Amp help please? Tubes Redplating.

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Steinmetzify

Steinmetzify

Well-known member
Got a buddy with this issue......Marshall JCM 2000 DSL......here's the problem:


I am looking for some feedback/info on an issue I am having. Hopefully you guys can shed some light for me. I recently had an issue where my amp kept blowing the main fuse and turning off. I tried replacing the fuse, but it just kept going out. I had the tubes "somewhat" tested and they all seemed to be OK to Good.

By the way - these are JJ tubes I am talking about. The whole amp is rigged w/ em - preamp and power. I am technically challenged and kind of hit a wall beyond testing the tubes and replacing the blown fuse. So, I took it in to have it looked at. Prior to taking it in, I noticed the logo on 2 of the 4 power tubes had turned from red to grey. I took it in and the guy right away knew those tubes were bad due to the discolored logo. I was told it was just the tubes - that JJs and these particular Marshalls do not play well together. The repair guy told me he had seen this several times and they were replaced with more JJs.

I am not a guy who plays out in a band, so I play when I can between work, etc. I played the amp a few times after getting it back and figured all was well - as it seemed. About a week ago, I noticed there was a slight volume drop while playing - which has happened previously right before the fuse would blow out. I put the amp on standby and spun it around to see if I could notice anything strange. I took the back cover off, took the amp out of standy and immediately saw the same 2 tubes "red plating" like crazy. I turned the amp back onto standby and saw that the new tubes now had a grey logo instead of red - like the ones that had been bad/replaced. I scrounged around my jam room and found a spare set of tubes. I powered the amp down, put the new tubes in, turned it on, and the same thing immediately happened again - red plating and grey tube logo.

Sorry if this is getting long-winded - just want to make sure you have all the details.

So, I took the amp back and told the guy it was doing the same thing - although the fuse wasn't blowing this time. I just got off the phone with the guy and he says there is nothing wrong with the amp - he went through and checked everything. He replaced the JJs with Svetlanas and had been running the amp/testing it for about 4 hours with no issue. He is sticking to his guns in that the only problem is the combination of JJs and this particular Marshall. Which makes me wonder - if that is the case - why put more JJs in there during the first repair?! But I guess that is a separate issue - lol. I am just kind of curious if you guys have heard of or experienced any pairing issues between JJs and the DSL amps? I appreciate your feedback!

What do the gurus say?
 
Because the guy kept playing it with redplating tubes which is not good he probably has another issue. Its usually one tube that plates and then the other of the pair will usually follow. The OT sees the four tubes as basically two. He most likely took out one of the screen resistors which is why it keeps happening. There is nothing that sounds like the hum you get when a tube plates. It is very distinguishable. If I hear it even for a few seconds I power down immediately and check everything, playing the amp while it is not in the headshell. First thing I always do as well is desolder one leg of the plating pair's screen resistors to check. Your guy is lucky he did not take out his OT etc..
 
So the 'tech' telling him there's nothing wrong with the amp, that it's just JJs that are causing this is wrong? I should point out that on this amp AFAIK, the inside tubes are a pair and the outside tubes are as well.....and it's the two on the left when looking at the back of the amp that are doing this, which means it's one of each pair. Is that normal?
 
Steinmetzify":9joqtoud said:
So the 'tech' telling him there's nothing wrong with the amp, that it's just JJs that are causing this is wrong? I should point out that on this amp AFAIK, the inside tubes are a pair and the outside tubes are as well.....and it's the two on the left when looking at the back of the amp that are doing this, which means it's one of each pair. Is that normal?
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Its usually one pair that plates. For some reason when one goes teh other one of the pair goes also. Did the tech lift the resistor and check it? JJ's not working is bull.
 
Far as I know, he didn't do anything.....called my boy and told him nothing was wrong with his amp. Of course, that's what he told him last time too, before it happened again.
 
Your "quad" might be 2 pairs, not matched evenly, and the higher rated ones are red plating
Did you check bias?
 
Steinmetzify":3g6azjpk said:
Far as I know, he didn't do anything.....called my boy and told him nothing was wrong with his amp. Of course, that's what he told him last time too, before it happened again.
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Well the tech sucks. :thumbsdown: He need to have his tubes checked for matching and biased properly business posted and he needs his screen resistors checked.
 
Pretty sure he biased it a while back......I'll check though.
 
Always bias when replacing tubes. Tubes run in pairs some guys pull 2 outer or 2 inner in 100 watters to "half" the volume (doesn't really halve it). Maybe the JJ's can't handle the plate voltage in that Marshall? Hard to tell without it in front of me. Start with the screen resistors. Definitely find a new amp tech. Lets see he tells friend JJ's don't work in the amp and puts JJ's back in. WTF?
 
The older DSL's have a problem with thermal runaway because of faulty material that the main power board was made of.
 
It can depend in what year DSL it is. The early ones had bias drift issues and I read somewhere that some boards had actually melted because of it. Supposed to be a major issue that they fixed after 06' I think? Could've been before that.
 
Keep in mind JJ tubes have smaller than standard pin diameters, it is possible the tube sockets are not making good contact and pin 5 is not getting a good negative bias supply.

Does the red-plating occur in the same socket positions.
 
baron55":1hrc17k7 said:
Keep in mind JJ tubes have smaller than standard pin diameters, it is possible the tube sockets are not making good contact and pin 5 is not getting a good negative bias supply.

Does the red-plating occur in the same socket positions.
Click to expand...

Yup.....same ones each time.

projectx102":1hrc17k7 said:
The older DSL's have a problem with thermal runaway because of faulty material that the main power board was made of.
Click to expand...

Jesus, that's lame.

Motorpud":1hrc17k7 said:
It can depend in what year DSL it is. The early ones had bias drift issues and I read somewhere that some boards had actually melted because of it. Supposed to be a major issue that they fixed after 06' I think? Could've been before that.
Click to expand...

Damn.....this is some fucked up repugnant shit right here....dude paid a decent amount of $ for this amp, too.
 
baron55":27w29pbq said:
Keep in mind JJ tubes have smaller than standard pin diameters, it is possible the tube sockets are not making good contact and pin 5 is not getting a good negative bias supply.

Does the red-plating occur in the same socket positions.
Click to expand...
This is what I told him to check. Forgot to mention the bias drift issue with some of those DSL's That plus the fact that JJ power tubes can be a crap shoot...
 
Pin 5 is usually where the problems with these start(especially early ones). Here's a shot of what needs to be done to isolate pin 5 from the heat and small amount of voltage that creeps over causing the negative bias suplply to change which in turn causes the runaway.
null-8.jpg
 
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