tube amp being weird -- any ideas

soundbadger · 8th March 2008, 05:15 PM

just finished building a single ended kt88 stereo amp and it works fine for a while then one channel goes very load and the other very quiet. it is consistent i doing this i can turn it off leave it for a while then turn it back on and get perfect stereo sound for a while. i am confused.
any ideas

audio_moksha · 8th March 2008, 08:51 PM

Hey soundbadger, what are you driving the KT88's with? A schematic and/or layout would be helpful. It is possible that the problem lies in the driver stage. Also go ahead and double check all your voltages and currents. Make sure nothing is shorted. I'm no expert, but I know how frustrating it can be to troubleshoot an amp.....

ray_moth · 8th March 2008, 09:27 PM

Yes, check the voltages and currents while it's working well and again when it's not, to see what is changing.

jechentau · 8th March 2008, 09:32 PM

if both channels use a single supply, especially if tube rectified,
it might be that one of the channels`s tubes and/or bias gets runaway.
what biasing method is used?
what value grid bias resistors are used?
resistor possibly going open when getting warm?
but just guessing now..

do you have and will post a schematic?

porkchop61 · 9th March 2008, 11:53 AM

I believe he's building Mikael's amplifier, is that right soundbadger?
I've built this amp myself, as well as many others, and it's a very simple design.
I'd do as Ray_moth suggested and take some voltage readings. I have voltage measurements on my page if this helps. It's got to be something simple.
Glenn

Mikael's KT88 SE amp

Glenn's KT-88 build

soundbadger · 10th March 2008, 02:01 PM

sorry have been away,,
yes it is Mikael's amplifier i am building.
i shall take readings tonight to try and see what happens when the problem starts to occur.

pmillett · 10th March 2008, 03:40 PM

Beware using a 470k grid resistor on the KT88. I've had modern KT88's draw enough grid current that 470k is too high. The result is that the bias point goes weird, sometimes even oscillates.

Just for kicks solder a 100k in parallel (it may hurt LF response) and see if the problem goes away.

If that fixes it you might need bigger coupling caps to restor LF response.

Pete

TubeHead Johnny · 10th March 2008, 05:15 PM

The great thing about stereo is you can always compare the voltages between the working/non-working channels. It's possible that one of the KT88's are defective.

soundbadger · 10th March 2008, 08:43 PM

THE BIAS VOLTAGE APPEARS TO DROP ON ONE CHANNEL WHEN THE PROBLEM IS occurring.
(sorry caps off now)
the bias resistors are also pretty hot, i measured them about 70 degrees centigrade , they are 12W resistors which i believe should be fine.
i am guessing that either one of the bias resistors or one of the bypass capacitors is not good.

the pre-amp stage seems fine even when the problem is occurring.

soundbadger · 11th March 2008, 06:09 PM

i changed the bias resistors for ones of greater power and changed the bypass caps, still the same problem after about 10 mins.

8th March 2008, 05:15 PM	#1
soundbadger diyAudio Member Join Date: Sep 2007	tube amp being weird -- any ideas just finished building a single ended kt88 stereo amp and it works fine for a while then one channel goes very load and the other very quiet. it is consistent i doing this i can turn it off leave it for a while then turn it back on and get perfect stereo sound for a while. i am confused. any ideas

9th March 2008, 11:53 AM	#5
porkchop61 diyAudio Member Join Date: Aug 2004 Location: Mansfield, Connecticut	I believe he's building Mikael's amplifier, is that right soundbadger? I've built this amp myself, as well as many others, and it's a very simple design. I'd do as Ray_moth suggested and take some voltage readings. I have voltage measurements on my page if this helps. It's got to be something simple. Glenn Mikael's KT88 SE amp Glenn's KT-88 build __________________ "Imagination is more important than knowledge"

10th March 2008, 05:15 PM	#8
TubeHead Johnny diyAudio Member Join Date: Jan 2005 Location: The Electric City, Schenectady, NY.	The great thing about stereo is you can always compare the voltages between the working/non-working channels. It's possible that one of the KT88's are defective. __________________ "No man really becomes a fool until he stops asking questions" - Charles Proteus Steinmetz

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8th March 2008, 08:51 PM	#2
audio_moksha diyAudio Member Join Date: Oct 2006 Location: Los Angeles	Hey soundbadger, what are you driving the KT88's with? A schematic and/or layout would be helpful. It is possible that the problem lies in the driver stage. Also go ahead and double check all your voltages and currents. Make sure nothing is shorted. I'm no expert, but I know how frustrating it can be to troubleshoot an amp.....

8th March 2008, 09:27 PM	#3
ray_moth diyAudio Moderator Emeritus Join Date: Jan 2004 Location: Jakarta	Yes, check the voltages and currents while it's working well and again when it's not, to see what is changing.

8th March 2008, 09:32 PM	#4
jechentau diyAudio Member Join Date: Feb 2005 Location: Gelderland	if both channels use a single supply, especially if tube rectified, it might be that one of the channels`s tubes and/or bias gets runaway. what biasing method is used? what value grid bias resistors are used? resistor possibly going open when getting warm? but just guessing now.. do you have and will post a schematic?

10th March 2008, 02:01 PM	#6
soundbadger diyAudio Member Join Date: Sep 2007	sorry have been away,, yes it is Mikael's amplifier i am building. i shall take readings tonight to try and see what happens when the problem starts to occur.

10th March 2008, 03:40 PM	#7
pmillett diyAudio Member Join Date: May 2005 Location: Dallas (but I am not a Texan!)	Beware using a 470k grid resistor on the KT88. I've had modern KT88's draw enough grid current that 470k is too high. The result is that the bias point goes weird, sometimes even oscillates. Just for kicks solder a 100k in parallel (it may hurt LF response) and see if the problem goes away. If that fixes it you might need bigger coupling caps to restor LF response. Pete

10th March 2008, 08:43 PM	#9
soundbadger diyAudio Member Join Date: Sep 2007	THE BIAS VOLTAGE APPEARS TO DROP ON ONE CHANNEL WHEN THE PROBLEM IS occurring. (sorry caps off now) the bias resistors are also pretty hot, i measured them about 70 degrees centigrade , they are 12W resistors which i believe should be fine. i am guessing that either one of the bias resistors or one of the bypass capacitors is not good. the pre-amp stage seems fine even when the problem is occurring.

11th March 2008, 06:09 PM	#10
soundbadger diyAudio Member Join Date: Sep 2007	i changed the bias resistors for ones of greater power and changed the bypass caps, still the same problem after about 10 mins.

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