JTM Blowing Main Fuse?

[cudamax2343]

I just got this Amp Mail order. It came with only 1 of the 2 EL34's working.

I pulled my 2 JJ EL34's from my good working Ashdown Amp and Installed them in the Marshall and on Stand-by it's fine, but with-in 1 min of playing, coming off of Stand-by, first you get some small popping sounds then it takes out the main 3amp Fuse. If I pull 1 of the 2 Power Tubes, it will go all day sounding like Dog Crap.

I do not know how long it was run with only one working power tube nor what has been Damaged because it was Run that way. I only know that pulling one power tube and not Blowing a Main Fuse tell someone Something?

Thanx's for any Input or Consideration, Dudes.

[52 Bill]

...If I pull 1 of the 2 Power Tubes, it will go all day sounding like Dog Crap.

Does it matter which one of the output tubes you pull out, or does it do this with either one pulled?

[cudamax2343]

It does not matter. Left or Right side Pulled. It does the same thing and now with it powered up on stand-by it also after about 30mins, makes a loud noise then pops the first boards 1/2amp--500ml volt fuse right next to the 2 blue things below in the photo. And when it Blows the Boards fuse, it stays Powered up but no output.


Any Idea's/What were you thinking?

[52 Bill]

It does not matter. Left or Right side Pulled. It does the same thing and now with it powered up on stand-by it also after about 30mins, makes a loud noise then pops the first boards 1/2amp--500ml volt fuse right next to the 2 blue things below in the photo. And when it Blows the Boards fuse, it stays Powered up but no output.

First off which model JTM is your amp? Which fuse number is blowing?

The 500mA fuse is probably the high tension fuse to the power tubes. It's strange that it would blow when the amp is in standby mode, as the high tension is shut off in standby mode.

Generally the usual reason for that fuse to blow is a shorted output tube. If it blows with no tubes in the sockets then there could be a problem with the sockets, caps or output transformer.

[cudamax2343]

Sorry, Its a JTM60 Head and on the Chassis its Labeled a JTM600. I pulled the tubes out of a perfectly good working Ashdown Amp and put them in the Marshall and it did all the funny things mentioned above.

So I put the tubes back in the Ashdown Amp and now the Ashdown blew the main fuse in the back within 1 min (2amp) and then after a second fuse was put in, the Ashdown Fallen Angel 60 took out the Board 500ml/1/2 amp fuse also. Just the the Marshall was doing.

So the Frick-in Marshall JTM60 what has ruined my good Ashdown tubes? and maybe something in the Marshall is Screwing up power tubes?

I'll try powering it up without the power tubes. Do I just pull the power tubes and just try it in stand-by mode only?

[52 Bill]

I'll try powering it up without the power tubes. Do I just pull the power tubes and just try it in stand-by mode only?

No, pull the output tubes replace the fuse and turn the amp on. After it warms up a little turn on the standby switch. If the tubes are the problem, the fuse will not blow. If the fuse blows, then there is something wrong with the circuitry.

[cudamax2343]

Thanx's Man, I'll try that. And that's some Damn good Advise.

[cudamax2343]

Well 10 mins with no power tubes in the sockets/warmed-up and off of
Stand-by it does not Blow any Fuses.
What I don't get is that I pulled 2 JJ Tubes out of a good working amp and when they are in this Marshall they don't work. And now they don't work in the amp they where in.

[52 Bill]

What I don't get is that I pulled 2 JJ Tubes out of a good working amp and when they are in this Marshall they don't work. And now they don't work in the amp they where in.

I don't know. It may be sheer coincidence that one of the JJ's decided to die when you pulled it to test the Marshall. It may be that one of the tubes got moved or shaken just enough to cause something to short.

Get yourself a new set of tubes for both amps and see what happens. Be sure to re-bias when you install the new ones.

[tim]

It's also possible that you have lost bias on one or both of your tubes / sockets. Check to make sure you're getting between 30-40 volts negative on pin 5 on the EL-34s.

Were either of the tubes red plating before the fuse blew? If you had lost bias and tried to run the amp like that from the get go, then it's probably why a Hi Tension fuse blew...if you stuck a good pair of el34's in there with a missing bias on one (or both) of the tubes then it would also destroy that tube in pretty short order as well.

Before you put another set in there, make darn sure that all voltages that are supposed to be there at the sockets are there before you install a fresh pair of tubes.

[52 Bill]

Good point Tim.

I assumed that he would notice and mention red plating tubes.

[cudamax2343]

Well you know what. It works now. I pulled the Chassis and replaced the 500ml-1/2 amp Fuse on one of the Boards again and put the old tubes in that came with it.

Powered it up and to my Surprise both tubes were lite-up on standby and now coming off of standby it does not blow any Fuses.

It does however on the left side (looking from the back) the power-tube looses power and I have been moving the power-tube around slightly until it lites-up.

Now It was shipped from New York to the Illinois/Wisconsin Boarder with the Tubes in it bouncing around in a truck.

So I got a "Tube Socket Problem" that I will try and Fix later after I play this thing real hard/loud and long and I am going to try the JJ tubes again to see if a Bias problem was blowing a fuse or not, but it is probably a Socket problem.

[cudamax2343]

It's also possible that you have lost bias on one or both of your tubes / sockets. Check to make sure you're getting between 30-40 volts negative on pin 5 on the EL-34s.

Were either of the tubes red plating before the fuse blew? If you had lost bias and tried to run the amp like that from the get go, then it's probably why a Hi Tension fuse blew...if you stuck a good pair of el34's in there with a missing bias on one (or both) of the tubes then it would also destroy that tube in pretty short order as well.

Before you put another set in there, make darn sure that all voltages that are supposed to be there at the sockets are there before you install a fresh pair of tubes.

I'll have to learn how to bias amps someday soon and how to check each Sockets pins for its values. Maybe someone/someday here will start a topic and walk guys that don't know through a step by step "How to Bias an Amp"
It would be nice for guys like me that just don't know what they are doing and what to learn.

[tim]

you probably have an intermittent contact on one of the pins in one of the power tube sockets or an intermittent solder joint under one of the tube sockets...you'll have to figure out which one by a process of elimination. You need to determine what you have because if you loose bias on one of the tubes, it will go into max conduction and fry the tube in pretty short order if it doesn't blow the high tension fuse first.



There are plenty of places on the web that explain how to bias amps and I think that if you search on this forum you'll also find information as well but Remember...All tube amps possess LETHAL voltages. They can kill you DEAD! If you feel you are not qualified to work on an amp, then by all means take it to a qualified technician! Don't prove Darwin right and end up eliminating yourself from the gene pool. Better be safe than sorry!!! TO PUT IT SAFELY YOU COULD DIE IF YOU MESS UP!!!

[trem]

As tim's warning states, make sure you check/drain the caps before you start to re-tension the tube sockets. Also, in regards to your bias query, there is a video on the Eurotubes website. It is not definitive, but if you have never biased an amp before the video will give you a basic understanding of the procedure.
Good Luck
http://www.eurotubes.com/euro-video.htm