Immortal Mods: additional fuses, where to put the fuse holders?

[mikepukmel]

Hi All,
I have a build that has slowed considerably due to grappling with where to put fuse holders for Immortal Mods extra fusing. he recommends fusing each leg of the HT off the power transformer, and each leg of the heater wiring.

This JTM45 type thing also has 5v for a tube rect.

After some trial and error, I found I can fit two screw down fuse clips on the side of the inside of the chassis, right over the PT. The HT wiring runs right around there anyway so at least I can't see a problem with this.

That leaves the 6.3v heater wiring fuses. The only place I could find in this chassis, and not much room in there, is between the power transformer and main board. There is just *barely* enough room mount the clips. The problem is running the wiring. I would need to run the heater wiring around the power transformer to one side of the clips near where the presence control is located (not so close but near there). This will make the heater wiring much longer, and also, running from the "output" side of the clips over to the first power tube runs right across the junction of HV wiring leading to the choke from the main reservoir cap.

The inline fuse holders I found didn't look to be much better a solution. I tried to find a place to put chassis mount (through chassis) holders, but, again not much room.

Any suggestions on where to put the heater wiring fuses, greatly appreciated.

[catalin gramada]

No need to put fuses on the heaters. Normaly the wire is so thick as any short here will determine the fuse in primary side to blow first and will not endanger the heater winding even a bit.

[Pedro Vecino]

Just a week ago an American guitarist (here in my city) accidentally connected his Vox AC30C2TV 120V to an output of the autotransformer with 230V. I thought the worst but fortunately only the mains fuse and filament fuse melt.
I think it's better to have more fuses than less

[pdf64]

No need to put fuses on the heaters. Normaly the wire is so thick as any short here will determine the fuse in primary side to blow first and will not endanger the heater winding even a bit.

I think you may be wrong there.
Problem being that shorts tend not to be perfect, such that any slight impedance limits current but allows the heater winding to deliver way more than its rated current without primary side fusing being stressed.

[loudthud]

I've seen the heater wires melt the insulation back to the transformer on a Fender blackface when a short occurs at the pilot light. On some years there are small resistors in series with the pilot light.

[pdf64]

Just to note that regular fuse types / fuseholders aren't really suitable to low voltage, high current applications such as tube heaters; with such a low 'wetting' voltage, any slight gunk build up on the contacting surfaces and things get way hot, drop voltage etc.
Whereas automotive fuses are a very similar application, so see what you can source.
Another option are fuses with leads that solder in place like a regular leaded component.

[R.G.]

If the he you were referring to was me, I've faced down some of the same issues. You're right - it is quite difficult to place the fuses.

I'm with pdf on the heaters - if you can find suitable blocks to mount them in, automotive blade fuses are the simplest to deal with. I googled "automotive fuse blocks" and found many hits. Here's one source: http://www.wiringproducts.com/fuse-blocks

In a typical amp, you'd need to fuse the primary (already done for you, probably), two high voltage leads on the secondary for B+, and one to three heater leads. That accounting comes from the idea that there may or may not be a 5V heater used for a rectifier tube, which would get one; one or two for the 6.3V heaters, depending on whether there is a hard CT used. If the hard CT is used, you need two. If there is a resistor CT, you only need one.

The four-place automotive block may be good for this. Use two for 6.3v heaters, which are held near signal ground by the CT, and one for the 5V rectifier fuse. Use your pliers and rip out one spot in a four-position block between the 5V heater fuse and the others, to get more HV insulation spacing as the 5V heater is sitting up at B+.

The inline soldered fuse is a good suggestion for sidestepping the problems with 5V being high current and low voltage, but sitting on top of several hundred volts of DC. You could encase this in shrink wrap, soldered in line with one of the 5V leads over to the rectifier tube socket, and also solve one of the other issues with fuses - how to run the wires so as to cause the least hum and noise. The 5V leads really need routed away from anything that might pick up rectifier hash.

[mikepukmel]

Thanks, yeah, mostly worried about a $260.00 transformer melting down from a bad tube, before the primary fuse goes out. I have fuse clips, for the HV, didn't know about low V high I problem, thanks for mentioning it.

I tried to find good inline holders but most look really crappy. Thanks for the suggestion about automotive. I can mount the HV clips on the side of the chassis, pretty much right in the path where the wiring is routed.

The heater wiring, akk, no good place to put holders, and the automotive blocks need some space as well. Will get dimensions on the smaller automotive clip.

So, I guess I know why the manufacturers didn't fuse everything properly, they would never be able to make any money.

The automotive pushin fuses have a really small footprint! Didn't know they could be used for heater wiring. How cool is that!

If I can't fit anything else in there, the solder-in idea, with heat shrink sounds great. Easy to test. Since its a low occurrence fail.

I think I understand the low V issue a little, the fuse holders e.g. the chassis mount, barely touch the fuse. The automotive fuses have a fairly high contact pressure between the blades and fuse. The chassis mount fuse holders I have, do have a fairly high resistance. I can't rotate the fuse once pressed in and it fits in fairly tight. Is this the main issue?

As mentioned above, solder in: I could push the fuse in and solder a very small bit of wire from the lug on the fuse holder to the metal part.

But, then, back to the wire routing/location issue. :O

How close do I need to keep the heater wiring from other stuff (presence control, the components on the end of the board for bias, HV wiring? Is there some minimum distance that I can use and say double?

[R.G.]

Thanks, yeah, mostly worried about a $260.00 transformer melting down from a bad tube, before the primary fuse goes out.[...]
I tried to find good inline holders but most look really crappy. Thanks for the suggestion about automotive. I can mount the HV clips on the side of the chassis, pretty much right in the path where the wiring is routed.

Fuses are tricky. They're in that class of things that seem to be absolutely straightforward, but get more complicated the more you try to apply them. They're sensitive to how much excess current flows, and for how long, and for snap-in fuses, you get the whole range of complexities of metallic contact metallurgy and contamination. Pure fuse issues aside, it seems you're having the contact and mechanical issues now.

Automotive fuses are intended specifically to interrupt low voltage currents. Fuses have a voltage rating, meaning that for voltages higher than their rating, they may fail to prevent a long lasting arc from forming as they melt. cartridge fuses are generally rated for 125vac to 250vac. Auto fuses may only be rated for 50V or so. One decent option is the 2AG 5x15mm mini cartridge fuse. It's not as small as automotive, but nearly. They also come in little single-screw mount blocks. See
MetricOMNIBLOCK here. Here's another: Littelfuse_Fuse_Block_254. They illustrate the singles, but they usually offer a 2, 3, 4 and more fuses wide block. The better fuse blocks are rated at 400Vac as well.

I think I understand the low V issue a little, the fuse holders e.g. the chassis mount, barely touch the fuse. The automotive fuses have a fairly high contact pressure between the blades and fuse. The chassis mount fuse holders I have, do have a fairly high resistance. I can't rotate the fuse once pressed in and it fits in fairly tight. Is this the main issue?

It's back to that "details matter" stuff. Good contacts require enough surface pressure between metallic contacts that the little microscopic hills and valleys on the surfaces grind together to get points of real metal to metal contact. The more points of contact, the better and lower resistance the total contact is. Each microscopic point contact can conduct X amps, and the total current carrying area is the number of contacting hills N times the current ability of each X amp contact. X depends on the metals in the contact wire and how flat it is. N depends on both how flat the contact surface is, and the area of the possible contact. Dirt and grease, sand, smoke particles, etc. fill the valleys until they start holding the hills apart and eventually N goes to zero and the contacts don't meet at all. Metal oxide films also interrupt hill-to-hill contacts. High pressure between the two metal surfaces breaks through the metal oxides and dirt, and re-insertion scrubs the two surfaces together and scrapes dirt and oxides off. Gold and other rare-earth metals don't oxidize in air, so they're best for premium contacts.

We know that all insulators have a voltage rating, where the voltage will literally punch through the insulation. Happens with dirt, grease and metal oxides too. High voltages can punch through minor contamination. Low voltages may not be able to. So the auto fuses use wide contact area and high spring force combined with long wiping action on insertion to make up for the lack of high voltage punch through. AC power line and higher, and expecially AC voltages can punch through any minor contamination easily enough. So AC line fuses get by with much less contact pressure and long through wiping on insertion.

For your B+ fuses, you want high voltage AC fuses. For your heater fuses, you want auto-style or similar. Auto style fuses in B+ lines may not clear on a fault.

As mentioned above, solder in: I could push the fuse in and solder a very small bit of wire from the lug on the fuse holder to the metal part.

It's a PITA, but it would work.

How close do I need to keep the heater wiring from other stuff (presence control, the components on the end of the board for bias, HV wiring? Is there some minimum distance that I can use and say double?

1. Heater wires carry low voltage AC. They can also carry AC line-borne junk. It is usually best to run heaters on twisted-pair wires which are run right along the metallic chassis. Even better, run shielded, twisted pair for heaters. Ground the shield at one end to the chassis. Twisting mostly keeps the AC voltage and line-borne junk inside the pair. The shield further attenuates any radiated capacitive coupling from the pair.
2. Coupling into signal wires depends on radiated magnetic field and capacitive radiation. Magnetic field radiation is maximum with high currents and big area enclosed by encircling currents. You can't necessarily lower the current in the heater loop except by star-wiring the heater pairs (!) but you can make the area between conductors smaller. Twisted pair is about the optimum possible way to do this. Shielded twisted pair heaters is about as good as you can do without going to DC heaters.
Coupling of hum into signal wires also depends heavily on the impedance of the signal wire being coupled into. Grid wires are the worst, as they're very, very high impedance. Tone/volume controls are also high-ish impedance runs, and you want them away from heaters; also not near the rectifier and first filter cap wires, Ideally, PT, rectifiers and first filter cap are at the far end of the chassis from any signal wires, and most especially away from the input jacks (which includes the reverb return, remember).
3.Parallel wires couple more than crossing wires. If you cross heater wires at 90 degrees, even directly touching, coupling is minimal, especially if the heater wires are twisted. Avoid long runs of signal wire in parallel with heater wires.
4. Given that your heaters are in twisted pair between tubes, you can usually stop worrying about distance from the heater wires after 1/4" to 1/2" from them.

At least that's what I think while going through my first two cups of coffee.

[Steve A.]

I always added automotive blade-type fuses when replacing 24VAC power transformers on HVAC equipment as added insurance... I used 1/4" crimp terminals to wire them in and would wrap them them with Scotch 33+ electrical tape.

In the old days I would use glass fuses in plastic holders but would have to charge extra for them...

Steve A.

P.S. Modern HVAC circuit boards often have 12VDC automotive fuses in line with the 24VAC control voltage supply but see RG's point in not using them for higher voltages.

P.P.S. I believe that "Immortal Mods" was the name of a Dan Torres article in Vintage Guitar magazine back in the 90's... I don't think that writers like him were actually PAID for their articles but would receive a free stack of magazines that they could sell in their shop.

[Justin Thomas]

I don't know about the "Immortal Mods," but "The Immortal Amp Mods" was a short series of articles written by R.G.
I don't know the publication for sure, but I believe it was Premier Guitar, as after having occasionally run across them online in part, I ran across several of them in the actual issues when a friend gave me a huge stack of his old GP & PG subscriptions... I may have cut them all out and filed away hard copies somewhere...

Yeah, I'm one of those odd folks who still likes to read actual PAPER...

Justin

[catalin gramada]

Interesting facts about low voltage fusible protection here. I wonder how low is low enough to consider regular 240v ac. fuses as a bad designation for low voltage dc. rail protection ?

[R.G.]

Probably depends on the current more than the voltage. Voltage is an issue for getting current into the fuse filament. The microscopic hills making metal to metal contact and having enough voltage to burn away small bits of crud still applies, but how low a fuse can go for current depends on the design of the filament alone, which is a different thing entirely.

Fuses interrupt AC better than they interrupt DC because the voltage and current go to zero - possibly at different times - twice per AC cycle. Any arc that forms has two chances per cycle to extinguish. But low voltage DC is easy enough to interrupt, as the voltage isn't high enough to form a self sustaining plasma arc.

Car fuses were specialized for conducting high currents. I can't remember an auto-style fuse lower than 5A, although they may well exist. Cars use 12Vdc, usually at 10A or more.

My best guess is that for currents in guitar amps, where 10A of heater current is high, don't sweat it. Either style will do. For solid state amps, if you have to put in a fuse, put in a 2AG or 3AG for last-ditch hail-Mary attempts to save things, and do solid state current limiters. Solid state limiting is the only thing that has a chance to save a high current device from overcurrent - fuses are way too slow and way too imprecise.

And the real answer is to do what I do: go find and read fuse application manuals.

[SoulFetish]

Interesting facts about low voltage fusible protection here. I wonder how low is low enough to consider regular 240v ac. fuses as a bad designation for low voltage dc. rail protection ?

Often you can use a particular fuse in a higher voltage application than it is rated for, particularly if there are no suitable options for an application. I know littelfuse recommends contacting themto get a recommendation for an appropriately de-rated selection.

[mikepukmel]

Thanks again everyone. Another great thread to get printed, and put in that big binder of great amp topics. The HV is covered, the line voltage, still don't have a perfect solution for the heater wiring but not for lack of your knowledge, its just that the blocks don't fit in this tiny chassis. Geez, this amp building stuff covers many very interesting topics. I know there are many old amps without all these fuses, but I do recall a few that went up in smoke back in the day.

Justin, yeah, that's the one, RG Keene, Premier Guitar. I made a few hard copies myself for my big binder of "stuff I would like to keep if I lose internet".

From all your links found 4 and up auto blocks, but haven't found a 2 block yet, still looking. Maybe a 4 block and a band saw with a fine blade would work. i think I have to get this one "right".