Oscillation in a JTM 50

[RCGPNY1]

O.K. so I changed the bright cap on my Jtm50 clone to a .005, as it is my
understanding that early seventies Marshalls had that value instead of the the .001. So If I turn the Presence and treble up too high I get oscillation. Funny thing is the first Marshall Super Lead I bought in "72 did the exact same thing, as did a freinds that he bough around the same time. The sound of the amp reminds me most of that original amp. Comes on quick and distortes wonderfully starting at about 4(Bright Channel) Thoughts? Can I comproimise with a .0039 and get the aggression without the oscillation?

[Littlewyan]

This is most likely lead dress or a grounding issue. Did you have this issue with the old bright cap? Can you post some photos please?

[Reeltarded]

Chopstick the area around the pres control. I build the whole circuit on the control and fly the wire across to the output. Never had a problem since.

[Teleguy61]

Chopstick the area around the pres control. I build the whole circuit on the control and fly the wire across to the output. Never had a problem since.

This is the way to do it.

[RCGPNY1]

This is most likely lead dress or a grounding issue. Did you have this issue with the old bright cap? Can you post some photos please?

Nope...no problem with the old value....
The amps breaks up earlier with the .005 and just sounds more like my early seventies Super Lead did.

[Littlewyan]

Did you ever have the volume on full with treble and presence up with the old value? Chances are the amp has always had this issue its just not been as obvious as before. Chopsticking is a gd way to solve this like Reeltarded said. Get the amp to oscillate and then move wires around with something non conductive to try and get it to stop. Do not use a screwdriver! I think even pencils are bad because of the lead.

[Roe]

try shielding the headcab. and keep the purple presence wire away from other wires

[Cameron]

O.K. so I changed the bright cap on my Jtm50 clone to a .005, as it is my
understanding that early seventies Marshalls had that value instead of the the .001. So If I turn the Presence and treble up too high I get oscillation. Funny thing is the first Marshall Super Lead I bought in "72 did the exact same thing, as did a freinds that he bough around the same time. The sound of the amp reminds me most of that original amp. Comes on quick and distortes wonderfully starting at about 4(Bright Channel) Thoughts? Can I comproimise with a .0039 and get the aggression without the oscillation?

Does it have the grid resistors on the power tubes....I don't think so.... Add them to each tube...use 10k ..if it does not .....that should fix it.

[RCGPNY1]

I'm understanding the troubleshooting suggestions...but again...this is something many Marshalls did right out of the box in the early seventies. so my question is...where they all victim of the same problem as they all had the same symptom? It would be quite a coincidence. BTW there is plenty of top end with this amp without using the treble and presence at the extreme settings....like Reeltarted says...it got much angrier with the .002 couple cap and the 5000 bright cap..will try the chopstick test this weekend. I do have the grid resistors...but there are no grid stoppers.

[Littlewyan]

Cameron is talking about the Grid Stoppers. In this Marshall they are normally either 1.5K or 5.6K. 5.6K is better. To be honest though its probably just wiring. Early Marshalls were pretty bad with wiring and grounding.

[Cameron]

I'm understanding the troubleshooting suggestions...but again...this is something many Marshalls did right out of the box in the early seventies. so my question is...where they all victim of the same problem as they all had the same symptom? It would be quite a coincidence. BTW there is plenty of top end with this amp without using the treble and bass at the extreme settings....like Reeltarted says...it got much angrier with the .002 couple cap and the 5000 bright cap..will try the chopstick test this weekend. I do have the grid resistors...but there are no grid stoppers.

Do the grid stoppers ...that will fix it. Its a well known problem....thats how you fix it....simple.

[Cameron]

Cameron is talking about the Grid Stoppers. In this Marshall they are normally either 1.5K or 5.6K. 5.6K is better. To be honest though its probably just wiring. Early Marshalls were pretty bad with wiring and grounding.

Yes..sorry ......grid stoppers..

[Roe]

use grid stoppers on v1 and v4-5 (or even v3)