power string wiring

[VacuumVoodoo]

Is it incorrect to think of resistors as fuses?

Yes, very incorrect. Unless you think of a special breed of resistors called "Fusible resistors" to be used as "non replaceable by user".

[PCollen]

Is it incorrect to think of resistors as fuses?

Yes, very incorrect. Unless you think of a special breed of resistors called "Fusible resistors" to be used as "non replaceable by user".

..or if you use a 1/2 watt resistor for the screen resistor

[Ken Moon]

One reason to design a power supply this way is to keep the preamp B+ voltages high, which is important in some designs.

Series RC filters necessarily cause the B+ to be lower at each stage.

I can't remember the amp, but I've seen one with 3 chokes, one for the plate and screen B+, and one each for two parallel preamp supplies, so each supply has its own CLC filter.

[Structo]

Is it incorrect to think of resistors as fuses?

While they may burn up saving other components, think about how hot those resistors will get every time you run the amp.

Then when the power tubes suffer a catastrophic failure like a dead short, those resistors will take a while to open, so then you have all that strain and current demand placed on the power transformer and output transformer.

I am really surprised that more amps don't use an HT fuse, such as a .5 amp fuse on the HT center tap.
That would open rather quickly if current demand all of a sudden went up.

[rp]

I am really surprised that more amps don't use an HT fuse, such as a .5 amp fuse on the HT center tap.
That would open rather quickly if current demand all of a sudden went up.

Which reminds me, I just read somewhere that the B+ fuse has to be a fast blow type, not slow blow, to be effective. That if a tube shorts the slow blow isn't fast enough to save an xformer. A fast blow would be ideal in all circuits but in practice will the fast blow in the HT hold up or will it just pop a lot at start-up or from stand-by? Fuses aren't that cheap when you start blowing one a day, and you don't always have them at hand.

I have slow blows in my HTs, are they doing nothing?

If it should be fast blow is this also true when using silicon diodes??

BTW how do you properly calculate the HT fuse? Just double the total load? Match the PT rating? Double the PT rating?

[Zippy]

A fast blow would be ideal in all circuits...

Except those circuits with a considerable current surge on startup (especially, for example, AC motors).