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Question regarding fusing the B+ rail

Brice

Brice

AK Subscriber
Subscriber
Hi guys,

I always do fuse the B+ rail on my creation.

I am a McIntosh guy, and all vintage MCxxx had an internal fuse on the high voltage PS, and of course one in the primary winding accessible for a common user.

Some will argue that it is useless, as by the time the fuse blow, damage is done.

Some others say it's useful if size and located correctly.

I am in the later camp, as I have witnessed a MC240 tube arc which took down the internal 2A fuse, with no damage to the amp.

On another occasion, an accidental speaker short did trigger the fuse so I do think those help.

Now my question is: is there a formula to size correctly this fuse?

I am aware that indeed such formula exists, backed up with long equation to model the behavior of a fuse.
But for a practical stand point, how can we be "almost good" in sizing this fuse.

Recently I experienced around with this. I have an amp with a 135 mA budget at idle and put a fuse after the PS HV caps to avoid the startup inrush of the charging caps.

During that exercise, I blew a 170, 180 and 200 mA fuses already and now am at 250 mA, all fast blow fuses. I do not want a slo-blo there as it would defeat the purpose.

So far so good but I am not confident that a value of 250 mA is good enough for that amp.

Any experts can en-light me on that subject?

Thank you
Brice.
 
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One such reference in link - it uses PSUD2 to help determine the worst-case turn-on peak current (to identify if fast or slow blow fuse is most appropriate) - but if you can actually measure the worst-case average rms current in the amp itself then there is information on what fuse to choose - the recommendation is to use a fuse in the secondary HV winding part of the B+ supply, rather than in the B+ DC feed, for a number of good reasons, and to use an appropriately specified/compliant fuse (rather than dig around in the old spares bin for something with the right value on it).

http://dalmura.com.au/projects/Valve amp fusing.pdf

Ciao, Tim
 
Just for perspective, Allen Organ fused the secondary on their 6550 amps with a 1/4 amp fast blow from the center tap of the HT (or B+) where it goes to ground. A neat trick is to use a CL120 between the CT and ground to slow down the HT voltage thus giving the power tubes a tiny bit of time to warm the plates and putting the HT on them slowly instead of hammering them when you hit the switch. I don't know what the voltage rating on that device is, but I've never had an issue. I think they would open like a fuse too even though it's rated at 1.1A, if you had a short circuit situation.
Mouser has them for a buck or so. http://www.mouser.com/ds/2/18/AAS-920-325D-Thermometrics-NTC-Inrush-031814-web-850596.pdf
 
The caps are going to produce a fair bit of inrush current when the rectifier starts to conduct and they begin to charge. Also, most push-pull amps will easily swing double the idle current at full power. Without modeling it, I'll take a WAG and say 1/2 to 3/4 amp might not be inappropriate. Just make sure the voltage rating is sufficient. AC and DC ratings on fuses are usually very different, DC being lower voltage than AC for a given fuse. An inrush limiter would help reduce how much you have to over-size the fuse to keep it from having nuisance failures as well.

No reason not to put it on the ground side either, functionally it comes out the same.
 
Putting the fuse in the AC has the benefit of helping to extinguish the arc across a blowing fuse because of the zero crossing.

In my last build I put a fuse between each leg of the PT and rectifier.
 
All that is good advice, thank you.

I forgot to add that I do use solid state rectification, so high voltage comes right away. However I placed it at the center winding of the OT so I don't get inrush effect at all as the tubes are not conducting at all at startup.

The startup is a non issue for me. But the dynamic usage of the amp, especially at low frequencies does.

Gadget, I am with you: I believe that if I bias the tube to let's say 50 mA per tubes, so the idle current in the fuse would be a little bit over 100mA. I blew a 200 mA fuse so that means that the amp did consume more than that.

I think it's more like 3x idle current,
 
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:
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.
 
Since I've been asked, here are some additional details.

I can't readily find the full Underwriter's Laboratories spec (UL charges for it), LittleFuse references it:
http://www.littelfuse.com/~/media/e...ittelfuse_fuseology_application_guide.pdf.pdf

A UL Listed fuse meets all the requirements of the UL/CSA/ANCE 248-14 Standard. Following are some of the requirements. UL ampere rating tests are conducted at 100%, 135%, and 200% of rated current. The fuse must carry 100% of its ampere rating and must stabilize at a temperature that does not exceed a 75ºC rise. The fuse must open at 135% of rated current within one hour. It also must open at 200% of rated current within 2 minutes for 0-30 ampere ratings and 4 minutes for 35-60 ampere ratings.​

So there you have it: running at 1.35 x rated current (35% overload) will cook your amplifier if you oversize the fuse to address the spike from inrush current. A decent transformer (output or power) can likely deliver that, but not for very long.

Better to do both of (a) and (b):
(a) Add an inrush limiter (as I above described)
(b) Size the fuse to be able to carry the typical current and not blow, but not size it for initial overload conditions or transient overload conditions.​

I'd rather blow a fuse now and then if the amplifier is turned up too loud (which I don't do) then blow an expensive output transformer, output tubes, or power transformer because a tube failed.
 
Thank you for your details.

Yes I am with you, that's why I added the fuse protecting the OT. I don't feel good adding a limiting device in the B+ path.
I guess I'll see if this 250 mA blow, as 1.15 x idle current roughly 185 mA blew before.
 
Brice said:
Yes I am with you, that's why I added the fuse protecting the OT. I don't feel good adding a limiting device in the B+ path. I guess I'll see if this 250 mA blow, as 1.15 x idle current roughly 185 mA blew before.
Click to expand...

Ummm, what's the problem with a fuse for B+? You already have at least one: the rectifier tube. Oh, yeah, it will go to Valhallah in a burst of fire if B+ shorts (output tube Chernobyls or a capacitor shorts) and the current becomes unlimited. Rectifier tubes make expensive fuses. Not sayin', just sayin'.

Or did you mean an inrush limiter? It protects the transformer and rectifier. It isn't in the signal path. I can't see the harm. (shrug)

You wrote, your amp runs "135 mA budget at idle". If you're blowing fuses at 2x that, that sounds like excessive startup current. When are these fuses blowing? Startup? High volume? Low volume after a period of time? How quickly? A 150 mA fuse should be able to take 250 mA to 300 mA in short bursts, although I would not want to push that envelope.

Can you measure the current when it blows?
 
Brice, if you have enough ability to set up PSUD2 for your amp, including measuring the power transformer winding resistances, then you can fairly accurately determine the hot start current pulses through a secondary side fuse, and then fairly accurately align them with a fuse rating, type (F or T/slo blo) and standard (IEC60127-2 or UL284-14). If you are going to buy in a fuse then the major suppliers are likely to have IEC60127 spec fuses, which give a better definition of performance and confidence that they will do the right job. But as you are in USA, then selecting a UL284 has reduced risk that someone will insert one of those in the future.
 
No rectifier. SS only. I have a thermistor but only in the primary of the PT, and I thought you meant inserting a thermistor directly into the B+ rail. I know some do that, but I don't.

It can not be the startup current as the fuse in inline with the center point of the OT only, and at startup the power tubes are not conducting.

Yes I run at idle 135 mA, and the fuses blew mostly after a startup, like 20 seconds later. I think too smaller fuse got abused during listening and them blow for any reason.

The amp is very dynamic so it is possible that the current swing exceed largely the 200 mA fuse, but not long enough to get him to the breaking point, that's why I am surprised.
That would be difficult to catch when the fuse blow. But I'll do the test and insert an Amp meter in the rail and see how much swing I get.

I modeled the PS with PSDU to see how it behaves on high current demand without seen anything abnormal, but because the fuse feeds only the OT, I can't really model anything, unless I am missing something.

So far 250 mA F fuse holds.
 
Brice said:
No rectifier. SS only. I have a thermistor but only in the primary of the PT, and I thought you meant inserting a thermistor directly into the B+ rail. I know some do that, but I don't.
Click to expand...

Well, you probably can't use a relay driven by a delay because DC at that voltage will arc. (Not when you close the bypass to remove the initial delay resistor, but perhaps when the power is off and the contacts switch open of their own volition to the normally-open state. Not sure about this, because the B+ is probably mostly gone at that point. I'm not calling this as either a problem or a non-problem, but experiments would be required to see, particularly what happened to the relay contacts.)

A high-voltage MOSFET with a delay could be used. Run B+ at 50% limit, then gradually back that off.

Limiting overall current into the power transformer may be your best option, then.

Brice said:
It can not be the startup current as the fuse in inline with the center point of the OT only, and at startup the power tubes are not conducting.
Click to expand...

Sure it can be. When empty the capacitors are dead shorts as are any chokes. Doesn't matter what kind of rectifier you have, the filtering stage is empty and it wants more and more electrons, all the electrons you can deliver. Hungry, soooo hungry for electrons.

Brice said:
Yes I run at idle 135 mA, and the fuses blew mostly after a startup, like 20 seconds later. I think too smaller fuse got abused during listening and them blow for any reason.
Click to expand...

Or you've got inrush issues. Like the typical tube amplifier power supply, even with silicon rectifier.
 
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The current from center tap to ground is equal to the current from the output of the rectifier to the filter cap. You'd have the same inrush currents, etc.
 
Yes thanks, and understood.
But the fuse is after the choke feeding and protecting the OT from the primary center tap. Here you can't have inrush startup current as the power tubes are not even conducting at startup, right?
 
Brice said:
I modeled the PS with PSDU to see how it behaves on high current demand without seen anything abnormal, but because the fuse feeds only the OT, I can't really model anything, unless I am missing something.
Click to expand...

If you can cope with PSUD2 learning curve, then the fuse sizing process in the link in post #2 may be understandable. Happy to take you through the process for a fuse in the AC part of the B+ power supply, although best to link to a schematic so we are all on the same page.

I wouldn't recommend keeping a fuse in the DC feed to the OT, as you can't safely use a common 250VAC fuse, and someone should really do an overvoltage assessment on the OT (what devices and where and whether they will manage a fuse rupture event).

Can you point to where in a schematic you have the fuse presently? Is your amp set up properly, with symmetrical clipping and balanced idle currents, and low distortion at rated output power (ie. the output stage valves are dynamically balanced as well)?
 
Brice said:
But the fuse is after the choke feeding and protecting the OT from the primary center tap. Here you can't have inrush startup current as the power tubes are not even conducting at startup, right?
Click to expand...

The filter capacitors must charge and chokes must build up flux. Load from tubes is irrelevant.

Upon initial turn-on these load the supply circuit and act like dead shorts.

You could simulate this with PSUD2 as was suggested. Use the sample supply and look at the various voltages and currents. It will demonstrate what happens in a power supply at startup.
 
I figure i know the least in the this discussion (probably not by a small margin).
I had a thought that may or may not in fact be a good idea.
As i understand the problem, they amp comes on fine and blows fuses playing program. (Please correct me if i have that wrong)
What i propose. Sort of a reverse time delay. Shunt fuse at turn on . After PS is at operating point open relay and let fuse carry load. It still brings you back to the what kind, what value. Where to insert issue but removes any PS surge issues from the equation :idea:
 
nj pheonix said:
I had a thought that may or may not in fact be a good idea.
As i understand the problem, they amp comes on fine and blows fuses playing program. (Please correct me if i have that wrong)
What i propose. Sort of a reverse time delay. Shunt fuse at turn on . After PS is at operating point open relay and let fuse carry load.
Click to expand...

That's a perfectly legitimate solution if it were for, say, low-voltage heaters.

I above described that solution as using a time-delayed NC relay to switch out a resistor which current limited B+. The problem is that when the relay closes the contacts may arc because of the voltage across them (contact closing jitter) and will become pitted or weld shut. Switching high voltage is tricky.

I'm also not sure what happens as the relay coil energizing stops when the unit is turned off if any B+ remains. As the contacts open with B+ across them this will cause a arc, damaging the contact and potentially welding them shut. It depends on how fast B+ declines. I would not want to base a circuit on that.

Instead of a mechanical relay it would need to be a high-voltage MOSFET. Which allows the conduction of the MOSFET to be varied with an RC constant.

So, yes, one may use a bypass, and its a perfectly good concept, but the implementation cannot be electromechanical.
 
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