Asking for a friend - wrong ohms

[AshInAmsterdam]

So the bassist in our band has a London City tube head that he loves very much. Due to speaker problems he plugged it into our rehearsal rooms cabinet at 4ohms (switching his amp accordingly). The following week he forgot to set the switch back to 8 ohms before plugging into his cabinet.

He played for about 3 hours into an 8ohm cabinet with his amp set to 4ohm output.

Suddenly his amp went quiet. Power appears to be intact - the valves were all still glowing and the blue lighting on the front panel was still lit.

Anyone want to give some suggestions of what this ohm mismatch might actually have done to his amp? All I can find searching is "it's bad mmmkay", or some people saying that 4ohm amp into 8ohm speaker is ok and others saying it is definitely not ok, without any solid info as to why.

Thanks.

 

[delta7fred]

Eliminate everything else first, try a different bass, speaker cab, speaker and input leads. (It is human nature to assume the worst and blame the most expensive part.)

If that makes no difference then it is time to get it to a tech as there is insufficient information to make an accurate diagnosis.

I ran my Carlsbro tube head (poor man's Marshall) on the wrong impedance setting for years with no damage. I had it set to 8 ohms when I ran 2 x 412s (16 ohms each) and forgot to change the setting when I went to one 412.

 

[AshInAmsterdam]

Thanks. Yeah we tried the other cab known to work and his bass and cables are fine. Was just trying to gather any info to calm him as he fears an expensive repair, having recently had the amp rebuilt.

Ho hum such is life.

 

[rodl2005]

Most. ..not all. .tube amps will handle this. Try it again with correct impedance (ohm) load & settings.
If no go. .THEN start troubleshooting

 

[agedhorse]

If he recently had the amp "rebuilt", that's the first place I would look to see if something was not done right. Then, consider other causes such as a bad preamp tube, etc.

 

[tzohn]

Why was it rebuilt? Check the electronic parts that were replaced or return it to the guy that rebuilt it for check. Also you mentioned he had speaker problems, was it fixed properly before using it again?

 

[Downunderwonder]

I believe you wear out power tubes if the impedance is double the tap and you go for gold on the volume.

Some people pootle away on an SVT with single 8 ohm cab for years with no problem.

So listen to Agedhorse.

 

[Gumps]

Definitely use the rebuilder's recent work to take it back for a look. What was rebuilt in the amp?

 

[Spidey2112]

I always thought that as long as your cab impedance was higher than what your amp wants to 'see', you'd be OK... you may not get full power, but it should still work. Also, 3 hours seems like a long time for a heat related problem to crop up, unless something heated to failure... any unusual smells, Ash, when it stopped working?

 

[agedhorse]

I always thought that as long as your cab impedance was higher than what your amp 'sees', you'd be OK... you may not get full power, but it should still work. Also, 3 hours seems like a long time for a heat related problem to crop up, unless something heated to failure... any unusual smells, Ash, when it stopped working?

With tube amps, you really want to match the amp to the cabinet impedance. Generally there's not much of a problem being off by one nominal impedance tap, but this is a generalization that is sure to have exceptions.

 

[Bassmec]

Is there a blue glow in the output tubes indicating the presence of HT.
Have you checked the HT fuse.
Worst case is its damaged the insulation in the output trans due to the very high voltages generated by running into too high output impedance.
Normally that only happens with an amp being driven into infinity ohms or open circuit.
Running a tube amp into the wrong impedance is not usually immediately fatal, just like driving a car coast to coast in second gear is not always fatal.
Just not recommended by people that don't supply replacement tubes and output transformers.X

 

[Old Garage-Bander]

Eliminate everything else first, try a different bass, speaker cab, speaker and input leads. (It is human nature to assume the worst and blame the most expensive part.)

I used to repair TVs on the side, (how they get on their side, I'll never know) without fail, people would always go right to, "Do you think it's the picture tube?" (This was before flat panel TVs.) "Can you take a look at my TV?" "The sound is out." "Do you think it's the picture tube?"
In all of my years of doing that I only replaced one picture tube. Ironically the new tube had a problem and I had to re-replace it.