KT-88 Music Angel problems with sound level

[wallagh]

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!!

[Svein_B]

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.

[wallagh]

Originally posted by Svein_B
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.

Tnx!. I will try this. The 100 ohm resistors: any specific requirements?

[Svein_B]

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

[wallagh]

Originally posted by Svein_B
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

Are you sure you mean pin 5 and 3?
Looking at the diagram and current wiring pin 3 and 4 seems to make more sence.

[Svein_B]

Originally posted by wallagh

Are you sure you mean pin 5 and 3?
Looking at the diagram and current wiring pin 3 and 4 seems to make more sence.

Of course it is
3=Andode, 4=Screen.

SveinB

[wallagh]

Originally posted by Svein_B


Of course it is
3=Andode, 4=Screen.

SveinB

And (hopefully last) question:
In the diagram it sais 51 Ohm.
In your mail you write that I should use 100 Ohm .
But in the amp itself there is a 2K7 between poin 3 and 4 (with the relay between).

Any thoughts on this?

[Svein_B]

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.

[wallagh]

I finished the "relay removal". *that* seems to work.

However, my original problem is not solved: suddenly 1 channel sounds louder then the other.
Exchanging tubes does nog change this.

Any other suggestions that I may try?

[Svein_B]

wallagh, could you try to explain in a little more detail;
- exactly what did you remove or add?
- before the mod, one channel was lower, now suddenly one is louder. Does this mean no change, or is the oposite channel louder than before?

SveinB