Blown 1 OHM current-sense resistors

[Gaz]

A lot of brilliant thoughts in this thread. I will be rereading for a while to come.

Here are my thoughts on the amp at hand...

The tubes are apparently fine, which means they could withstand a surge/short that the resistors could not. To me this is a design flaw. It seems in this case fuses would be much easier to replace, and if it was in fact a tube failure, then the fuse would likely just blow again, rather than having to change out the resistor. Another serviceability aspect is that fuses are much easier to find that precision resistors at your local Radioshack.

Since the amp blew both resistors, I really have to doubt that they were both defective and simultaneously popped.

I thought I mentioned it before in a post, but I've actually seen this happen before, once in an Orange that had cathode fuses, where a tube was having a lightning storm inside, and once in Marshall JCM2000 with small 1R resistors. In that amp the resistor was replaced and the amp went along. I never found the initial cause for that. So while Enzo doesn't see it a lot, it seems somewhat common to someone like me who doesn't fix amps for living.

I'm well aware of the consequences of overrating parts thoughtlessly. A long time ago in a Fender type build I once added 5W screen resistors (hey! worked for Marshall, right?!). Well, a screen shorted, and I lost the PT because, unlike Marshall I did not have an HT fuse. Normally a Fender's small screen resistors would have opened up. I'm trying not to do that again

[Gaz]

Oh, and another attraction to the the cathode fuses is fusing the pair so that I could 'finish' the gig on the remaining pair

[Enzo]

That of course assume that the tubes will suffer that specific failure. A short to control grid will still kill bias to the other tube, and so on.

[Gaz]

That of course assume that the tubes will suffer that specific failure. A short to control grid will still kill bias to the other tube, and so on.

Yes, of course. Same with a heater short. Let me have dreams, Enzo Another side benefit would be an easy half power option by just pulling one of the fuses. I don't really see myself doing that, however.

The only thing I'm undecided about is whether or not to use the same 1W resistors with 1N4007s across them, or the 3W Wirewounds. I don't which is more likely to survive different overload conditions. Opinions?

[g1]

3W must be more likely to survive overload than 1W, that's no opinion!
Some may think 3W is overkill, that's an opinion.

[Gaz]

3W must be more likely to survive overload than 1W, that's no opinion!
Some may think 3W is overkill, that's an opinion.

g-one, there is another arrangement I was referring to where a diode is placed across a low value... wait, you already know about this, I learned about it from you in an old thread!

Are you saying '3W is greater than 1W' because the 1N4007 has a 1A rating? I know some new Fenders use a 1N4007 across a 1W, and I assume it's because of the 1N4007's 30A surge rating as others have pointed out. It's hard for me to know whether Fender picked the 1W/1N4007 set up because it was actually more robust that the 3W (or larger) wirewound, or just because it was cheaper. That's why I'm still looking for advice on the matter.

[Enzo]

Nothing to do with the diode. a 3w resistor is larger and will therefore withstand more current. So regardless of whether it is needed or efficient or whatever, a 3w will always be more likely to survive a tube failure above it. All by itself, 3w is larger than 1w. A 3 egg omelet is larger than a 1 egg omelet.

Putting a diode in parallel with a resistor limits the voltage across the resistor. If the current through the resistor, and hence the voltage across it, gets up to the point the diode conducts, that is the maximum voltage that resistor will ever see. Any greater current will be carried by the diode. You can calculate the resistor load. Take the diode junction voltage times the resistor, which was 1 ohm. As far as the resistor is concerned, the diode type doesn;t matter. A 1A diode limits the voltage across the resistor to half a volt just as well as a 3A diode.

I am being informal. The junction voltage curve is not flat. At very low currents like from your meter, a diode might start out at .500v. When you get a whole ampere flowing it does rise to more like .9v, and for the 1N4007, at 2A of current the junction rises to 1v even. The 1N5408 has similar curves, starting at half a volt or so, but it takes about 9-10A to get the junction drop up to 1v. But it is darn unlikely your B+ will be able to provide such current. On the other hand, the hotter the part gets, the lower its junction voltage goes. That quality is why solid state amps use them in the bias circuit as thermal compensators. So I suppose a 3A diode has a teeny bit larger junction voltage than the 1A diode. And would thus put a teeny bit more stress on the resistor, if I ignore the rest of the context.

[g1]

Sorry Gaz, I think I misunderstood your question. I thought you meant 3W or 1W, both with diodes. Upon rereading, I think you meant 1W and diode versus 3W with NO diode. Can't say I have a preference either way, except the Fender method (resistor & diode) is tried and true so that's good enough for me.

[defaced]

Not sure if this has been suggested, I've only been casually following along. Replace the resistor with the exact same kind/rating and see if the problem repeats. That will differentiate between an outlier event and a true problem that needs further consideration. Also, this will help further diagnose the root cause. If the same resistor blows, then something is wrong with that tube/associated circuitry. If another 1R resistor blows, it means something is wrong with the resistor rating or the general approach.

[Gaz]

Nothing to do with the diode. a 3w resistor is larger and will therefore withstand more current. So regardless of whether it is needed or efficient or whatever, a 3w will always be more likely to survive a tube failure above it. All by itself, 3w is larger than 1w. A 3 egg omelet is larger than a 1 egg omelet.

Putting a diode in parallel with a resistor limits the voltage across the resistor. If the current through the resistor, and hence the voltage across it, gets up to the point the diode conducts, that is the maximum voltage that resistor will ever see. Any greater current will be carried by the diode. You can calculate the resistor load. Take the diode junction voltage times the resistor, which was 1 ohm. As far as the resistor is concerned, the diode type doesn;t matter. A 1A diode limits the voltage across the resistor to half a volt just as well as a 3A diode.

I am being informal. The junction voltage curve is not flat. At very low currents like from your meter, a diode might start out at .500v. When you get a whole ampere flowing it does rise to more like .9v, and for the 1N4007, at 2A of current the junction rises to 1v even. The 1N5408 has similar curves, starting at half a volt or so, but it takes about 9-10A to get the junction drop up to 1v. But it is darn unlikely your B+ will be able to provide such current. On the other hand, the hotter the part gets, the lower its junction voltage goes. That quality is why solid state amps use them in the bias circuit as thermal compensators. So I suppose a 3A diode has a teeny bit larger junction voltage than the 1A diode. And would thus put a teeny bit more stress on the resistor, if I ignore the rest of the context.

Enzo, thanks for going into some detail about the turn-on voltage curves. I understand the concept of the diode, and it seems like if I'm going to use diode in parallel, going up to a higher resistor wattage is pointless since the diode will turn on long before the resistor reaches it's max power dissipation. Please correct me if I'm missing something here.

Since the diode is 'taking over', my question is more about which might be tougher, the diode itself vs. the resistor. The diode has a 1A rating with a 30A surge as you pointed out, while the 3W wirewound can dissipate 3W with about 1.73A flowing thorough it. I do not know what kind of surge a resistor like that can handle.

[Enzo]

The resistor cares about dissipation. Look up the data sheet for your resistor, and see if they have curves.

[Gaz]

The resistor cares about dissipation. Look up the data sheet for your resistor, and see if they have curves.

The sheet says that for the resistors I ordered that they can withstand 5x their rated power for 5s, which is about 3.9A. http://www.mouser.com/ds/2/427/cw-239747.pdf

Here's the sheet for the old resistors I was using, but I'm not sure how to glean the same information (because they don't spell it out like the other sheet )
http://www.mouser.com/ds/2/427/cpf-64917.pdf

I have a feeling I'm missing something about the 1N4007's 1A rating. I mean, if there is a short to the cathode for any amount of time that would cause the 1W to blow at 1A, shouldn't I have the same concern about the diode? I understand that it can tolerate a very large surge for , but what about a sustained surge?

Thanks!

[g1]

Where will the current come from to sustain the surge? It would have to also go through the HT fuse and cathode fuse, if present. And indirectly, the power comes through the mains fuse.

[Gaz]

Where will the current come from to sustain the surge? It would have to also go through the HT fuse and cathode fuse, if present. And indirectly, the power comes through the mains fuse.

This is the part I'm still trying to wrap my head around; understanding the power supply as a whole system. I was assuming the big fault current could come from a heater short. My Ht fuse is also on the AC secondary side of the rectifier, not after the B+ cap, so it's not immune from it either. If I do the cathode fuses, then resistor and or diode just has to be robust enough to let them actually pop (the fuses), which isn't much really if their fast-blow.
Still I'm genuinely curious about the diode vs. resistor thing, and I suppose no one could hazard a guess without doing some torture testing. You're right though, if Fender keeps using the arrangement it must be worth something. The boutique stuff seems to use fat resistors where money is of less concern, then again, a lot of boutique builders have no idea what they are doing

[Enzo]

May I humbly suggest you forget Fender. Their needs are not your needs. They face a variety of legal and regulatory hurdles that you do not. Some of those hurdles require certain parts be added or whatever. If you want to add the diode protection, then just do the math. Fender may have had something totally different in mind. It may be academically interesting to ponder those things, but that is a separate issue from your intent to protect the cathode resistor in your amp.


a 1 ohm resistor with 1A flowing through it means 1 amp of current. All those 1's make the math easy, that is 1 watt. So:

if there is a short to the cathode for any amount of time that would cause the 1W to blow at 1A

But it won;t make the resistor blow, it is a 1W resistor, it will handle 1W OK.

If you ignore the resistor and flow the entire 1A through the 1A diode, well it is a 1A diode, and will conduct 1A all day without complaint.

SO if you have 1A of failure current flowing through those parts, it just isn't going to burn them up no matter what route it takes through them.

what about a sustained surge?

What about a skinny fat girl? If it is sustained, it is not a surge. Look up the data sheet for 1N4007 or other diode of choice. Should be curves on how long it can withstand how much current.

I was assuming the big fault current could come from a heater short.

OK, to the other voltages in the tube, the heaters look like ground. Either the VERY low resistance path through the center tap, or at most a couple 100 ohm resistors. Just WHAT do you expect to be shorting to the heaters? And if the heaters are the path to ground, how is the cathode resistor threatened by it? If you fuse the cathodes, that won;t protect anything against a short to heaters. A heater to B+ short would blow a B+ fuse, but not a cathode fuse. Gonna need both type fuses for this.


Decide how much current your B+ can provide before blowing fuses and how long it can do it. Chart a curve I suppose. Select a diode that can handle the entire amount. If it takes 10 seconds for a fuse to blow (I am making up the example) at 2A of failure current and a 1A diode can take 2A for 5 minutes, then the 1A diode is sufficient. Or go nuts and use a 3A diode. We are talking cheap parts, so take your pick. Now the resistor. Most of the time, your diode will not even be in the picture. SO your resistor will handle the operating current. The diode will only turn on when excess current flows. Look at the forward voltage drop curves for your diode and look at your predicted failure currents. What voltage drop will be across the diode then? And so across the 1 ohm as well. What dissipation will that voltage across the resistor make? That is what you have to allow for.