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.
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.
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.
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.
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.
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.
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!
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!
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 (I’m 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, you’ll 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 you’ll 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?
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 (I’m 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, you’ll 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 you’ll 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?
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.
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.
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?
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?
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.
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.
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.
pm
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.
pm
Mach1,
Thanks for some alternate perspectives. I was hoping to provoke more discussion by “tossing a turd into the punchbowl”. Like I said, there will be exceptions to my generalizations.
I can’t comment on Thorsten’s “bee”; I have had differing views from his on a few audio topics, although I do respect his technical ability, if not his cocksureness. I haven’t used a 5842 as a driver, but I have used it in the first stages of RIAA and line amps where signal levels are smaller.
Tubes such the 5842/417A have a high figure-of-merit: having both high gm and at least moderately high mu. To achieve the tube’s gm potential, plate currents must be pretty high. High plate currents and high mu translate into small values of Vgk, where grid currents can then become an issue. I’m not referring to operation in Class A2 where the grid is driven more positively than the cathode; I’m referring to the Vgk region between 0 and -1.3 volts (or so). In these cases, we’re operating on the edge of incipient grid current, and at the transition point from negative to positive grid current phenomena. Grid signal excursions will cause the grid current to change at various points of the waveform. Especially under large voltage swings such as you’d see in driver duty, additional distortion can be expected, even if Vgk always stays negative. Of course, this is dependent upon the source impedance. I think that grid-current-caused distortion needs more airplay. Grid current effects might explain Thorsten’s findings. Any one of several other tube types would occur to me for driver duty before the 5842.
On the subject of low mu tubes, as an example, I have used the 12B4A with plate load resistors in the 6.2K to 8.2K range. The sound quality of the 12B4A can be quite stunning with these values, as long as plate current is pretty rich (20 to 30mA). 6.2K may seem low, but if the plate resistance is about 1K, the ratio Rl/rp = 6, which is just adequate in my view. For perspective, a 12AX7 set up with Rl/rp = 6 would need a plate load resistor around 400 to 500K, which is rarely seen. BTW, is it any coincidence that low mu tubes also have large negative values of Vgk?
I haven’t read SP14 'Feral Eye' article. Can you summarize it in a sentence or two?
Very often tubes will sound more “tubey” at lower currents, IMO. This character might be due both to higher levels of second-order distortion and to lower bandwidth. Guitar amp designers certainly understand this. I do think that many audiophiles prefer that sound for “hi fi” amps. If it makes them happy, that’s great. When people make claims that a design choice “sounds better” (like THAT never happens) I’m never sure if I would agree with them, since they may define “better” in a different way than I do. I do prefer a more neutral perspective, and in general, I get that with richer current settings in triodes.
Thanks for some alternate perspectives. I was hoping to provoke more discussion by “tossing a turd into the punchbowl”. Like I said, there will be exceptions to my generalizations.
I can’t comment on Thorsten’s “bee”; I have had differing views from his on a few audio topics, although I do respect his technical ability, if not his cocksureness. I haven’t used a 5842 as a driver, but I have used it in the first stages of RIAA and line amps where signal levels are smaller.
Tubes such the 5842/417A have a high figure-of-merit: having both high gm and at least moderately high mu. To achieve the tube’s gm potential, plate currents must be pretty high. High plate currents and high mu translate into small values of Vgk, where grid currents can then become an issue. I’m not referring to operation in Class A2 where the grid is driven more positively than the cathode; I’m referring to the Vgk region between 0 and -1.3 volts (or so). In these cases, we’re operating on the edge of incipient grid current, and at the transition point from negative to positive grid current phenomena. Grid signal excursions will cause the grid current to change at various points of the waveform. Especially under large voltage swings such as you’d see in driver duty, additional distortion can be expected, even if Vgk always stays negative. Of course, this is dependent upon the source impedance. I think that grid-current-caused distortion needs more airplay. Grid current effects might explain Thorsten’s findings. Any one of several other tube types would occur to me for driver duty before the 5842.
On the subject of low mu tubes, as an example, I have used the 12B4A with plate load resistors in the 6.2K to 8.2K range. The sound quality of the 12B4A can be quite stunning with these values, as long as plate current is pretty rich (20 to 30mA). 6.2K may seem low, but if the plate resistance is about 1K, the ratio Rl/rp = 6, which is just adequate in my view. For perspective, a 12AX7 set up with Rl/rp = 6 would need a plate load resistor around 400 to 500K, which is rarely seen. BTW, is it any coincidence that low mu tubes also have large negative values of Vgk?
I haven’t read SP14 'Feral Eye' article. Can you summarize it in a sentence or two?
Very often tubes will sound more “tubey” at lower currents, IMO. This character might be due both to higher levels of second-order distortion and to lower bandwidth. Guitar amp designers certainly understand this. I do think that many audiophiles prefer that sound for “hi fi” amps. If it makes them happy, that’s great. When people make claims that a design choice “sounds better” (like THAT never happens) I’m never sure if I would agree with them, since they may define “better” in a different way than I do. I do prefer a more neutral perspective, and in general, I get that with richer current settings in triodes.
Brian Beck said:
I'll admit to being one who prefers lowish currents. Every tube I've tested save for one demonstrated falling higher harmonics with decreasing current and rising B+, within reason. A Shushang 813 follows this pattern until the Vp is over 1 kV. It's true the second rose in conjunction and I can't get a handle on whether the sound is due to that or the first. However re: bandwidth last I tested a CCS loaded 6C45 at my preferred ~5 ma it was still flat - too flat - at 200 kHz. Hammond iron limits the top end of the same tube directly driving the 813's grid. The current driver in my daily listen is a 9004 (955 equiv) at < 4 ma and the frequency response is limited by the 96 kHz sampling of my sound card.
This is in the context of power amp drivers however and probably not applicable universally.
But maybe circuits can sound less tubey with a driver tube at lower currents. I would think that the lowest distortion point for a stage is not always the lowest distortion point for the the amp, due to cancellation effects. It seems to this neophyte that generalizations on single tube operation points can only really be made for stages, not amps, and only then limited by qualifiers such as lowest distortion, greatest swing, etc.. Sy's analogies re racing cars seem apt.Brian Beck said:
Sheldon
Well, Brian, you proved my point about tailoring the operating point to the application. The "run it hot for linearity" rule is why the 12AT7/ECC81 has such a poor reputation.
Hi Sheldon. An age related failure to differentiate orange from yellow once resulted in a 10 kohm grid resistor ahead of an output stage. The amp's distortion plummeted due to cancellation, <~0.15 % THD @ 1 watt for a NGFB SE EL84 triode. Unfortunately that 0.15% was almost equal parts 3rd, 4th, 5th and 6th. With driver properly loaded the 2nd increased an order of magnitude, third was about 0.03% as I recall and no harmonic above the fourth was over -100 dB, or 0.001%.
That raises the big question of what is the best definition of 'least distortion', or perhaps least audible distortion. Possibly wrong but I've consistently ignored the 2nd in single ended stages and worked at reducing everything higher, and been happy so far with the results. Incidentally, some of the common operating points for various tubes have generated a disturbing number of harmonics on my bench. 2nd might be low but visible harmonics past the 20th of a 1 kHz fundamantal still visible, the KT100 one example I recall.
That raises the big question of what is the best definition of 'least distortion', or perhaps least audible distortion. Possibly wrong but I've consistently ignored the 2nd in single ended stages and worked at reducing everything higher, and been happy so far with the results. Incidentally, some of the common operating points for various tubes have generated a disturbing number of harmonics on my bench. 2nd might be low but visible harmonics past the 20th of a 1 kHz fundamantal still visible, the KT100 one example I recall.
Fair enough rdf, wouldn't dispute anything you say. I was careful not to generalize, only point out what might occur. Fortunately, with programs like RMAA and computers, it's pretty easy to measure the outcome and see the results of changing the operating points.
I like Steve Bench's site. http://members.aol.com/sbench101/
He has lots of circuits with unconventional operating conditions.
Sheldon
I like Steve Bench's site. http://members.aol.com/sbench101/
He has lots of circuits with unconventional operating conditions.
Sheldon
Good lord Sheldon, dispute away. I'm no authority. Being demonstrated a blockhead is a normal learning process for me.
It's just my experience. 2nd cancellation may be a valid technique in certain special circumstances but it's well beyond my skill to make work cleanly and, more difficult still, over a wide range of operating levels single-ended.
It's just my experience. 2nd cancellation may be a valid technique in certain special circumstances but it's well beyond my skill to make work cleanly and, more difficult still, over a wide range of operating levels single-ended.
Good catch on the inprecise language. I should have said; "I wouldn't dispute what you said in the previous post". Of course, I reserve the right in the future to dispute anything, including anything I've said. 😀
Sheldon
Sheldon
Figured I'd resurect this thread and see if anyone would like to ad anything else.
Lately I've been using quite a few 12bh7's with different operating points.
What seems to work out fairly well is a 30k plate load, 8ma and 170v on the plate.
Straight grounded cathode voltage amp.
Any comments?
Then trying to use them as LTP phase splitters get's a little trickier to get same values.
Been running them again with a 30k plate load 4.7ma and 200v on the plates, seems to work well.
(Driving push pull UL KT-66)
Not sure what properties are more important for this application, neg grid volts are 3.5.
Lately I've been using quite a few 12bh7's with different operating points.
What seems to work out fairly well is a 30k plate load, 8ma and 170v on the plate.
Straight grounded cathode voltage amp.
Any comments?
Then trying to use them as LTP phase splitters get's a little trickier to get same values.
Been running them again with a 30k plate load 4.7ma and 200v on the plates, seems to work well.
(Driving push pull UL KT-66)
Not sure what properties are more important for this application, neg grid volts are 3.5.
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