KT90 to KT88 with autobias

pof@ · 21st August 2011, 03:19 PM

I have a SE amplifier running KT90.

The builder tells me that due to the auto bias system it should be able to run KT88 as well. But whatever autobias circuit he has used I guess it will always try to reach the same particular bias current no matter if I put in a KT90 or a KT88.

Are the two tubes so similar so this doesn’t matter?

My idea was to make a schematic from looking inside but this is not easy at all after first look.

costis_n · 22nd August 2011, 06:18 AM

No, autobias is a negative feedback mechanism. It reacts to cathode current. So the bias will be OK more or less when you install KT88.

pof@ · 22nd August 2011, 06:22 AM

Quote:

Originally Posted by costis_n

No, autobias is a negative feedback mechanism. It reacts to cathode current. So the bias will be OK more or less when you install KT88.

Will the bias be the same for KT88 and KT90? I think it will?

costis_n · 22nd August 2011, 06:24 AM

No, and it does not have to!

pof@ · 22nd August 2011, 06:33 AM

Seems like I need some basic knowledge on autobias. Is there anyone who can give me a good explanation?

Skorpio · 23rd August 2011, 09:51 AM

Auto bias is just as simple as a resistor in the cathode. It will mate within some limits to different tubes...

Try to look at Tubelab.com, there you will find examples of designs using a variaty of tubes...

21st August 2011, 03:19 PM	#1
pof@ diyAudio Member Join Date: Nov 2008	KT90 to KT88 with autobias I have a SE amplifier running KT90. The builder tells me that due to the auto bias system it should be able to run KT88 as well. But whatever autobias circuit he has used I guess it will always try to reach the same particular bias current no matter if I put in a KT90 or a KT88. Are the two tubes so similar so this doesn’t matter? My idea was to make a schematic from looking inside but this is not easy at all after first look.

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22nd August 2011, 06:18 AM	#2
costis_n diyAudio Member Join Date: Aug 2008	No, autobias is a negative feedback mechanism. It reacts to cathode current. So the bias will be OK more or less when you install KT88.

22nd August 2011, 06:24 AM	#4
costis_n diyAudio Member Join Date: Aug 2008	No, and it does not have to!

22nd August 2011, 06:33 AM	#5
pof@ diyAudio Member Join Date: Nov 2008	Seems like I need some basic knowledge on autobias. Is there anyone who can give me a good explanation?

23rd August 2011, 09:51 AM	#6
Skorpio diyAudio Member Join Date: Mar 2004 Location: Denmark	Auto bias is just as simple as a resistor in the cathode. It will mate within some limits to different tubes... Try to look at Tubelab.com, there you will find examples of designs using a variaty of tubes...