JCM900 on the bench again

Messages
322
HI
I have a JCM900 reverb head on the bench again. It has brand new JJ Ecaps, brand new Mojo output transformer, brand new matched quad of JJ 6L6 output tubes and new preamp tubes.
The amp works fine, it does not blow fuses. However, when chunking hard on low bar chords there is a very noticeable crackling/unnatural distortion going on, alomost like an arcing sound. Master more than half way up, gain more than half way up.
I have tried the following:
1- retested all the new tubes, they test good
2-removed and replaced each tube one at a time- same problem
3- isolated the head from vibration - same problem
4- tried many different speaker cabinets and cables - same problem
5- visual inspection of the output tubes under stress shows nothing unusual
6- tried different guitars and cables - same problem
7- disconnected reverb pan, turned reverb off- same problem
Anyone have any ideas???
Thanks in advance
Jack
:bow
 

WaltC

Member
Messages
2,129
and... is the noise with the amp head on top of the speaker cab? If so does the noise disappear if you separate the cab and the head to keep the cab from vibrating when you play?

either way the normal stuff still applies (it's why it's the normal stuff <G>), check the tubes, make sure they're in tight and the sockets are ok, swap in known good pre-amp tubes, and if the noise persists then it's time to take it to a tech to check the insides (especially any new solder joints) to make sure there are no cracked resistors, bad solder joints, etc.

(careful, lots of killer high voltage DC in there...)
 

WaltC

Member
Messages
2,129
oh, should'a read ahead further before I typed <G>! Looks like it's time to have a tech crack the case and look inside...
 

Balok

Member
Messages
3,534
The first thing is to check the most recent work done.
Then I would check for cold solder joints first, or cracks in the solder where the component legs enter the pcb.
Also - clean the effects send/ret jacks.
 
Messages
322
Did it do that before the upgrades? (you know where I'm going...)
The amp is used in a rehearsal studio. I was called in when it started blowing fuses. Previous to that, there was no mention of the noise problem. I did all the work on the amp and I can assure you it was done in a most professional manor.
Thanks to those who have contributed.
Jack
 

StratTone

Member
Messages
959
The first thing is to check the most recent work done.
Then I would check for cold solder joints first, or cracks in the solder where the component legs enter the pcb.
Also - clean the effects send/ret jacks.

I agree totally with the effects/send jack. If they get dirty they can do strange things.

Now that being said you said it didn't do it before so I would just for the hell of it double check your work and make sure all solder joints are good as well as make sure others are all good as well.
 

Dickie Fredericks

Gold Supporting Member
Messages
10,754
Im going with a bad solder joint. I have a 900 and it made the sound you describe. It was a bad solder joint in the FX loop. resoldered and fixed.
 

JJman

Member
Messages
994
My JCM900 came with 6l6/5881's and even had a "5881" sticker on the front. I sold it because I was intimidated by the complexity of the internals. I did notice some strange behavior went away when I starting using the loop jacks.
 

gtrmech

Member
Messages
3
I've had 3 of them in the shop in the last 8 months that all did the same thing. They were all OTs... Try a fresh OT. Also all of them were the 6L6/5881 version.
 




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