Not to side track on your troubleshooting,
But it is funny how the two amps (MK6 & MK8)
Reverse roles when modding for TOTL level...
Best tubes for MK6 remain same as if not molded, and can be quite expensive for the power tubes. .
While best tubes for MK8 reverse from the very expensive 6n30dr supertubes,
To the ultra low costing 6n6p tubes talked about here :
http://www.lampizator.eu/UPGRADE/upgrade noval tubes to 6H6P.html
Those tubes I run at a slightly higher bias...
For the driver stage, the MK8 is the same circuitry as the MK6, except for using 9pin tubes (probably for looks).
So I after alot of testing, switched to MK6 choices of using 8-pin driver tubes...
I finally settled on the 6C8G (6SL7 equiv) type for driver stage:
And this is after trying many many many 9pin and 8pin varieties of types, not just brands...
For this amp driver stage, the best 8-pin always were better than the best 9pins.
I have tested as many driver tubes as I could before I blew out my transformer heater coil output winding to get that opinion.
So just recently I went online to see if any correlation to this,
And I found out that my findings actually did correlate to what was known already in general about some 9pins and Octal pin tubes.
Specifically the tubes used for these amps driver stage...
Hey, great work Maxx! These are pretty cheap too!
Which ones do you like, KenRad, RCA, National Union... not the TungSol's at $2000 I hope .
As soon as the amp is up and running......
I am finding out a right combination is best...
Like brighter driver tube will liven a thicker powerube down the line...
So its a balance .
I'm struggling to understand this. I've looked at the circuit diagram. I've also taken readings from every component that can be reached in the circuit.
There is another way...
If you really want to see if it is your opamp,
Just swap that specific one which controls your defective stage,
To the other channel on the same stage (positibe or negative stage)
Then try and see if the meters are different .
Maxx good to hear that you like the 6C8G. I have bought few pairs of them. I did not have an opportinity to test to many types of the driver stage tubes. So hearing from you that I just have done one of the best choices make me feel good
. Very simiral are the 7193 KenRad with adapter. They are faster and tighter than 6C8G but less musical. Recently I have 7193 pluged in.
And
The 'king' of the 6SL7 / 12AX7 high mu high rp product line was the special quality 6SF5. It's got pretty much perfect 4P1L level curves, and a mu of 100. The top cap can be a pain though, I personally never got it completely quiet. Pretty tube though.
I am specifically not dealing with single triode tubes,
because for me the only gain would be more linearity(non issue becaue, as MrCurwen stated we have feedback),
And the slight improvement from possible isolation of triodes.
The issue is that in this driver circuit it doesn't really apply because we are still within the same channel.
So the theoretical & possible gains of triode isolation would result in maybe better noise cancellation ,
wich is already non detectable..
And in this specific amp " balanced " design,
the driver tube actually NEEDS the two triode sections as similar as possible , so within the same tube glass,
is actually a better chance of uniformity.
Better still would be a driver tube with common cathode.
I tried some but haven't found any noticeably good ones.
Of course in your application this is not very important; as a follower inside a NFB loop. The low Pa puzzles me though.
Yes, you are correct as usual lol.
I am glad your interest about the tube.
Yes there is lower power output, although I am running them near max bias.
What I found was that although all these type tubes have the annotation of "P" for pulse,
Which is an advantage for increased power output..
In my trials upon listening, the music dynamics & on transient attacks, it seemed exaggerated ..
I found that the music dynamics seemed artificially expanded in minute levels at times.
Of all the 6n6p or 6h30 or 6n30dr types I tried,
The bottom line was the russian 6n6p (oldest version without the gold grids)
Were the most natural sounding... In this amp.
They had both the most natural, realistic and most holographic sound of the bunch... Even over the "supertube" version.... All hype.
But you are absolutely correct in that the 6n6p are a weaker tube for power output.
I lost a few DB of power as a result in my amp.
Remember since we are powering headphones , my application is not as critical to have lots of power,
So I still have just enough power on tap for my HEKV2(20ohm) to be acceptably loud , but not tremendous...
And my HD800 can play tremendous loud still.
I have the amp on high gain as well, with higher bias on both driver and power stages, and using the higher gain type driver tubes.. All to help with "gain".
It just all works out, lucky for me.
Also the increase MU of the 6SL7 type or 6C8G type , for me gives more life and detailing , wich would or could be lost in any non-optimal circuit or bias situations..
Remember the stock design is using a super low bias point which is already not optimal..
So I prefer the choice of a higher gain driver tube .
.
