and a switch, but the switch only kills the speakers, the phones are always on... Safe to run without speakers connected or turned on? Seems obvious but I just want some encouragement!
 
Why not play it safe and always keep a load on it when you turn it on. If by chance you should forget, and the volume is not too high you still might be safe, but I wouldn't chance it. I never new that it could be damaged if driven hard with no load untill I read it myself. I don't have a Tube amp, but might somedy induldge myself, so I want to know something about them.

My own ignorance about Old tube radio's many years ago is the reason my Old Plastic Tube AM Radio EXPLODED when I connected the the Antenea ground to a water pipe. I had no Idea the chassis was live.
 
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hmmmm......I guess I am a skeptic.....when we say we shouldn't leave the output trannies unloaded becuase voltages will climb cuasing arch overs and the like....I can't help to wonder why arch overs and the like does not occur with unloaded iso-taps and other devices that have unloaded windings....even my monster AVS-2000 when plugged in and if not loaded....no cautions from manufacture about loading.....what about unused taps (16 ohm tap) no arch over problems? so if we assume there is a problem...maybe with reflected inductance or something...then maybe a small zobel network tuned to a frequency below potential speaker ranges.....if we say speakers can play at 20hz (more like 30 to 40), so a zobel network tuned to less than 20hz would take care of reflected inductance without disapating power like using a 60 or 600 ohm resister would. just scratching my head and thinking out load. it's not like these things are thelsa coils.
 
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