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Author Topic:  tube amps and speaker load
Tommy Boswell

 

From:
Virginia, USA
Post  Posted 27 Jan 2022 6:59 am    
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What's the downside of using an 8 ohm speaker with a tube amp designed for a 4-ohm load? My Fender Bassman 70 sounds great with Eminence PF400. Any chance of damage to the amp over the long term?
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Bill Hatcher

 

From:
Atlanta Ga. USA
Post  Posted 27 Jan 2022 8:45 am    
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it wont hurt that amp at all. a bit less efficient, but dont worry about it.

if the scenario were reversed....then there could be issues.
8ohms out into a 4ohm speaker would not be advised.
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David Ball


From:
North Carolina High Country
Post  Posted 27 Jan 2022 11:55 am    
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Old Fenders with an external speaker jack run the internal and external speakers in parallel, so two 8 ohm speakers would result in a 4 ohm load to the amp. You lose some efficiency, but tube amps are pretty resilient.

Dave
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Carl Gallagher

 

From:
New York, USA
Post  Posted 27 Jan 2022 12:19 pm    
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Fender amps are very forgiving when it comes to impedance mismatch,100% in either direction is safe, so an 8ohm output can use 4, 8 or 16 ohm load.You probably wouldnt even notice the mismatch.Other amps may not, like Marshall, they are notorious for blowing the OT with any mismatch.
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David Ball


From:
North Carolina High Country
Post  Posted 27 Jan 2022 4:58 pm    
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Fenders are forgiving. Don't know much about Marshalls I'm afraid.

Fender amps had shorting sockets on the output so that if no speaker was attached, the output would be shorted to ground. Safer than having it open in those circuits. The very opposite of solid state amps where open is safe, but shorted is sudden death!

Dave
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Dave Mudgett


From:
Central Pennsylvania and Gallatin, Tennessee
Post  Posted 27 Jan 2022 6:02 pm    
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Fender output transformers were relatively robust when new. The issue with an old Fender amp that I would be concerned about is what a transformer has been subjected to in the last 50+ years.

And definitely - a typical tube amp tolerates too low an impedance than too high an impedance. The issue is flyback voltage from the power tubes back to the output transformer, which can cause arcing and destruction of the transformer. For example, see these threads:

https://bb.steelguitarforum.com/viewtopic.php?t=334784

https://bb.steelguitarforum.com/viewtopic.php?t=228139

https://bb.steelguitarforum.com/viewtopic.php?t=197354

https://bb.steelguitarforum.com/viewtopic.php?t=181272

Finally - I think it matters just how hard one is gonna push an amp with a mismatched load. A lot of players push their amps pretty damned hard. In this case, I would suggest running the correct load impedance. And be careful extrapolating this 'robustness' to other amps. For example, old Marshalls are well known to be sensitive to load matching.

With all of this said, I personally prefer to run my old tube amps, regardless of make, with the correct load impedance. YMMV.
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Tommy Boswell

 

From:
Virginia, USA
Post  Posted 28 Jan 2022 6:34 am    
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Wow, thanks for the answers!
Rolling Eyes

I guess I'll be looking for a second PF400. For now I've got a PF350 I can run parallel.
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