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Old 17th August 2006, 05:47 PM   #1
kegger is offline kegger  United States
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Default Preferred operating points of small signal triodes

Preferred operating points of small signal triodes? Do's and don'ts?

I'd like to see if we could get a discussion going on how best to run these tubes.
And for now keep it based on the 20-30 gain varietys like the following tubes maybe.
(Would Just like to keep it minumized for now to get more focused discussions)

6DJ8
6922
6CG7
6SN7
6J5
12AU7
12BH7

Any other varietys in there I didn't mention that someone wants to expand upon?
If this goes well we could try an expand it to others or just start another thread.

Starting with the 6DJ8 (Any variants someone wants to expand on is good by me to)
This tube has the least amount of voltage it can tolerate and is usually run with a good
amount of current through it. Also the plate resistance is rather low at 2650 compaired with
the other tubes in this area with the exception of the 6922 which is basically a beefier 6DJ8?
Judging by sheets on this tube I come up with suggested plate loads of 10K-30K roughly?
And it would seem you really want about 90volts on the plates, ma of 10, neg grid of 1.3
or so in that area anyway? Are there any best for sonics suggestions on these #'s?

Now the 6CG7 is pin compatable with the 6DJ8 and is with others as well if you redo the
heater connections, but that's about all it has in common with 6DJ8 after that, as it has a
more liking of it's #'s to the rest of the tubes including the higher plate voltage it can take.
Plate resistance is right around the 7200 mark (given certain voltages) and it's plate load
would suggest in the 25k-45K range, 250 volts on the plates, ma of 8, and neg grid of 8?
Could run the lower voltage and high current of like a 6DJ8, suggestions for best sonics?
How would they run if a situation was made for them and a 6DJ8 to run in the same place?
(Meaning would it still perform sound wise when run at the lower voltage, high current)

6SN7, 6J5 basically the same values as the 6CG7 and how to run them? Of all these tubes
if you can't get the perfect #'s for your curcuit is there any do's and don'ts? Or anything you
should strive for if you have to sacrifice in some area, on all of the tubes discussed here?
(Yes you can keep them inside the posted plate curves but any issues in there anywhere)

12AU7 and 12BH7 pin compatable, with the BH7 drawing more heater current and a little
less plate resistance, 5500 verse 7200 but both would basically be at home with same #'s
as the 6CG7 mentioned above for plate loads, current, plate voltage?

Do we want to run most of these tubes up around that 250V plate voltage with the current
draw as suggested? How do they run at the lower plate voltage of say 100 and current that
is near 10ma with basically no neg grid voltage? Any one that you want to run harder?

I've been doing a lot of research and circuit design lately in spice format but find no definative
results of what is best or accepted for these tubes as far as propper operation and sonics go.

Any and all thoughts are welcome.
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Old 17th August 2006, 07:04 PM   #2
SY is offline SY  United States
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What's the best rpm to shift gears? It depends on what you're trying to do. Same here; there's no "universally good" operating point for a tube or everyone would do their circuits the same way.
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Old 17th August 2006, 07:26 PM   #3
kegger is offline kegger  United States
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OK is there any bad operating points, ones you should stay away from?

Also I've heard some say that 6SN7's can be microphonic when not run in a certain criteria.

So there's got to be some do's and don'ts know?

If were racing our car and speed is the thing then yes there are certain points were we should
shift for best performance.
I would think the same holds for tubes, if sonics are our choice then were should we run them
if that's what were after.
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Old 17th August 2006, 11:08 PM   #4
SY is offline SY  United States
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Quote:
Also I've heard some say that 6SN7's can be microphonic when not run in a certain criteria.
Since there are dozens of variations of 6SN7s, that doesn't really follow as a general case.

Operating points to avoid always would be those where the tube has excessive dissipation. For low distortion operation (hifi), you want to keep load lines as horizontal as possible. For an instrument amp, this may not be true. For a phono input stage, swing and linearity are secondary to noise. For an output tube or a driver, it's just the opposite.

Your racing analogy illustrates the point perfectly. First, you've already limited the field, so that considerations of fuel mileage and long term reliability have been excluded. But even then, the question is too broad. Straight line drag or a curvy track? Rally or Formula 1? Road or dirt?

Really, it's best to try to understand the concept of load lines, tube curves, and how to derive distortion and max swing; then, in any given design situation, you know what to do with the tube curves and a ruler, just like a pro driver will instinctively know what to do with the shifter and clutch in different driving situations.
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Old 17th August 2006, 11:16 PM   #5
kegger is offline kegger  United States
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Thanks for the post.

Understand the load lines and the curves, no problem there.

Not concerned with how long they will last.
Ultimatly the concern is with sound quality as regarded to hi fi use only!
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Old 17th August 2006, 11:27 PM   #6
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OK, so how about two big sweeping generalizations that will likely create flames? Being generalizations, there will be exceptions. Lots of them. So please don't get all hissy if you drool over the sound of your AudioEcstasy Model One that doesn't follow these guidelines.

1.) I don't think people run enough plate current through small signal tubes as a general rule. Many, if not most, small signal designs appear starved to me (Iím not addressing power stage tubes here). Good things happen at higher currents: transconductance goes up, noise drops, plate resistance falls, and mu usually becomes more constant (more linear). A 6DJ8 trickling along at 2 or 3 mA is a waste of a good tube, IMO. Data sheet currents are a starting point. How many people run their 6DJ8 at 15mA? Cons? Unless plate voltage is dropped by a corresponding amount, plate dissipation goes up and tube life will decrease. If you drop plate voltage far enough, youíll run out of grid range and into the grid current range starting at about Vgk = -1 volt (or less in magnitude). So, figure out how much grid swing youíll need, keeping the grid always 1 volt or more below the cathode (exceptions duly noted), and adjust plate voltage to keep dissipation at 60% to 80% of rated value. Enjoy life.

2.) Regarding the load line, most designs don't provide a high enough plate load. Triodes need Rl>>rp for the gain to become solely dependent on mu, and for it not to be dependent upon rp and gm. Mu is more constant (linear) than rp and gm, so that's a good thing.

Rule number 1 and rule number 2 do collide. If you run a lot of current AND want a large plate load, you have three choices: a high B+ voltage with high-value high-power load resistor (think wire-wound), a plate choke, or a low-capacitance plate CCS. Sorry, just tough it out. What price glory for audio DIY?
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Old 17th August 2006, 11:46 PM   #7
kegger is offline kegger  United States
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Thank you Brian that is just the sorta post I was looking for.

Not saying I agree or disagree but that is the kinda info I was looking for.

I am torn between the higher voltage less current verse the less voltage more
current approach and generally end up somewhere in the middle of those 2.

Not quite sure I follow where your at with respect to the plate load though.
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Old 18th August 2006, 04:46 AM   #8
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Three popular tubes that I think should probably be included in your list are the high mu trio: 6SL7, 12AT7 and 12AX7.

An aspect that can be discussed, as you say, is which operating conditions to avoid. For example, the 6SN7:

I'vre seen "good" advice, from those who should know, that 6SN7 plate current should be > 8mA, plate voltage should be > 200v and plate load should be 47k. The problem is, most people wouldn't want to have a B+ of around 600v so as to meet all of these conditions. So, it is quite common to see compromises on the plate current and/or voltage and/or load resistance in a typical circuit using 6SN7 as a voltage amp or driver. The question is, which of these compromises does the most damage to the sound (forgetting about OP signal amplitude or headroom for now)? Or would it be better just to choose a different tube?
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Old 18th August 2006, 06:44 AM   #9
kegger is offline kegger  United States
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Good point RAY I'm in that boat at the moment (with 6SN7, 6J5's)

With only having a supply of 425v on one that I can get decent #'s on.

Then on another rebuild only having 315v really gets me in trouble there.
(Problem is I have 2 octel sockets to design the front end with on here)

So I to wonder if I NEED to go another tube route or which way can you sacrifice and still get decent sound.
(With or without having to do a CCS)

Really that was the point of this thread, trying to determine where can you run tubes in a circuit whether it be
the perfect situation and you get the voltage/current you need/want or a situation that is not ideal for sonics.
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Old 19th August 2006, 03:31 AM   #10
mach1 is offline mach1  Australia
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No flames Brian, however I will play devil's advocate and articulate the case for the negative.

Thorsten always had a bee up his *ss about running the triode drivers on DHT SET amps at much lower currents than convention would dictate. I believe he favoured about 10mA for 5842.

Herb Reichart and the NY triode mafia recommended running low mu triodes with as low a plate load (see SP14 article 'Feral Eye') as possible to attain the best sound (in both resistive and inductively loaded cases).

Personally, I have no philosophical preference and prefer dialling in the current I like for any given circuit.

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