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Recommendatons bias settings for KT88 and KT90



 
 
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  #1  
Old September 11th 07, 06:58 AM posted to rec.audio.tubes
Wessel Dirksen
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Posts: 11
Default Recommendatons bias settings for KT88 and KT90

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

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  #2  
Old September 11th 07, 12:08 PM posted to rec.audio.tubes
Peter Wieck
external usenet poster
 
Posts: 2,259
Default Recommendatons bias settings for KT88 and KT90

On Sep 11, 1:58 am, 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


http://hereford.ampr.org/Tube4.php?tube=6550

A start.

Peter Wieck
Wyncote, PA
Kutztown Space 338

  #3  
Old September 11th 07, 12:20 PM posted to rec.audio.tubes
Wessel Dirksen
external usenet poster
 
Posts: 11
Default Recommendatons bias settings for KT88 and KT90

I forgot to mention they will be setup in triode mode.

Also, what is a good value of grid leak resistance for these? I am
using 220K for 2 EL-34's in parallel. Can I raise this for KT88/90?

Wessel

  #4  
Old September 11th 07, 12:55 PM posted to rec.audio.tubes
Ian Iveson
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Posts: 960
Default Recommendatons bias settings for KT88 and KT90

Wessel wrote

> Also, what is a good value of grid leak resistance for these? I am
> using 220K for 2 EL-34's in parallel. Can I raise this for KT88/90?


Hi Wessel

Datasheets generally give a value for max grid leak. e.g.

http://www.tubezone.net/pdf/kt88new.pdf

Note at least two values are usually given for max R(g1-k), one for
cathode bias, the other for fixed.

Mode makes no difference, AFAIK. I see that datasheet has different
values depending on total (anode + screen) power dissipation. That's a
bit interesting...it's quite a big difference.

Ian



  #5  
Old September 11th 07, 03:44 PM posted to rec.audio.tubes
Patrick Turner
external usenet poster
 
Posts: 3,934
Default Recommendatons bias settings for KT88 and KT90



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.

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.

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.
And choke loading if you can use it; there is info on this at my site,
see the pages on 8585 amp.
6CG7 are very good in the input stages, like 6SN7.

Patrick Turner.
  #6  
Old September 11th 07, 05:27 PM posted to rec.audio.tubes
Wessel Dirksen
external usenet poster
 
Posts: 11
Default Recommendatons bias settings for KT88 and KT90

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?

FWIW, the current config (experiment) is a two input stage amp with 2x
6CG7's
A CCS non bypassed grounded cathode stage biased at about 6.7mA is DC
coupled (+ voltage PS only) to a long tail phase splitter (& driver)
biased at about 14,5 mA. I'm using different Rl values to offset the
balance problem. It's cool because there is only one capacitance in
series with the signal into the power stage (f-3 about 15 hz). Both
stages are good for a gain of 24. With 2VRMS input it seems to drive
it all fine. OPT is an older torroidal Plintron with Raa of 2400.
Currently I'm using the Blumlein "garter" method to crossbias the big
tubes. It takes a 20V B+ hit but distortion is lower.

I don't have the most eloquent measuring gear for beyond 96khz. But
it's very linear with a speaker load (+/- 1.5 db) and overall THD and
IMD are about 1% at 3/4 full output with EL34's running at 65mA. I'm
kinda proud of this first contraption of mine in it's current state.

> And choke loading if you can use it; there is info on this at my site,
> see the pages on 8585 amp.
> 6CG7 are very good in the input stages, like 6SN7.
>
> Patrick Turner.- Tekst uit oorspronkelijk bericht niet weergeven -


Thanks Patrick


  #7  
Old September 15th 07, 01:33 AM posted to rec.audio.tubes
Patrick Turner
external usenet poster
 
Posts: 3,934
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.
 




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