• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Fusing speaker outputs

I’m 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.
 
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.
 

adason

Member
Paid Member
2004-11-10 8:31 pm
Maryland
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.
 
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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Hammond organ did it in a vacuum tube model. Saved some transformers. Totally enclosed fuse holder in plastic, open clips gather too much dust. As above, totally insurance compliant types will put a 3 phase mains fuse holder in, measuring 2"x2"x2.5". I have never seen a single phase 600 v fuse holder for sale. Have to leave room for the handle to swing out to replace fuses. Has to mount on a separate bracket sold in 8' pieces. Total p***. Fuse is rated 600 VAC, no DC rating.
Tried AGC 15 fuses (250 vac rating) in a Peavey 1.3k on flying rails which go to 180 v when faulted. Dodgy output transistors shorted, fuse blew. Not guarenteed by Buss to work every time.
Appliance safety is guarenteed by the mains fuse at the input of the transformer. Prevents building fires. The output fuse only protects the transformer, which does not catch fire if the output is shorted. Shorts only burn the insulation inside. $80 problem. Human safety? - serviceman should never touch B+ with his hands until it is measured at below 1 v.
 
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baudouin0

Member
2019-03-22 5:41 pm
UK
If you do put a fuse on B+ make sure its one of the longer ones, and put a catch diode to ground on the transformer side of the fuse. The primary inductance is enough to cause several KV across the fuse if it blows which will cause it to arc. The catch diode will conduct the primary current away until it reaches zero.

I think it can save other components including the output transformer in certain situations. Try and use an enclosed holder if possible.
 
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Fusing the B+ to the output transformer is nono for a penthode amp. If the screens still
have voltage when the fuse blows the tubes will be destroyed.


A fuse for B+ to the output transformer is ok with ul and triode coupled penthodes though.


A better alternative is to connect the cathodes to ground with a small ( 1w) metal film
"flameproof" resistor of 10 ohms. If a tubes flashes the resistor will evaporate faster

then a fuse protecting the output transformer.
 

kodabmx

Member
Paid Member
2011-10-31 1:00 am
Toronto
+1 for the metal film resistor method but wirewound actually blow faster in my experience (if you have a high current tube amp anyway -- they are 2W) not the cement ones, this kind: WNC10RFET Ohmite | Resistors | DigiKey

They are probably already there for setting bias current (my designs all use them anyway). They also make a good fuse for the screen grid if say a plate cap loses connection (220R instead of 10R though).
 

disco

Member
2006-04-17 6:27 pm
Holland
2W/220R=95mA... 100% instant overload capability=180mA. What would go first, G2 or the "fuse"?

George mentioned a current limiting part last month, I don't recall the name but they are disk shaped and often blue :)
So, fuse in the ground return and one of those go from secundary to ground.
 
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Fusing the B+ to the output transformer is nono for a penthode amp. If the screens still
have voltage when the fuse blows the tubes will be destroyed.


OK for ultra linear as the screens come off the transformer -- Sorry you said that.


A better alternative is to connect the cathodes to ground with a small ( 1w) metal film
"flameproof" resistor of 10 ohms. If a tubes flashes the resistor will evaporate faster


The only reason I did not do that is that is leaves the metal base of the valve (6550A) at HT.


Actually I should have wired pin 1 to ground rather than pin 8 I think.
 
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kodabmx

Member
Paid Member
2011-10-31 1:00 am
Toronto
2W/220R=95mA... 100% instant overload capability=180mA. What would go first, G2 or the "fuse"?

George mentioned a current limiting part last month, I don't recall the name but they are disk shaped and often blue :)
So, fuse in the ground return and one of those go from secundary to ground.

220R 1/2W, the 2W are 10R. But I use 220R 2W on 6P45S and I've blown them without tube damage.