Class 5 blown fuse T125ma L250v help!

[markophonic]

just bought a new Class 5.

started experimenting with new tubes and put in 2 new Tung-Sol 12ax7's and a new JJ el84.

Sounded better. I also bought 2 new Mullard reissue 12ax7's and a Sovtek el84m. I decieded to try these today.

swapped the tubes and let it warm up a minute...started playing at low volume and it sounded fine. slowly increased volume incrementally and playing...all is fine until I get to about 1 o'clock on the volume and it cuts out...absolutely no sound whatsoever...everything still powered and lit up. I shut it down and pull the chassis out and sure enough there is a blown fuse. T125ma L250v I couldn't find this fuse anywhere locally so I ordered a few from Mouser electronics.

In the meantime I find a fuse at true value hardware. Its 250ma 250v.

I'm guessing that the new Sovtek el84m went bad and shorted out blowing the fuse.

would it be ok to try the 250ma 250v fuse with the JJ el84 tube that previously worked fine? Or should I just wait for the new fuses to arrive vial mail in a few days.

I just want to know if the JJ tube works...I don't want to play through it until I get the proper fuse...but I'm wondering about firing it up to see if i get sound and then shut it down.

What is this fuse protecting and will the additional 125ma potentially harm something?

I'm pretty sure what the correct answer is...but I'm really impatient

[SteveD]

Welcome to the forum markophonic.

Wait for the correct fuses please!

The T125mA fuse protects the high voltage HT circuit and increasing its value may negate that protection causing far more serious damage to more expensive components. It sounds like one of the new valves you installed (probably one of the EL84s) was faulted and caused the fuse to blow. However, there's always the chance that something else may have been disturbed during the swapping process.

Always best to fit the correct value and type of fuse in anything.

[markophonic]

Thanks Steve!

I'm hoping it was the brand new Sovtek EL84m that was the problem.

The fuse let go about 2 minutes after it was installed and the amp turned on.

I'm waiting for my new fuses. I ordered 5 each from 3 different electronic supply distributors so I'll have a few spares!

What do you think my first course of action should be? Replace the fuse + try the valves that were known to be good and try the amp. or should I replace the fuse + try the Sovtek that was replaced just prior to the fuse blowing to see if it blows again? What is your advice as to how to proceed.

If all goes well after the fuse change...can I assume everything is all right? worst case scenario is I have to send this amp back...which I really don't want to do.

Thanks for your reply...I was hoping you would jump in!

-- Wed Dec 07, 2011 2:16 pm --

Okay...I received some of my fuses today and put one in. installed the set of tubes I had in it prior to swapping in the mullards and sovtek. I put the tung-sols and JJ back in it.

Fired it up and it works

I want to try the sovtek back in there and see if it might be the reason the fuse blew??? but then again I'm afraid to.

SteveD or anyone have any ideas how I should proceed.

Do I just risk blowing another fuse or could it be a recipe for disaster?

[SteveD]

It's up to you. If the fuse protected the amp the first time then it should do so again. However, now that everything is working you might just want to leave well alone.

[markophonic]

Thanks again Steve!

I spent the day rolling tubes through the Class 5 and after trying a multitude of combinations of new, old, and NOS tubes I've settled on 2 RCA 7025's that I pulled out of my 1970 Fender Silverface Deluxe Reverb (bought them a NOS about a year ago) and a New JJ EL84. This combo sounded best to me. A new set of Tung-Sol 12ax7's really didn't sound too bad either. I selttled with the JJ EL84 that I purchased recently...I tried a bunch of different power tubes and the sound was all over the place. I bet I tried about 12 different Sovtek EL84's and they all sounded different...most were much brighter...too much so.

I had one NOS RCA EL84/6BQ5 that really sounded kinda blah/dull. I've got a NOS Sylvania that i'm going to try today.

One thing I didn't try but will soon is a 12xay7 or 12xat7...I have a bunch of old ones that I use in my old Fender amps.

[TommySmallz]

I too just blew the same fuse on my Class 5 after installing a Sovtek EL84M. Does the Class 5 not "like" the EL84M? Im seeing several people posting about the same issue. But other posts stating to use the EL84M for increased headroom.

[markophonic]

I too just blew the same fuse on my Class 5 after installing a Sovtek EL84M. Does the Class 5 not "like" the EL84M? Im seeing several people posting about the same issue. But other posts stating to use the EL84M for increased headroom.

I was going for the increased headroom with the EL84M...but shelved the idea after my blown fuse.

I was wondering if some of the EL84M's just didn't agree with the Class 5 for some (unknown to me) reason. I have pursued the issue since blowing the fuse.

[FuzzMonkey]

I've had the same issue in my C5-01 combo and gave up on the EL84M after two tries with two tubes. Have used a JJ EL84 ever since without any problems.

[GuitarBuilder]

The EL84M draws a higher plate current than a standard EL84, clearly more than that fuse can handle. I've used a Mullard EL84 with good success.

[ClassicRock88]

+1 on the Mullard. I have one in my C5 and it's a great little tube in that amp. For more headroom you can also try a lower gain tube such as a 12au7 in V2.

[ID10T]

Hello All -

Can someone please post an exact link where the fuse can be purchased? I have been searching on T125ma L250v and I'm coming up with too many option that I don't know which one to order. I'm getting ready to change my tubes and I want to have extra fuses on hand.

Thanks in advance!

[JCM900MkIII]

Hello All -

Can someone please post an exact link where the fuse can be purchased? I have been searching on T125ma L250v and I'm coming up with too many option that I don't know which one to order. I'm getting ready to change my tubes and I want to have extra fuses on hand.

Thanks in advance!

I guess you buy your tubes at a music store?
My local store has fuses. Maybe you should try asking there (your local store).

[ID10T]

You guessed wrong. :) I purchased my tubes online. The thing is I don't want to guess when I purchase the fuse. My plan is to have everything in place when I swap the tubes. It never hurts to measure twice and cut once. :)

Thanks for the response.

Anyone have a link?

Thanks in advance!