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Re: Marshal JTM60's having trouble...


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9/22/2004 11:36 PM
Enzo
Re: Marshal JTM60's having trouble...
Just a thought, but if you have two amps fading out after a brief period, the first thing I would ask a customer at this point is, does your guitar have active pickups? And if so, how old is the battery?
 
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9/23/2004 11:23 AM
Mark Black

Enzo - that's a great question and a wonderful example of thinking outside the box!  
 
You've been doing this a while, haven't you...  
 
-Mark
 
9/23/2004 6:51 PM
Enzo

Yeah, I been here since 6:00, and I am about ready for a break.
 
9/23/2004 10:37 PM
scott

i have a jtm30 with the same problem. the sounds just fades away after 5 minutes or so. i opened her up and saw she has sovtek 5881/6L6WGC for output tubes. is this stock or a mod ? the solders on the sockets look suspicious too. not shiney clean like the rest. anyone know what the stock marshall tubes should be ? the preamp tubes all have marshall on them.  
 
thanks,  
 
scott
 
9/24/2004 7:25 PM
Chad

Good thinking as well Enzo, but this happens regardless of the guitar hooked on either amp, but certainly a good question.  
 
Scott, EL34 type tubes are typically found in most modern Marshalls, the brand is up to you, Marshall use "Marshall" branded Svetlana's, JJ's are usually decent as well (for new makes).  
 
Still plugging away when time permits on the JTM's.  
 
Chad.
 
9/25/2004 12:23 PM
scott

i started with the power tubes because they were the most obvious thing i saw. the solders look different than the others . they have lots of brown goo on/in them (flux,rosin core ? ) so i started my suspicions there. the schematic sez 5881 and literature sez marshall uses sovtek sometimes so they could be original ( 5881/6L6WGC not marshall branded ). the solders could have been redone and/or with a tube replcement ( improper biasing, incorrect values in replacement tube ( these seem correct) ) they could be deteriorating due to faulty soldering technique, or improper current/voltages through that part of the circuit causing other components to break down. the amp plays fine for a few minutes, then volume drops down and out. since it starts fine, this points to something breaking down with heat once it reaches critical temps ? still probing.  
 
thanks,  
 
scott
 
9/25/2004 4:55 PM
Mark Black

OK so I went through the JTM60 from my previous post and did find another idea to report.  
 
On this particular amp the complaint was "Tubes don't light up" and what I found was a bad fuse (T6.3A) in the heater supply circuit. Now the reason the fuse went bad is the interesting part which may have some bearing on your problem: the fuse clip on one end was a little oxidized and making a poor contact, which led to heating of the connection and failure of the fuse. I could visualize a poor connection at that point creating a similar symptom to what you both have described but different in that your fuse hasn't gone yet. The key would be whether the heaters are still glowing when your volume tapers off.  
 
I was also reminded of the need to retouch solders as even though this amp was in great shape and there were no other reported problems I did find cracked solders at the AC inlet, fuseholder, and at the jacks on that same rear PCB. Maybe they weren't bad enough to manifest themselves audibly yet but I'm sure they soon would have.  
 
This particular amp had EL34's in it, and if I remember correctly from previous research both 5881's and EL34's were used in that model at different times, with the 5881's being earlier (I could be wrong on that). The brown flux residue at the power tube socket solders is pretty common in my experience and doesn't necessarily mean anything bad - maybe somebody has already retouched those.  
 
Another thing I was reminded of is that the bias control (at least on this one) is capable of adjusting way too low so bear that in mind when setting bias. I ended up at about -42V on this amp with its existing tubes using the scope method of adjustment.  
 
If indeed your heaters are dying and it's only on the preamp tubes take a good look at the bridge rectifier and associated circuitry/solder/traces as the preamp tubes get DC heater voltage through that rectifier while the power tubes get AC taken off before the rectifier.  
 
It has not been my experience that tubes would fade over a short time due to a defect or improper choice but rather work/not-work or in some cases start arcing.  
 
I'll be interested in what you both find...  
 
Good luck!  
 
-Mark
 

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