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Leben CS300 tube amplifer without load / speakers

I was doing some cable rolling on my system and removed the speakers cable from my Leben CS300 to plug the cables to another amplifier. Then I switched the Leben on for a while to warm it up....but forgot to plug the speaker cables again.... what an @#$^$% (repeated 1000x)



I left the Leben for approx. 60-90 min without the speaker cables plugged back. Positively there was no signal going to the Leben.


1.Should I expect any damages to the trafos or any other parts?
2.Any damage on the tubes?
3.Does the Leben has a protective circuit when no load is applied?
4. How can I check the internal damages?

FYI, it is soundwise ok, I didn't notice any impact on sound quality.

Please reassure-me (or make fun of me), I had too much sleepless nights already
cydest
viridian's avatar
viridian
4,184 posts
 
You have no damage to the transformers, or the tubes, or anything else for that matter. If there was a problem you would certainly hear it.

Tube amps are actually quite robust and generally not subject to the types of catastrophic failures that you see in solid-state gear. Rest soundly, you were stupid but got away with it.
rrog
1,958 posts
 
You have severely stressed the output tubes by turning the amplifier on without a load.
viridian's avatar
viridian
4,184 posts
 
I thought that you might enjoy this article. Note that it addresses tube amps that are actually driven without load, not just sitting there like yours was. So the article looks at worst case issues:

http://www.tronola.com/html/unloaded_vt_amps.html

BTW I love your amp and really wanted one, but it was just way beyond my budget. A less daunting situation was that it did not have a phono stage.

So, being good at working on amps, I purchased a Stromberg-Carlson ASR-433, which is a 12 watt EL84 amp from the golden age, with phono. I updated and upgraded it to assure reliability and proper performance.

Which lead to getting my hands on an Eico HF-81, yet another EL84 amp from the golden age........which lead to a Scott 299C, 7591 tubes and higher output.

And now I am working on a power amp, Acrosound Stereo 20-20 that too uses the EL84 tube. I love them all......but it was the little Leben that was my inspiration. Inspiration that has lead me down the path to perdition.....
gregm
3,021 posts
 
down the path to perdition.....
Fie on't! But the cause is sacred...:)
paulfolbrecht
586 posts
 
"You have severely stressed the output tubes by turning the amplifier on without a load."

This sounds like a straight-up guess.  In any case, it's completely wrong.

A tube amp's *output transformers* can be damaged (overheated) if the amp is *played* with no load.  If there is no SIGNAL (as the OP states), then there is NO problem.  In either case, there is no such thing as "stress" to the tubes.

Think about it: When the speaker wires are connected, there is nothing happening if there is no input signal to the amp - if there WAS something happening - some voltage present - you'd hear it out the speakers.  So how could merely disconnecting those leads make any difference to the amp?
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