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

[SoulFetish]

You ever run across amps that fuse the bias supply?

[dstrat]

You ever run across amps that fuse the bias supply?

That would be unwise...imo

but im not going to fuse any 6.3vac heaters either.

[R.G.]

You ever run across amps that fuse the bias supply?

As dstrat says, bad, bad, bad idea. The bias supply may be thought of as a separate power supply voltage that is itself a protective mechanism. It protects the power output stage from destroying the output tubes, output transformer and possibly the power transformer too. You never want the bias supply to shut down.

It's generally unlikely that it will shut down, as the bias supply is a low-power, very low current thing, so it's easy to simply use massively derated parts to run up the reliability of the parts. It has a high impedance output to the tube grids, so there's not much chance that a shorted wire to a tube grid will kill the bias supply. Realistically, a fault in the bias supply rectifier or filter cap will let the output tubes run wild. The outputs serve as an "indicator" that the bias supply is having problems by getting too hot, overloading the main PT, sending up smoke signals and other amusing and interesting effects.

In fact, along with the ideals of the Immortal Amp, it might be a good idea to have some circuit sense the presence of the bias supply and turn the entire amp off if it was not of sufficient magnitude.

but im not going to fuse any 6.3vac heaters either.

It's a personal risk assessment we all have to make.

[dstrat]

Yes It will likely bite me sooner or later with modern tubes

[Leo_Gnardo]

You ever run across amps that fuse the bias supply?

Doesn't SEMKO demand a fuse on the bias supply? (Safety certification for European Community + Switzerland = SEMKO)

[trobbins]

Maybe if it is sufficiently current limited, and of low enough voltage, then all the hazards are removed - unless you play at a gig requiring explosion proof gear!

[loudthud]

When you look at some of the more recent Fender schematics, it seems like some versions do have a fuse for nearly every power transformer winding.

This schematic shows fuses for export models at the far right.

Link: http://el34world.com/charts/Schemati...verb_schem.pdf

[Leo_Gnardo]

When you look at some of the more recent Fender schematics, it seems like some versions do have a fuse for nearly every power transformer winding.

This schematic shows fuses for export models at the far right.

Link: http://el34world.com/charts/Schemati...verb_schem.pdf

There 'tis: brown secondary for bias is fused at 250 mA. Thanks for posting the schemo Loud Thud. I thought I'd seen similar on some Peavey models starting in the early 90's.

[R.G.]

Doesn't SEMKO demand a fuse on the bias supply? (Safety certification for European Community + Switzerland = SEMKO)

First off: I have no idea. (I felt it only fair to get that out first. )

But it does strike me that it would be very strange if they did. Bias supplies have very low potential for hazards to humans or property. Loss of a bias supply through a fuse blowing is much, much worse.

[ric]

First off: I have no idea. (I felt it only fair to get that out first. )

But it does strike me that it would be very strange if they did. Bias supplies have very low potential for hazards to humans or property. Loss of a bias supply through a fuse blowing is much, much worse.

Man, there's a good point- keeping in mind what could be made more dangerous by a fuse in the wrong place.

[g1]

Some of the later Ampeg V4's did that with the fuse on the bias winding, I always thought it a bad idea.

[R.G.]

Some of the later Ampeg V4's did that with the fuse on the bias winding, I always thought it a bad idea.

Adding protection stuff is always more complicated than it appears at first. For each mod you do to "protect", you have to ask whether the cure is worse, or more expensive, or more frequently broken than what it's curing.

In the case of a fuse on the bias winding, you have to go a few levels deep and ask what it is you're protecting. If it's the output tubes, a fused bias winding is a bad idea. If it's the power transformer, may be a good idea if incompletely thought through. The PT is usually the single most expensive part in the amplifier electronics. Protecting that lump of iron and copper and sacrificing other things may be justifiable. In my mind, it's OK to fuse the bias winding to protect the PT if you also use some sideways sensing to turn off the power to the output stages at the same time, because unbiased output tubes will overheat, die, and maybe take the OT and PT with them. Maybe the designer that put that fuse in the bias winding thought that the unbiased tubes would always pop the mains fuse. I disagree with that idea, but then I don't get to do the amp design.

[loudthud]

I think what we are dealing with is some blanket regulation in some countries that requires that every transformer winding be fused, not some engineers notion of protecting a transformer. That's why only one version of the amp has the fuse in the bias winding.

Remember that most fuses are there to prevent the appliance from catching on fire. R.G. makes a good point about loss of bias, but explaining that to some bureaucrat in a foreign country is not something that average engineer or company wants to take on. You've got the marketing guys who want to sell in that country and the legal guys telling you that the amp has to pass regulations. So you install the fuse in the version of the amp that has to pass the regulation, and hope for the best. Maybe you even test the amp by removing the bias fuse to see if the other fuses protect the amp from catching on fire. The regulators probably will.

[SoulFetish]

I've had some amps come in the shop because a bias fuse goes open and causes the output tubes to loose bias, causing over-current conditions and cascading problem as a result. I remember wondering if the designers really thought this through.
Then I had an *oh shit* moment and realized that my output tubes are directly coupled to the driver with a separate (fused) independent power supply. Working backwards in my head at the time, I overlooked the fact that if the winding goes open... then where do the output tube grids get their voltage reference? But I ended up getting lucky in that I put bleeders on the filter caps in my +/- driver supply and the grids end up with a grid leak reference within their design specifications to the CT ground point. Sometimes you can almost pass off being lucky for being clever.

[trobbins]

The most common general purpose standard I've had the 'joy' of working through is IEC 60950. It has a requirement that a linear power supply type transformer has to have over-current protection, or include a thermal cutout, or have a current limiting design. Each winding is overloaded to test compliance, and needs to remain hazard free - a winding that open-circuits during such a test can be considered safe - but of course a fuse is typically the cheapest compliance path.

For DIY, it is worth while aiming to make a bias winding as fault tolerant as practical. The simplest technique I know of is to connect a filter resistor right at the winding - even before any diode(s) and with insulation/heatshrink to avoid any form of accidental short across the winding itself. That resistor would be part of an RC (or RCRC) filter, and if a short was to occur downstream then the winding is somewhat current limited and may well be damage tolerant (of course protecting the output stage valves and output transformer from a bias supply fault is a different matter, which generally leads to fusing the HV winding used for B+).