Resettable fuses in 200A Wurlitzer amps -- What do people here think?

[DocWurly]

Resettable fuses are being sold to replace the 1.5 amp glass ones in Wurlitzer amps.  They are spec'd to trip at minimum 1 amp current.  Apparently they can take a max of 4 seconds to trip (according to spec sheet, if I am understanding it correctly).

The glass ones are a pain, as we know.  At the very least, one replaces such blown fuses with an inline fuses setup for easier switching.

But what of these Polyfuse resettable PTC fuses?  I've heard arguments that if your amp is blowing fuses, we don't want the thing reset, because that's a sign of a problem.

That makes sense.  What I am wondering though:  Are these fuses basically safe, assuming you don't keep endlessly resetting them?  Or do they take too long to trip, compared to the glass fast-blowing fuses?

I already have one opinion that I tend to trust, but I'd like to get a roundtable on this one.

[pianotuner steveo]

As little no as you find and solve the issue that caused it to trip in the first place, it sounds like a good idea to me, but the up to 4 seconds bit scares me a little. I believe my old Rhodes suitcase had one of these circuit breakers on the power supply.

[Tim Hodges]

Good article which covers the differences:

Fuses vs PTCs

Choosing between a PTC and a Fuse

Overcurrent circuit protection can be accomplished with the use of either a traditional fuse or the more recently developed resettable PTC. Both devices function by reacting to the heat generated by the excessive current flow in the circuit. The fuse melts open, interrupting the current flow, and the PTC changes from a low resistance to a high resistance to limit current flow. Understanding the differences in performance between the two types of devices will make the best circuit protection choice easier.

The most obvious difference is that the PTC is resettable. The general procedure for resetting after an overload has occurred is to remove power and allow the device to cool down. There are several other operating characteristics that differentiate the two types of products. The terminology used for PTCs is often similar but not the same as for fuses. Two parameters that fall into this category are leakage current and interrupting rating.

Leakage current: the PTC is said to have "tripped" when it has transitioned from the low resistance state to the high resistance state due to the overload. Protection is accomplished by limiting the current flow to some leakage level. Leakage current can range from around a hundred milliamps at rated voltage up to several hundred milliamps at lower voltages. The fuse on the other hand completely interrupts the current flow and this open circuit results in "0" leakage current when subjected to the overload.

Interrupting rating: the PTC is rated for a maximum short circuit current at rated voltage. This fault current level is the maximum current that the device can withstand but the PTC will not actually interrupt the current flow (see LEAKAGE CURRENT above). A typical PTC short circuit rating is 40A. Fuses do in fact interrupt the current flow in response to the overload and the range of interrupting ratings goes from hundreds of amperes up to 10,000 amperes at rated voltage.

Resistance: Reviewing product specifications indicates that similarly rated PTCs have about twice (sometimes more) the resistance of fuses. Time-current characteristic: comparing the time-current curves of PTCs to fuses shows that the speed of response for a PTC is similar to the time delay of a Slo-Blo® fuse.

For me I prefer fuses. PTC's make sense to me if I were to have a power surge but didn't want to replace a fuse each time however if there was a fault in the amp I would rather there be no current running through after the fuse or PTC has been tripped.

[DocWurly]

Thank you for this!

[Tonewheel]

I called Ken Rich (Kenrichsounds) about this because my understanding is that he has been a guru in the area of Wurlitzer repairs and restoration. He says that he has installed these in many units, and has not had a problem.

I blew one fuse attributed to a loose solder to the speakers just after delivery of my 200A, and then another recently because of a loose C31 (not located in a good spot because it is right beside the screw holding the amp to the plate). And there may be operator errors that will be responsible (reed touching the pickup?)

I think most sentient beings would not keep "rebooting" the unit if the fuse keeps blowing. So the real issue is replacing the fuses in the first place. I know that an online business sells the exact tiny copper-coiled pigtail, but who wants to go that route even once? And I now have 2 fuse holders that will get torqued a little when replacing a fuse, so I have ordered the pair of resettables from KR. $5.50 plus shipping.

I will certainly let you know if this does not work.

[cinnanon]

I use a light bulb limiter and 25W bulb to troubleshoot. Saves me many many fuses! I also just replace the pigtail fuses with inline fuse holders.

[Tonewheel]

I now have two fuse holders, too. But their wires are thicker and tend to put torque on the pins when manipulated or the top is closed. And once I flipped the board up, it was easy to tell that someone had soldered in an "OEM" replacement in the distant past (this was an estate sale item where the owner had passed on years before, and had not been played).

I also replaced the main .5 Amp pigtail with a small fuse block using a self-tapping metal screw (careful with length because a wooden key is just below)


I like the clean board with lots of real estate on the Retrolinear replacement.

[Tonewheel]

Got the resettables and this really cleaned up the top of the board. My problem with the fuse holders is that replacing a fuse that way will inevitably put torque on the soldered leads and then cold solders start showing up.

Also have the variable vibe from Retrolinear installed, and it sounds like it should have always been there. I especially like the slower frequency for ballads. Very quiet board.