Help with KT88 biasing

woodturner-fran · 26th January 2009, 08:38 PM

Hi all,

I need to prevail on your help guys!

I've been asked by a friend to give a hand with a KT88 amp he has just bought. He has some audible distortion in one channel.There is also a low hum from this channel that is not present in the other one. The amp is fixed bias and each pair of valves is biased using a balance pot and a bias pot. Actual bias is given by a dial on the front panel (its an airtight ATM2).

So when I have a look, if I switch the bias display from one valve to the other, I get widely different bias readings, but if I try to adjust the balance pot I find its already all the way to one extreme.

So am I right in thinking:

1) One of his KT88 valves is suspect?
2) If I had a new KT88 I could drop it in there, first adjust the balance so that both valves read roughly the same bias, then when balanced, adjust the bias upwards to the recommended range?

He is willing to spring for a new quad of KT88s (USD250) but I would like to be sure they are the problem before committing the $$.

What you guys think?

Any help/advice is much appreciated. He has dropped a fairly large wad of cash for this amp so really don't want to do any harm!

Fran

HollowState · 26th January 2009, 08:45 PM

Since you have to start somewhere, an easy test is to switch the KT88's around and see if the problem moves with the tube. It it does, then it's the tube. If it does not, then there is something wrong in the circuit.

Ty_Bower · 26th January 2009, 08:46 PM

Start by swapping tubes around...

woodturner-fran · 26th January 2009, 11:14 PM

Ok, thanks guys. I will be calling over to see him on wednesday so will do exactly that. Really hope it ain't the amp.... this is a serious piece of kit!

Fran

woodturner-fran · 28th January 2009, 11:49 PM

Just to let you know this was just that one tube was bad. Luckily my friend had a single russian KT88 from a previous amp and I could sub that in. It biased up AOK and runs very sweet.

He does have a bit of hum in one channel, independant of volume, I would like to get rid of it. Anyone out there have experience of these amps?

fran

26th January 2009, 08:38 PM	#1
woodturner-fran diyAudio Member Join Date: Jan 2006 Location: Ireland	Help with KT88 biasing Hi all, I need to prevail on your help guys! I've been asked by a friend to give a hand with a KT88 amp he has just bought. He has some audible distortion in one channel.There is also a low hum from this channel that is not present in the other one. The amp is fixed bias and each pair of valves is biased using a balance pot and a bias pot. Actual bias is given by a dial on the front panel (its an airtight ATM2). So when I have a look, if I switch the bias display from one valve to the other, I get widely different bias readings, but if I try to adjust the balance pot I find its already all the way to one extreme. So am I right in thinking: 1) One of his KT88 valves is suspect? 2) If I had a new KT88 I could drop it in there, first adjust the balance so that both valves read roughly the same bias, then when balanced, adjust the bias upwards to the recommended range? He is willing to spring for a new quad of KT88s (USD250) but I would like to be sure they are the problem before committing the $$. What you guys think? Any help/advice is much appreciated. He has dropped a fairly large wad of cash for this amp so really don't want to do any harm! Fran

26th January 2009, 08:45 PM	#2
HollowState diyAudio Member Join Date: Aug 2006 Location: Taxland, New Jersey	Since you have to start somewhere, an easy test is to switch the KT88's around and see if the problem moves with the tube. It it does, then it's the tube. If it does not, then there is something wrong in the circuit. __________________ "The supercomputer is technologically impossible. It would take all of the water that flows over Niagara Falls to cool the heat generated by the number of vacuum tubes required." ~ Professor of Electrical Engineering, New York University

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26th January 2009, 08:46 PM	#3
Ty_Bower diyAudio Member Join Date: Mar 2007 Location: Newark, DE	Start by swapping tubes around...

26th January 2009, 11:14 PM	#4
woodturner-fran diyAudio Member Join Date: Jan 2006 Location: Ireland	Ok, thanks guys. I will be calling over to see him on wednesday so will do exactly that. Really hope it ain't the amp.... this is a serious piece of kit! Fran

28th January 2009, 11:49 PM	#5
woodturner-fran diyAudio Member Join Date: Jan 2006 Location: Ireland	Just to let you know this was just that one tube was bad. Luckily my friend had a single russian KT88 from a previous amp and I could sub that in. It biased up AOK and runs very sweet. He does have a bit of hum in one channel, independant of volume, I would like to get rid of it. Anyone out there have experience of these amps? fran

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