Question regarding fusing the B+ rail

[Retrovert]

If I add a 125mA fuse at each cathode, this would work at protecting the OT of a suicidal tube.

Nope.

It's better to fuse the output transformer because you aren't protecting from everything that can fail on that side of the tube. Remember, you have high voltage on the output transformer and are switching it to ground through the tube to complete the circuit. So something can fail in the tube other than the cathode, or outside of the tube, such that full current may pass though the transformer.

Same issue exists for fusing windings with a center-tap. Each leg typically gets a fuse.

Think global, fuse local.

Do they need to be high voltage here? I do not believe so as the internal resistance of a conducting tube never goes to 0. Unless we have a catastrophic short of course.

Why would it matter if the tube shorted or nearly shorted? What matters is that current through a device (the output transformer in this case) exceeds a devices maximum rating. A biasing failure can cause this, but so can lots of other things. Using a fuse on the cathode protects against bias failure, but your output transformer can be cooked by other causes. The internal resistance of the tube has nothing to do with the high voltage being interrupted.

A summary of what I have explained:

+ Arcs can jump across a gap depending upon the voltage. Every insulator has a breakdown point, and, yes, this includes the gap inside a fuse.
+ Higher voltages more readily arc across a gap than do lower voltages.
+ Fuse manufacturers clearly warn that using higher-voltage on a lower-voltage fuse will cause the fuse to internally arc, preventing it from blowing or causing it to catastrophically self-destruct. (See below)
+ Fuses are used in failure mode. They protect equipment when it all goes horribly, terribly, absolutely wrong.​

Conclusion: Don't cheap out on fuses. Safety first!

For a different take than LittleFuse, here's what Bussman has to say:

www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/solution-center/technical_library/BUS_Ele_Tech_Lib_Voltage_Rating.pdf

Most low voltage power distribution fuses have 250V or 600V ratings (other ratings are 125, 300, and 480 volts). The voltage rating of a fuse must be at least equal to or greater than the circuit voltage. It can be higher but never lower. For instance, a 600V fuse can be used in a 208V circuit. The voltage rating of a fuse is a function of its capability to open a circuit under an overcurrent condition. Specifically, the voltage rating determines the ability of the fuse to suppress the internal arcing that occurs after a fuse link melts and an arc is produced. If a fuse is used with a voltage rating lower than the circuit voltage, arc suppression will be impaired and, under some overcurrent conditions, the fuse may not clear the overcurrent safely.​

Why would you potentially risk blowing up hundreds of dollars of equipment to save a few dollars in fuses?

Sure, such failures are rare, but if they never happened you wouldn't be asking about fuses, now would you?

 

[trobbins]

Brice, there are a few things to consider if fusing a cathode of output stage.

One aspect is to limit the cathode voltage when a fuse blows, as the fault current may not be due to a faulty tube and the tube could be damaged if cathode voltage goes too high - a 150V 5W zener in parallel with the fuse is the simplest means for that protection.

If the fuse value was determined decades ago then it may need a quick assessment, especially as the fuse spec may be different. For a 'normal amp' the tube operating curves for Vg=0 (class AB1) and screen voltage identify the max current level through the fuse as worse-case, and a 50% duty cycle for cranked operation. That worst-case average current has to be handled by the fuse - so it can't be more than 70% of a UL284 fuse rating. A F fast blow is appropriate for that application. For a mac amp, the screen-cathode voltage is not normal, so a bit more effort may be required, or just wing it and use the vintage rating in a UL284 F fuse. If a fault does occur, then the fuse is opening a DC supply as there is substantial OT primary inductance, so my preference would be to use a 250VAC fuse (rather than say a 125VAC fuse).

The fault current through a cathode fuse depends on OT primary resistance, and possibly tube effective resistance, and B+ sag. For cathode fuses in a simple PP stage, there is likely not much over-current available for the fuse to operate in a short time - that would need to be assessed a bit more with measured resistances and an actual circuit. For parallel tubes in larger power amps, individual cathode fusing has better discrimination.

 

[gadget73]

There isn't any inrush issue when fusing the cathode, whereas the high voltage side has to charge up the filter network. The voltage on the cathode is much lower too, typically well under 50 volts. Its a lot easier to pick fusing for that sort of thing vs fusing 400 odd.

Yeah the cathode voltage will float high, but if the fuse blows there is a fair chance that the tube has some other problem anyway, and its not at the top of your concerns. Of course this will do nothing to protect the power transformer or output transformer in the event of a short to ground, though to be fair the only reasonably low impedance path to ground through a tube is through the cathode or perhaps the heater. There is also primary side fusing which will blow in the event of some horrible failure of the supply.

 

[Brice]

Yes all these are good points. Thank you.

Fusing each cathodes will present an advantage over the center tap of the OT, as this solution is more sensitive to a tube problem.

I realize that this will not catch all failure mode, but at least the most likely one like a lost of bias as rightly pointed out, and run away tube (it did happen to me with a new KT88 on this amp and the fuse did go off ).
That is good enough for me.
And I have the primary PT winding as a last resort if something terribly wrong happens elsewhere.

I am not trying to have a bullet proof solution, but just adding some measure to protect the OT from the most frequent problematic events, that's all.

 

[trobbins]

There isn't any inrush issue when fusing the cathode, whereas the high voltage side has to charge up the filter network.

Some caution may be needed with ss rectifier power supply, especially when coupling cap to output stage has a long time constant, and/or output stage is cathode biased with large bypass cap.

 

[gadget73]

Not sure what the SS supply would have to do with it since the output tubes would still start to conduct relatively gradually. Same with regards to the cap, the tubes will start to conduct as they come up to temperature which should pretty much eliminate any big surges to charge up the cap. Have you seen otherwise? I don't own any SS rectified cathode bias amplifiers. All of mine are tube rectifiers, though most of them are directly heated tubes.

 

[Brice]

Yes me too. I don't get it. Tube don't go online in an instant: unless there is a time delay switch for the B+ after preheating but that's another story.

And the coupling cap issue, do you mind giving us more detail ? Thank you.

 

[trobbins]

The concern would be for a hot re-start of the amp, where valve cathodes are hot, but power supply and coupling caps have discharged during the short time that an amp was switched off.

If mains power is reapplied fairly quickly, B+ could rise quickly in the order of a few mains cycles (less than 100ms). The coupling cap may have a longer time constant to reach normal bias voltage, especially for a hi-fi amp where coupling cap could be approaching a uF and grid leak is 100's of kohm. An output stage valve grid-cathode voltage could then be stuck at or near 0V for a short time until the coupling cap charged up sufficiently towards normal bias voltage. If screen voltage has also risen quickly, then the anode current rise would only be constrained by the L/R time constant of the OT primary and valve conductance (likely to be much less than 100ms), and so there is a chance of a short initial cathode current surge.

For a cathode biased output stage, grid-cathode voltage bias needs to charge the cathode bypass cap up initially from 0V, and the cathode bypass cap charges with an RC time constant related to the tube conductance, which could be more than circa 100ms and so there is a chance of a short initial cathode current surge.

 

[gadget73]

OK, so not something that you'd get on every day power ups, but maybe following a brownout or power flicker. I can see that.

 

[Brice]

Yes trobbins. You are correct. That is not something I am looking forward to. Although it did happen to me. since I have all my tube equipment on manual reset GFI type of device which when AC line power is lost, it's off. You need to manually reset it.
Thank you.

 

[pjrengineer]

Do Not Use Generic AC Fuses for DC Applications.

It ends badly for everyone, except for the fire department which has an exciting night out, instead of watching TV, and gets to meet your neighbors, who (hopefully) give them cookies. Your cookies, at this point, are all carbonized or soggy. (NB: Hyperbole. Your amplifier is probably slag, though.)

Safety is complicated and it really does matter what kind of fuse is used. Some won't work at all in DC applications.

A fuse is a non-linear device, and the datasheets explain the delays involved before it blows. The fuse essentially works by overheating and then burning out a segment. That's fine for AC, but for high-voltage DC? HV DC doesn't care about your steenkin' fuse. It just arcs across the burnt pieces and keeps on going. Remember, the zero crossing on the AC cycle extinguishes the arc. With DC there's no crossing, only continuous current flow. This is why fuses have either an AC only rating, or AC with a substantially lower DC rating. And that DC rating can be about 50% lower than the AC rating. Same thing goes for relays. Interrupting (i.e. switching) high-voltage DC is hard. You can do it with a MOSFET, though. Fast, too.

DC fuses have special internal construction to avoid problems at higher voltages. The fuses to use are ceramic body types filled with sand so the ablated material can't sputter on the interior glass and then arc to that.

For example, here is one of the Littlefuse pages on fuses:

www.littelfuse.com/products/fuses/industrial-power-fuses/midget-10x38mm-fuses.aspx​

Your attention is called to some fuses being AC only and others giving different ratings for voltage type, like 500 VAC / 300 VDC.

The magic phrase in high-voltage DC fuses you'll see is "High Rupture Capacity" (HRC), "High Breaking Capacity" (HBC), or "Interrupting Capacity" (IC). That capacity must be higher than the short-circuit current which can be, as the kids say, like, majorly awesome, dude.

You need several fuses to do properly fuse B+, of different but related values. At a minimum (assuming multiple windings on common core, so the overall current through the transformer may be insufficient to blow the mains fuse):

(1) Power Supply Protection.

(a) A fuse goes after the B+ rectifier but before the first capacitor. If something downstream goes this saves the rectifier tube (or fancy diodes) and power transformer.
(b) A fuse goes after the transformer (on both secondaries) and before the rectifier to save your transformer if the rectifier welds shut.​

Both (a) and (b) are sized at the max current which, if you lack inrush limiters, will be higher than steady state (powering into dead short capacitors and inductors) and may need to be tweaked. Upon startup the inrush (if no inrush limiters are installed) can exceed the nominal fuse rating. But, maybe not, because fuses are not far from instantaneous (read the data sheet) so you have some amount of time at that load. You may need to adjust the value up if you're constantly blowing fuses because of inrush. (This is your amp pleading with you to add an inrush limiter.) Because of initial current spike, and the delay in blowing the fuse, it may not blow in time if it is over-rated to deal with inrush current.

If you're lucky this fuse might (key word is "might") be fast enough to save your output transformer from Chernobyling if a tube loses its bias and runs amok. But probably not (depends on the total current demand which can be low enough to cook the output transformer and not blow the fuse) so don't count on it. That's why (2) is added.

(2) Output Transformer Protection. A fuse goes inline with the output transformer center tap. (Could add to both secondaries as well.) This protects the output transformer (obvious) and the tube and cathode bypass capacitor and resistor (if cathode bias) (less obvious). But you'll also need to add flyback protection to the output transformer in order to avoid an arc should the fuse blow and the primary flux has nowhere to go and gets lonely. So very lonely it just wants to go absolutely everywhere all at once. The last thing you want is an arc from the output transformer going back through the tube or across the socket, through the bypass capacitor and through the cathode resistor (although this is the least expensive piece to be vaporized) to ground, as well as back through the grid (or cathode) to the previous stage. Arcs are funny things and go places nobody needs them to go or wants them to go.

(3) Cathode. The cathode bias resistor acts as a sort of fuse if one uses special fusible resistors. Normal resistors are not fuses and tiny resistors can sometimes withstand surprisingly high wattage before they vaporize. This is a "what can it possibly hurt?" sort of fuse to add in addition to the others. This may save your output transformer (if it has flyback protection) if the tube runs amok, but not necessarily so if it doesn't go fast enough.​

That's the basics, but it simplifies a bunch. Paths for B+ through bias supplies, etc. I omitted screen protection but it's basically the same. You should also fuse the rectifier filament and the heater circuit.

A transformer generally takes a temporary overload without permanent damage, but the fuse should not be too far past the normal current. Again, read about the delay in opening.

I suggest reading fuse application notes. Works as both an insomnia aid and to ensure nobody talks to you on the train.

I’m using both 1b and 2 fuses on the hk 250 amp I’m working on. Will need to get the proper fuses I guess. I only have cheapo 30mm fast blow agc type from Amazon. Thinking I’ll start with those and 1 amp…what do you guys think for push pull ss rectified 395 v b+ amp?