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Patrick Turner Patrick Turner is offline
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Default Recommendatons bias settings for KT88 and KT90



Bret Ludwig wrote:

On Sep 11, 11:27 am, Wessel Dirksen wrote:
On 11 sep, 16:44, Patrick Turner wrote:



Wessel Dirksen wrote:


Hi Guys,


I've been droping in again as I have some time and ideas. Even if my
amps sound good, after awhile you still want to do something different
and start taking them apart again. LOL


I was hoping to get some opinions about the "sweet spot" for KT88 and
KT90's. I bought a set of both. PP 4x power tubes at AB1 with B+ 425V.
I want to run a bit rich on class A if possible. What sounds best with
a descent life span?


BTW, I also got some NOS EI 6CG7's. Wow, what a diference a tbe makes!


Thanks,


Wessel


If you want lots of class A and blameless fidelity, you must use at
least 40% UL,
or triode, and bias the tubes up a bit, and keep Ea low,
and RLa-a high, so this takes care of the output stage.


But didn't tell us what schematic you have, so whatever we might say may
be of zero benefit to your music.
If you have cathode biasing, Ek with UL/triode will be about 45V, so Ea
= 380V approx,
so adjust Rk to get Ik = about 80mA, for Pda = 32 watts.


Yeah, forgot to mention details. I really like the triode sound (+/-
30 watts is enough for me) and use cathode bias.
I was already thinking of about 80mA as a target. Healthy amount of
Class A with what I guess in can infer from your input that this
won''t have too much wear and tear.



Then you might get up to 28 watts of pure class A and maybe 10 more
watts of AB in UL.
Less for triode, about 27 watts.


Loading is important, and expect Va swing at each anode = +/- 300V pk in
UL,
and if the Ichange = +/- 75mA pk in class A, the load each tube works =
4k, so
load a-a = 8k and PO = 22 watts at low thd and will all be class A.
Lower loads such as 4ka-a will be tolerated OK and give more power, AB.


But if you have a 8ohm and 4 ohm outlets on the back of the amp, try the
4 ohms regardless of what your speakers are.
This will reflect the highest possible load to the tubes depending on
the OPT TR.
The highest load for UL or triodes give the lowest THD/IMD because the
high RL gives lowest THD/IMD, and the higher gain, therefore increasing
applied global NFB.
But about 12dB of NFB with triode is all you need, maybe 15dB with UL,
so perhaps you may adjust NFB downwards after maximizing output tube
gain with class A action into high RL
when Ia is high and gm is thus high, and Ra low.


I'm only using about 5 db of global NFB. Great soundstage!



NOS Seimans 6CG7 are supposed to be the best sounding, but for blameles
dynamics,
use EL84 in triode each side of the LTP driver with 15mA per tube.


Hummm, interresting. EL84 better than 6CG7 as a driver? Robust yes,
but also better sounding?


EL84 is a bit big for driving receiving tubes except in Class B. I
would look at some of the smaller power pentodes (there are several,
including 6AQ5) or triodes.


6AQ5 is like a 6V6 in a small package, and a 6V6 in your estimations
would be a "bit big" for driver purposes unless its for class B.

Why so?

I think EL86, EL86, EL95 6V6, 6AQ5, 6AR5, 6CL6 et all are all very good
driver tubes in triode.
Also 6EJ7, 6BX6, 6AU6 etc, but EL84 is amoung the very best, and
its because you can still maintain high linearity and great sounding
dynamics with
low bias resistor values and low dc supply resistors. And gain is high,
Ra low.
One EL84 is about like 5 halves of a 6CG7 or 6SN7 all in parallel.


In SE amps I routinely use EL84 and 6V6 to drive large output tubes.

Patrick Turner.