I'm goint to install a fuse (or two. or three.) in my KT88SE amp since I'd hate to fry an output transformer. I see a lot of amps placing the HT fuse on the CT of the power transformer, but would it be a good idea to fuse the output tubes separately in addition to this? Something like 500mA-1A, depending on inrush current, on the CT and around 200-250mA on each tube?
You should use a slow blow fuse rated at the maximum current your power transformer was designed to provide.
The Valve Wizard
Placing standard fuses in the DC part of the circuit (as well as in CT) is generally not a great idea for the reasons stated in the above article. But if you stil want to go for it, at least make sure you've protected second grids too (if your KT88s run in pentode mode) - having the HV on G2 without having it on the plate is REALLY BAD.
Placing standard fuses in the DC part of the circuit (as well as in CT) is generally not a great idea for the reasons stated in the above article. But if you stil want to go for it, at least make sure you've protected second grids too (if your KT88s run in pentode mode) - having the HV on G2 without having it on the plate is REALLY BAD.
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OK, I'll try to install one on the CT first and just figure out how small I can go by testing. I only run it in triode or UL, and I also have silicon diodes in series with the tube rectifier, so if one rectifier fails short I have a backup.
I thought about having a smaller fuse on each of the output tubes, so if one fails I'd know which one it is. The problem I see with it is that the voltage would go up a bit on the remaining tube, but I don't know if it would be unsafe levels.
I thought about having a smaller fuse on each of the output tubes, so if one fails I'd know which one it is. The problem I see with it is that the voltage would go up a bit on the remaining tube, but I don't know if it would be unsafe levels.
If you fit an HT fuse to the CT add a reverse diode to ground. This will stop the voltage going a few KV negative if the fuse blows causing an arc inside the fuse.
Use a proper high voltage DC rated fuse - if a fuse is arcing at DC, it may not stop until the unit's on fire. With hundreds of watts available from a PSU a fireball of some size can be sustained.
Such fuses are not as cheap.
Such fuses are not as cheap.
Fuses in the HT side definitely affect SQ so that's another reason to put them in the AC part before the rectifiers
Now that I've thought about it, I think I'm going to put a pair of fuses before the rectifier as in the link. I have a 25 ohm NTC on each leg of the primary so inrush current shouldn't be so bad, I'll just have to experiment with how low I can go with the amp rating.
I'd feel safer still with a fuse on the B+ of each output tube though. Is it a good idea, and if it is, how should it be implemented on a cathode biased amp? Maybe I'm just being paranoid about it since I just fried a catode resistor 🙂
I'd feel safer still with a fuse on the B+ of each output tube though. Is it a good idea, and if it is, how should it be implemented on a cathode biased amp? Maybe I'm just being paranoid about it since I just fried a catode resistor 🙂
Could you explain how a few kV would develop?
If one side winding was conducting and the CT fuse started to open, the fuse won't extinguish easily until the next current zero crossing - which would be close to the voltage zero crossing due to the heavy current for a capacitor input filter.
A choke input filter would be different, and more difficult, but the winding halves get hit for six anyway in normal operation during commutation.
Ipso facto, don't fuse the B+ feed, fuse the CT.
Showdown, you can end up selecting a fuse rating by trial and error. You can measure the the CT rms current by inserting a low value current sense resistor, and choose something higher as a starting point. You can also estimate the CT current using simulation, and use some design guidelines, as per link below.
https://www.dalmura.com.au/static/Valve%20amp%20fusing.pdf
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Sorry as usual misread the thread. I put a fuse not on the power transformer but on the centre tap of the output transformer for each channel. It was a 1KV part but even so I added as reverse diode IN5404 from CT to ground to catch the current flow from the inductance of the output transformer if the fuse were to blow.
Use proper (not cheap) fuse holders with ceramic fuses. There are a few better ones available for small 5x20mm fuses that are rated for 600V AC/DC (Schurter for instance). If you go with larger fuses 10x38mm, most fuse holders are rated for at least 600V and 4kV dielectric strength.
Johan, there is no need to use a ceramic (high rupture rated) fuse for most valve amps as the prospective fuse current doesn't exceed 35A. A 250VAC fuse is also often quite acceptable to use in a typical valve amp HT secondary due also to the current limited nature of the secondary side windings - some of the fuse manufacturers provide that advice in their tech notes, such as by Littlefuse:
https://www.littelfuse.com/~/media/electronics/application_guides/littelfuse_fuseology_application_guide.pdf.pdf
https://www.littelfuse.com/~/media/electronics/application_guides/littelfuse_fuseology_application_guide.pdf.pdf
Why? From the fuse's viewpoint it doesn't matter where in the current circuit it is positioned. It's just a matter of the holder's insulation capability.
Best regards!
I'm sure quality fuse holders rated for 250VAC are fine. It is cheap brands that could be susceptible. But it can't hurt to have quality ones that are rated for even higher. Someone on this forum had problems with breakthrough to chassis. He solved the problem by putting the fuses on ground side of B+.
I've installed 250VAC fuse holders on each of the secondaries now. They're the cheap kind but I've insulated them well from the chassis to prevent arcing. A lot of guitar amps I've seen use this kind so I guess they do fine. I've also mounted a 1R/4W resistor on the CT so I can monitor the current draw when I fire it up again.
A fuse has a much harder time rupturing when the current is DC (as in a filtered B+ feed), as compared to when the fuse is passing pulses of current where the current and voltage periodically reach zero (as in the CT connection, or a diode anode connection).
Ubiquitous 250VAC rated fuses are not rated for DC applications.
Fusing the B+ feed, say to an output transformer, may cause a high voltage transient on the output transformer winding if such a fuse ruptured, due to the DC current flowing in such a high inductance path.
Ubiquitous 250VAC rated fuses are not rated for DC applications.
Fusing the B+ feed, say to an output transformer, may cause a high voltage transient on the output transformer winding if such a fuse ruptured, due to the DC current flowing in such a high inductance path.
This wasn't clear in your post #11. One could place the fuse between the rectifier output and the 1st filter cap. In this case it doesn't matter where exactly - besides the holder's insulation capabilities.
Best regards!
Best regards!
Yup I should have clarified that although a fuse in the CT line is most common, it's function is the same in the rectifier + output going to the first filter cap, and that location can be the preferred location when a tap or take-off is used for a bias supply from the HV winding.
I guess some countries may often have 115VAC rated fuse holders, and some may unknowingly use them for this application.
I guess some countries may often have 115VAC rated fuse holders, and some may unknowingly use them for this application.