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Marshall JTM60 broken or just tubes?

Carlos Pérez

Member
Messages
55
Hi
On 2007 I bought from eBay a nice JTM60 1X12 with extension cabinet. Then ordered 2 Winged "C" EL34 & 4 JJ Tesla ECC83-S from www.thetubestore.com and replaced stock tubes. Everything worked ok.

On my first gig amp went silent, because of a hidden fuse gone. Replaced some days after.
Since then this amp is rarely used, even for room practice, but when used everything works well.

Now, since a friend of mine is interested on buying, I plug my strat in order to a full check and found the sound was lowering an coming back so I turned it off and after waiting the tubes not to be hot, check the sockets and found one EL34 may not be full inserted. Inserted correctly, turned the amp on and the volume remain going and coming back so then.....

I Impulsively decided to sligtly turn the BIAS pot only to find the two EL34 turned from red to a full white glow (almost blue) along with a bzzzzZZZZZZZ sound.

Immediately turned the amp off, back the BIAS pot to it´s original position. Checked minutes later and now the sound was totally gone. Amp and tubes turn on, but no sound from the amp.

Someone can resume the exact point where I am, and what I´m supposed to do?
Thank you in advance.

Carlos
 
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TweeDLX

Member
Messages
3,756
Have you replaced the tubes to see if that made a difference? That would be the first step.

Mike
 

rockon1

Platinum Supporting Member
Messages
12,794
Replace the power tubes,clean the sockets while your at it and set the bias to the proper range. If that doesnt do it its time for a tech. Bob
 

Carlos Pérez

Member
Messages
55
Replace the power tubes,clean the sockets while your at it and set the bias to the proper range. If that doesnt do it its time for a tech. Bob
There´s any simple recommended procedure for socket cleaning?

Besides, I don´t have any idea how can I assume the right spot for the BIAS pot, right now it´s back on it´s original position, but that was the position it arrived from the eBay seller. How can I be sure THAT´s the correct BIAS pot position this amp should have?
 

rockon1

Platinum Supporting Member
Messages
12,794
Carlos Pérez;5791292 said:
There´s any simple recommended procedure for socket cleaning?

Besides, I don´t have any idea how can I assume the right spot for the BIAS pot, right now it´s back on it´s original position, but that was the position it arrived from the eBay seller. How can I be sure THAT´s the correct BIAS pot position this amp should have?
A bit of electrical contact cleaner on the tube pins then work the tubes in and out of the sockets a few times. Radio shack has it. Yeah guys will say its not the best but it works.
The bias needs to be set for the particular set of tubes installed. Just putting the pot back to its original position does no good.

Heres a simple procedure you can follow. You will need a multimeter. Bob

http://www.amparchives.com/album/Marshall/Schematics%20&%20Layouts/Marshall%20Amp%20Schematics/JCM%202000%20Series/DSL50%20&%20DSL100/slides/JCM2000%20%20Series%20Bias%20Procedure.html


....
 

Carlos Pérez

Member
Messages
55
The HT fuse would be the first thing on my list.
Finally had time to take a look and check inside the amp, found a T500L 250V fuse blown.
Cannot find the exact same type, just F500mAL 250V that seems thinner than original.

I guess that´s not the correct replacement part, right?
 
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