JCM 2204 Protection Fuse value question

[NSB_Chris]

Hello All!

I am building a JCM 2204 clone and am close to starting to wire it up.  I have planned in protection fuses but am now struggling with the values.  I want to establish them now so that I can put labels for replacement.  I have the chassis prepped and ready for plates and labels.  I have consulted Rob Robinette's amp protection page which references R. G. Kleen's article, but still struggle to dial in fuse values.  My power section schematic is attached

F1 is the AD input and the original Marshall circuit specifies 2A slow blow

F2 is the B+1 fuse which is after the first filter can.  Should this be 500mA slow blow?

F3 is the fuse on one of the 6.3VAC heater leads.  Am I correct that I should use a 4A slow blow fuse on just one lead because I have 3 12AX7 tubes at .3A and two EL34 tubes at 1.5A?  The transformer has a heater center tap.

F4 is the main PT center tap, which I have at 500mA slow blow.

Any guidance you have will be greatly appreciated!!!!

[pdf64]

T = time delay = anti-surge = slow blow, F = fast / quick blow.
Re F1 the mains fuse, the original spec looks to be T3A for 120V operation https://drtube.com/schematics/marshall/2204specs.gif
and F2 spec is T500mA. However, I think that a F500mA fuse could be used here (assuming that a standard JCM type choke is used), which would offer better protection, ie will blow quicker if a tube shorts.

If the heater winding is to be fused (big if), with a CT to 0V, both legs need to be fused. Low voltage high current supplies can be a problem for regular fuses / fuseholders and tend to overheat. RG Keen suggests the use of automotive type blade fuses as being better suited to this application. I think that 6A should be ok, I'm not sure of the fusing characteristics of regular blade fuses, not aware of T or F options, but assume they have some degree of surge capability.
F4 yes T500mA should be good.

[NSB_Chris]

Thank you!  That is a big help.  I did not plan on two heater winding fuses although Rob shows that in his site.  Got confused between that and the Keen article.  Thank you very much for the clarification.  Maybe I will just leave out the fuse protection of the heater windings.

[sluckey]

Why not just follow the Marshall plan?

[NSB_Chris]

You mean the mains fuse and the B+1 fuse only and don't bother with any other fuse protection?  Certainly an option.

[sluckey]

HT center tap fuse basically duplicates the function of the B+ fuse. You have a fuse in the B+ side of your car battery. But there ain't one in the ground side.

Why would you even want to fuse the filaments?

[shooter]

Why would you even want to fuse the filaments?

because they short before they open   

[pdf64]

Historically, amps had minimal fusing and most builders follow that pattern. However many modern amps have  much more extensive fusing. I’ve read R G Keen making the case that the PT is the most expensive part in an amp, and best practice is to fuse every winding, except, if applicable, the bias.
The issue being that with multiple secondary PTs, a single fuse in the primary can’t feasibly protect each seconary should a fault develop. Heater windings being a prime example of a (usually) much lower capacity minor windings, in which fault current would not necessarily result in fault current in the primary. Hence the heater winding might overheat and fail, rendering the transformer useless.