Hello!
Im not a native english speaker so please try not to laugh OR laugh as much as u like but try to answer my question as well.
So, the problem is with a MB bass amp m-pulse 360. It was on a 4Ohm load cabinet and it wasnt cranked up when it just got like quieter. Not dead silence, but half as loud as it was before. I dont have a schematic, so its a bit harder to find the fault but im 90% sure that the end on mosfets is ok. Tried to change the tubes in preamp but nothing. The tubes are 2 ecc83. The strange thing is that
the voltage in kinda wierd on the valves.
According to this image http://en.wikipedia.org/wiki/File:EIA-9A.png
the volts are on the first tube starting from one 305, 0 , 0, 9, 0, 250, 0, 1 ,4
and the other ones are 350, 210, 210,4, 0, 211 ,0, 1, 4
Seems like the second tube grid and cathode are getting too much volts or am i wrong? Where to look, what and how to mesure?
Im not home with tubes but if someone shows me the way, maybe im able to find the faulty component and change it.
Thanks
Im not a native english speaker so please try not to laugh OR laugh as much as u like but try to answer my question as well.
So, the problem is with a MB bass amp m-pulse 360. It was on a 4Ohm load cabinet and it wasnt cranked up when it just got like quieter. Not dead silence, but half as loud as it was before. I dont have a schematic, so its a bit harder to find the fault but im 90% sure that the end on mosfets is ok. Tried to change the tubes in preamp but nothing. The tubes are 2 ecc83. The strange thing is that
the voltage in kinda wierd on the valves.
According to this image http://en.wikipedia.org/wiki/File:EIA-9A.png
the volts are on the first tube starting from one 305, 0 , 0, 9, 0, 250, 0, 1 ,4
and the other ones are 350, 210, 210,4, 0, 211 ,0, 1, 4
Seems like the second tube grid and cathode are getting too much volts or am i wrong? Where to look, what and how to mesure?
Im not home with tubes but if someone shows me the way, maybe im able to find the faulty component and change it.
Thanks
So the gain drops a lot but the voltages all still look reasonable?
I think a cap might be "open". Could be a cathode bypass cap. These are typically parallel with a resister. Its a likely to fail part too, electrolytic cap. If you remove one of those the DC voltages don't change but the gain on the stage is reduced. If a coupling cap opens you'd have no sound at all.
You are going to have to get a schematic, put a signal on the input and trace the signal level through the amp.
With the given information we are guessing. But my bet is on a cathode bypass cap.
I think a cap might be "open". Could be a cathode bypass cap. These are typically parallel with a resister. Its a likely to fail part too, electrolytic cap. If you remove one of those the DC voltages don't change but the gain on the stage is reduced. If a coupling cap opens you'd have no sound at all.
You are going to have to get a schematic, put a signal on the input and trace the signal level through the amp.
With the given information we are guessing. But my bet is on a cathode bypass cap.
Even if those voltages are weird, it is never a good idea to think of reasons NOT to check something.
Your very first tube reading of 305v on the plate and 0 volts on cathode and grid tell me that that side of the tube is not conducting current. The other half of that tube sees 250v on its plate and 1 volt on its cathode - THAT side is working.
Possible reasons for that are:
The heater not working on that side (that is why I asked)
Faulty tube socket - I bet you measured that 305v on the tube socket pin rather than on the tube pin itself. If that socket pin is not making contact with the tube pin, no current flows.
The pins in a tube socket have a solder tab sticking out the bottum, and a female part on the top that grips the tube pin. This socket pin can break in two, leaving the solder tab soldered in underneat, and the female part gripping the tube very nicely, but the two halves are no longer connected to each other. Result - open circuit.
Open cathode resistor - same posible socket failure, open resistor, failed ground connection for that resistor. Measure resistance from pin 3 to chassis ground. You should get some reasonable cathode resistor reading - something like 1.5k to 3.3k I'd guess.
On the second tube you found 350,210,210. When I see plate voltage that high on a 12AX7, and then grid and cathode at 210v, I immediately assume this is a cathode follower stage and those voltages look perfectly normal.
SO I don;t think the second tube has too many volts, I think your first tube is not funtioning on the first three pins.
Your very first tube reading of 305v on the plate and 0 volts on cathode and grid tell me that that side of the tube is not conducting current. The other half of that tube sees 250v on its plate and 1 volt on its cathode - THAT side is working.
Possible reasons for that are:
The heater not working on that side (that is why I asked)
Faulty tube socket - I bet you measured that 305v on the tube socket pin rather than on the tube pin itself. If that socket pin is not making contact with the tube pin, no current flows.
The pins in a tube socket have a solder tab sticking out the bottum, and a female part on the top that grips the tube pin. This socket pin can break in two, leaving the solder tab soldered in underneat, and the female part gripping the tube very nicely, but the two halves are no longer connected to each other. Result - open circuit.
Open cathode resistor - same posible socket failure, open resistor, failed ground connection for that resistor. Measure resistance from pin 3 to chassis ground. You should get some reasonable cathode resistor reading - something like 1.5k to 3.3k I'd guess.
On the second tube you found 350,210,210. When I see plate voltage that high on a 12AX7, and then grid and cathode at 210v, I immediately assume this is a cathode follower stage and those voltages look perfectly normal.
SO I don;t think the second tube has too many volts, I think your first tube is not funtioning on the first three pins.
Here are the results.
//Even if those voltages are weird, it is never a good idea to think of reasons NOT to check something.
Agree. So i used only the power amp and it sounds nice.
//The heater not working on that side (that is why I asked)
Still seems like it works, ill make a pic later. There is a good chance that im just an idiot.
//Faulty tube socket - I bet you measured that 305v on the tube socket pin rather than on the tube pin itself. If that socket pin is not making contact with the tube pin, no current flows.
Tested it. The sockets are fine.
//Open cathode resistor - same posible socket failure, open resistor, failed ground connection for that resistor. Measure resistance from pin 3 to chassis ground. You should get some reasonable cathode resistor reading - something like 1.5k to 3.3k I'd guess.
Both cathode resistors are fine and give a reading about 83k. Both are also grounded nicely. On the second tube tho the ground and pin 3 test gives a "not connected" sign as the first tube gives 2,2k as u predicted. Ill try to find why. AND in the schematic the second tubes cathode(pin3) is connected straigt through a orange drop to power amps input.
//On the second tube you found 350,210,210. When I see plate voltage that high on a 12AX7, and then grid and cathode at 210v, I immediately assume this is a cathode follower stage and those voltages look perfectly normal.
so its also normal that the power amps signal is taken from pin 3 through a cap into power amp?
//SO I don;t think the second tube has too many volts, I think your first tube is not funtioning on the first three pins.
How to check that properly so that it helps to predict my problems? Tube in or tube out?
//Even if those voltages are weird, it is never a good idea to think of reasons NOT to check something.
Agree. So i used only the power amp and it sounds nice.
//The heater not working on that side (that is why I asked)
Still seems like it works, ill make a pic later. There is a good chance that im just an idiot.
//Faulty tube socket - I bet you measured that 305v on the tube socket pin rather than on the tube pin itself. If that socket pin is not making contact with the tube pin, no current flows.
Tested it. The sockets are fine.
//Open cathode resistor - same posible socket failure, open resistor, failed ground connection for that resistor. Measure resistance from pin 3 to chassis ground. You should get some reasonable cathode resistor reading - something like 1.5k to 3.3k I'd guess.
Both cathode resistors are fine and give a reading about 83k. Both are also grounded nicely. On the second tube tho the ground and pin 3 test gives a "not connected" sign as the first tube gives 2,2k as u predicted. Ill try to find why. AND in the schematic the second tubes cathode(pin3) is connected straigt through a orange drop to power amps input.
//On the second tube you found 350,210,210. When I see plate voltage that high on a 12AX7, and then grid and cathode at 210v, I immediately assume this is a cathode follower stage and those voltages look perfectly normal.
so its also normal that the power amps signal is taken from pin 3 through a cap into power amp?
//SO I don;t think the second tube has too many volts, I think your first tube is not funtioning on the first three pins.
How to check that properly so that it helps to predict my problems? Tube in or tube out?
"so its also normal that the power amps signal is taken from pin 3 through a cap into power amp?"
Look up "cathode follower" and learn how it functions.
I may have been in error here, Without a schematic I am guessing. Can you link to a schematic? The dead side of your tube could also be a cathode follower, in which case it too would have the 200v on the cathode. If pin 3 has a cap to the power tube grids, that is its cathode. Does pin 8 also have a cap to a power tube grid? If both cathodes show 85k to ground (probably 100k resistors?) that would be consistent with that.
In either case, if there is zero volts on the cathode, the tube is not conducting .
If you have already tried a diffderent tube in that socket, then I could suggest pulling the tube and probing the pins of the socket. There should be 6VAC between pins 9 and 4, and also between pins 9 and 5. You apparently already have B+ on both plates. Cathode resistance - with power removed - to ground to be checked.
Look up "cathode follower" and learn how it functions.
I may have been in error here, Without a schematic I am guessing. Can you link to a schematic? The dead side of your tube could also be a cathode follower, in which case it too would have the 200v on the cathode. If pin 3 has a cap to the power tube grids, that is its cathode. Does pin 8 also have a cap to a power tube grid? If both cathodes show 85k to ground (probably 100k resistors?) that would be consistent with that.
In either case, if there is zero volts on the cathode, the tube is not conducting .
If you have already tried a diffderent tube in that socket, then I could suggest pulling the tube and probing the pins of the socket. There should be 6VAC between pins 9 and 4, and also between pins 9 and 5. You apparently already have B+ on both plates. Cathode resistance - with power removed - to ground to be checked.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.