KT-66 Rk

TUBEDUDE · Post by **TUBEDUDE** » Mon Sep 21, 2020 9:54 pm

Anyone know why the KT tubes require separate cathode resistors? Their tendency to oscillate easily maybe?
I'm looking at designing an amp and as my first KT66 foray I see they are an oddity in the 6L6 grouping.

pdf64 · Post by **pdf64** » Mon Sep 21, 2020 11:01 pm

I don’t think they have to, it’s just good practice.
As modern tube sets can be bought matched, a shared bias would seem to be fine.

Post by **Colossal** » Tue Sep 22, 2020 1:24 am

Have fun with your build. KT66s sound great and they look cool, I can tell you that.

Stevem · Post by **Stevem** » Tue Sep 22, 2020 10:12 am

They don't require seperate ones, it just more stable!

The KT66 has in general more head room then a 6l6 which means it has less gain, unlike the EL34 and 84 type tubes, so this should mean a lesser tendency to oscillate.

If anything the KT66 could better use a smaller value grid stopper resistor then let's say a EL34.

Now a question for someone!

The KT designation stands for Kinkless Tetrode, so can someone explain to me why this tube ( KT66 ) atleast in the JJ catalog is called a beam Pentode like the 6L6 family of tubes are ?

wpaulvogel · Post by **wpaulvogel** » Tue Sep 22, 2020 10:56 am

I think it was all about the Mullard patent and they named it kinkless Tetrode to avoid patent infringement. It contains 5 elements which makes it a pentode and it uses beam forming plates vs a suppression grid so it’s truly a pentode. The fact that the plate structure and beam plates are different than a suppression grid pentode does make them operate different. The screen grid alignment sets them apart also.

TUBEDUDE · Post by **TUBEDUDE** » Tue Sep 22, 2020 3:27 pm

Previous convention dictated tubes that had beam deflectors internally connected to the cathode were Beam Tetrodes, not pentodes, as they can't be separated. Some designers prefer to have a negative voltage on the suppressor grid, this can only be done with a true pentode. Forming plates are helpful but still a tetrode.

The data sheet and application notes say high value grid stoppers are needed, tens to hundreds of kiloOhms.

Stevem · Post by **Stevem** » Wed Sep 23, 2020 10:15 am

Thanks for the info gang!

mhuss · Post by **mhuss** » Thu Sep 24, 2020 4:01 pm

They're both identical in basic function, I think that "kinkless tetrode" is British for "beam tube" (or vice versa).

Both were developed as an alternative power tube design to get around the Mullard patent on the pentode tube (specifically the suppressor grid).

martin manning · Post by **martin manning** » Thu Sep 24, 2020 5:06 pm

https://en.wikipedia.org/wiki/6L6

pdf64 · Post by **pdf64** » Thu Sep 24, 2020 5:12 pm

I’ve noticed that GE describe the 6550a as a ‘beam pentode’.
https://tubedata.altanatubes.com.br/she ... /6550A.pdf
On investigation, likewise for the 5881 / 6L6 and 6V6
https://tubedata.altanatubes.com.br/she ... 5/5881.pdf
https://tubedata.altanatubes.com.br/she ... /6L6GC.pdf
https://tubedata.altanatubes.com.br/she ... /6V6GT.pdf
Tube manufacturers don’t seem to use the term ‘beam tetrode’, seem to prefer ‘beam power’ or ‘beam pentode’ - who’d have thought it!

TUBEDUDE · Post by **TUBEDUDE** » Thu Sep 24, 2020 8:38 pm

Guess it's just good to know which tubes have real supressor grids.

wpaulvogel · Post by **wpaulvogel** » Thu Sep 24, 2020 9:12 pm

That would be EL34’s and el84. Real 6ca7, 6l6, 6v6, 6550, KT88’s, KT66 and even Russian 6p14p have beam forming plates.

TUBEDUDE · Post by **TUBEDUDE** » Fri Sep 25, 2020 1:10 am

Correct.

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