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My friend killed my amp

8/4/1999 11:21 AM
Tobias Karlsson
My friend killed my amp
Hi all of you!  
I was stupid enough to lend one of my old tube amps to a friend (a HagstrŲm 50w) and now he phoned me up and told me itís dead. I may only be a fuse thatís burned out, he hasnít returned it yet so I havenít checked it out but as he says, itís nothing wrong with the fuses.  
The thing is that the speaker output is 8 ohm and my friendsí speaker cabinet is a 4 ohm so I told him that he shouldnít use my amp before he could fix his cab or get another one wired to a 8 ohm setting. HE DIDNíT DO THAT!  
Anyway, I donít now much about tube amps but couldnít the amp be damaged really easy with an 8 ohm output into a 4 ohm cabinet, this canít be healthy for the output transformer...or?  
The other way around you just loose some effect but shouldnít cause any problem for the amp, right...?  
I would be very grateful if someone could bring me some info on this and in what ways this mismatch of impedance could damage my amp (...or maybe it didnít?). Iím really mad about this as the amp is a ďhard to findĒ type of amp.  
Tobias Karlsson in Sweden  
8/4/1999 12:49 PM

While your friend may have killed your amp through some means, it's unlikely that the use of a 4 ohm cabinet on an 8 ohm tap did it. So you may have to kill him for something, but ignoring the rule isn't it.  
Tube amps in general are completely tolerant of mismatched speaker loads of double or half. All this does is move the loading on the output tubes to where they are less efficient and so the amp limits its own power out, and nothing dies. Most tube amps are fine into a short as a matter of fact. Loads that are too high - like a 32 ohm load on a 4 ohm tap, maybe , or worse yet an open output - are the ones to watch for in tube amps. This lets the output transformer develop damaging transient spikes that can punch through the internal insulation or arc across the output tube sockets.  
Solid state amps are the opposite - they are completely tolerant of higher resistance loads, even open circuits, but intolerant of lower load resistances.
8/4/1999 11:19 PM
Tobias Karlsson

Thanks RG!  
I guess I canít blame my friend, itís just pure bad luck.  
BTW, these HagstrŲm amps were made not far from where I live. My amp is with a tremolo but I donít think the overall sound is so good. They also made guitars that I think is rather expensive in the US.  
Now when I have to fix it I can also do the service that it has been in the needs of for a long time. A good thing comes out of a bad thing.  

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