Hello!
I have a Marshall 1959 Plexi clone, modded for bass.The problem is every time i switch it on and play for like 5-10 minutes, the amp suddenly drops the volume and after 2-3 sec there is no sound at all.When i turn the volume up there is very, very low level, distorted sound.This happens on both channels.But when i pluck the strings really hard, i.e. increase the input level, the volume is back to normal, but then again, after several seconds it goes dead.Hard hitting the amp
does not help, so it's not a bad solder point IMO.The fact that input level brings the sound back leads me to bad cap?Or bad recto tube?HELP!
p.s. sorry for the bad english...
I have a Marshall 1959 Plexi clone, modded for bass.The problem is every time i switch it on and play for like 5-10 minutes, the amp suddenly drops the volume and after 2-3 sec there is no sound at all.When i turn the volume up there is very, very low level, distorted sound.This happens on both channels.But when i pluck the strings really hard, i.e. increase the input level, the volume is back to normal, but then again, after several seconds it goes dead.Hard hitting the amp
p.s. sorry for the bad english...
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Can you check the output tube voltage and bias while the problem is occurring?
You've eliminated the input gain stages, as the problem occurs on either channel...isolate the problem further. Really would help to plug in a signal generator and scope the signal from the input onwards until you find the bad point in the circuit.
goodluck!
You've eliminated the input gain stages, as the problem occurs on either channel...isolate the problem further. Really would help to plug in a signal generator and scope the signal from the input onwards until you find the bad point in the circuit.
goodluck!
Some of these have an effects loop in and out jack socket. Try cleaning the series contact on the effects in.
After Bass Mod
This only started after the amp was modified?
Well it may be that the amp is going into oscillation?
Who made the amp?
Please post some pics of it.
This only started after the amp was modified?
Well it may be that the amp is going into oscillation?
Who made the amp?
Please post some pics of it.
Well, "modified" is a strong word, since the only replaced components are in the tone stack (with the Super Bass values).Tonight i'll try to measure the AC voltage to the speakers when ok and when faulty...
What you are describing does really sound like a bad connection.
No matter what, the best way to find the problem is to start from the grids of the power tubes and work your way back to the inputs. Use your multimeter on AC volts and inject a signal into the input with a sig generator - which can be as simple as using an Iphone playing an MP3 of a 400Hz tone in loop mode. A dummy load/power soak will save your ears but you probably need to hear the amp.
So check each stage's grid AC voltage with the amp working and not working. When you find the stage that has good input, but bad output to the next stage's grid, closely inspect for bad solder joints.
Good luck!
..Joe L
No matter what, the best way to find the problem is to start from the grids of the power tubes and work your way back to the inputs. Use your multimeter on AC volts and inject a signal into the input with a sig generator - which can be as simple as using an Iphone playing an MP3 of a 400Hz tone in loop mode. A dummy load/power soak will save your ears but you probably need to hear the amp.
So check each stage's grid AC voltage with the amp working and not working. When you find the stage that has good input, but bad output to the next stage's grid, closely inspect for bad solder joints.
Good luck!
..Joe L
You were right, it is a bad connection.After trying to measure the output voltage when there was a volume drop i accidentaly touched the send-return jacks and the sound was back.Several taps on them cleared the problem - dirty jacks.I also played 30 mins trough the return with my Zoom B3 and no problems...so it was the "little devil" as we say in my country - just a jack 🙂
Thanks for the time guys!
Thanks for the time guys!
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