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Fusing speaker outputs
Fusing speaker outputs
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Old 16th November 2019, 07:10 PM   #1
OldHector is offline OldHector
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Default Fusing speaker outputs

Iím not exactly sure why, but I have thought it was necessary to add fuses on the speaker outputs on the two amps I managed to get into a reasonably finished state. Could be that I had used an old PA as the donor chassis, and that had fuses.

I have read more on this topic since then, and seen finished amps, and it does not seem that anyone else fits protection for the speakers.

Does anyone fit fuses? I use 1amp fast blow, which was the consensus on another thread, but most of the people are using higher power solid state amps, and have parties, neither of which is applicable to me. My amps are 35W max.
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Old 16th November 2019, 07:18 PM   #2
Richard Ellis is offline Richard Ellis  Argentina
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Last I heard, you're not suppose to run tube amps without a load on the outputs...


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Old 16th November 2019, 07:23 PM   #3
petertub is offline petertub  Sweden
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Fusing speaker outputs
Quote:
Originally Posted by Richard Ellis View Post
Last I heard, you're not suppose to run tube amps without a load on the outputs...


-----------------------------------------------------------------------Rick........
This is very true. Fusing speakers without securing some load is generally a bad idea for a tube amp thad uses an output transformer.


To secure the amp one could parallell the 8 ohm output with a 100 ohm 5w resistor. This will secure from overvoltage without any audible costs.
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Old 16th November 2019, 07:25 PM   #4
baudouin0 is offline baudouin0
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Fusing modern transistor amps is a good idea as a fault can result in large DC on the speaker and cook the coil.

Valve amps have an output transformer which prevents a valve failure from putting DC on the speaker.
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Old 16th November 2019, 11:09 PM   #5
Galu is offline Galu  Scotland
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If it is of any assistance, Bose recommended a 1.5A slow-blow fuse for their model 301 speaker if the amplifier exceeded the speakers maximum 60W rms rating (for reproducing speech and music).
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Old 16th November 2019, 11:23 PM   #6
adason is offline adason  United States
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Quote:
Originally Posted by baudouin0 View Post
Fusing modern transistor amps is a good idea as a fault can result in large DC on the speaker and cook the coil.

Valve amps have an output transformer which prevents a valve failure from putting DC on the speaker.
DC sensing circuit is way better (faster) to protect the speakers then fuse. Before the fuse burns, your coil will see excesive dc and membrane will move excesively in one direction, may be mechanically damaging speaker.
Fuse is good when you overload speakers with too much power, like in disco or party.
No need for tube amp though. There will likely be no DC.
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Old Yesterday, 12:31 AM   #7
audiopro is offline audiopro  Canada
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Since you posted in tubes, I can only assume these are tube amps.
DO NOT fuse the speakers. What you want to do is use that fuse on the plate voltage for the output tubes. Put in the B+ of the output transformer. 500mA is plenty big enough.
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Old Yesterday, 01:21 AM   #8
PRR is offline PRR  United States
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I can't remember seeing a fuse on the speaker side of any transformer-coupled tube amp. Even my 150+W beasts.

These are TUBES, not unreliable transistors. A failed tube can smoke a transformer, but rarely hurts a speaker.
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Old Yesterday, 02:13 AM   #9
indianajo is offline indianajo  United States
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Always put 1k 3 w resistor or so INSIDE the chassis parallel speaker ports. Infinite impedance is very bad for transformer windings. Causes voltages at the terminals greater than the insulation voltage rating of the windings. I've tripped over ST70 speaker wire & pulled it off before. Back before the internet was invented: the source of all knowledge was the RCA Tube Handbook at the library, plus the '59 Ford Car service manual chapter 11.
Yeah, B+ fuse to output tube plates is fine. 1 1/4" AGC types usually don't arc over, although they are not rated that high voltage. Keep them clean, dust will arc over that close. Tube amps at sea, use 600 v rated fuses costing $10 apiece.
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Old Yesterday, 02:20 AM   #10
rayma is offline rayma  United States
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Quote:
Originally Posted by audiopro View Post
What you want to do is use that fuse on the plate voltage for the output tubes.
Put in the B+ of the output transformer. 500mA is plenty big enough.

Most common fuses are not rated for significant DC voltage. This suggestion is unsafe.
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