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Old 4th June 2008, 02:33 PM   #1
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Default Output transformer protection fuses

Can anyone explain to me the mod to protect my output transformer on my amp with fuses? I'm just not sure what wires the fuses would go in line with. Also what value fuses would be the best to use?

Thank you
ERic
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Old 4th June 2008, 05:23 PM   #2
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If it's a P-P output, put the fuse in the center tap B+ feed lead. Usually red. If SE, put it in the B+ side of the primary. I use a slow-blow type of around to amp to allow for transients. If it's a loud rock guitar amp, you may need an even higher current one. These are the standard 3AG size mounted in a through chassis holder for easy changing. Wire the B+ to the bottom connector and the transformer to the side terminal of the holder. Don't be concerned about the 250 volt rating of these fuses as they're good for much more.

Victor
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Old 4th June 2008, 07:13 PM   #3
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Quote:
Originally posted by HollowState
If it's a P-P output, put the fuse in the center tap B+ feed lead. Usually red. If SE, put it in the B+ side of the primary.
That may not be such a swell idea. If using pentodes with a separate screen supply, poofing that fuse will remove the plate current, and the screens will melt down shortly thereafter. Your best bet is to fuse the cathodes. You can use a small cathode resistor (10R is a good value) that's poofable at a current that isn't likely to ruin the OPT. The cathode resistor can also serve as a current sense resistor for bias adjustment when everything's working as it should.

If using triodes or Ultralinear pentodes where there either is no screen supply, or the screen voltage disappears when the Vpp does, then the only concern is the voltage handling capability of the fuse itself. What you don't want is a fuse that can strike an arc when the metal strip melts. That "fuse" ain't no fuse at all, and will only serve to provide a false sense of security. Given the HV nature of hollow state, that's worse than no fuse at all.
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Old 4th June 2008, 07:24 PM   #4
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I would rather use diodes to protect transformers than fuses: tubes limit well max current, but without load connected they may create huge voltage that kills transformers.

Use diodes backward in parallel with each output tube. You may pull some out of old TV or a computer monitor.
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Old 4th June 2008, 08:34 PM   #5
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What are you protecting the OPT from?

I use B+ fusing in the line to the OPT in my triode amps.

My reason is to protect the OPT from accidents like hitting the anode connection with a grounded probe, etc.

I use a KTK style 600 volt rated fuse because in my humble experience the 250 volt fuses will sometimes arc when interrupting high voltage DC.

http://www.grainger.com/Grainger/ite...0Fast%20Acting

If you notice the price, you might surmise that the real protection is
that with a $20 fuse at stake I don't go anywhere near the B+ with
anything grounded!

For class A amplifiers I don't see the need for much over-rating. 2X or 3X the standing current ought to be OK. I'm using 200mA fuses on 100mA B+ with no problems. The fuse will respond to the average audio power which in class A is pretty much constant over signal levels.

How big are current transients in the output stages of class A amplifiers anyway?

For class AB/B it would be the current draw under full signal with probably the same multiplier, though I haven't tried it.

Cheers,

Michael
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Old 4th June 2008, 09:40 PM   #6
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Quote:
Don't be concerned about the 250 volt rating of these fuses as they're good for much more.
The fuses might be, but the through-chassis holders are not. I've had a holder arcing through from the connnector to the chassis with 400v B+. Made sparks both above and below the chassis and a hell of a row from the discharging of electrolytics in the power supply! It's better to use this type of fuse holder in the common/ground line.
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Old 4th June 2008, 10:11 PM   #7
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Fuses if used should be connected between filtering caps, especially in SE amps where load inductance is high, otherwise instead of protection you'll get a big trouble.
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Old 5th June 2008, 07:20 PM   #8
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Thanks guys for all the quick replies. My reasoning for doing this mod is to protect my output transformer from an accidental impedence mismatch or being run without a load for a time. I thought I remembered hearing that having a fuse inline with the OT somewhere could do this.

Thanks
ERic
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Old 6th June 2008, 03:07 PM   #9
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Quote:
Originally posted by Michael Koster
What are you protecting the OPT from?

How big are current transients in the output stages of class A amplifiers anyway?

For class AB/B it would be the current draw under full signal with probably the same multiplier, though I haven't tried it.

Cheers,

Michael

I use 630mA fast blow 20mm types (i.e roughly 1.5x full p-p load current) for my parallel-UL p-p amp which is adequate for hefty music bass beat note surges. This fuse rating may seem "close" but as I incorporate a 15Hz HP filter to rid sub audio signals, the fuse rating margin can be lowered.
A fuse might protect tubes but will save the more expensive o/p tranny windings from an unforgiving lo imp power supply and electrode s/c.

Fuses don't effect audio quality in centre tap B+ because the signals at this point are continuous duty. The effective resistance of the fuse can be ignored when compared to the resistance of the pri winding.
Not all tube amp designs can cope with signals below o/p tranny cutoff when tube currents increase (hammering) as core permeability runs out on smaller cores, as core goes towards saturation and waveform clips. This is where fuse blowing can be persistent, but is telling you something.

This can be serious problem. To avoid saturating the o/p tranny, the iron core area has to be roughly doubled for each halving in frequency.

richj
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Old 6th June 2008, 07:27 PM   #10
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Quote:
Originally posted by signalpaths
Thanks guys for all the quick replies. My reasoning for doing this mod is to protect my output transformer from an accidental impedence mismatch or being run without a load for a time. I thought I remembered hearing that having a fuse inline with the OT somewhere could do this.

No, fuses won't help when running without a load. Use diodes to shunt output tubes in reverse polarity. A fuse "inline with the OT" may do the opposite thing.
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