Distortion in amp after warm up

[mjock3]

I have a Sonic Frontiers SFC-1 integrated amp. Once it warms up pretty well it distorts out of one channel when played at moderate levels. At low levels all seems fine. Any ideas what would cause this?

Thanks,

Mark

 

[adason]

have you swapped the tubes between channels? to see if the distortion follows certain tube?

 

[mjock3]

No I will give that a try. I was wondering if tubes or bias could cause that.

Thanks

 

[DF96]

Yes, tubes or bias (or leaky caps?).

 

[mjock3]

I have swapped the tubes and it did not change anything. Have also checked bias and all looks well there. How would I go about checking for leaky capacitor?

Thanks,

Mark

 

[DF96]

Check the grid voltage on the next stage. If the coupling cap is leaky then the grid voltage will rise.

 

[diesel_tech]

Check the sockets pins to see if they are dirty/corroded. Could also be a resistor or cap that is changing with heat.

 

[wa2ise]

Old high resistance value resistors have been known to drift to higher resistance. resistors like 470K going to 700K or more. Enough to cause distortion, even with perfect vacuum tubes. With the power off, measure each resistor to see if it's within its marked tolerance value.

 

[diesel_tech]

Old high resistance value resistors have been known to drift to higher resistance. resistors like 470K going to 700K or more. Enough to cause distortion, even with perfect vacuum tubes. With the power off, measure each resistor to see if it's within its marked tolerance value.

Make sure you dissconect one lead of the resistor from the rest of the circuit during you check.

 

[wa2ise]

Make sure you dissconect one lead of the resistor from the rest of the circuit during you check.

Of course, but if you have the schematic, and you see that one end of the resistor only connects to a tube element (grid, plate or cathode) and also a capacitor, but no other resistors or coils, you won't need to.

 

[mjock3]

Thanks guys, I will look into that. Really do appreciate your help.

Mark