Fuse on both windings of a center-tapped secondary

[R.G.]

I never knew fuses could be so controversial!

They're not. Gary has been stepping into technical discussions and flinging spears to show how clever he is and how clever others aren't, or just continuing a grudge match where he feels he's been wronged in some way.

That's tolerable (if unpleasant) if the flinger-of-spears is in fact possessed of better technical knowledge and experienced. It's not very tolerable when the flinger is merely being self-aggrandizing.

I think he probably has some good ideas that his mannerisms and (lack of) communications skills won't let him get out. My personal approach is to adopt tough love, trying to get him to talk about topics on a technical level and to support his one-liner assertions with numbers and logic. There was a long discussion on fusing transformer secondaries between Gary and others, myself included. It's in the forum here. I did my best to get Gary to discuss computing currents, inrush, and energy as regards fuse applications. I believe that may be what the reference to

I've already posted comments on fusing low current secondary windings... Too steep for this crowd, so I'm just going to leave this one alone....

was about. I find it amusing that this translates as
"I've already told you guys the right stuff, but you're too slow or stupid to understand, so I'm just not even going to mention it any more - even though I just did, to be sure you knew that I knew."

So far this hasn't helped much, but I have hopes that he can learn to interact well someday.

[Chuck H]

It was an unprovoked attack. Gary always makes it a point to imply his brilliance and our stupidity. It's so bad lately that he seldom posts to try to help anyone except to essentially say "I know how. but I won't tell you because I don't trust you not to claim it for yourself and print it in a book. And you wouldn't understand it anyway". Other than these he only pops in when he wants to say something negative about the forum or it's members. What the hell is the point in such posts?

[Diablo]

Ouch.

[Chuck H]

Ouch.

Sorry to place your post out of context. I had to edit for moral reasons...

[imaradiostar]

All fighting with Gary aside, I tend to think that a pair of internal oversized secondary fuses (one per HT leg) are the way to go. From my own experience abusing transformers it seems like they'll drive a dead short and make an angry sound well before they melt down and self destruct. When driving a dead short any high voltage primary will put out current far in excess of their rated output for a reasonable amount of time. While I agree with Steve's .707 logic, I wouldn't bother. I'd probably just double up on the closest easy-to-acquire size.

For that matter, assuming a transformer isn't already piping hot during use, a correctly sized mains fuse will guard against many errors. This logic only fails when someone stuffs an oversized fuse in the holder. This is where the aforementioned (in another thread) INTERNAL fuse can be a transformer saver.

Of course, nothing I build ever fails so why would I need fuses?

jamie

[big_teee]

I thought we only had these Knock Down Drag Outs in the Pickup Forum!
B_T

[R.G.]

All fighting with Gary aside, I tend to think that a pair of internal oversized primary fuses (one per leg) are the way to go. From my own experience abusing transformers it seems like they'll drive a dead short and make an angry sound well before they melt down and self destruct. When driving a dead short any high voltage primary will put out current far in excess of their rated output for a reasonable amount of time.

That's a good way to look at it, and works really well as long as (a) there is only one secondary and (b) the coupling between primary and secondary is tight.

Fuses blow based on self heating, with I²T being the metric they use for how much heating happens in a short time.They'll last semi-forever at just over their rating, but go more quickly on overloads much more than two times the rating, depending on the time-rating of the fuse. If there is enough overcurrent, they go fast, before the transformer can heat up, even locally.

In the case of the original discussion, heater windings may be shorted, but the step down ratio from primary to secondary is so big that the primary current seen from a heater short can be small enough to let the primary fuse hang in there for a long..long time. The heater current can then overheat and burn before the primary fuse pops.

On a high voltage winding, the primary fuse pops quickly if the wiring resistance of the high voltage winding is small enough to let the primary see a large current. This is probably true in most tube amp power transformers, but I have seen some high voltage transformers where the many turns of thin wire caused a resistance that limited the secondary current in a short. So a short could go on forever without popping a primary fuse. Another version of this happens in extra-small transformers where the primary has to be wound of zillions of turns of very thin wire, and the primary resistance is large enough to limit the primary current even if the secondary is shorted.

But for most tube amp power transformers, the B+ winding will cause a primary fuse to pop.

For that matter, assuming a transformer isn't already piping hot during use, a correctly sized mains fuse will guard against many errors.

Yes, it will. However, it is important to remember why the primary fuse is put there. The guy who designed the primary circuits put that there because it's an accepted way to keep an internal electrical failure in the amp from starting a fire. And he kept raising the fuse values on the prototype until he was sure no start-up transient would make it fail. He probably felt really clever at having done some elegant design work when he found that it also kept the power transformer from burning out in many cases. But that was very much icing on the cake. It is a happy coincidence that a primary fuse protects the power transformer for some faults.

But as the saying goes, if you gotta choose skill or luck, take luck.

[imaradiostar]

Hah- in truth I'd completely forgotten about the need to fuse the filament windings in a truly bulletproof amplifier. I could see how the filament windings are a small enough percentage of overall current draw that they could result in a failure if shorted. I suppose an additional fuse per winding is a good idea.

jamie

[big_teee]

You guys are going to have a amp full of fuses.
Where do you put all those fuse holders, or do you use the solder in kind?
My Marshall Clone has the basic Mains and B+ fuse.
T

[imaradiostar]

Yeah, it gets out of hand quickly. On a small/low power El84 or 6v6 amp I don't bother fusing anything other than the mains- I've had a few major failures and the mains fuse has been enough to save the transformer. On a much larger/higher power amp where tubes and transformers get pricy fusing starts to become a lot more important.

I favor the small Keystone fuse holders like THIS.

jamie

[big_teee]

Yeah, it gets out of hand quickly. On a small/low power El84 or 6v6 amp I don't bother fusing anything other than the mains- I've had a few major failures and the mains fuse has been enough to save the transformer. On a much larger/higher power amp where tubes and transformers get pricy fusing starts to become a lot more important.

I favor the small Keystone fuse holders like THIS.

jamie

Those look good, the holder that takes 2 fuses would be Ideal for the double primary fuse.
Who sells those?
T

[imaradiostar]

on my original reply I said a pair of primary fuses when I meant secondary- sorry about that! I suppose that changes things!

Mouser.com has a good stock of Keystone products. A quick search yielded this:

Keystone Electronics Mouser Electronics - Electronic Component Distributor fuse holder 3ag

Jamie

[big_teee]

on my original reply I said a pair of primary fuses when I meant secondary- sorry about that! I suppose that changes things!

Mouser.com has a good stock of Keystone products. A quick search yielded this:

Keystone Electronics Mouser Electronics - Electronic Component Distributor fuse holder 3ag

Jamie

If I had 2 of those, one for HT, and one for filaments, then leave the Other ones in place I would be set.
I just blew a KT77 Tube last week, and the .5A HT exploded, which has me gun shy now.
Also my B+ Fuse is after the Diodes, and after the Caps, and after the Standby switch.
So the way it is now if the Cap Shorted it would take out the rectifiers, or worse the PT.
If you put the B+ fuse before the Caps, it would probably blow when the Caps are flat, when they charge.
I figure that is why they are after the Caps.
So if I put the double fuses on the HT Secondary.
I could make them a little big, and if the cap ever shorted, It should blow the fuses before it ruined the PT.
The 1/2 Amp B+ fuse would protect everything else.
T

[imaradiostar]

Yeah, I've had rectifier failures which lead me to think you need to fuse each half of the secondary to avoid destroying the transformer. I had a good scare with a more or less irreplaceable transformer and it made me think through all of this stuff!

jamie

[R.G.]

You guys are right on top of it. Good reasoning.

The number of fuses for real bulletproofing does grow a bit. You need
- one for the primary, being sure it's in the line/hot side, not neutral.
- one for each secondary winding you want to protect
- two for a CT winding, one in each half.

I like the primary fuse being in an IEC power entry connector with a fuse holder built in, and a fuse block with all the secondary fuses right after the PT, but that's just me.

An amp being bulletproofed might use:
- primary (protects from a random failure inside the trannie or a wiring break/short before it)
- two in HV (protects from shorts of one side to ground and half-to-half
- one in a heater winding, two (one in each half) of a CT heater winding
- MAYBE a fuse in the bias winding or bias tap.

The bias winding thing gets tricky. If you lose a bias winding through a short, the output tubes go critical. They do the same if you lose through an open fuse, and then you're worried about the HV fuses opening. One could make arguments on both sides of a heater winding fuse.

I used to design power supplies for a living, so I got used to thinking about what happened if this died or that died, and what consequential damage would result. If you are really wanting to protect something, you sense the events that could kill it, and turn the power off some way before the thing goes up in smoke. There is a lot that can be done to a tube amp to make the world safe for tubes and transformers, but a huge inertia preventing this. The good old iron, copper, and glass tube amp is fortunately pretty rugged as is, but we now have things the tube amp designers of the Golden Age could not even dream of.