Author Topic: amp problems (Read 2000 times)

bucketshred · « **on:** July 24, 2007, 10:53:13 AM »

Right, after gigging over the past few months, I'ne noticed one little niggly problem with my amp: The mighty JCM800 Reissue (100w).

Now, I use the gain on full and boost the cr@p out of it with my HBE Big (Miracle Man Pickup in my Jackson btw). The problem is that at high volume I get this horrible squeal when I'm not playing - not sure if it is microphonic or not but its annoying as some of our tunes we have a stop/start rhythm in them and I don't want to have to keep clicking my OD pedal on and off and on and off and on and off etc.

Any solutions?

Andy!

simonp · « **Reply #1 on:** July 24, 2007, 11:02:33 AM »

Sounds most likely to be microphonic feedback, and most probably from the pickup. Is is worse with the guitar close to the amp?

indysmith · « **Reply #2 on:** July 24, 2007, 11:19:44 AM »

my jcm800 does it too. when i took ym amp for a service at the Marshall factory they said it was just a trait of the jcm800, and that the only way to stop the squealing is to lower the volume or move a loooong way away from it. He also said to get used to rolling back the volume control whenever you stop playing. Pisses me off. In a few days i should have my Tonelab back, and i can try it with the noise reduction. That should help. At the moment i never use it at high volumes because it hurts my ears squealing, and that's annoying.

EDIT: what's really quite strange is that it only ever seems to do the squeal really badly when you boost it = without the boost it's fine... It does it all the time aswell, with whatever guitar and whatever valves through whatever cabinet.

simonp · « **Reply #3 on:** July 24, 2007, 11:24:28 AM »

It is not surprising that it does it when you boost it, as you are adding more system gain using a boost pedal, the more system gain the more risk of feedback there is.

indysmith · « **Reply #4 on:** July 24, 2007, 11:29:22 AM »

but the thing is you only have to boost it the tiniest bit for the squealing to begin... It seems sort of like it's a fault with the pedal, but it does the same with every boost i've tried.

Orkestra · « **Reply #5 on:** July 24, 2007, 11:38:46 AM »

buy an ISP Decimator

HTH AMPS · « **Reply #6 on:** July 24, 2007, 01:57:36 PM »

I've seen this in quite a few 2203/2204s, it's an oscillation in the extreme top end. The solution is VERY simple - solder a 100pF cap between the plate and cathode of the first gain stage.

This is a mod that Marshall did themselves from the factory on some original JCM800 2204/2203s, you RI amp might not have this cap.

It's worth checking with an amp tech and getting this done. It's only a five minute job, should get away with the basic bench charge. Might as well get him to check the bias and clean the pots while he's at it (you're paying for that first hour afterall).

:twisted:

bucketshred · « **Reply #7 on:** July 25, 2007, 10:47:16 AM »

so marshall have put a dodgy amp on the market - wonderful