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Old 6th May 2008, 02:14 PM   #1
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Default Fuse between B+ and output transformer?

Does anyone use a fuse between B+ and output transformer? One-electron recommends it in this link as a safety measure in case output tube is shorted or lost bias. Would it be more of a concern for amps with fixed bias?
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Old 6th May 2008, 04:08 PM   #2
alejo is offline alejo  Argentina
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I´try this in my RH-807 Single End Amp. and the amplifier lost all bass.
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Old 6th May 2008, 04:11 PM   #3
kevinkr is offline kevinkr  United States
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I use a fuse between B+ and the output transformer center tap in most of my PP amplifier designs. To limit collateral damage to the amplifier it is a good idea to make sure that the power supply caps are rated to comfortably withstand the unloaded supply voltage when the fuse blows - it is also a good idea to put a snubber network from the output transformer center tap to ground to reduce the likelihood of arcing when the primary connection goes open. (the magnetic field collapses generating a potential flyback discharge through the weakest link.)

It is not a bad idea with cathode bias amplifiers either, although all of my designs use fixed bias. In that case it might prevent the cathode bypass cap from exploding if the tube fails - while not common I have seen this happen, witnessed it on my bench unfortunately, both scary and rather messy. The tube, a KT88 experienced an open grid or grid to cathode short, the tube was too damaged to determine which, and it was altogether a little more exciting than I care for. . FWIW the tube was a screened, burned in JJ KT88 with close to 0 hours on it.
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Old 6th May 2008, 04:19 PM   #4
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by alejo
I´try this in my RH-807 Single End Amp. and the amplifier lost all bass.

Not sure why this would be, the fuse resistance is tiny relative even to the dcr of the transformer primary. In a class A design the dynamic modulation of the fuse resistance caused by variations in plate current shouldn't normally be a concern. To be conservative you can always put a small cap across it to deal with any potential HF ac issues. (Distortion mainly)

There is so much talk about fuses coloring the sound, and the availability of tweak audiophile fuses has not helped the situation. I can't see spending $20 or more for a single fuse that I will probably blow at some point. (E.g. HIFI tuning fuses, etc.)

In the case of a cathode biased 807 SE it is very doubtful the fuse is needed, I only use them in PP amplifiers running at very high voltages, where the potential for a serious safety hazard and amplifier damage exists..
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Old 6th May 2008, 04:51 PM   #5
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I've used it between B+ and O/P transformer in an 6550SE. Was recommended to use one to minimize damages in case something went very wrong.
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Old 6th May 2008, 06:06 PM   #6
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In every p-p design I use a 20mm length fast blow fuse rated 630mA for parallel pairs (lowest resistance) in the centre lead of o/p tranny prim and a 1uF cap on the tranny side. Simple and very effective. It won't save the tubes unless a real surge comes, but fuse must handle peak signal excursions or effective rms rated.

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Old 6th May 2008, 07:08 PM   #7
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Default Fusing

Fuses operate on I squared R...they have a resistance value, They are a non-linear device, they heat up to break. Fuses rarely act fast enough to provide much protection....don't be using a glass type as they will vaporize and coat the inside of the tube & STILL conduct (found this out the hard way) ...use sand/ceramic types.
Consider the inrush current upon start-up...........too unpredictable for me....I just put them on the primary BEFORE the main switch.
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Old 6th May 2008, 07:12 PM   #8
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Using a slo-blo?!
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Old 6th May 2008, 07:48 PM   #9
kevinkr is offline kevinkr  United States
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Default Re: Fusing

Quote:
Originally posted by Richard Ellis
Fuses operate on I squared R...they have a resistance value, They are a non-linear device, they heat up to break. Fuses rarely act fast enough to provide much protection....don't be using a glass type as they will vaporize and coat the inside of the tube & STILL conduct (found this out the hard way) ...use sand/ceramic types.
Consider the inrush current upon start-up...........too unpredictable for me....I just put them on the primary BEFORE the main switch.
_________________________________________Rick..... .....

Usually no significant inrush current in the output stage unless a standby switch is being used in which case there could be an issue.

I have had the repeated experience of arcing fuses, but only with 20mm types, the larger 63mm ones generally haven't with one or two exceptions around 600V.. Sand/ceramic types are recommended.

Fast blow types will not save the failing tube, but in my experience they do reduce or even eliminate the colateral damage in every instance I have been able to make the comparison.

They aren't perfectly linear, but OTOH their resistance which is temperature dependent is relatively low at normal design operating currents. Despite this I don't generally use them in my own gear, and only fit them in certain large PP amplifiers I built that have owners who tend to do a little too much tube rolling and aren't as careful setting the bias as they should be.
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Old 7th May 2008, 04:46 AM   #10
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Quote:
Originally posted by soundbrigade
Using a slo-blo?!

The mains inrush is a big headache, using a 500VA transformer without inrush circuitry one has to rate an input fuse around 8-10A extra slow anti surge, such a value is hopeless for circuit protection other than a s/c. With inrush control (i.e timer and switched out ballast resistor) the fuse value can be far more realistically set much lower to proper requirements. The B/H curve with large toroids so close to saturation makes proper mains input fuse rating a hopeless case without inrush control.
In Europe with mains input distribution over-current trip relays common practice, these get "knocked out" each time such high VA transformers get switched on.

The fact that duty cycle of audio signals in output transformers operates in continuous current mode, with slow dv/dt and high Z compared to fuse inductances and resistance, it is doubtful that they have any sonic effect.
Bit like OFCable, a complete brain-wash fad.

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