MrCurwen
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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)
I'm sure most have heard me quote the old "NFB is trading gain for linearity". There is another level to this; within this linearity that is acquired via NFB. It trades lower overall THD to higher proportion of higher order products.
So let me clarify.
If you have a circuit that produces 1% THD consisting of only 2H and 3H and you apply gNFB, you then get lower (let's say 0.1%) THD, but the spectrum of harmonics changes so that you now have very small amounts of 4H, 5H, 6H, and 7H.
What this means to you is up to your ears and brains. Just something to think about. (I'd like to reiterate that "the SS sound" in my opinion is not caused by SS, but by NFB.)
To bring it back to the quote above, it's not completely irrelevant how inherently linear your gain stages are, even if they are inside a loop. It is mostly irrelevant, but not completely, and for BEST RESULTS HIGH END AUDIO it definitely is not at all irrelevant.
My personal opinion is that is is always best to not create any distortion in the first place than to try to fix it after the fact.
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.
For balanced SS circuits it is very important to have the devices heat coupled (same heat sink is best). For tubes, heat does change the specs a bit, but not as much as SS components. Having the tubes inside the same glass envelope is not important at all for the matching, especially since 99.99% of the heat inside a high rp low current tube is from the heater anyway, not the actual Ia or Pd.
I'd say single triodes are for best results in circuits where tube matching is critically important, because you cannot get double triodes with identical tube sections.
I don't think in practise this makes much difference in your circuit, but just a thought.
Better still would be a driver tube with common cathode.
I tried some but haven't found any noticeably good ones.
6SL7 has a shared cathode version (for radio duty) 6SC7.
http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6SC7.pdf
http://www.mif.pg.gda.pl/homepages/frank/sheets/127/6/6SC7.pdf
It has essentially identical specs and identical linearity.
Very nice metal envelope, JAN, super high quality for military: http://www.ebay.com/itm/1-x-Ken-Rad...894410?hash=item5b349f88ca:g:2mIAAOSwMTZWR08f
These are a bit more expensive than I'd like to pay for a tube, since they are common in early 1950's Tweed style guitar amplifiers. I love tweed amps, I simply use russian 6SL7 for sensible price. Quality just as good.
Russians also make a copy of 6SC7, it's 6N10 but it's not available for sale. Probably only a few were ever made.
Why do you think having the cathodes connected together inside the tube envelope makes any difference compared to connected on the outside?
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 .
Yep. My own philosophy would be "don't mess with the frequency response in the first tube stage, so you don't have to try to fix it in the second tube stage".
Yes there is lower power output, although I am running them near max bias.
What's your Ia?
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..
Kind of. Pulse capability doesn't increase RMS power at all, but it can (in a properly designed circuit) increase transient response which brings about the sensation of power in the listener.
In a circuit that doesn't speficically take into account the pulse capability, they will probably sound worse. Let's go into that:
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.
Yes. I find this easy to believe.
Back some time ago I wrote about the grid parasitics, with specific mention to high pulse capability tubes (such as these russian tubes and also TV sweep tubes which I've been dealing with).
Basically the bigger the pulse capability of the tube, the bigger the electron cloud hovering around the cathode. Now there electrons hit the grid, and they must find a way to escape then. The circuit around the grid must accomodate this.
Worst case scenario, there is a large resistor from grid node to ground (or other reference that is applicable). This trapped current must then go thru that resistor to escape; current thru resistor causes voltage (Ohm's law). So that shifts the voltage at the grid.
What is the voltage at the grid? It's the audio signal.
This grid current phenomenom happens a little bit on zero signal situations, depending on bias point. In high mu high gm supertubes like 6E5P and other frame grid tubes it most definitely happens even at zero signal conditions. This is because the frame grid is positioned very very close to the cathode.
But of course with signal peaks you understand is where most of this grid current happens. In audio terms it affects transient response.
Now your circuit is not feeding the grid from a low impedance source (i.e. a source that is able to deal with the current easily) so it is no wonder that pulse tubes cause transients to sound 'wrong'. What you are probably hearing is extra distortion on transients, this makes your ear catch them more, not in a good way.
They had both the most natural, realistic and most holographic sound of the bunch... Even over the "supertube" version.... All hype.
100% circuit dependent.
I find that pulse types (such as TV sweep tubes) make THE BEST audio output tubes, provided they have adequate grid drive. Source followers in practise. If the transients are clean, they don't catch your ear in a bad way, instead they make the music more 'lively' and dynamic.
My current workhorse amp has 6E5P output tubes. Incredibly dynamic and also superbly clean. Have a look at the triode curves, sure is a sight for sore eyes aren't they.
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".
I always tend to end up either with too little or way too much gain. Current amp has a voltage gain of 3500 (12AX7 -> 6E5P) which is simply stupid and an invitation for oscillation problems. I need my lab set up so I can build a new amp...
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 situations..
So I prefer the choice of a higher gain driver tube .
Yes all things being equal everybody prefers the louder one. That's why A/B tests need to be strictly volume matched.