[coinmaster]

I am thinking the values on the driver resistors to was set that way because of the Limited PSU so I will look into that

Yeah, if you want proper voltages with high impedance resistors then you would need more then double the voltage the power supply offers (minimal). I think this was the best balance of efficiency and sound while maintaining a price point. The power supply is usually the most costly part of the amp.
 

I was told that using a pot was a bad idea

Oh, did you take your stock one out?

 

[Maxx134]

The input stage power supply is unregulated.
The resistor values are poor
The bias is poor, depending on the tube you use
The balance between the tubes is poor
Resistor loading in general is poor due to the impedances and the way the signal travels along the load line.

Just because you say it's poor doesn't make it so..
Edit :
If you give it an F,
I will give it a C+

Obviously it looks like it could be better..

I already showed you the diagram of what's happening in our stage ..

You keep sidetracking being stuck on my words that's why I had to show you the diagram.

Also certain driver tubes datasheet actually makes note & specify same values we have for resistors with near same voltage,

So This needs to be looked into further

 

[Maxx134]

Quote:
, did you take your stock one out?

That was not attempted by me,,
but definitely looking into this,
because experienced to designers know where the sweet spots are,
when dealing with limited PSU..

 

[Maxx134]

Yesterday I was sidetracked with a WCF cap not making connection causing a slight hum...

 

[coinmaster]

Just because you say it's poor doesn't make it so.

Unless you ignore the facts of electronic design it is poor. Of course everything is relative.
 

I already showed you the diagram of what's happening in our stage ..

You keep sidetracking being stuck on my words that's why I had to show you the diagram.

I've already showed you that diagram so many times and explained how it works.
That explanation on that diagram literally just explains how two individual SE tubes function when they are fed by anti phase signals, because that's what it is, two amplifiers feeding two signals.
 
As I went over previously the anti phase currents maintain a stable voltage on the cathode but that's only true if they are both drawing the same current, which isn't the case in real life when tubes cannot be perfectly matched and are often 10-20% different from each other.
 
This is why it explains that you lose gain when the signals are not antiphase because it starts acting more like a cathode follower and you lose cathode-grid potential.
Plus there's the fact that they will be operating at slightly different points on the load line.
 
As MrCurwen and I have been saying, a current source cleans this mess up and balances them unconditionally with other benefits.
 

Also certain driver tubes datasheet actually makes note & specify same values we have for resistors with near same voltage

Depends on tube and design. For a 6sl7 the plate resistors are okay but it doesn't change the impedance ratio issue of the cathode resistor that MrCurwen was explaining, no practical design with resistors is going to beat a CCS here because a CCS is close to a 1meg impedance to AC.
 

 

[Maxx134]

As an example about Sweet Spots in tubes,

I was able to increase my tube performance in sound stage with a slight increment of bias to rear power tubes..

They are different tubes than MK6,
butt just showing an example.

The bias point is still below what datasheet states optimal,
but I am finding out that the tubes Heat and longevity is compromised with higher setting ...

We only need what works.
We don't need what datasheet states..

 

[coinmaster]

As an example about Sweet Spots in tubes,

I was able to increase my sound stage with a slight increment of bias to rear power tubes..

We only need what works.
We don't need what datasheet states..

You have to remember there is a reason why things improve when you do things. It's not magic.

 

[Maxx134]

QUOTE:

As MrCurwen and I have been saying, a current source cleans this mess up and balances them unconditionally with other benefits.

no practical design with resistors is going to beat a CCS here because a CCS is close to a 1meg impedance to AC.


As a solution,
I will repeat the words "current source" till it sinks in..
That may take a while. .

Edit:
Meanwhile,
I will listen to my amp,
which "shouldn't work" as good as it does.
I remember you stating Yours was excellent also..

 

[Maxx134]

As an example about Sweet Spots in tubes,


I was able to increase my sound stage with a slight increment of bias to rear power tubes..

We only need what works.

We don't need what datasheet states..

You have to remember there is a reason why things improve when you do things. It's not magic.

**,
Of course.
I placed bias point more in midde of the optimum part of the curve,
But optimal value, was stated higher,
And the datasheet doesn't tell me what sonic gains were to be had. .

edit:
Also the decision of the designer to a place the bias point lower. .
why?
I think because of heat and longevity and no fans on bottom MK8 unit!

 

[coinmaster]

 Of course. I placed bias point more in midde of the optimum part of the curve, But optimal value, was stated higher,

Datasheets and reality are far different from each other. Most datasheets were done almost a century ago and some were hand drawn. This is why the only way to really know what your curves are is with a curve tracer.
Even a tube tester has a considerable margin of error so even when you buy matched tubes you aren't really buying matched tubes most of the time.
 
Datasheets are a general reference, even if they were accurate they would only be accurate for the tube that was tested since tube specs vary between each other.
 
I've been contemplating building a tracer but I am not yet sure the extent to which bias shifting affects the sound so I don't want to go overkill for no reason.
 

and the datasheet doesn't tell me what sonic gains were to be had. .

Well, maybe not down to the last detail but it does give you an idea. More linear parts of the load line = more accurate reproduction. Less linear = more distortion.
 

Also the decision of the designer to a place the bias point lower. . why?I think because of heat and longevity and no fans on bottom MK8 unit! 

I don't know what your tubes are biased at stock but I don't think it the designer would compromise bias rather then buy some fans.
 

Meanwhile,
I will listen to my amp,
which "shouldn't work" as good as it does.
I remember you stating Yours was excellent also..

Yeah I can't argue with how good it sounds, I'm detaching myself from what I heard and am looking for design improvements regardless.
The amazing sound is why I became interested in improving it to begin with since a few component upgrades made such a huge huge difference.
 
Although technically I have no real passion for the subject of electronic design and I'll be glad when its over. But since I'm already down the rabbit hole I might as well explore.

 

[Maxx134]

There is no distortion cancelation in the design, it is not a common cathode input stage.

Just to clarify your statement with some corrections..


Yes it is not a common cathode, 

So misunderstanding of names on my part. 

But the design our amp has does cancel distortion
Look here: 
http://www.atma-sphere.com/Resources/Our_Basic_Circuits.php

Optimally, it would have these advantages:

 

[coinmaster]

But the design our amp has does cancel distortion

You're right, it does cancel second order harmonics which is sometimes considered the "good" kind and is responsible for SE sound but the common cathode topology cancels out all harmonic distortion. It's on another level.

 

[Maxx134]

Hmm.
Also this seems to match why the driver PSU is not so well executed. .

Also;
Common Cathode like you say would require double the tubes!
Ha!

 

[Maxx134]

Something tells me this may have been it.. (!):

 

 

[coinmaster]

Yeah I was thinking about that when you showed it to me a while back.
But it has 1 tube too many for common cathode per channel.
 
I'm not really sure what that's about.