Tweet
A friend brought the JMP 50 over that was quite noisy. Turned out to be first preamp tube. It only has two inputs not four and marked in between them is Sensitivity. Looking at the Faceplate it never had four inputs. When plugged into the high input the amp can be turned up quite loud, when plugged into the low input the volume is cut by half or more. I have checked a few things and found nothing wrong yet. I did notice that on the circuit board it does not have and appears to never had the 330UF electrolytic cap installed in the Low input circuit. The 10K resistor is there but no cap. Would not having the cap be be normal for this amp and be the cause of the volume drop? A quick Internet search is coming up as that's the way it is supposed to be, but I figure I would ask here to be sure. Looking for the schematic but have not found a 50 Watt yet only 100 Watt.
-
It's all about the bass. Lock in the groove and stay out of everyone else's way. -
Some of them had an extra gain stage for the 'high' input. Sounds like maybe the case for this one. Look at JCM800 schematics for examples.Originally posted by Enzo
-
Like upper right preamp schematic here:Originally posted by Enzo
Comment
-
Thanks. I have been looking at few schematics and all I have seen show the 330UF. I looked a the JCM 800 as you suggest and don't see the 330UF on it. I would guess that it must be the reason for the volume difference. If it's OK and If he wants to maybe I'll put the 330UF in and see what happens.It's all about the bass. Lock in the groove and stay out of everyone else's way.Comment
-
Was looking at this one.Attached FilesIt's all about the bass. Lock in the groove and stay out of everyone else's way.Comment
-
-
This schematic looks like what you're describing. The 'Low' Sensitivity jack in this case would indeed be much lower in output level..Logic is an organized way of going wrong with confidenceComment
-
When you plug into high, the signal goes into V1b grid, out V1b plate, then through the switch contacts of the low jack on to the pre-volume pot.
If you plug into low, it bypasses the entire V1b circuit and goes straight into the pre-volume pot.
Because it has bypassed the first stage, it is much weaker. This is normal, and not a result of the 330uF cap which is not used in this model.Originally posted by Enzo
Comment
-
Haven't checked this amp against the JCM 800 schematic yet.It's all about the bass. Lock in the groove and stay out of everyone else's way.Comment
Comment