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Thread: Secondary Fuse Value

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    Secondary Fuse Value

    Trying to determine a good fuse value for fusing a 100W amp. I am putting the fuse into one leg of the bridge rectifier (per Valve Wizard and others' recommendations). I've seen both 1A slow blow and 2A Fast fuses used in this spot in commercial amps, but not sure what is the best choice for my amp. I took some measurements with my Fluke meter using the Max function. Here are my readings:

    From cold at start up: 800mA
    With 1kHz sine wave signal at full volume: 1.3A

    What do y'all advise?

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    It's hard to answer this without seeing a schematic or at least more info. There are lots of different 100W amps. We don't know how many preamp tubes and a host of other info. Please post a schematic.

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    Is your Fluke a 'true rms' type? As the current waveforms won't be very sinusoidal, which is what non true rms meters will be 'calibrated' for.

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    In case you know the PT's rated secondary current, Merlin states that the fuse is typically rated for twice to 3 times the rated transformer current.

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    Quote Originally Posted by The Dude View Post
    It's hard to answer this without seeing a schematic or at least more info. There are lots of different 100W amps. We don't know how many preamp tubes and a host of other info. Please post a schematic.
    I don't have a full schematic, but basically hot rodded Marshall, I just want to move the fuse before the bridge rectifier.

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    It is true rms, so readings should be close enough for rock?

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    I do, actually it's 550mA. I guess that's why I was kind of stuck between 1A slow blow or 2A fast. I think I could be okay with either one, but not sure...

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    You could try with 1A slow, if you get any nuisance blowing move up to 1.5A slow.

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    Assuming its a capacitor input main B+ filter then a slow-blow fuse type is appropriate.

    It's preferable to identify what fuse you can purchase, as they typically come in two models - one made for US market (UL-284-14) and one for most other markets (IEC60127-2), and those differ by the max rated current they can handle continuously.

    For example if you can only get US model fuses then they would have to be at least 1.3/0.75 = 1.7A rated fuse, whereas an IEC model has to be at least a 1.3A model. Given you have taken a 1.3A reading on the secondary HT winding, then you don't need to size up too much - eg. a 2A fuse from US range or a 1.5A from the IEC range should be ok. If you oversize too much, then the fuse has less chance of protecting the secondary winding.

    There is a bit of measurement inaccuracy with a general true-rms meter as the current waveform being measured likely has a very high crest factor that is above the meter spec level (which is typically 3x).

    A standard 250VAC fuse is appropriate for the PT secondary even if the winding voltage is substantially more than 250VAC, as protecting that side of the transformer is much more benign that protecting a mains AC supply.

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    Quote Originally Posted by trobbins View Post
    Given you have taken a 1.3A reading on the secondary HT winding,
    He did not state specifically that the reading was at the HT winding. The HT winding is rated for 550mA.
    Not sure exactly what he measured.

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    Yeh I had my doubts too which was why I led with the presumption of 1.3A measured on the secondary winding, given the topic is about fusing the secondary. Hopefully the detail will come out :-)

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    Quote Originally Posted by trobbins View Post
    Yeh I had my doubts too which was why I led with the presumption of 1.3A measured on the secondary winding, given the topic is about fusing the secondary. Hopefully the detail will come out :-)
    Yes, I broke the connection to one leg of the bridge rectifier to measure the current on the HV winding. It is rated at 550mA, so my meter must giving an inaccurate reading? It seemed high to me, but I trusted it, not understanding the crest factor was higher than the meter could handle.

    I've seen schematics of Peavey using a 2A fast blow in this spot or 1.5A slow on older models. My guess is that the fast blows, well, faster, and is an easier value to find, especially on the road.

    Thanks for the help, and thank god for amps during self-isolation. Hope everyone is well out there.

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    Rated transformer secondary current is the max averaged RMS current the winding should be able to supply 100% of time without overheating.
    Amplifier current demand at full power may be higher than rated PT current, as full power won't be delivered all the time.

    Crest factor is a meter limit for RMS accuracy. It means the ratio of peak value to RMS value. For a sinewave the crest factor is 1.414 (square root of 2). The crest factor of the transformer current feeding a rectifier with capacitor load is typically much higher as the current consists of short high peaks.

    I think your current readings are too high. A 100W amplifier typically draws not more than 500mA DC current at full power. The corresponding transformer RMS current should be less than twice that value or less than 1Arms.

    The primary purpose of the secondary fuse is to protect the PT winding from overheating. From the reasoning above it makes sense to double rated transformer current for fuse rating.

    A slow blow fuse generally makes sense as it won't be triggered by short turn-on surges and because winding heat-up takes time. With a fast blow fuse a higher current rating will be necessary. Without monitoring of inrush surge (which depend on specific PT and amp parameters) with a DSO that provides RMS conversion, it's hardly possible to predict the optimum rating for a fast fuse.

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    Last edited by Helmholtz; 03-18-2020 at 09:51 PM.
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    Thanks for thanks, explanation, Helmholtz. My readings were confusing me, and now I understand why. It seems like a 1A slow blow would be reasonable on the AC side of the rectifier.

    Would it be more accurate to measure voltage across a 1 Ohm current sense resistor?

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    Would it be more accurate to measure voltage across a 1 Ohm current sense resistor?
    Only if your meter allows for a higher crest factor with voltage measurements than with current. Check your meter manual. If I had to guess I would think that a crest factor of 5 should be sufficient. Maybe someone knows more exactly?

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    You could estimate the fuse current by simulation - that requires a few measurements made on the power transformer for its effective resistance and voltage parameters, as well as an understanding of the power supply and amp circuitry. I use PSUD2 program for that assessment. That can also indicate the crest factor that a meter would need to cope with. The link is to an article that shows how that can be done:
    https://dalmura.com.au/static/Valve%20amp%20fusing.pdf

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    Quote Originally Posted by Helmholtz View Post
    Only if your meter allows for a higher crest factor with voltage measurements than with current. Check your meter manual. If I had to guess I would think that a crest factor of 5 should be sufficient. Maybe someone knows more exactly?
    The manual for my Fluke reads:

    "AC crest factor can be up to 3 at full scale, 6 at half scale. For non-sinusoidal wave forms add -(2% Rdg + 2% full scale) typical, for a crest factor up to 3."

    Would that explain my high readings?

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    Quote Originally Posted by trobbins View Post
    You could estimate the fuse current by simulation - that requires a few measurements made on the power transformer for its effective resistance and voltage parameters, as well as an understanding of the power supply and amp circuitry. I use PSUD2 program for that assessment. That can also indicate the crest factor that a meter would need to cope with. The link is to an article that shows how that can be done:
    https://dalmura.com.au/static/Valve%20amp%20fusing.pdf
    Thanks, Tim. I really appreciate that article - it's the best and most detailed I've found by far. I was actually going over and over it before getting frustrated and posting here. I just simply don't have enough electronics background to put it all together. I've simulated my own PS, but am having trouble choosing a fuse value based on the results. Perhaps you can help me. Here are the screenshots of my sim results. Thanks!

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    "AC crest factor can be up to 3 at full scale, 6 at half scale. For non-sinusoidal wave forms add -(2% Rdg + 2% full scale) typical, for a crest factor up to 3."
    If I interpret that statement correctly, it doesn't give any information about accuracy for crest factors above 3.

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    Isn't crest factor just Peak/RMS ? In the first example it looks like the crest factor is just less than 3. T1 peak current 1.7697A, RMS .61188A . The assumption is that the numbers for the transformer are correct. You need primary and secondary resistance and the no-load secondary Voltage. Is 50/60 Hz set correctly ? Two Ohms for ESR of C1 is the default number and is probably on the high side.

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    Can you confirm which parts of the model you have measurements for? Do you measure 340V on the secondary unloaded? Did you use the secondary winding effective resistance calculator (buried in PSUD2) to get the 14.5 ohm value (ie. insert the primary mains voltage and the primary and secondary winding resistances)?

    Do you effectively have 265mA max of loading on B+ (ie. are you able to measure and sum the various currents drawn from B+, such as the Output transformer or output stage cathodes, and any screen and preamp feeds)? And what is the B+ DC voltage at 'full volume'?

    Do you have similar measurements for idle conditions (sum of currents, and B+ DC voltage)?

    Measurements for the above questions allow the simulation model to be verified to get some confidence that the model is adequate for the task at hand, given all the other assumptions and margins that are being applied.

    If your model was adequate, then fuse continuous current = 612mArms with a crest factor of 2.9. The following estimated levels indicate a 0.75A T type IEC fuse would be fine - a 0.75A fast blow would not. The fuse current rating is fairly high, so the steps between ratings is fairly large (given that an IEC fuse would have to be at least 0.62A).

    Click image for larger version. 

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    Assessing a UL248-14 model fuse, it would have to be at least 0.83A rating, so likely a 1A Slow blow. There are no minimum multiplier requirements, only maximum, so not much help for design. You may find UL248-14 compliant fuses with typical characteristic curves - which you could use, but you don't have a guaranteed minimum performance to design with.

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    The total DC current I(R1) demand of a 100W amp with a B+ of 400V is typically around 500mA at full power. So actual load resistance must be much lower than 1.7k. Total DC current at full power can be found by inserting a 1R resistor right after the reservoir cap and measuring DC voltage drop.

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    I took some measurements with my Fluke meter using the Max function. Here are my readings:

    With 1kHz sine wave signal at full volume: 1.3A
    What do you get at full output power (just before clipping into rated load) in normal meter mode (not using the Max function)?
    With a 100W Marshall I'd expect around 1.0Arms.

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    Quote Originally Posted by Helmholtz View Post
    In case you know the PT's rated secondary current, Merlin states that the fuse is typically rated for twice to 3 times the rated transformer current.

    I've become very skeptical recently of this rule of thumb after meeting the Masters of Doom Metal (fake name) and the Incendiary Power Transformers. They play their amps at full tilt. A 100W amp came in from them with a blown power transformer. 1A fuse was fitted to HT secondary for a 420mA winding. Seemed reasonable enough. Fitted new PT, checked it out and sent it on it's way. Comes back a few weeks later with a burnt out power transformer again, and I mean burnt. The heater winding was being used at it's limit so I added an auxiliary heater transformer and off it went. Came back again, albeit after a few years, with the same "flaming" problem. Questions as to mode of use followed. Reproducing those conditions on the bench gave a 950mA RMS secondary current. A 1A fuse would never blow and so the transformer would over heat. Not a good situation.

    So, if you fit a fuse that is bigger than the transformer rating you risk a fire under the right conditions. Therefore, how can we in all good conscience advise using a 2 x or even 3x rating ? The theory is the mains fuse blows to prevent a fire, and it would, but not until it's too late for the poor transformer.

    A time delay fuse ran at 1.5 times it's rating will take something like 60 mins to fail. This seems to me a perfectly reasonable arrangement given the long thermal time constant of a transformer and so allows for irregular peak loudness of more normal use. If that fuse keeps blowing the transformer is not going to last very long.

    On the other hand, there is the surge current when you close the standby switch. I suspect that the reason for the 2x-3x rule of thumb. So we have opposing requirements which leads me to conclude that the fuse rating should be close to the transformer rating and that the issue of surge current needs to be dealt with by other means for example a negative temperature thermistor.

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    I agree and and need to clarifiy that Merlin's statement seems to be an observation rather than a recommendation.

    http://www.valvewizard.co.uk/fuses.html

    (Actually PT manufacturers should know best and I think they should be bound to specify the fuse ratings.)

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    Quote Originally Posted by nickb View Post
    1A fuse was fitted to HT secondary for a 420mA winding. ..... Reproducing those conditions on the bench gave a 950mA RMS secondary current. A 1A fuse would never blow and so the transformer would over heat. Not a good situation.
    Yikes - Agreed - but the transformer rating is the problem. No fuse can solve that. That amp simply can not be played safely.

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    Maybe add some protecting resistance in series with the winding, in order to limit the max current?
    Similar to what would be done with a tube rectifier, should the effective HT winding resistance be too low.

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    Quote Originally Posted by uneumann View Post
    Yikes - Agreed - but the transformer rating is the problem. No fuse can solve that. That amp simply can not be played safely.

    ..as is self evident But perhaps you should tell that to Hammond. It's their JCM800 100W transformer 290HX. I'll leave it at that to keep this thread on topic (fuse ratings).

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    Quote Originally Posted by nickb View Post
    But perhaps you should tell that to Hammond. It's their JCM800 100W transformer 290HX.
    You know, might not be a bad idea. The model has failed in this fashion enough times to determine a pattern. If I were Hammond, I would want to know.
    I have a question(s) as it relates to this topic. I can use this hammond transformer as an example.
    This is essentially a problem of heat, with the breakdown of the insulation causing the failure, correct?
    Could hammmond keep the same transformer specs, as it’s currently designed, and change only the class of insulation to an appropriately higher temperature, and have the transformer work normally at the load you mentioned?

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    I checked the specs of a few more 100W amp PTs. All of them have a rated B+ winding current below 50% of the amps' current demand at full power (using PSUD2).
    It seems that these PT ratings/specifications are based on the assumption that max power is typically only demanded for a period of time that is short compared the thermal heat-up time of the PT with ample low power cooling periods between power peaks.

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    Quote Originally Posted by Helmholtz View Post
    I checked the specs of a few more 100W amp PTs. All of them have a rated B+ winding current below 50% of the amps' current demand at full power (using PSUD2).
    It seems that these PT ratings/specifications are based on the assumption that max power is typically only demanded for a period of time that is short compared the thermal heat-up time of the PT with ample low power cooling periods between power peaks.
    Exactly. They typically design using a crest factor of 10 to 20dB and so can justify an economical transformer. This is is fine 99% of the time. The other 1% of the time you are looking at a charred and very sad power transformer. The Masters of Doom had a crest factor of about 2.5dB, which is a square wave with approx 50% duty.

    Since not blowing up transformers is a priority, I say use a time delay fuse equal in rating to the transformer secondary and no harm will be done. If the fuse keeps blowing you might need to address the surge current otherwise you need a bigger transformer.

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    Quote Originally Posted by trobbins View Post
    Can you confirm which parts of the model you have measurements for? Do you measure 340V on the secondary unloaded? Did you use the secondary winding effective resistance calculator (buried in PSUD2) to get the 14.5 ohm value (ie. insert the primary mains voltage and the primary and secondary winding resistances)?

    Do you effectively have 265mA max of loading on B+ (ie. are you able to measure and sum the various currents drawn from B+, such as the Output transformer or output stage cathodes, and any screen and preamp feeds)? And what is the B+ DC voltage at 'full volume'?

    Do you have similar measurements for idle conditions (sum of currents, and B+ DC voltage)?

    Measurements for the above questions allow the simulation model to be verified to get some confidence that the model is adequate for the task at hand, given all the other assumptions and margins that are being applied.

    If your model was adequate, then fuse continuous current = 612mArms with a crest factor of 2.9. The following estimated levels indicate a 0.75A T type IEC fuse would be fine - a 0.75A fast blow would not. The fuse current rating is fairly high, so the steps between ratings is fairly large (given that an IEC fuse would have to be at least 0.62A).

    Click image for larger version. 

Name:	Fusing example 1.jpg 
Views:	13 
Size:	43.0 KB 
ID:	57505

    Assessing a UL248-14 model fuse, it would have to be at least 0.83A rating, so likely a 1A Slow blow. There are no minimum multiplier requirements, only maximum, so not much help for design. You may find UL248-14 compliant fuses with typical characteristic curves - which you could use, but you don't have a guaranteed minimum performance to design with.
    OK, thanks for clarifying trobbins and Hemholtz. I took some more measurements that will hopefully clear some ambiguities up.

    - Found the source res calc, and now the correct value is 25 Ohms
    - 340V is unloaded voltage
    - total cathode current at full crank is 700mA, and B+ sags to 395vdc
    - Idle secondary current is 292mA and 952mA at clipping (just about what Hemholtz guessed)

    I think I have everything needed to use PSUD2 correctly, except I don't know how to find the load. I was mistakenly using the OT primary impedance.

    Thanks!

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    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.

    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.

    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.
    Click image for larger version. 

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  34. #34
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    total cathode current at full crank is 700mA
    I am surprized at your high DC current of 700mA. I would have expected a value below 600mA. The EL34 datasheet gives a total cathode current of 580mA at a "clean" output of 110W. This is based on the recommended idle cathode current of 34.4mA per tube.
    A DC current of 700mA can't be consistent with a transformer current of 952mArms. Were operating conditions identical for both measurements?

    How did you measure total cathode current? Does "full crank" mean lots of clipping?


    except I don't know how to find the load
    Equivalent load resistor (R1) is found by loaded B+ = V(R1 divided by total DC current = I(R1) (Ohm's Law). R1 varies with the amp's operating conditions (idling, full power etc.) and must be chosen accordingly.


    Do you have a specsheet of your PT? It seems to be more powerful than typical replacements for 100W Marshalls.

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    Quote Originally Posted by Helmholtz View Post
    I am surprized at your high DC current of 700mA. I would have expected a value below 600mA. The EL34 datasheet gives a total cathode current of 580mA at a "clean" output of 110W. This is based on the recommended idle cathode current of 34.4mA per tube.
    A DC current of 700mA can't be consistent with a transformer current of 952mArms. Were operation conditions identical for both measurements?

    How did you measure total cathode current? Does "full crank" mean lots of clipping?
    Sorry for the confusion. I thought "full power" meant most power out of the amp, not clean power. At clipping, DC cathode current is 440mA (B+ sags to 410vdc), and AC secondary current is 950mA. I figured maximum output would be more useful.

    Quote Originally Posted by Helmholtz View Post
    Equivalent load resistor (R1) is found by loaded B+ = V(R1 divided by total DC current = I(R1) (Ohm's Law). R1 varies with the amp's operating conditions (idling, full power etc.) and must be chosen accordingly.
    Thanks this is helpful.

    Quote Originally Posted by Helmholtz View Post
    Do you have a specsheet of your PT? It seems to be more powerful than typical replacements for 100W Marshalls.
    I don't, but it's closer to a Hiwatt PT. I do know secondary is rated at 550mA.

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