adding fuses to secondaries

[gsr]

I am in early stages of my second amp build. The first came out great with the help I found here. I ran across an old thread where RG mentioned adding fuses to the secondaries on the pt for protection and how this might be preferable/better than the 100 ohm virtual heater center tap would provide. Is this a common thing to do? If so, what size/type of fuses should I use. Thanks

[Gaz]

Could you post a link to the thread? I'm confused because I don't see how fusing the high voltage secondaries would have anything to do with the virtual center-tap for the heaters.

[pdf64]

A tube short may connect the B+ circuit to the heater circuit.
Amps do turn up in which those 100 ohm artificial centre tap resistors are burnt up. The only way I can see that could happen is if such a short occured.
Once those resistors have fused and become open circuit, the short could then elevate the entire heater circuit to B+ Vdc.
I've never seen an amp where every tube has had it's cathode/heater insulation damaged, but that's the worst case scenario.

[gsr]

Could you post a link to the thread? I'm confused because I don't see how fusing the high voltage secondaries would have anything to do with the virtual center-tap for the heaters.

I'm sorry, I'm new and don't know how to link yet. The thread is about six pages back on this "Theory" forum and is titled, "Heater supplies; real compared with artificial center tap."

[R.G.]

... RG mentioned adding fuses to the secondaries on the pt for protection and how this might be preferable/better than the 100 ohm virtual heater center tap would provide.

Actually, that's not exactly what I meant. I'm with pdf - I can't see that 100 ohm heater center taps have any protection effect at all. Maybe, under an unusual set of circumstances, maybe.

What resistor centertaps provide is accuracy of the CT, which is something you may or may not get out of the transformer. This is worthwhile in a studio-quiet amp all on its own. But it's not a particularly useful protection technique. The idea of a resistor centertap and protecting transformer secondaries are not connected.

Fusing transformer secondaries is how you protect the secondaries from burning out.

Is this a common thing to do?

No. It's not common, just right. Putting in a fuse for secondaries pretty much stops the transformer from being overheated by secondary currents. But it takes a handful of fuses and some more wiring. So commercial manufacturers don't do it because they can get by without it in many cases, and the warranty costs are limited. Cheaper to let'em fail and pay for the few which do fail within the warranty period.

The labor/price calculation is reversed if you're building your own amps.

And after all - what does "common thing to do" have to do with it? Building your own amp at all is not a common thing to do. Neither is having an amp that's quiet and does not burn out its PT under some kinds of failures.

If so, what size/type of fuses should I use.

Again leaving aside whether it's a common thing to do as irrelevant, you size the fuse by the rated current of the secondary on the PT, and you use slow-blow fuses.

If you buy a new PT, the PT will likely have a current rating listed. Make the fuse that current, close as you can. If you're reusing a PT, use the fuse value which is just big enough to not blow under full operating loads. In the case of the heater winding, you add up the heater currents for all the tubes in your amp - this is specified in the tube datasheet! - and make the fuse the nearest size bigger than that. Note that heaters pull a big current surge when they are cold, so you may have to use a fuse slightly bigger than this calculated size to not blow on the surge. And use "slow blow" fuses to avoid the fuses opening on transients.

What you're trying to do is get a fuse that's just big enough to not blow with the worst treatment you give the amp while it's not failing, but small enough to blow if there is a fault. Most of the time, this is not too hard.

[GainFreak]

Again leaving aside whether it's a common thing to do as irrelevant, you size the fuse by the rated current of the secondary on the PT, and you use slow-blow fuses.

I've seen Peavey schematics where fuses are 2-3 times above what secondaries current rating seems to be.
I fuse all my secondaries and sometimes I have situations where a secondary that draws ~300mA would blow a 500mA fuse at start up or another that draws ~800mA will blow a 1A fuse (both fuses slow blow). Changing them to 750mA and 1.6A respectively solves the problem but maybe this is also due to the fact that I'm using a toroidal PT.

[gsr]

RG, Thank you for your response! I guess I should have written, "Is this what someone who is still learning, but wants to build the best possible amp, should do? I really meant to ask if it is common for master builders.

[R.G.]

RG, Thank you for your response! I guess I should have written, "Is this what someone who is still learning, but wants to build the best possible amp, should do? I really meant to ask if it is common for master builders.

It's uncommon even for master builders. I got ribbed a bit when started proposing it. The only drawbacks are
(1) you have to find a place to put the fuse holders where you can get at them.
(2) you have to figure out fuse values; this will necessarily be a bit experimental.
(3) you have to pay for the fuse holders, fuses, and extra wiring.
I believe Kevin O'Connor, author of the "Ultimate Tone" series of books is an advocate of secondary fusing.

You may never have to replace them. I put fuses in all the secondaries of our line of amps. We've had maybe one or two people blow heater fuses. No one has popped a B+ fuse and no one has killed any of the other fuses. I was sure enough of what I did with them that I had the fuses soldered in. The Golden Age amps didn't use fuses. But then, some of them died from tube shorts too. But not all. If you ever do have to replace a secondary fuse, you can snicker a bit about the hundred dollar power transformer you're NOT buying.

I have a power supply design background. Where power is concerned, you can control it if you think about what happens when X or Y shorts or opens. The trick is to figure out what you want to happen. A considerably neater trick than fuses is to sense overcurrents and shut down or limit the OC. But I'd never convince people that anything more complicated than a fuse works - in spite of fuses being really, really complicated to make do what you want reliably.

They'll have no effect on tone, done properly. My definition of "best amp possible" includes things like "tough as anvils" because I personally don't think nails are tough enough. 8-) Things you made by hand and slaved over should be durable if possible.

[mooreamps]

I've never fused any of the secondary windings. But, I would always fused the primary's. Line fuses are designed to work that way. Fusing the secondary windings provide no benefit. It would be the only "amp trick" that I would not just come out and say it's another one of those stupid amp tricks since they would provide some protection say, during the build, and you make some wiring mistake. But that's about it.




-g

[R.G.]

But, I would always fused the primary's. Line fuses are designed to work that way. Fusing the secondary windings provide no benefit. It would be the only "amp trick" that I would not just come out and say it's another one of those stupid amp tricks since they would provide some protection say, during the build, and you make some wiring mistake. But that's about it.

Sir, I respectfully disagree.

There are explicit situations where a failing tube will kill the power transformer without blowing the line fuse. I have done this experimentally, just for my own curiosity. If a tube has an internal filament short, the short on the heater winding will sometimes burn out the heater winding, or melt the wire insulation inside the transformer, without blowing the line fuse.

In this case, a secondary fuse on the heaters will prevent the destruction of the power transformer. The line fuse cannot protect against this in all cases.

Note that this is not a fault of the build, it is a tube failure. This may not be common, but I personally have seen tubes do this. It is, of course, possible to do this during a build, but that's not the only way it can happen.

The obvious response is "Well I've never seen it happen." Maybe not. Mother Nature does not "bless" us all with the same experiences.

But $0.50 of prevention against a $100-$200 bill for a new PT if it does happen seems like pretty reasonably priced insurance.

Line fuses are not there to save components from damage, although they may do that sometimes. They are there to prevent fires. It's good that you always put in a line fuse. Safety standards REQUIRE that.

[mooreamps]

A line fuse on the secondary for the filaments is not a bad idea, given the low winding impedance. For this application, I would recommend the bus fuses rated at 32 volts, instead of the one's rated for 120/240, which I would recommend being used on the primary side.


-g

[R.G.]

A line fuse on the secondary for the filaments is not a bad idea, given the low winding impedance. For this application, I would recommend the bus fuses rated at 32 volts, instead of the one's rated for 120/240, which I would recommend being used on the primary side.

I'm confused. Can you explain what distinction you're making between "line fuse" and "bus fuse" if you're making one? I think I may be misunderstanding some subtlety you're making.

My memory from doing the math on fuses is that there is no particular need to seek out lower voltage rated fuses for lower voltages. The voltage rating on a fuse is the rating for the highest voltage it is rated for interrupting, and the kind of voltage, AC or DC; AC is easier to interrupt because it reverses, going through zero each cycle.

I went off searching to find what I remembered from decades ago when I first dug into fuse applications. I found one reference here: http://www.belfuse.com/Data/UploadedFiles/fuseterm.pdf.

Quoting from that note,

Voltage Rating
Fuses may be used in any application where the open circuit voltage does not exceed the fuse's voltage rating ... A fuse can be used at any voltage below the fuse voltage rating (i.e. a 250V fuse may be used in a 125V circuit.)

So there's no reason to go find 32V fuses for 6V heater circuits. A 125/250V fuse will work fine. The voltage rating is a maximum.

One could make the case that since a heater winding fuse is a good idea, and since there's already a DC fuse in the B+ line in many amps, it might be a good idea to fuse the AC side of the rectifiers in the B+ line. Doing that prevents rectifier shorts (ask AC30 fans!) and filter cap failures from taking the power transformer with them as well as protecting the B+ DC line.

My personal approach would be to electronically current limit the DC side of B+, as this will protect the power tubes and output transformer from bias failures, and protect the OT from tube shorts with less stress on it than a DC fuse. Contrary to popular imagery, fuses do not hold right up to their rating and then vaporize instantly at 100.001% of their rating.

Again quoting from the Bel Fuse link:

The UL/CSA 248-14 Fuse Standard requirements are much broader in scope in terms of characteristics. ...
Only fuses designated "Time Delay must meet a specified minimum clearing time requirement. All others need only comply with the broad criteria of : 135% Rating- one hour maximum to clear; 200% rating - two minutes maximum to clear (one minute maximum for Micro fuses).

The trick in using fuses is to use them where they will split the difference between normal use plus surges, and still blow when massive overloads would (a) cause fires for fuses in AC line/primary situations or (b) kill the components you're trying to protect. The soggy, indistinct blowing characteristic of fuses in mild overloads means that if you have a sharp threshold of damage to a component, you're unlikely to be able to protect it with a fuse.

Bottom line: if you want to protect a power transformer from over currents on the secondary, use secondary fuses, especially if it's driven by tubes.
Fortunately, both transformers and tubes are notable for being able to withstand overloads for quite some time before being damaged. This makes them a much better match for fuse protection than semiconductor power devices.

[oc disorder]

Hands up how many people have replaced the heater fuses in Marshalls ?
Well I have replaced a few after tube failure.
Not as messy as some without fuses which can burn up other surrounding components.
I thought the fuses marshall fitted in the cathode circuit were a good idea as they had a LED indicator to show fuse failure.As they paired the tubes from either side ie 1 & 3
2 & 4 it meant you could still limp home on 2 tubes.
However that seems to be dropped in favour of internal circuit board fuses which means
less user servicable ,probably a good idea considering some of the "user replaced fuses" I've found!
Unfortunatly it seems these are a necessary fixture with the unknown quality of todays
production tubes.

Have seen 2 x Mesa's with a blown power transformer with the primary fuse still intact !
With the cost of the replacement that hurts !
Mind you I resisted the temptation to fit secondary fuses and so far so good !

[pdf64]

'Mind you I resisted the temptation to fit secondary fuses and so far so good !'
That seems like a 'lightning never strikes the same place twice' type of rationale.
Why resist experience and continue with Mesa's penny pinching arrangement?
I'm more of a 'once bitten twice shy' mind set.

[R.G.]

Hands up how many people have replaced the heater fuses in Marshalls ? Well I have replaced a few after tube failure.
Not as messy as some without fuses which can burn up other surrounding components.

It does happen.

I thought the fuses marshall fitted in the cathode circuit were a good idea as they had a LED indicator to show fuse failure.As they paired the tubes from either side ie 1 & 3 2 & 4 it meant you could still limp home on 2 tubes.However that seems to be dropped in favour of internal circuit board fuses which means less user servicable ,probably a good idea considering some of the "user replaced fuses" I've found!
Unfortunatly it seems these are a necessary fixture with the unknown quality of todays production tubes.

In another part of this forum I mentioned a circuit I did which sensed current in power tube cathodes and opened the cathode circuit when the current was big enough for long enough. I'm still digging out the original schematic - this was over 10 years ago. Unlike the uncertainty with fuses, you could actually set the trip current and allowable overcurrent time. The opening device was a high current/high voltage MOSFET in the cathode, which also did standby. I'll put that up here as soon as I find it.

Have seen 2 x Mesa's with a blown power transformer with the primary fuse still intact ! With the cost of the replacement that hurts !

I knew I could not be the only person who'd ever seen this.

Mind you I resisted the temptation to fit secondary fuses and so far so good !

To the music... I ain't afraid of no ghost...