Secondary Fuse Value

[nickb]

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.

[Gaz]

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).

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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.

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!

[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.

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.

[Helmholtz]

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.

[Gaz]

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.

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.

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.

[Helmholtz]

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.

[Gaz]

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

[Gaz]

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.

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?

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.
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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)?

[Helmholtz]

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.

[Gaz]

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?

[Helmholtz]

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.

[Helmholtz]

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.

[Gaz]

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.

[Gaz]

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

[Helmholtz]

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.