[GreatDane]

Hello,

I've noticed a very interesting issue with my KT88 based power amp. The amp sounds great 99% of the time, BUT, if I leave the amp on for an extended period of time (say about 4 hours or so), it begins to sound very different. Thin sounding with noticeable distortion. I've taken everything else out of the loop and can still consistently reproduce this problem, so I know it's the amp.

Anyone ever heard of this happening? I have recently re-tubed it (power tubes and pre tubes), so I know it is not a tube issue. Perhaps a thermal/capacitor issue?

[G Georgopoulos]

Hello,

I've noticed a very interesting issue with my KT88 based power amp. The amp sounds great 99% of the time, BUT, if I leave the amp on for an extended period of time (say about 4 hours or so), it begins to sound very different. Thin sounding with noticeable distortion. I've taken everything else out of the loop and can still consistently reproduce this problem, so I know it's the amp.

Anyone ever heard of this happening? I have recently re-tubed it (power tubes and pre tubes), so I know it is not a tube issue. Perhaps a thermal/capacitor issue?

that is not possible,however i suspect the tubes if that's the case!

kind regards

[GreatDane]

Fair enough, but it's having the same issue with all different tubes in it.

If you could help me understand how it's not possible to be a thermal/capacitor issue, I'd appreciate it.

[Freo-1]

Need more information.

1) Is this one channel or both?

2) How old is the amp?

3) Who made the amp? 


I would first look at voltage rectification.  A stressed resistor or diode bridge can indeed degrade the sound. 

[GreatDane]

Both channels

About 10 years old

Chinese amp - Rhymes with "Wholida".

[Freo-1]

OK.  A couple of points:

1) I would change out the diode bridge and resistors in the power supply section.

2) Look at changing out the fuse, and possibly fuse holder.  I have had dodgy fuses that started acting like a resistor, and did degrade the sound.

3) At 10 years old, may want to change out electrolytic caps in the power supply.

[G Georgopoulos]

Fair enough, but it's having the same issue with all different tubes in it.

If you could help me understand how it's not possible to be a thermal/capacitor issue, I'd appreciate it.

i dont know,if it was a capacitor issue there would also be hum...
check the new tubes ,might be the problem...

kind regards

[JakeJ]

First I must ask if you are aware of the voltages and currents that are designed into this amp?  I ask because it might be helpful if you could take off the bottom cover and take a picture or two for us.

But I only want you to do this if you are comfortable with doing it and understand the inherent risks of exposed electronics and especially in a tube amp.  Amp off, unplugged, and cool.  Flip it over and take off the bottom cover and look for abnormalities (dark colored resistors, leaking or pooched out capacitors) take some photos and post them to this thread.

It could be a capacitor that is internally shorting only when the temperature gets hot enough.  What ever it is we just need enough info to figure it out, well...unless it needs to be put on a test bench to truly get to the bottom of it all.

[kentajalli]

Or it could be slightly over biased, and it is heating up the output transformers or something, try to gently cool it using a slow spinning computer fan, blowing heat out, not blowing cold air in.
See if it helps, if it does, find out what bias currents you need ( as suggested previously) - check to see if one tube has drifted.

[ltr317]

I'm with Kentajalli on this.  I have a 15 year old 302a, and the bias pots at that age can drift.  Check each pot when you encounter the problem.  I also bias about 10% below the low end of the recommended range, which prolongs tube life and doesn't affect sound quality.

Paul Mah

[G Georgopoulos]

I'm with Kentajalli on this.  I have a 15 year old 302a, and the bias pots at that age can drift.  Check each pot when you encounter the problem.  I also bias about 10% below the low end of the recommended range, which prolongs tube life and doesn't affect sound quality.

Paul Mah

bias wont cause distortion,it will reduce o/p power if you reduce it...
anyway is it se or pp?

[Freo-1]

bias wont cause distortion,it will reduce o/p power if you reduce it...
anyway is it se or pp?

Agree on the bias.  Probably need to put a o-scope on the power supply output and see how it looks.  Still think it is a dodgy component in the power supply once it heats up. Assume the bad sound is out of both channels equally?

[kentajalli]

Agree on the bias.  Probably need to put a o-scope on the power supply output and see how it looks.  Still think it is a dodgy component in the power supply once it heats up. Assume the bad sound is out of both channels equally?

True!
but overheating as the result of an over bias (or is it under bias?) can play havoc with many parts, capacitors on feedback loops, saturated output transformers, etc.
Hey we are guessing blind (since very little information is given) - mine was over heating.

[ltr317]

True!
but overheating as the result of an over bias (or is it under bias?) can play havoc with many parts, capacitors on feedback loops, saturated output transformers, etc.
Hey we are guessing blind (since very little information is given) - mine was over heating.

On my amp, some of the pots drifted over recommended range and was overheating also.  Since there is very little info with GreatDane's issue, we can only guess.