Amp fuse suggestions

[Mini-Strat_Maine]

Now this one is more "Wiring" than "Tube Amp," so here goes: I thought if my Musicmaster was opened up for repairs, replacements, and improvements anyway, it might be a good time to add a fuse to the 110 VAC supply path. (Apparently the earlier MMs were UL listed in spite of not having a fuse.)
Ampwares www.ampwares.com/index.html (for one) has a multitude of fuse holders, but I wanted to ask your suggestions for the amp rating and type (slow vs. fast, etc.) of fuse.

[sumgai]

Doug,

Make no mistake, this is a field whereof I speak from more than a little experience. The deal is, either MusicMan did not put out an amplifier for sale, or else they had a UL rating or listing. Cut and dried, no argument. Laws are in place at many stages along the manufacturing path that work to ensure the safety of a moderately functional human being who might purchase said amplifier, let alone any other device that uses electrical power in any fashion.

But fortunately for you, that's not what you're dealing with here. If you open your amp up, you'll find the fuse is sitting in a clip style holder on the printed circuit board. If you want to move it to the back panel of the chassis, I encourage you to do so, I've done it often for my customers, and I don't believe there is any downside to doing this. Why MM didn't do the normal thing in the first place is beyond me, but here we are, so we're gonna make the best of it, right?

For general information, here's the answer to your question. When you first turn on a tube device, there is an inrush of current that is trying to heat up the filaments of each tube. Since they are cold, their collective resistance is low, thus the larger-than-normal demand for current. As things warm up (in a second or three), the demand slacks off until it reaches the normal current draw.

To guard against blowing a fuse during that large initial flow of electrons, we use a Slo-Blow fuse. Why? Because it does not blow at the rated current draw until that draw exceeds a certain amount of time. How long is that time? Interestingly enough, about the same amount as it takes the filaments to settle down to normal current draw. Kinda like it was meant to do that, eh?

Yeah, the fuse was designed just for that reason, with that time element in mind. When the initial rush is over, the fuse has to be able to tell when a warmed-up amp is misbehaving, and it needs to blow now, not after a few seconds. The timing of this isn't mission critical, but during the manufacturing process, the tolerances are held pretty close.

So where do we use a Fast-Blow fuse? Easy, in most non-tube circuits. But truth to tell, they're still too bleeping slow to protect expensive output transistors. In fact, there's even an old saying in the technician community that goes "A 10 dollar transistor will blow up in order to protect a 50 cent fuse every time." ;D One of Murphy's Laws, believe me.

HTH!


sumgai

[Mini-Strat_Maine]

Okay, I'm thinkin' 1-amp, 1¼" x ¼" Slo-Blo fooz and appropriate holder.
It's actually a '71 Fender Musicmaster Bass amp. Looking at the schematic that I'd forgotten I have, it does show a "CSA THERMAL PROTECTOR 010872" between the wall plug and the AC switch. 'Magine that!
So now I know not only what kind of fuse is best for the MM, but why. , SG!

[sumgai]

Doug,

I'm not editing my previous post to correct the obvious error, I need to take my lumps. But you did say 'MM', and to me, that meant MusicMan, as in Leo's second company. Sorry.

Now, if I'd paid better attention the first time around, I'd have known to tell you that some Fender solid-state amps had circuit breakers in them instead of fuses. While you can still convert to fuse, the question now becomes, why bother? The circuit breaker will likely last as long as you do, barring unforeseen circumstances like, say, a lighting strike right on your house. That would toast it for sure, but at that point in time, I'd lay heavy odds that the amp's problems would be one of your least concerns!

Still, it's your call. Talk it over with your tech before you make the jump, that's all I can suggest.


sumgai

[Mini-Strat_Maine]

sumgai said:

But you did say 'MM', and to me, that meant MusicMan, as in Leo's second company. Sorry.

No, that's my error, for once again thinking everyone else knows what I'm babbling about. (When some days, I don't know, either. )

Now, if I'd paid better attention the first time around, I'd have known to tell you that some Fender solid-state amps had circuit breakers in them instead of fuses. While you can still convert to fuse, the question now becomes, why bother? The circuit breaker will likely last as long as you do, barring unforeseen circumstances like, say, a lighting strike right on your house. That would toast it for sure, but at that point in time, I'd lay heavy odds that the amp's problems would be one of your least concerns!

Yup.

I'll have to take a closer look at it tonight, but I don't recall seeing a reset button anyhwhere on it. If a thermal breaker resets itself when the temperature goes back within limts, then it's not such a big deal. I just thought replacing a fuse would be easier than resetting (or replacing) an internal part that I haven't seen or located yet.
Also had this comment from somebody on another forum: "I don't understand how Fender allowed the amps to be sold with just a thermal fail-safe, as a power-tube failure usually won't overheat anything."
Wouldn't a tube failure act as an "open," thereby shutting everything down? Or would it cause an inrush of curent that a fuse would protect against?
Just looking for a little more protection against surges and other rude surprises.

[sumgai]

Doug,

In the kindest words I can manage, would you please take a picture of the back of this thing, and post it here? I also cannot believe that a Fender tube amp made it out the door without any kind of overload protection. A circuit breaker would work, but it would be much more expensive to design and use one that had the same characteristics as a Slo-Blow fuse (handles large current inrush).

Fender did put out a MusicMaster Bass amp that was solid state, and like several other solid state models in those years, they used a circuit breaker. They must have figured it would be more profitible overall (lower costs of assembly), or something like that.

I really gotta see this, my imagination is running wild here.


sumgai

[Mini-Strat_Maine]

sumgai said:

Doug,

In the kindest words I can manage, would you please take a picture of the back of this thing, and post it here?

I just went back and looked at the pic I attached with my post about the loose screw on the output transformer, and it's now tiny compared to what it looked like when I first posted it. Apparently I was seeing a cache of the image as it was first linked, while you and anyone else were seeing the 25% version that Photobucket eventually changed it into.

I also cannot believe that a Fender tube amp made it out the door without any kind of overload protection. A circuit breaker would work, but it would be much more expensive to design and use one that had the same characteristics as a Slo-Blow fuse (handles large current inrush).

I'll try to get a good clear shot of the back of it, if I don't already have one from the pics the previous owner posted on eBay.
I got that amp around the end of January, so you were spared the barrage of questions I bounced off all the other members, and any raving I did when I got it.
It's a 1971 vintage, so maybe all UL was concerned with in those days was that there weren't any exposed wires anywhere. This one has a 12AT7 and a pair of 6V6GC's in it, plus an upgrade of its speaker.

I really gotta see this, my imagination is running wild here.

I've deleted the now-too-small pic that I took of the loose screw, and am throwing in two that the seller took of the back:


The screw that really isn't holding the OT in place is visible just to the left of the right-hand 6V6. The black spot at the top left corner, next to the UL label, is where the Tolex has come loose. The power cord enters the chassis just under there. No fuse holder anywhere in there, nor on the front panel:
Other than a few warts like that, it's a pretty good amp, not to mention being the upper limit of what I'm willing to lug around.

[sumgai]

Doug,

I confess, I can't see anything that would lead me to believe that this amp is kosher. No fuse, no circuit breaker, it looks like you got bupkis. Only one possibility left, and that is that one of those items is mounted inside. Mighty strange for a Fender, but then again, I guess there are people who say that about me! (No, not that I'm a Fender..... ;D)

Shag butt down to your tech, post haste. If there truly is no protection inside, then have the tech document the mods he makes. You should also take a few pics before and after. No big deal, but in a world full of lawsuits, it never hurts to CYA.


sumgai

[Mini-Strat_Maine]

sumgai said:

I confess, I can't see anything that would lead me to believe that this amp is kosher. No fuse, no circuit breaker, it looks like you got bupkis. Only one possibility left, and that is that one of those items is mounted inside.

I'll admit, I don't know an awful lot about schematics, but I can't spot anything designed to "cut the juice" in any way, other than that "thermal protector" and the "A.C. SWITCH." There's nothing in the line cord, so it's got to be buried behind the sheet metal.
The schematic is at www.schematicheaven.com/fender.htm. Look for CFA-7010, just a little ways down the center column.

Everything between me and the wall outlet is going to be plugged into a GFCI adapter, but sometimes redundancy is good.
Thanks for looking that over.

[sumgai]

Doug,

I see it all now! ;D That's a "thermal" protector in the sense that it depends on too much electron flow, too quickly, thus creating heat in the coil of the unit. Said coil, carrying the line current, becomes too hot, and flexes (it's made of two different metals - different heat transfer co-efficients). When the flex is at maximum range, the current flow is broken (the "switch" is open). Then the strip of bi-metal cools down, the contact is re-established, and current again flows. If the fault has not been removed, then the cycle repeats all over again.

Personally, I don't care for them, but the UL (and other safety laboratories around the world) has signed off on them, so who am I to fly in the face of such worldly-wise people? <need sarcastic smilie here> Leave it in, but stick a fuse in there too. Belt and suspenders may get you laughted at, but you'll be alive to be laughed at, right? ;D


sumgai

[UnklMickey]

ministratmaine said:

...I just went back and looked at the pic I attached with my post about the loose screw on the output transformer, and it's now tiny compared to what it looked like when I first posted it. Apparently I was seeing a cache of the image as it was first linked, while you and anyone else were seeing the 25% version that Photobucket eventually changed it into....

i must have wasted hours when i first started using photobucket, trying to figure out why my edited / altered images wouldn't "take".

image the head-slap, when i realized, all i needed to do was hit "refresh" on my browser!


unk

[Mini-Strat_Maine]

sumgai said:

When the flex is at maximum range, the current flow is broken (the "switch" is open). Then the strip of bi-metal cools down, the contact is re-established, and current again flows. If the fault has not been removed, then the cycle repeats all over again.

I thought that was how them li'l monsters worked. No replacing or manual resetting needed, but if you keep hearing "PLINK!" in between everything going off and on, something needs to be troubleshot.

Leave it in, but stick a fuse in there too. Belt and suspenders may get you laughted at, but you'll be alive to be laughed at, right? ;D

My thoughts 'zackly.

[Mini-Strat_Maine]

unklmickey said:

must have wasted hours when i first started using photobucket, trying to figure out why my edited / altered images wouldn't "take".
image the head-slap, when i realized, all i needed to do was hit "refresh" on my browser!

Yeah, it finally dawned on me to try that button in Firefox' toolbar, the one with the circling arrows on it. Wow, what a concept.
But now I've raised my standards beyond where I'm just happy to not have smoke emitting from anywhere on the computer.

[sumgai]

Doug,

.....
But now I've raised my standards beyond where I'm just happy to not have smoke emitting from anywhere on the computer.

What, you finally passed the Environmental Test - no fire, no explosions, no women and children fleeing the scene in terror? Congratulations! Here, have one on me:




sumgai