Fused heaters

[joesatch]

PT 5V heater wires to tube rectifier. Supposedly it is highly recommended to fuse them. Are two fuses (one for each wire) used or just one of the wires?  Also what value? Slow blow or fast acting?

[sluckey]

PT 5V heater wires to tube rectifier. Supposedly it is highly recommended to fuse them. Are two fuses (one for each wire) used or just one of the wires?  Also what value? Slow blow or fast acting?

Who is recommending to fuse the 5VAC? I've never seen a commercial amp with fuses for the rectifier filaments. Are you aware that there is B+ voltage riding on the 5VAC filaments?

[joesatch]

here:
http://www.valvewizard.co.uk/fuses.html

and here:
https://robrobinette.com/5F6A_Modifications.htm
Rob states in his notes only one fuse is needed but that is unclear as the image he refers to has two sets of fuses (filament heater and rectifier heater).

[sluckey]

Yeah, well Rob is just being too clever.   

And the Valve Wizard never even mentions fusing the 5VAC. In fact he has this to say about fusing the 6.3VAC...

An argument can be made for not fusing the heater winding, provided it supplies nothing but AC heaters and lamps. You know, simple stuff. There is very little to go wrong with a simple AC heater supply, and the likelihood of a fault creating a permanent short is slim. Also, the inrush to heaters at switch-on is so heavy that it may be difficult to find a fuse that will withstand it. I've never seen a 20mm fuse rated at more than 10A. Beyond this you might consider automotive fuses. Traditional amp designs don't use heater fuses, and I have never heard of it being a problem.

[joesatch]

Yeah, well Rob is just being too clever.   

And the Valve Wizard never even mentions fusing the 5VAC. In fact he has this to say about fusing the 6.3VAC...

An argument can be made for not fusing the heater winding, provided it supplies nothing but AC heaters and lamps. You know, simple stuff. There is very little to go wrong with a simple AC heater supply, and the likelihood of a fault creating a permanent short is slim. Also, the inrush to heaters at switch-on is so heavy that it may be difficult to find a fuse that will withstand it. I've never seen a 20mm fuse rated at more than 10A. Beyond this you might consider automotive fuses. Traditional amp designs don't use heater fuses, and I have never heard of it being a problem.

Thanks Sluck, i knew you were the voice of reason. This was a headscratcher to me as i haven't seen heaters fused on any layouts

[pdf64]

I’ve seen some modern amps that fuse protect the heaters (5V & 6.3V) yet not the HT.
eg https://el34world.com/charts/Schematics/files/Fender/Fender_57_bandmaster_reissue.pdf
Which seems nuts to me.

I suppose if there’s a problem in the HT, it’s likely to blow the primary fuse.
Whereas there just isn’t enough VA in the heater windings to ensure that, thereby allowing a problem to damage the PT winding.

[Willabe]

I’ve seen some modern amps that fuse protect the heaters (5V & 6.3V) yet not the HT.
eg https://el34world.com/charts/Schematics/files/Fender/Fender_57_bandmaster_reissue.pdf
Which seems nuts to me.

I suppose if there’s a problem in the HT, it’s likely to blow the primary fuse.
Whereas there just isn’t enough VA in the heater windings to ensure that, thereby allowing a problem to damage the PT winding.

I'm thinking Marshall, (which you seem to lean towards?) fused the main HT because they were using EL34's which have a true screen grid, which can be a problem when you push them, dcv/current. Where as Fender used 6L6GB/GC's which have beam forming plates.   

[PRR]

I understood that some national safety code wanted no-fire even if random guts were shorted. For some cases, fuses may be the answer.

It certainly was uncommon before obsessive safety codes.

[pdf64]

I’ve seen some modern amps that fuse protect the heaters (5V & 6.3V) yet not the HT.
eg https://el34world.com/charts/Schematics/files/Fender/Fender_57_bandmaster_reissue.pdf
Which seems nuts to me.

I suppose if there’s a problem in the HT, it’s likely to blow the primary fuse.
Whereas there just isn’t enough VA in the heater windings to ensure that, thereby allowing a problem to damage the PT winding.

I'm thinking Marshall, (which you seem to lean towards?) fused the main HT because they were using EL34's which have a true screen grid, which can be a problem when you push them, dcv/current. Where as Fender used 6L6GB/GC's which have beam forming plates.   

Marshalls are just Fenders with a HT fuse 
Well, tweed era, specifically the 5F6A Bassman.
Bear in mind that Marshall fitted a HT fuse even to the very first prototype (I suppose Dudley etc, being ham radio buffs, may have thought it was good practice?)
And used beam pentodes (5881, KT66) for the first 2 or 3 years production.
Until the price  / availability advantage of EL34 become overwhelming 

And a new era was born

I admit that when a likely, reasonably foreseeable fault occurs eg an output valve or something develops a big problem, I much prefer a fuse to blow. And preferably immediately or ASAP, not after subjecting the transformers etc to fault current for maybe 30 seconds.
It just seems good engineering to me 
For that to be, something a bit more specific that a primary fuse is necessary. An appropriately value F type HT fuse can work pretty well.