Early rev A 2550 replacement caps

[Fantomaz]

Hi guys,

My original 1987 early serial rev A Marshall 2550 silver jubilee keeps on blowing the 2 amp fuse.

I suspect the original 1987 LCR caps to be the culprits, but these aren't made anymore.

I have little technical knowledge, basically I know how to discharge caps, solder stuff, rebias but not much more.

The amp is 100% original, sounds stupidly good, has been gigged/recorded quite a bit.

There is no tech for 200 miles around where I live.

What do you think?

Should I replace them? with what?

Thanks in advance!

2550 caps.jpg

[xtian]

Those caps are dated 1987--36 years old! Definitely replace, along with the very important bias capacitors. Be aware there can be other reasons an amp draws too much current and blows fuses. Do fuses blow immediately when you flip power switch? Or after warming up/throwing standby switch?

For the filter caps, I prefer F&T and JJ cap cans, like these: https://www.tubesandmore.com/products/c ... c3110a3205

There may be other things that need to be replaced, like the B+ dropping resistors. Let's see some photos of the circuit.

[Fantomaz]

Thanks for your reply!

I dont have the pics with me, but last time I took the chassis out I had a good look at the whole circuit and everything looked absolutely stock.

The only thing I ever did to the amp is replacing worn out output tubes by the same svetlana EL 34 that came in it when I got it 20 years ago and biasing to around 35mv then by ear. I also switched a microphonic preamp tube but other than that I am pretty sure the amp is exactly as it came out of the factory.

The 2 amp fuse blows when going from standby to on. That said, this amp has been blowing that fuse on a few occasions over the years, sometimes for no apparent reason, sometimes because of a blown output tube.

I still have one last matched pair of svetlanas.

I will send you a picture when I take it out tomorrow.

Thanks for the help!

[Roe]

pull tubes and see if it still blows fuses. tube failure is more likely than cap failure. The mains cap may blow before the screens cap though because it filters much more current. Keep the original caps if possible. they sound better than F&T.

[bepone]

blue F&T are ok for marshall circuits, JMP build with them , sound is excellent, so that cap didn't degrade anything
https://www.tube-town.net/ttstore/f-t-5 ... ction.html

[WhopperPlate]

…assuming filter caps without a proper diagnostic protocol is just asking for tail chasing … tubes are almost always number one suspect … especially with an amp suddenly blowing fuses …

I have been inside much older amplifiers that still had caps happily working…the whole “filter caps must be changed after (x) years “…. As the saying goes: “ Lies , damned lies, and specifications.” It really depends on who and how the amplifier was stored and utilized over its lifetime.

All that said…if you do end up replacing the caps…F&t, especially in a Marshall , can be thinner and brighter . I remember playing the “f&t are the best” song and dance for years , but I never got results I wanted until I left them out of my builds .

They lend themselves to a forgiving elastic feel, but they don’t have the balls I find with other caps . If you a bedroom playing or small stage bar crawling with your cover band then it will probably be the best sound you have ever heard , but when I needed to cut over a real band with a real rock drummer and bass player and an extra rhythm guitar on ten , then you might be like me and get lost in the mix.

That being said…try ARS… if they actually need to be changed .