Best driver for KT88 Triode SE.

[Kenneth Zhu]

I am planning a KT88 triode connection SE. To cancel the distortion, a driver triode with curves similar to those of KT88 in triode connection should be found. I have compared ECC81 and ECC85 and find ECC85 a better option.

But from esthetic viewpoint, an octal would be more ideal. But it seems hard to find an octal triode or twin triode that is so distorted as ECC85 (as octal usually have better linearity than noval). Any suggestion?

Thanks in anticipation.

[Tubes4e4]

Hi,

just one tip:

Just only comparing statical curves doesn´t help much in this case. Instead, comparing transfer functions under actual load will be much more useful.

Regards,

Tom

[Cycline3]

Ni Hao Kenneth!

I personally only consider tubes in current production. You may find something that would work better, but what about 10 years from now? Can you still get that tube? In octal that is the 6sn7 and 6sl7, at least at the very top of the list. I would think you could make a fine driver with these, as both are great tubes and have plenty of selection in brands to choose from.

[Kenneth Zhu]

Usually 6SN7 or 6SL7 is not considered to be the ideal driver for KT88 triode connection, a "triode" that exhibits much poorer linearity than a real triode. 6SL7 and 6SN7 has good linearity, and if being coupled with KT88 triode, in particular in the form of two-stage driving, will explicitly show the unlinearity of KT88 in triode connection, leading to high distortion.

The idea of THD cancellation requires a driving stage that has similar distortion features to those of KT88 triode connection (as under particular load, such as 3.5K, as floor 2 indicates).

I am searching for such a triode, in octal. I am an amateur, so tube availability doesn't matter much to me, and the performance matter more.

[Cycline3]

Kenneth,

Maybe you are focusing too much on a detail you aren't sure is really important? I've driven quite a few KT88 and 6550 tubes and have with the tubes mentioned and others.

I am confident you can put together a very nice sounding amp with the above mentioned tubes; triode, pentode or UL connected outputs.

I also feel that any perceived value gained by your description of "distortion cancellation" is more than outweighed by the benefit of different tube brands and being able to tube roll in the best valve for the amp at hand.

You have to realize that your idea of having the 2 tubes cancel out each others bad properties (as they are out of phase) is assuming that they are operating on the same voltage levels or that the deficiencies are in ratio to the voltage ratio of the driver to output, which of course, they are not, or at the very least, extremely unlikely.