KT90 to KT88 with autobias

[pof@]

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]

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@]

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]

No, and it does not have to!

 

[pof@]

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

 

[Skorpio]

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...