# Thread: Secondary Fuse Value

1. How did you measure total cathode current?
I was asking this as cathode current consists of DC and (a lot of) signal AC. Your meter might not be able to correctly read averaged DC.

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2. Sorry, across a 1 Ohm resistor from power tubes cathodes to ground.

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3. Originally Posted by trobbins
A 700mA B+ loading and 395V aligns well with the measured transformer parameters. For my PSUD2 sim I tweaked the load resistor R1 to 570 ohm, and the results table gave I(R1) = 703mA and V(R1) = 401V.
Yes, I can see now how the sim aligns with my measured values! In response to Hemholtz I also gave my values at full clean power as well (instead of of maximum output with lots of clipping), and plugging those into PSUD2 lined up as well.

Originally Posted by trobbins
Note that PSUD2 also provides an estimate of filter capacitor ripple current, so best to check your filter capacitor datasheet to make sure it is rated for something approaching 1A. You may find the ripple rating is down below 0.5A at 120Hz for some caps, although the 1A is a 'metal' level.
This is interesting and something I never considered. Right now my caps are rated at 720mA, but I have two 400V caps in series. Does the rating double like voltage does in series?

Originally Posted by trobbins
Based on IEC fuses, the table below indicates the minimum limit fuse capability for 0.5A, 0.63A and 0.75A fuses. A bogey fuse sits in between min and max compliance levels, so even the 0.5A fuse would be quite unlikely to blow on an in-rush, and given the compliance level is at least 1.5hr at 150% rating, then the 0.5A fuse would seem reasonable unless the metal gig went on for a lot longer.
Attachment 57527
I looked at all my fuses and they are UL. You say in your article: "the circuit operating current must be no more than 75% of the fuse current rating" for UL fuses. Since my absolute maximum current through the T1 is about 1.3A, does that mean my fuse should be rated at 1.5A (which is the nearest standard value)?

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4. Originally Posted by Gaz
Sorry, across a 1 Ohm resistor from power tubes cathodes to ground.
You may try to bypass the cathode resistor with a 220µF or higher value cap for smoothing/averaging. Does this change the DC reading?

But I recommend to take the total DC current reading directly after the reservoir cap (where some amps have the standby switch), as this would be the real "I(R1)" value. Beware of high voltage!

When I mention "full power" I always mean output power just before or at the onset of clipping as results with clipped output are not predictable.

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5. Originally Posted by Helmholtz
You may try to bypass the cathode resistor with a 220µF or higher value cap for smoothing/averaging. Does this change the DC reading?

But I recommend to take the total DC current reading directly after the reservoir cap (where some amps have the standby switch), as this would be the real "I(R1)" value. Beware of high voltage!

When I mention "full power" I always mean output power just before or at the onset of clipping as results with clipped output are not predictable.
I tried the smoothing cap, but results were the same.

I also tried taking a measurement after the first reservoir cap (before the choke and screen filter cap), and got 90mA at clippping, which seems odd. I tried across 1 Ohm resistor as well. Same - 90mV. In any case, the PSUD sim lines up with the measurements I have taken so far at both full output and with maximum clipped output. That seems like a good thing, ya?

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6. Since my absolute maximum current through the T1 is about 1.3A, does that mean my fuse should be rated at 1.5A (which is the nearest standard value)?
Don't forget that the main purpose of the secondary fuse is to protect the HT winding. So it's the PT's rated current and not the amp's max current demand that matters.
I am not familiar with UL type fuses, but I think you should not go above 1A.

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7. I also tried taking a measurement after the first reservoir cap (before the choke and screen filter cap), and got 90mA at clippping, which seems odd. I tried across 1 Ohm resistor as well. Same - 90mV.
90mA at that point is not possible. Sure you measured DC not AC?
Could you post a drawing of the power supply showing where you measured current?

I tried the smoothing cap, but results were the same.
That seems to confirm the original measurement.

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8. Originally Posted by Helmholtz
90mA at that point is not possible. Sure you measured DC not AC?
Could you post a drawing of the power supply showing where you measured current?

That seems to confirm the original measurement.
Yes, I screwed up and was just measuring screens + preamp.

Full DC current draw at clipping is 520mA.

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9. Both of these sims are now accurate with my measurements for T1 and R1 both just before the onset of clipping and with the maximum power the amp can draw. This has been a great learning experience using PSUD and measurements from the amp.

I'm still a bit confused with what fuse value to choose based on all the technical and anecdotal info in this thread lol

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10. Both of these sims are now accurate with my measurements for T1 and R1 both just before the onset of clipping and with the maximum power the amp can draw.
Different R1 values, same results - not possible.

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11. I can suggest you first identify a part supplier that provides datasheets for the fuses sold, unless you are lucky enough to have previously purchased fuses and know specifically the manufacturer and fuse model. If you need to buy in a fuse, and can purchase an IEC specified fuse, then I would recommend that type of fuse be purchased, rather than a UL (unless the fuse manufacturer provides sufficiently detailed datasheet).

That doesn't mean a generic no-name fuse won't be just as good - one would never achieve that guarantee because there are so many variables as to what faults can occur, and what tolerance/time a particular fuse will perform with, but like buying new valves - confidence of knowing what you are using can have benefit.

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12. Originally Posted by Helmholtz
Different R1 values, same results - not possible.
Sorry, forgot to hit "simulate" again after changing R1. I reuploaded the files.

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13. Originally Posted by trobbins
I can suggest you first identify a part supplier that provides datasheets for the fuses sold, unless you are lucky enough to have previously purchased fuses and know specifically the manufacturer and fuse model. If you need to buy in a fuse, and can purchase an IEC specified fuse, then I would recommend that type of fuse be purchased, rather than a UL (unless the fuse manufacturer provides sufficiently detailed datasheet).

That doesn't mean a generic no-name fuse won't be just as good - one would never achieve that guarantee because there are so many variables as to what faults can occur, and what tolerance/time a particular fuse will perform with, but like buying new valves - confidence of knowing what you are using can have benefit.
Tim, didn't think they were listed, but easily found the datasheet on the suppliers' site:

https://www.cedist.com/sites/default...files/f-zs.pdf

It seems like every fuse I have is UL, so probably smart for me to design around those. I guess I should be clear that my main objective is to prevent the HV winding from getting damaged under fault conditions and not blowing even with totally clipped output based on my measurements and the simulations.

Nickb's story about the doom band frying Hammonds is interesting, but I know a lot of bands that play like that and don't fry PT's... unless they are Jet City's or other cheap imports that can't take the heat. My understanding of the HV winding's rating was what it was the amount of current it voltage could supply while staying within it's 5-10% regulation, not the maximum amount of current before failure. Maybe I'm wrong about that, but I just think the story shows that Hammond transformers are not designed as well as they used to be, not that most transformers will fail when pushed past their current rating.

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14. You'll notice at full tilt that the transformer RMS current is 1.3A for an output average current of 0.718A. Pretty much the in the same ballpark is I reported earlier, this is why you should not use a secondary fuse rating that is higher than the secondary rating, unless you are prepared to accept the consequences.

Manufacturers have a different rules as they are required to prove that the equipment is protected from causing a fire by the primary fuse, but the transformer will be damaged. They can afford that as it allows them to safely use a smaller transformer.

PS: Gaz, whose transformer and part number are you using?

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15. Originally Posted by nickb
You'll notice at full tilt that the transformer RMS current is 1.3A for an output average current of 0.718A. Pretty much the in the same ballpark is I reported earlier, this is why you should not use a secondary fuse rating that is higher than the secondary rating, unless you are prepared to accept the consequences.

Manufacturers have a different rules as they are required to prove that the equipment is protected from causing a fire by the primary fuse, but the transformer will be damaged. They can afford that as it allows them to safely use a smaller transformer.

PS: Gaz, whose transformer and part number are you using?
I appreciate the thoughts and totally understand the logic there, but I have never seen a large or boutique manufacturer use a fuse rating that matches the secondary rating closely. It's always at least 2x the rating. Does it means that every manufacturer of guitar amps big and boutique alike are using the wrong size HV fuses for their amps (serious question)? I really do think that maybe Hammond's insulation is not up to snuff. Because lots of people completely dime small amps all the time and the PT's don't fry. I think the rating is not the maximum limit before meltdown, but I will ask Heyboer how they rate them (if they are still open!).

By the way, I am using a Heyboer unit a bought second hand that is supposed to be a clone of a Hiwatt DR103 PT. They said it had a 550mA secondary so I do know that for sure.

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16. If you play "normally" through a dimed amp the secondary current will have high peaks but, due to the usual crest factor of the guitar input (10 to 20dB), the average is less than the transformer rating so there is no problem. Given the average is low there is no justification for using a bigger fuse. As I mentioned above a slow blow fuse with 1.5x it's rating will take around 60 mins to open so most of the time there's no issue. The problem comes if you drive it really hard, in your case the current will 2.35 (1300mA/550mA) times the fuse rating and it will blow after just s few seconds and that is why people use bigger fuses.

It seems maybe we're looking at it the wrong way. If your max current is 1.3A and you want to be able to handle that without ever damaging the PT and without fuses blowing, I suggest you need at least a half that i.e. a 650mA transformer with a 630mA fuse. The fuse would blow in 10 secs at 1.3A which, without trying it, seems reasonable. Just because everyone is doing it a certain way doesn't make it right.

Yes, a bunch of boutique manufacturers use a 2 x fuse but I suspect they are using the rule of thumb without really thinking it through. It also matters what primary fuse rating is so we'd need to look at it in more detail. I suspect they'll have the usual 2 x expected max current there too in which case I guarantee I can blow up the PT in about a hour or two. I have learned that very few people realize how the power supply causes massive transformer current. If you look at the Hammond site you will find they claim for a bridge rectifier that the average DC current is 0.65 the transformer current (i.e. secondary is 1.6 x DC current), This could mislead a lot of people. You yourself have demonstrated a factor of 1.8 and I'd say use x2 if you want a rule of thumb.

The amp I mentioned actually burned out a total of three PT''s, the original and two replacements so I don't think you can blame Hammond. They have to demonstrate compliance to regs in any case. I would say that the 420mA JCM 800 transformer seems very inadequate in light of the above, but they have been in use for donkeys years and are fine for "normal" use. I didn't mention it, but they did they same to another amp. This had 6 x EL34's and a 900mA PT.

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17. Originally Posted by Gaz
I appreciate the thoughts and totally understand the logic there, but I have never seen a large or boutique manufacturer use a fuse rating that matches the secondary rating closely. It's always at least 2x the rating. Does it means that every manufacturer of guitar amps big and boutique alike are using the wrong size HV fuses for their amps (serious question)?
I'm not exactly sure how you would know. I mean, you would have to know the OEM design specs for the transformer model to make that assesment.
This is not information that is readily available, and in my experience, can be quite difficult to get a hold of; and This is for OEM data on any individual model.

I really do think that maybe Hammond's insulation is not up to snuff. Because lots of people completely dime small amps all the time and the PT's don't fry.
Hammod has been supplying transformers for the industry for decades. They're one of the most well know manufacturers in the buisiness. I find it hard to believe that this would be the case.

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18. Just to throw some numbers out, although it's coming out of the rectifier rather than going in, standard Marshall value is 1AT for 100W and 500mAT for 50W. Rough specs for their PT's should be easy to find.

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19. I'm not exactly sure how you would know. I mean, you would have to know the OEM design specs for the transformer model to make that assesment.
This is not information that is readily available, and in my experience, can be quite difficult to get a hold of; and This is for OEM data on any individual model.
Tons of replacement transformers out there with specs. Definitely enough to make that statement. Also find one 100W with an HT less than 1A. I can’t, and I guarantee no one is using transformers rated for 1A secondary current.

Originally Posted by SoulFetish
Hammond has been supplying transformers for the industry for decades. They're one of the most well know manufacturers in the buisiness. I find it hard to believe that this would be the case.
Ok, I’m not so confident in huge brands. The only other PTs I’ve seen shit the bed are cheap imports. I just thought perhaps the insulation may not have been up to real world demands. Nick pointed out in his last post that the original blew too, so probably not the Hammond specifically, but I don’t just don’t let companies rest on their laurels.

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20. Originally Posted by nickb
If you play "normally" through a dimed amp the secondary current will have high peaks but, due to the usual crest factor of...
Nick, thanks a lot for your thoughts on this. Food for thought. I think you’re right that a lot don’t think it through and just use the rule of thumb approach. It does protect against most common failures. I just can’t believe I haven’t seen more PT’s overheat learning what I have from this thread. Sure big amps don’t get played full bore all the time, but little amps do, and many don’t even have HT Fuses. Why aren’t they all overheating?

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21. Originally Posted by Gaz
Nick, thanks a lot for your thoughts on this. Food for thought. I think you’re right that a lot don’t think it through and just use the rule of thumb approach. It does protect against most common failures. I just can’t believe I haven’t seen more PT’s overheat learning what I have from this thread. Sure big amps don’t get played full bore all the time, but little amps do, and many don’t even have HT Fuses. Why aren’t they all overheating?
They aren't all overheating because 99.9% of people don't push them that hard.

OTOH, a leaky cap in the bias supply could cause a high DC current that could easily overload the transformer.

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22. Hey just wanted to follow up in this thread because I emailed Heyboer about the 550ma rated PT. Thoughts?

“Now as to current rated hv winding. We usual just go by what the wire is rated to carry full duty. In this case is 24 ga. we sort of round it off. It can really handle about 600mA all day long. This also depends on length of, and total turns comes into play with size of core for total VA . that winding is 200 VA or can say it like 100 volts at 2 amp or 200 volts at 1amp. the wire itself is rated to 130c. for a fuse on the font end of this transformer , id think you need total ampers the primary will see. yea? the total is 285 VA ( give or take a few ,is sort of rounded) so divided by primary volts 120 . primary will see 2.375 Amp if all windings are put to max current draw at their voltage. If any one of those windings shorts or cooks, the primary current will spike way up and pop the fuse, provided you dont overkill on the fuse. the old farmers trick , well ill just shove a coin in here because the fuse keeps popping. Its got to be a bad batch of fuses, not bad or shorted wire or something else running off the chain. am joking here a bit, but you get my drift. Id think go by total VA at full duty for fuse. But im not an amp builder.”

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23. 130C insulation?!
That sounds like even Hammond are now using cheap wire
See J M Fahey's run down on the topic https://music-electronics-forum.com/...l=1#post306712
Explains how I managed to frazzle a 1650G OT last year; I'd been using it on the basis that the 16XX range could be used a little above their rated hifi power in a guitar amp, so I was pushing it to nearly 40W clean, 55W heavily overdriven.

Originally Posted by Gaz
...Sure big amps don’t get played full bore all the time, but little amps do, and many don’t even have HT Fuses. Why aren’t they all overheating?
These Marshalls are somewhat unusual (for vintage type amps at least) in that they used FWB, rather than biphase rectification; hence the winding current can be much higher.
Does the lowish reservoir cap value mean that at high demand, the conduction angle is very wide?

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24. 130C insulation?!
That sounds like even Hammond are now using cheap wire
This is Heyboer.
What is known about the temperature class used by Hammond?

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25. Oops, I’ll try and blame the small character size on my phone, I saw the ‘H’, seemed to remember it being a Hammond PT previously, and read it as ‘Hammond’

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26. Originally Posted by Helmholtz
This is Heyboer.
What is known about the temperature class used by Hammond?
I wanted to correct some information here. I emailed Heyboer to confirm what temperature class magnet wire they use and he responded that the temperature class 155C is Typically used. This is fairly common in transformers. Motors, relays, and other coils. Obviously, 180C is “better”, in that I would want my transformer to have it. But it comes down to a cost/benefit analysis.

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27. Originally Posted by SoulFetish
I wanted to correct some information here. I emailed Heyboer to confirm what temperature class magnet wire they use and he responded that the temperature class 155C is Typically used. This is fairly common in transformers. Motors, relays, and other coils. Obviously, 180C is “better”, in that I would want my transformer to have it. But it comes down to a cost/benefit analysis.
Looks like contradictory statements on the part of Heyboer.

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28. Originally Posted by Helmholtz
Looks like contradictory statements on the part of Heyboer.
Ok, so it looks like the mixup with the temp was the rating of the lead wire, which is PVC "appliance wire". It's rated at 130c, but the gauge of that is 22AWG. Strange mixup

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