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6ck4 sweetspot

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I'm looking for suggestions on good bias spot for 6ck4 in SE. Been running such an amp for a while but it's time to look for better sounding spots; currently running at 250V/43ma 550ohms Rcathode, 5k Zout which sounds a little thin; before it had about 270v on the plate with same Rcathode which sounded great, much warmer. I know some of you have toyed with the 6ck4, suggestions are most welcome!!

Klimon said:
I'm looking for suggestions on good bias spot for 6ck4 in SE.

Unforch, there really isn't one. The 6CK4 works better as a push-pull than a single ended amp.

Here's a pretty good PP loadline. The h3 estimate comes in at 3.7%, which is not good and not bad. The actual h3 will be considerably less since the graphical estimation method comes in high, due to the simplifying assumptions needed to make it work at all. (I used the same method to estimate h3 for the 6BQ6 and the actual measured h3 was almost half of the estimate.) This just might be OK. Of course, there is no indication as to what it does with higher order harmonics. Even though there are graphical methods, it'd be quicker to breadboard the thing and measure directly. As for what it sounds like, I dunnow. Different VTs (triodes and pentodes alike) have their own "sonic personalities" depending on whether or not they generate those dissonant high order harmonics, which, in turn, determines how you use corrective feedback.

I almost did a 6CK4 project, but had the OPT and power xfmr, and a lot of other junk box pickin's for a 6BQ6 amp instead, so that's what I did.

There are lots better triodes out there for SE use than this one.
Thanks for your remarks gentlemen. 30mA / 10w is a welcome guideline (I can't seem to open the pdf though); I'll experiment and listen a little to see what I like best --- I do believe 6ck4 has alot of potential in SE; sounded really sweet before I messed things up by implementing the loadline that looked good on paper.

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I've run them as high as 330V at 30mA and around 275V @ 35 - 40mA or so in SE (as a driver) with pretty good results. Not all 6CK4 are good, I tend to prefer the Sylvanias with the box plate with wings from the center of the plate structure on either side.. The narrow ends have three perforations and you can see the grid support rods. IMO The best sounding ones have grey plates, not the black shown here.


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Re: Re: 6ck4 sweetspot

Miles Prower said:
The 6CK4 works better as a push-pull than a single ended amp.


I'm curious to know a little more about the analysis behind this. Is there something specific in the plate curves or the resulting distortion profile that you look at, i.e., high single ended distortion that would cancel in PP?


Well, I was considering a project based on the 6CK4. Everything looked pretty good here, at first. You had a real power triode with a pretty decent Pd rating which is conveniently an indirectly heated type. I figured this might make a decent SET.

Then I drew some loadlines, and got some pretty horrible results. THD estimates that came in at ~7.0% if there was a decent amount of output, and nothing much under 5.0% with shallow loadlines and very poor efficiency.

Now there is something wonkey about the plate curves at Vgk= 0. Even if you didn't take the loadline all the way to Vgk= 0, this didn't much improve the THD estimates either. If you take a look at some specific Vpk= const (what you use to determine g(m) ) this graph goes way up in the positive direction, but down in the negative not so much. I've seen this type of characteristic before: the 6BQ7A, an RF dual triode that doesn't even pretend to be an audio tube at all. Even though the spec sheet says nothing about it, I suspect a variable-u characteristic for the 6BQ7A. The 6CK4 seems to have the same sort of characteristic.

Of course, that doesn't mean that you can't work around it. 6BQ7A's sound pretty good operated as cascodes in an LTP. 6CK4's operate better as PP finals. That way, the balance helps to reduce distortion. Types that produce a lot of h2 will operate with little distortion if the distortion it does produce is balanced out.

Look around the 'Net and you can find PP designs based on the 6CK4, but very few SETs designed around this type. For SE use, there are better TV vertical PAs.
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