Distortion in amp after warm up

mjock3 · 16th April 2011, 02:37 PM

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 · 16th April 2011, 03:02 PM

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

mjock3 · 16th April 2011, 03:22 PM

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

Thanks

DF96 · 16th April 2011, 07:16 PM

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

mjock3 · 28th April 2011, 08:06 PM

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 · 28th April 2011, 09:03 PM

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

diesel_tech · 28th April 2011, 09:09 PM

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

wa2ise · 29th April 2011, 07:33 PM

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 · 29th April 2011, 07:39 PM

Quote:

Originally Posted by 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.

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

wa2ise · 29th April 2011, 08:34 PM

Quote:

Originally Posted by diesel_tech

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.

16th April 2011, 02:37 PM	#1
mjock3 diyAudio Member Join Date: Jan 2011 Location: Wisconsin	Distortion in amp after warm up 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

29th April 2011, 07:33 PM	#8
wa2ise diyAudio Member Join Date: Apr 2005 Location: NJ	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. __________________

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16th April 2011, 03:02 PM	#2
adason diyAudio Member Join Date: Nov 2004 Location: home sweet home	have you swapped the tubes between channels? to see if the distortion follows certain tube?

16th April 2011, 03:22 PM	#3
mjock3 diyAudio Member Join Date: Jan 2011 Location: Wisconsin	No I will give that a try. I was wondering if tubes or bias could cause that. Thanks

16th April 2011, 07:16 PM	#4
DF96 diyAudio Member Join Date: May 2007	Yes, tubes or bias (or leaky caps?).

28th April 2011, 08:06 PM	#5
mjock3 diyAudio Member Join Date: Jan 2011 Location: Wisconsin	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

28th April 2011, 09:03 PM	#6
DF96 diyAudio Member Join Date: May 2007	Check the grid voltage on the next stage. If the coupling cap is leaky then the grid voltage will rise.

28th April 2011, 09:09 PM	#7
diesel_tech diyAudio Member Join Date: Dec 2006 Location: Sterling, CO	Check the sockets pins to see if they are dirty/corroded. Could also be a resistor or cap that is changing with heat.

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