[Wild Bill]

Short fuse or long fuse?

The prefix with Bussman Fuses (major brand and everybody can recognise and convert their numbers) will be MDL.

So you're gonna ask for an MDL 4 amp and MDL 5 amp.

The "T" in the spec means slow-blow. Most of these glass fuses are AGC series which means fast blowing. The slo-blow series is MDL. My memory was better before the kids came but I believe GMA refers to a 5mm x 20 mm European size of fuse, a bit smaller than you usually find in American made vintage amps.

Many tube amps use slo-blow fuses. When you first turn on the amp there's a big surge to get things started but only for a fraction of a second. The filter caps in the power supply are drained and look like a dead short until they start to charge up. The tubes are cold and they also draw gobs of extra current until they start to warm up and glow.

A normal fast blow fuse will panic and crap out too quickly. Slow-blow fuses are more laid back and allow the initial surge to pass through without getting upset, only blowing when there is a sustained overload.

Any home owner with a sump pump in the basement knows that you need a time-delay fuse to allow the motor to start up. It's the same principle here.

You wouldn't believe how many Marshalls I get in where the player changed his own fuses and bought fast blow 'cuz he didn't know the difference! I usually just charge him for the fuses and quickly send him on his way. I don't make any money on that call but it buys a lot of good will when he really has a problem...

 

[Wild Bill]

Too much info...

Thanks all

I'm still a bit confused.

Bogoboy

Yeah, that was a lot thrown at you all at once. Let's see if I can pare it down to the essentials.

Fuses are chosen by amperage rating. 5A means 5 amps.

Fuses come in many different shapes and sizes. Yours are 1/4" x 1 1/4".

There are 2 overall types - fast blow and slow blow. The "T" in your part number means slow blow.

Ask the store clerk for a glass fuse, 1/4" x 1 1/4", slow blow in 4 amp and in 5 amp. The 100 ma is the exception. It should be fast blow.

Good luck!

 

[Wild Bill]

No news is good news...

Thx once again for your help. So let me see if I got this correct... T means slow blow-is there a code for fast blow?

Bogoboy

Fast blow is the "normal" type. If they don't tell you then it's always a fast blow.

Slow blow fuses are for "special" cases, like tube amps! So they have to make a point of indicating it to you.

If you look at the insides you'll quickly see that the fast blow type have a solid and probably thin wire inside. The slow blow kind will have the wire wound into a thin and springy looking spiral.