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 ½ to ¾ 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
Victor
HollowState said:
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.
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/it...use, Interrupt, 200K Amp At 600V, Fast Acting
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
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/it...use, Interrupt, 200K Amp At 600V, Fast Acting
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
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.
Michael Koster said:
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
The "Valve Wizard" recommends placing a fuse at each outer leg of the PT secondary, before the rectifier. This way if the rectifier shorts, the PT will be protected.
http://www.freewebs.com/valvewizard/fuses.html
http://www.freewebs.com/valvewizard/fuses.html
av-trouvaille said:
Cathode of the diode to anode of the tube, anode of the diode to ground, so if a speaker got suddenly disconnected diodes shunt the primary saving the tranny from overvoltage that can kill it.
TubeHead Johnny said:
Yep, good idea!
I once bought a good mixer console from a guy who failed to fix it changing capacitors, rectifiers, transistors in PS, but could not get rid of hum.I bought it very cheap, then replaced a fuse. It was nice console from Tascam, barely used.
signalpaths said:
What is the amplifier in question? Many amplifiers with no global feedback won't mind being unloaded. I've seen a couple with 200-600 ohm resistors across the speaker posts, perhaps as "temporary loading" or something along those lines. Impedance mismatch is not anything to worry too much about, especially on a single ended amp. Such mistakes will generally not be pleasant to listen to, and your ears will likely make you shut the amp off before any problems occur. If you load the amplifier too lightly, you might actually prefer the sound (8 ohm speake on the 4 ohm tap). Most folks employ protection devices for runaway tubes, which can happen for many reasons. In this case, a low wattage 10 ohm resistor under the cathodes (for fixed bias) of your output tubes will give INDIVIDUAL protection for each tube.
Impedance and output transformer damage
Dears -
The left output trafo of my VT-86PP was damaged after few hours of running. The impedance is OK (I am using an Acoustic Energy EVO 1 speakers, 8ohm impedance).
The amp begining with a "hummm" and scratches noise at the left channel, and after the grid of an EL84 melt. I imediately turn off the amp, but too late. The left output trafo was dead.
I searched at the Mableaudio website in the parts secction a new trafo to repair the amp, but I am still confused. The original trafo showed in the website appear weak, bad shaped.
Anyone can help me what may caused the trafo damage? Bad quality of the trafo or other cause?
http://www.mableaudio.com/www-web/shopview.asp?id=69&typename=output tranformers
http://www.mableaudio.com/www-web/shopview.asp?id=71&typename=EL84 amplifier
Thanks
No Noise
Dears -
The left output trafo of my VT-86PP was damaged after few hours of running. The impedance is OK (I am using an Acoustic Energy EVO 1 speakers, 8ohm impedance).
The amp begining with a "hummm" and scratches noise at the left channel, and after the grid of an EL84 melt. I imediately turn off the amp, but too late. The left output trafo was dead.
I searched at the Mableaudio website in the parts secction a new trafo to repair the amp, but I am still confused. The original trafo showed in the website appear weak, bad shaped.
Anyone can help me what may caused the trafo damage? Bad quality of the trafo or other cause?
http://www.mableaudio.com/www-web/shopview.asp?id=69&typename=output tranformers
http://www.mableaudio.com/www-web/shopview.asp?id=71&typename=EL84 amplifier
Thanks
No Noise
Kindly enlighten me
Dear Wavebourn,
I found your suggestion in the Tubestore forum.
I wanted to protect my expensive output transformer with a fuse , in case the tube draws sudden excessive current.
I am not a EE , so need your guidance " how to use diodes backward in parallel with each output tube."
The amp I uses is a single ended one.
Thanks
Paul Yahya
email addy removed to prevent spam
Dear Wavebourn,
I found your suggestion in the Tubestore forum.
I wanted to protect my expensive output transformer with a fuse , in case the tube draws sudden excessive current.
I am not a EE , so need your guidance " how to use diodes backward in parallel with each output tube."
The amp I uses is a single ended one.
Thanks
Paul Yahya
email addy removed to prevent spam
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