[MrCurwen]

As far as caps I think we are open minded for whatever works best.

That's a good attitude to have!

However, there was a categorical claim made about russian caps being poor. At least the categorical claim of them being good has (in addition to a growing pool of DIY guys with extensive history of comparing caps getting on board with them) actual technical specs supporting that at least they are very good.

Anyway I should not be getting more people into the MBGO / KBG / SSG / K73 stuff. The prices are slowly but steadily climbing, and that's not good.

the question of Tubes was going to be my next one.
But not the single triode types.
I was wondering which "dual triode" types tubes would be best for our amps, in which space is the limiting factor. .

I think what you have is good. There are better ones, but they would basically need a lot of redesigning and a bigger chassis. These are very good tubes anyway. Tubes are not the bottleneck; the design is.

You re-made the diagram so you should know instead of asking coinmaster..

What did I ask? I don't understand, please clarify.

It is incorrect to say nothing is balanced.

Alright, a clarification;

It is not very well balanced. It is not balanced to the degree that I would consider "high end". Whether or not that matters to you is up to you, but I'll present my opinion with some reasoning.

We already have force balanced in driver stage by the "common cathode tail".

No.

Forced balance means that the outputs are of exactly equal amplitude in antiphase, even if

1) the inputs are not of exactly equal amplitude (the other input could be grounded for example, so completely nil)

2) the tubes (or tube sections) are not identical in specs (they never really are, +-10% is the norm, +-20% not hard to find)


Without going into a long explanation of how a LTP works (unless there is interest), the forcing of the balance is dependent on what are the relations between

1) the tail impedance

2) the tubes' plate resistance

3) the plate load impedances

Three main rules;

1) the more dominant the tail impedance is, compared to all other impedances in the current route, the better the balance

2) the branches from the shared cathode node (top of the tail) must be as closely matched as is possible

2) considering the previous, the more dominant the plate load impedance is compared to the tube plate resistance, the less the tubes' spec variance matters to the balance


So let's round up and say the plate load impedances are about 4.5 times the plate resistance. The tail is 1.5k, so each branch on top the tail is about 180 times the impedance.

Even guitar amps which don't look for fidelity of any kind, have better ratios. Guitar amps are conserned with balance in some cases, but not because of fidelity but because of power efficiency.


Looking at guitar amps;

Using similar enough tubes like 12AX7 you have 100k plare resistor (bad for linearity, but good for guitar amp) and 10k or 22k or 47k tails!

This amp has 1.5k tail.

It is relying heavily on the fact that the input is perfectly balanced. And, of course, tube selection.

If you put into your amp an signal that is 100% on the other side, and 80% amplitude on the other side, the balance is not going to be pretty.

And also, if you put in a tube that has a (very normal) 10% section mismatch, the balance is not going to be high end pretty.


If you want forced balance, the tail impedance must be AN ORDER OF MAGNITUDE bigger than the COMBINED impedances of both the branches.

Also the plate load impedance should be the dominant one in each branch; 50 or so times greater than the plate resistance. This plays the tube variance out of the picture completely; increases fidelity significantly.

As I said, if this general tube theory is ok in this thread, I can elaborate.

Then, the "tuning" of the opamps controlling DC offset also means power tubes are balanced.

The servos don't contribute to the signal balance between the phases in any way. The servos control DC conditions, they don't affect the signal in any way.

Also they are looped one servo feedback loop per phase. Not connected.

Also everyone forgetting the two 56k that I believe help the opamps achieve zero offset before the headphones are plugged in.

They are dummy loads. They are there for stability and to prevent pops in turn on and turn off conditions and like you said plugging the headphones. Not relevant in any circuit analysis.

Yes, I agree and why Electrolytics are optimal in PSU stage.
So I was wondering why would you put film caps in your PSU to try hear any difference.

Indeed. I used film caps just because I happened to have them (I bought them for a +800 or +900 V supply, so for the voltage rating). I did not expect to hear any difference, for reasons I explained earlier.

A lot of people use very expensive film caps in PSUs because "they are better". A smarter and a lot cheaper solution would be to understand electronics and design, and design around the problem; such as not having the PSU in the signal loop.

If you throw a rock up into the air, and close your eyes, you don't have to see it fall down to know it is in fact going to fall down.

A cap that doesn't charge and discharge in accordance to the signal doesn't affect the sound. One doesn't need to try this out to understand it.

If it happens to affect the sound, then you know you do not in fact have a constant current draw amplifier.

Too much PSU & cap rambling in your posts so whatever cap works for you is fine with me.

I'm sensing I'm going off topic and losing your interest. I'm sorry.

Not shure why you explaining so much about the caps though

Well, when I was trying out different caps for my traditional DIY stuff (mostly single ended and some old timey push pull amps) I was very much interested in how and why the caps seemed to do what they did. General knowledge is applicable to many situations, including modding your amp. This could be off topic though so I'll stop.

"...to big film caps in the PSU would be to add even a simple 2 FET regulator..."
Interesting ideas but hard to follow your point because it seems you are not referring to our amps.

Your amps could have regulators in the PSU. At least designwise, maybe not in the space that is available. Line between DIY and modding is drawn in water. I will of course respect the purpose and confines of the thread so please just say if it's of not enough relevance.

And we already have 2 power transistors operating in our PSU.

Yes. It is a voltage setter, not a regulator.

This is Interesting for other designs and should be made more clear that, that is what it is.

It could be that none of this stuff can be made with the space available or with the PCB. It is for general interest and increased knowledge on how your amp works. If it's off topic I'll leave it alone.


[qoote]So then, what alternatives can we implement for coupling?[/quote]

Output from input stage -> cap (of maybe only modest quality) -> source follower -> direct coupled to output follower grid

This is probably not practically possible in the available space. Anyway if there is interest I'll elaborate.

 

[MrCurwen]

So, you think the 6SL7 and 6SN7 gets same amount of current in same type of circuit? That they bias exactly the same? I'm thinking that's not the case. They're very different tubes spec wise. The only thing they have in common is plate voltage (for typical operation) and same pin-out.
Perhaps @MrCurwen
 knows something about this?

Misusing equipment by installing tubes that it was not designed for is possible in circuits with autobias i.e. cathode bias done with resistors and using a resistor as a plate load.

Let's say you have such a circuit set up with 6SN7 and 10mA bias current. The plate resistance of the 6SN7 is part of the equation that establishes the op point (ie. plate voltage & plate current).

Once you change the tube to another type (or, even, another piece of the same type with +-20% variance in specs!) the plate resistance of the tube in the circuit is different. So the equation works out differently, and the plate current is not the same anymore.

If the plate resistance is substantially larger, the plate current will drop substantially.

This kind of misuse (or misdesign) might be damaging to the tubes, or it might not. It might result in exiting new kind of distortion for the effects-seeker, or it might not. If the original stage was horribly designed, it might even lead to an objective improvement.

If you have a system that doesn't have this autobias function (ie. set plate voltage, fixed grid bias) you can not do this kind of thing. There will be smoke and sparks. The op point is fixed, it's set.

With the resistors on the plate and the cathode, ohms law will do it's thing. This might be enough to land the new tube into a safe operating area, or it might not. Depending on all the variables (resistor values, B+, tubes in question etc.).

Do you understand load lines SonicTrance?

 

[coinmaster]

the more dominant the tail impedance is, compared to all other impedances in the current route, the better the balance

So if we used a very high value resistor between B- and the the cathodes it would make it more balanced?
 
Let's say I ran a resistor between B- (-100v) and the cathodes using the 6SN7 with the goal of 10ma@150v. 
 
I would need a 10300 ohm resistor on the cathode and a 7k resistor on the anode. This works well in spice, however if I wanted to use a plate resistor that is 4.5x plate resistance then all of the voltages would go negative except for the grid because the plate resistance would be too high to allow that current to flow in series with the plate resistor.
 
The only solution that I see in this situation is to increase the B+ to around 500v or something similar. Or obviously a current source since they are effectively an "infinite" impedance but a current source, at least your version, probably isn't a good idea as a general mod here. It depends on how far people here want to go into circuit creation and alteration, if they even have the space for it.
 

 the branches from the shared cathode node (top of the tail) must be as closely matched as is possible

I assume you're talking about wire resistance?
 
Quote:

Yes. It is a voltage setter, not a regulator.

It's a series pass regulator by definition.

Never heard of a voltage settler and google hasn't either, is that some slang?
 

Output from input stage -> cap (of maybe only modest quality) -> source follower -> direct coupled to output follower grid

This is probably not practically possible in the available space. Anyway if there is interest I'll elaborate.

Probably doable, all you'd have to do is solder in the source pin where the cap leg was and solder the cap to the gate with a resistor and some wire. Should be a relatively easy and safe mod, there should be enough space even in a build like max is using. Although I imagine max would dread the thought of it given all that hot glue and 3D architecture he already has in there 

 

[SonicTrance]

If you want forced balance, the tail impedance must be AN ORDER OF MAGNITUDE bigger than the COMBINED impedances of both the branches.

Also the plate load impedance should be the dominant one in each branch; 50 or so times greater than the plate resistance. This plays the tube variance out of the picture completely; increases fidelity significantly. 

Hmm, we have 220K Ra in our amps, which is spot on 5 times 6SL7 44K. However, if we use 6SN7 it's about 31-32 times the internal Rp of the tube.
 

Misusing equipment by installing tubes that it was not designed for is possible in circuits with autobias i.e. cathode bias done with resistors and using a resistor as a plate load.

Let's say you have such a circuit set up with 6SN7 and 10mA bias current. The plate resistance of the 6SN7 is part of the equation that establishes the op point (ie. plate voltage & plate current).

Once you change the tube to another type (or, even, another piece of the same type with +-20% variance in specs!) the plate resistance of the tube in the circuit is different. So the equation works out differently, and the plate current is not the same anymore.

Yes, I'm beginning to understand this now.
 

If the plate resistance is substantially larger, the plate current will drop substantially. 

Yes, I recently modded my amp to give the 5998/421A output tubes more current by reducing the cathode resistor value from 330 ohms to 165 ohms. With the lower cathode resistance the current doubled. From 30mA to 60mA (same amount of current that the 6AS7 gets with stock resistors)
However, I didn't change the plate resistors, only cathode resistors. We have another mod for the plate resistors. 
 

This kind of misuse (or misdesign) might be damaging to the tubes, or it might not. It might result in exiting new kind of distortion for the effects-seeker, or it might not. If the original stage was horribly designed, it might even lead to an objective improvement. 

Easy now MrC. That's like saying most of the head-amps that's discussed on this forum are horribly designed. Just look at the different tube rolling threads for different amps, you'll see that many prefer non-stock tubes with adapters and so on.
 
I'll give you an example.
I have four different types of Tung-Sol driver tubes. 6SL7, 6SU7GTY, 6F8G and 6C8G, all with same construction with the round plates and black glass. To me they all sound identical in my amp. Sure, I notice the increase in volume when using the tubes with higher mu, but that's it.
 
Also, the Mullard ECC35/33/32. I did a comparison of these tubes some time ago and found them to sound very similar in my amp. Yet, spec-wise they're very different. Perhaps the ECC33 for example would outclass the ECC35's with proper biasing. I don't know.  
 

Do you understand load lines SonicTrance? 

Yes, well for the most part. Honestly, I find that those graphs in old datasheets can get confusing and hard to read. But then again, I'm a newbie after all.

 

 

[coinmaster]

However, I didn't change the plate resistors, only cathode resistors. We have another mod for the plate resistors. 

The plate resistors on those tubes are too low to limit the current, it's only an issue if the plate resistor value is too high, such as my above example. As a general rule the cathode resistor sets the current.
 

Easy now MrC. That's like saying most of the head-amps that's discussed on this forum are horribly designed

In all fairness, they probably are. It doesn't take an amazing design to create an expensive product, a running theme I've noticed as I've looked around as popular amps.
A lot (most?) of the commercial tube amps run a generic, commonly known design that has been used for ages without any true innovation.
 
Like in the MK6 amp, it's just a basic resistor loaded SE input stage, it doesn't get any simpler than that. I have to admit, after modding the amp sounds mind blowingly good and I can't help but wonder whether MrCurwen has an idea of just how good it actually sounds, but from a technical perspective the input stage is not that impressive and the output stage is an ancient design other than the servo usage.

Yes, well for the most part. Honestly, I find that those graphs in old datasheets can get confusing and hard to read. But then again, I'm a newbie after all.

 

 

The horizontal numbers on the bottom are the voltage difference between the plate and the cathode.
The vertical numbers on the left is the current flow through the tube.
The curves indicate the voltage difference required between the grid and the cathode in order to achieve the desired current at the desired voltage.
 
So if you want 10ma @ 150v then you imagine a diagonal line between 10ma and 150v and see where that lands you on the grid voltage. This is the line that the signal will follow.
With a current source it will be a horizontal line because the current would remain constant. You always want your grid to be more negative than the cathode otherwise the positive grid potential will sling shot more current to the plate and possibly destroy the tube among other bad things.
 
I'm unsure what happens if you run a tube with positive grid voltages and still stay within the thermal limit of the tube since the charts don't have that data. However it does seem to be a typical theme that the more positive the grids are relative to the cathode, the more linear the tube is. In order to run positive grids you would need to use a source follower or something on the grids because the grids will draw current at that point.

 

[SonicTrance]

The horizontal numbers on the bottom are the voltage difference between the plate and the cathode. The vertical numbers on the left is the current flow through the tube.
The curves indicate the voltage difference required between the grid and the cathode in order to achieve the desired current at the desired voltage.
 
So if you want 10ma @ 150v then you imagine a diagonal line between 10ma and 200v and see where that lands you on the grid voltage. This is the line that the signal will follow.

Yes, that much I know. Was more referring to graphs like this one. Which can be messy and hard to read.

 

[baronbeehive]

  ..............
Like in the MK6 amp, it's just a basic resistor loaded SE input stage, it doesn't get any simpler than that. I have to admit, after modding the amp sounds mind blowingly good and I can't help but wonder whether MrCurwen has an idea of just how good it actually sounds, but from a technical perspective the input stage is not that impressive and the output stage is an ancient design other than the servo usage.
 
...............

I was wondering the same. It would be interesting if MrCurwen had a listen and told us what he thought compared to one of his amps for example. We can carry on with the component mods for a while yet and then perhaps switch to some circuit mods if possible. Obviously the circuit is the critical thing but the component changes have been positively compared by Maxx for example to some of the top amps out there I believe. Maxx may like to respond if I have made an error in my last statement.
 
BTW MrCurwen, please carry on with your theoretical posts, I don't know about the others but I don't see them conflicting with the aim of the thread at all and should throw up some good ideas.
 
Below is a pic of the finished mods on my amp carried out by the engineer who tested it. As you can see some rearrangement of one of two components had occurred with the board back but I can see that I could redo these to make everything fit properly including rerouting the wire below the wimas. Then with the bigger fans the cooling should e sufficiet, hopefully! It will be interesting to see whose amps are still working in a years time!
 
 

 

[MrCurwen]

Hmm, we have 220K Ra in our amps, which is spot on 5 times 6SL7 44K

Rp is dependent on the op point that is in use.

The op point that your amp has for the input stage 6SL7's is about 400µA at 85 or so volts Va. This op point has rp of about 75k quickly calculating from the curves. Real tube sections will have anything from 60k to 90k.

. However, if we use 6SN7 it's about 31-32 times the internal Rp of the tube.

If you look at 6SN7 curves, you wouldn't want to use it under 4mA or so. 5mA with a flat loadline is very good though. Depends on the load line, always.

Easy now MrC. That's like saying most of the head-amps that's discussed on this forum are horribly designed.

Well, I said I'm not going to be that guy so I guess I'm not going to defend my assertion further in this thread. But I will say this; if the design sounds better with another type, why didn't the designer use it in the first place? What does that say about the designer?

Just look at the different tube rolling threads for different amps, you'll see that many prefer non-stock tubes with adapters and so on.

Adapters don't necessarily mean you'll be using "another type" in the sense that I meant.

For example take 6SN7 and it's military rugged brother, 2C22 or 7193. They are basically the same tube, except different envelope and connections. 2C22 fits just fine into any circuit with 6SN7 despite having a bit lower rp (I don't remember by heart what the datasheet says, but all the 2C22 I've got are lower than 6SN7).

There are plenty of examples like these.

But if the tube is really dissimilar, one must ask again; why didn't the designer use that tube to begin with?

Of course some very simple designs can accomodate a number of tubes that have a similar enough op point available.

Yes, well for the most part. Honestly, I find that those graphs in old datasheets can get confusing and hard to read.

It's all about the curves. The secret life of tubes!

 

[MrCurwen]

Yes, that much I know. Was more referring to graphs like this one. Which can be messy and hard to read.

What's hard about it, looks fine to me? =)

Quite the pulse capability that tube has.

 

[coinmaster]

 It would be interesting if MrCurwen had a listen and told us what he thought compared to one of his amps for example.

Yeah, that would save me a lot of skepticism. Plus all the time I've been putting into tests of progressive technical improvement over the modded MK6, using the MK6 as a standard.
 

We can carry on with the component mods for a while yet and then perhaps switch to some circuit mods if possible

My tests will tell us what mods are worth doing. I'll be testing many different changes to the design including a variation of MrCurwens suggestions.
It's annoying how my project keeps getting put on hold by small things like the wrong size washer and such though.
 

Obviously the circuit is the critical thing but the component changes have been positively compared by Maxx for example to some of the top amps out there I believe

I can't speak for Max but I've listened to the flashship amps of Moonaudio, Hi-Fi man, Mcintosh, and some I forget the name and they all sound garbage compared to the modded MK6. I remember one of them costing over 10k and my MK6 still wipes the floor with it.
 
 

BTW MrCurwen, please carry on with your theoretical posts, I don't know about the others but I don't see them conflicting with the aim of the thread at all and should throw up some good ideas.

Indeed, this is good stuff and is filling in the gaps of my understanding, which saves me time and money, plus it is good for potential mods.
 
 

 if the design sounds better with another type, why didn't the designer use it in the first place? What does that say about the designer?

 

Cost I'd imagine, the MK6 is extremely cheap for what it is and so are the tubes they come with. Plus it's china 

 

 

[MrCurwen]

I was wondering the same. It would be interesting if MrCurwen had a listen and told us what he thought compared to one of his amps for example.

I was thinking about what route to take with this one.

I have in fact built "all of the classics" (not really all, but sufficient amount to know different most used topologies from the retro times), such as maybe a dozen or so circuits with the kind of input stage that this amp has.

I've done a number of things with tubes attempting to handle current like in the WCF output of your amp. An excellent analogy is the road I took with regs; I started out with different tube regs.

I've built a number of cathode followers, and also the Aikido.

While I have not heard your amp, I have built and listened to most of it's constituent components. I am sure that the amp does in fact "good sound", but I am as sure it is not high end as such. Of course all of these titles depend on what is the comparison.

The engineer always sees the weak points, and thinks of ways to improve them. The customer wants to think highly of his investment. These two can cause friction.

But, again, not everybody wants to or even can DIY a whole amp.


I'm not going to go on the route of explaining the transparency of my amp via words. I should probably just build a rugged build and mail it to someone in Europe. I don't know, sounds like a lot of trouble for small gain. The flagship amp is way too big to mail, but maybe one of the smaller ones with IDH tubes (6SL7 & 6SN7 and a current buffer)...

We can carry on with the component mods for a while yet and then perhaps switch to some circuit mods if possible.

Is the amp in one or two chassis'?

BTW MrCurwen, please carry on with your theoretical posts, I don't know about the others but I don't see them conflicting with the aim of the thread at all and should throw up some good ideas.

Thanks. Alright!

 

[coinmaster]

While I have not heard your amp, I have built and listened to most of it's constituent components. I am sure that the amp does in fact "good sound", but I am as sure it is not high end as such. Of course all of these titles depend on what is the comparison.

The engineer always sees the weak points, and thinks of ways to improve them. The customer wants to think highly of his investment. These two can cause friction.

I've been disappointed the majority of times I've tried a high end product, including thing's I've spent a lot of money on, like the HD800 and TH900 initially, plus numerous other things. I'm not afraid to call my investment crap if it is because I don't want to waste money on crap.
 
However, from an objective standpoint, after modification, the MK6 sounds unspeakably good, like my brain actually gets confused because it sounds like the singer is standing in the room but I cannot see them. After upgrading to my Duelunds alone all aspects of the sound quality increased hugely and the sound stage went from frontal to almost 360 degrees. The Bypass cap upgrade increased the bass quality 5-10x.
I cannot find fault in the sound it's full, it's fast, it's holographic, it's realistic, it's euphonic, yet still neutral, even with my HD800s the base quality and quantity is like wow.
Obviously you can't imagine a color you have not seen so maybe it is "bad" compared to what you have heard but I cant imagine it getting much better, I didn't even upgrade the WCF cap either before I tore my amp apart and that is supposed to be a huge improvement.
 
I'm comparing it to the standard of real life non-reproduced sound and it sounds in some ways even better than real life. However, there are soundstage and bass limitations due to it being headphones.
 

Is the amp in one or two chassis'?

One, some of us have been using wooden/metal extensions to create more space.
 

Thanks. Alright!

Yes, moar, moar.
 
At the very least the source follower mod on the grid seems easily doable without annoying complications and should probably be one of the first things looked into at this point, especially if we want to test the "good caps are better" theory.

 

[Maxx134]

Hey!
I dissappear for 2 days and now I have all this to read? !

Quote :
"BTW MrCurwen, please carry on with your theoretical posts, I don't know about the others but I don't see them conflicting with the aim of the thread at all and should throw up some good ideas."

We will plan to tune these amps further with these long winded discussions on tubes & bias.


I agree with MrCurwen about the attention our balance amp needs to be made optimal.
The design on driver stage is autobias and common cathode.
Driver tube Cathode resistance is low, for a different reasons like PSU voltage & input *edit "tube type" & impedance of the driver stage.

Yet for driver stage such small signal, we actually only looking at a tiny part of those big graph/curves..

So we need to put things into perspective.
Your driver tube is not going to change in tonality with volume adjustment,
and
A coupling cap in a high impedance part of a circuit is not going to a be a "bad choice".
Yet they will be more dependant on quality, to perform as intended.

So it's not the design as flaw,
As reading MrCurwen posts may imply.
But the cost cutting measures used to carry out the design objectives in our amp.

Yet,
The idea of designing out component dependancies is the clever/interesting topic that MrCurwen provides.

"Output from input stage -> cap (of maybe only modest quality) -> source follower -> direct coupled to output follower grid

This is probably not practically possible in the available space. Anyway if there is interest I'll elaborate."

Hmm ..
yes there is interest but let's keep the modest quality out and at least say "good quality" !
Lol

So..
A cheapo cap > to transistor > to the grid...
Blasphemy!.. Lol

I do not want any SS device in my signal path! (as a choice.)

Although I do not disagree the theory I don't know,
just the components.
Looks complicated :

http://www.bartola.co.uk/valves/tag/schade-feedback/

"Quite the pulse capability that tube has."

One of the many many reasons I have read tubes are a superior devices to SS.
That's another topic offtopic,
but a reason we like tube amps.

"At the very least the source follower mod on the grid seems easily doable without annoying complications and should probably be one of the first things looked into at this point, especially if we want to test the "good caps are better" theory."

It will probably be one of last things attempted,
Only because we are backed up with focusing on optimal tubes and bias now, and finishing overdue mods.

We are currently testing what works best, not just in theory.

Also, and as example,
is coinmaster's real world testing and discovery of an occurrence of " a sonic improvement", of "mixing powertubes", but only "in one direction".

You see I don't dismiss anything that is found in real testing.



So far I am establishing data about which sonic characteristics are being affected, and which sonic characteristics are predominant in each tube socket position.
Which is not easily explainable on paper,
But nevertheless easy to find out in testing.

So far, I have successfully tried and noted coinmaster observations as a true occurrence.
There "can" be sonic benifit of combining strengths of two slightly different tubes based on socket position,
But,
Having better tubes in general is optimal solution "in theory"..

So far have not done anything crazy, Only with same parameter tubes to decipher the "predominant" traits according to the "positive or negative signal" socket position.

Further testing is needed so I am in process ordering more tube types.

"I can't speak for Max but I've listened to the flashship amps of Moonaudio, Hi-Fi man, Mcintosh, and some I forget the name and they all sound garbage compared to the modded MK6. I remember one of them costing over 10k and my MK6 still wipes the floor with it."

I made it a choice to never talk about such controversial topic of which tube amp actually sounds best, as it is non-productive.
Simply because they all have different levels of euphoria and realism.

People like to have choice.
I chose this amp as a hobby.
I wanted a fully balanced tube amp.
Although I can afford a top tube amp "now",
I have no reason to with this one.

So far,
I what "best" amps I did hear were similar in high realism,
yet different tonalities...

Also, of interest to me, is Soundstage...
They all have great separation,
But I found Soundstage to be the key in separating the best amps from the others.

So far I must say this amp always had great Soundstage,
But the latest sonic improvements I have noted in tuning powertube bias & tube selection was the "realism within" the soundstaging...
The "holography"...

It is relying heavily on the fact that the input is perfectly balanced. And, of course, tube selection...:

Exactly, and is testament to the designer that is works this well..

Actually, driver tube selection is not even a choice,
As only one dualbtriode tube per channel makes it already up to the circuit to balance the two triode sections within ONE tube,
which are by default already close to matching being identical within the same tube,
and so should be easier for the common cathode tail to achieve.

"MK6 is extremely cheap for what it is and so are the tubes they come with. Plus it's china "

The Chinese market has much competition and a lot of very beautiful looking designs.
Yet they all seem to have their main goal on price over anything else.

I have in fact built "all of the classics" (not really all, but sufficient amount to know different most used topologies from the retro times), such as maybe a dozen or so circuits with the kind of input stage that this amp has.

Impressive! but wondering,
have you built a balanced tube amp?
Not many balanced amps out there.
I realize it is mostly a customer preference, but
This is the only balanced tube at the thousand mark in cost.

"I should probably just build a rugged build and mail it to someone in Europe. I don't know, sounds like a lot of trouble for small gain. The flagship amp is way too big to mail, but maybe one of the smaller ones with IDH tubes (6SL7 & 6SN7 and a current buffer)"

That does sound excellent. .
The "thread starter" is in Europe
and we had played around with the idea last year,
about what topologies would be of interesting for a future project.

But so far, these MK6 & MK8 amps keeps getting better with mods,
so I stopped looking for at least another year to enjoy..

 

[Maxx134]

..
Below is a pic of the finished mods on my amp carried out by the engineer who tested it. As you can see some rearrangement of one of two components had occurred with the board back but I can see that I could redo these to make everything fit properly including rerouting the wire below the wimas. Then with the bigger fans the cooling should e sufficiet, hopefully! It will be interesting to see whose amps are still working in a years time!

Very nice!
have you tested it yet?

 

[coinmaster]

just the components.
Looks complicated :

The source follower grid drive mod would be simple and easy to implement unless MrCurwen has some fancy pants version of it.
I would be extremely interested in someone trying this out because I am very interested to see if the claims that a source follower does not effect sound are true. Source follower have often been said to sound better than cathode followers as well.
I won't be able to test this my self until next month since I've realized that I'm missing about $100 in components I need for my power supply and I can't pay for it atm.
 

but a reason we like tube amps.

Technically it is current that drives headphones and transistors are as good at current duties as tubes as at voltage duties.
 

It will probably be one of last things attempted,
Only because we are backed up with focusing on optimal tubes and bias now, and finishing overdue mods.

Force balancing tubes is an even worse idea to try first. If we want to use 4.5x resistor value of plate resistance then that doesn't leave us an easy option with our 6SN7 either.
 

 
So far I am establishing data about which sonic characteristics are being affected, and which sonic characteristics are predominant in each tube socket position.

Every aspect of the sound can be affected. One socket position can sound bland and boring and the other can sound lively and realistic. It's best to try with as many different tubes as you can because the variation in improvement is large between them. Since the signal must travel between both tubes I suspect it is shifting the over all linearity of the "curves" to the highest impedance of either tube at any one point on the load line.
 

 but maybe one of the smaller ones with IDH tubes (6SL7 & 6SN7 and a current buffer)"

I'm curious to how those sound, it seems like a common cathode input stage with a simple current buffer is a really good design for its simplicity.