Hello,
I have sudenly a problem with my Music Angel XD-SE.
This is a (simple) KT-88 tube amp. It can be switched between Trido and Liniair mode.
It always worked fine, but sudenly the right channel has a much lower volume then the right channel (on all inputs). This is only the case in Triode mode (which a always use). In Liniair mode, there is no problem.
Questions:
1. I recently decided to connect my speaker to the 4 Ohm tap in stead of the 8 Ohm tap. Can that be the cause?
2. More important: has any body an idea how I can fix this?
The Schema of the amp is:
PLease help!!
I have sudenly a problem with my Music Angel XD-SE.
This is a (simple) KT-88 tube amp. It can be switched between Trido and Liniair mode.
It always worked fine, but sudenly the right channel has a much lower volume then the right channel (on all inputs). This is only the case in Triode mode (which a always use). In Liniair mode, there is no problem.
Questions:
1. I recently decided to connect my speaker to the 4 Ohm tap in stead of the 8 Ohm tap. Can that be the cause?
2. More important: has any body an idea how I can fix this?
The Schema of the amp is:
An externally hosted image should be here but it was not working when we last tested it.
PLease help!!
Attachments
A possible (likely) source of the problem is the relay that switch between triode and pentode operation. Those contacts can easily get stuck, resulting in partial shorting of the OPT primary.
If you prefer triode mode, I would strongly suggest permanently connecting the screen (pin 5) to anode (pin 3) with a 100 ohm resiostor, directly on the tube socket. Remove the wires from screens to the PCB.
Edit: If the relay contacts are stuck, you may also need to remove the relay or resistor to fix the problem.
SveinB.
If you prefer triode mode, I would strongly suggest permanently connecting the screen (pin 5) to anode (pin 3) with a 100 ohm resiostor, directly on the tube socket. Remove the wires from screens to the PCB.
Edit: If the relay contacts are stuck, you may also need to remove the relay or resistor to fix the problem.
SveinB.
Idellly, I would suggest 2W AllenBradley carbon composite resitors for the screenstoppers. However any resistor of about 100 to 300 ohm value should probably be OK.
You could re-use those presently mounted on the PCB which are probably 270 ohm (do not trust the values on the published schematic)
SveinB
You could re-use those presently mounted on the PCB which are probably 270 ohm (do not trust the values on the published schematic)
SveinB
For triode operation, the screen is connected to the anode. In principle no resistor is needed. It is common practice to use a screen-stopper. A recommended value by members on this forum for this class of tube is around 100 ohm. It should normally not need to be higher to stop parasitic oscillation.
2K7 is much higher than common practice. I have no experience with the possible effect this could have on the sound.
SveinB.
2K7 is much higher than common practice. I have no experience with the possible effect this could have on the sound.
SveinB.
Svein_B said:
1. I removed the wire(s) from pin 4 (1 wire per tube). I cpnnected pin 3 and 4 with a new 2K7 resistor.
I also removed the 2 power connections to the relays. SO in short I think I now have a fixed triode amp.
2. Sorry about hte confusion: No, there is no change in sound level: their is still the same difference in sound levels betwene the two channels.
wallagh said:
Which two power connections did you remove, B+ or the low voltage to operate the relay?
Now consider what could have caused the problem:
The relay at idle is in the pentode position (the factory diagram is wrong). When you use the amp in triode mode, the relay will release to pentode position when you turn power on. You can hear this as a momentary increase in level if you have signal when you power off.
This creates a spark between the relay contacts which can cause them to become "stuck" (velded together). This can produce a short across the OPT primary as shown in red on the diagram below
SveinB
Svein_B said:
Which two power connections did you remove, B+ or the low voltage to operate the relay?
Now consider what could have caused the problem:
The relay at idle is in the pentode position (the factory diagram is wrong). When you use the amp in triode mode, the relay will release to pentode position when you turn power on. You can hear this as a momentary increase in level if you have signal when you power off.
This creates a spark between the relay contacts which can cause them to become "stuck" (velded together). This can produce a short across the OPT primary as shown in red on the diagram below
SveinB
I remove the low voltage that operates (through the p/t switch) the relay.
I checked: the relay themself are not stuck.
Could it be the Bias?
Well, if you are certain there are no problems with the relay - it can be a bit difficult to verify for sure. You may want to check again that there is no short between pins 3-4 when counting from the back of the amp. Better bleed off the B+ before doing this, as it will usually keep close to 200V for days.
You should check the bias since the bottom is already off. The cathode resistor is probably 5 ohm if there is a 10 ohm on both sides of the PCB. Measure the value to be sure. Adjust the corresponding bias pot to get a voltage across the cathode resistors corresponding to a current of 50-65mA.
SveinB
You should check the bias since the bottom is already off. The cathode resistor is probably 5 ohm if there is a 10 ohm on both sides of the PCB. Measure the value to be sure. Adjust the corresponding bias pot to get a voltage across the cathode resistors corresponding to a current of 50-65mA.
SveinB
Attachments
Maybe the problem in triod mode is in the LIFE of tubes on every channel, and in using
4 ohm output instead 8 ohm.
Ra=k.Ri.....Ri in triod mode is 2-3 time less than in penthod mode!!
/Ra go down also..../. If You used 4 ohm, k decrease also, Ra decrease more......and the volume
decrease. And this is not the same in both channels, maybe.....the life is problem!! You must adjust tube curents ! Ri is not the same.....
4 ohm output instead 8 ohm.
Ra=k.Ri.....Ri in triod mode is 2-3 time less than in penthod mode!!
/Ra go down also..../. If You used 4 ohm, k decrease also, Ra decrease more......and the volume
decrease. And this is not the same in both channels, maybe.....the life is problem!! You must adjust tube curents ! Ri is not the same.....
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