What size fuse to put in O.T. center-tap?

[Humbucker]

The previous owner of my Fender Bassman had installed an in-line fuse holder (with a 200mA fuse in it) and this was wired into the centre-tap wire for the O.T. This was a Torres mod BTW. This 200mA tube burnt-up a couple of weeks ago due to a power tube blowing.

Well, I was reading in Dan Torres' book and he suggested a 160mA fuse for two 6L6 tubes(which is what I've got). I was only able to find a 175mA fuse and when I took the amp off standby, this fuse popped right away. I then tried another 175mA and naturally that one blew too, so I guess that I'll have to go to at least a 200mA or 1/4 amp fuse to avoid this problem.

I'm wondering if Torres meant a Slo-Blow fuse when he recommended a 160mA fuse? Do you fellows have any suggestions here?

Right now I have a 1/4 amp fast-acting fuse in there and it hasn't popped but I'm wondering if I shouldn't go back to a 200mA, what do you guys recommend?

[J Martin]

Only use a fast blow. 250 sounds safe. Marshalls use 500 for two tubes. There is an extensive treatment of this subject in one of the O'Connor books, but I can't find it at the moment.

[Enzo]

Most Marshalls I can think of use slow blow fuses. That is what the T means in T500ma - timed fuse. I would in fact recommend only slow blow fuses for this. Otherwise turn-onm surges will pop the fats blow fuse way too often. WHat we call a nuisance blow.

Since the Fender Bassman got along for decades without this fuse, and as far as I know the Bassman has never been known as a tube eater or transformer killer, just what exactly is the problem this added fuse prevents? Like the man says, Marshall used 500ma for a pair of tubes. Now you want to use only a third as much of a fuse. A fuse that small is not only expensive, it also doesn't allow much headroom over normal operation.

A fast fuse that close to power peaks is likely to blow from nothing more than a slapped note or other loud transient.

[kg]

The previous owner of my Fender Bassman had installed an in-line fuse holder (with a 200mA fuse in it) and this was wired into the centre-tap wire for the O.T. This was a Torres mod BTW. This 200mA tube burnt-up a couple of weeks ago due to a power tube blowing.

Well, I was reading in Dan Torres' book and he suggested a 160mA fuse for two 6L6 tubes(which is what I've got). I was only able to find a 175mA fuse and when I took the amp off standby, this fuse popped right away. I then tried another 175mA and naturally that one blew too, so I guess that I'll have to go to at least a 200mA or 1/4 amp fuse to avoid this problem.

I'm wondering if Torres meant a Slo-Blow fuse when he recommended a 160mA fuse? Do you fellows have any suggestions here?

Right now I have a 1/4 amp fast-acting fuse in there and it hasn't popped but I'm wondering if I shouldn't go back to a 200mA, what do you guys recommend?

i think the fact that the original fuse installed in the amp served its purpose admirably should be a primary factor in sizing a replacement. :)

in my experience fuse sizing is an iterative process. buy a couple of boxes of various sizes and keep going down in value until it blows unnecessarily, then go one step back. you've basically done most of that testing already!

fwiw i use fast blow 500ma fuses in the baga center taps. it is designed in such a way that the bias supply is always up before the main B+ (625vdc :D ) though, so there is no turn on surge across those fuses to accommodate.

[Humbucker]

Yeah, it seemed like the original 200mA fast-acting fuse was holding up okay before I had a tube failure but I'm thinking that that might be a little too close for comfort. I think that I'll just go with a 250mA time delay fuse and call it a day.

[kg]

Yeah, it seemed like the original 200mA fast-acting fuse was holding up okay before I had a tube failure but I'm thinking that that might be a little too close for comfort. I think that I'll just go with a 250mA time delay fuse and call it a day.

no matter which way you look at it, replacing a 200ma fast blow with a 250ma slow blow is putting more stress on components during a failure.

you have to really study your motivations for taking on the additional risk of turning a $20 output valve and 50c. fuse replacement into a $20 output valve + $200 transformer replacement. :)

personally i wouldn't change the value. i'd put the 200ma fast blow back in the amp, and i would also tape a 250ma fast blow to the cabinet for a last ditch "just in case" during a gig or something. if the amp blew both 200 and 250 then it would clearly be at fault and it should be opened up to determine what happened.