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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
12BH7Any 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 6DG8, 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.
Follow Ups:
6DJ8 and 6922 are the same tubes. 6J5 is half a 6SN7 in octal config and 6CG7 is basically the same them in 9 pin.Tubes should always be operated in their most linear region according to their plate curves and always (to a greater or lesser extent) operate better when lightly loaded ie higher plate resistances. This is why I prefer CCS, but plate chokes and IS coupling can achieve similar results. Distortion is greatly reduced in this mode of operation eg can come down from a few percent to a fraction of 1% (even <0.1% without feedback).
Now, generally biasing tubes to higher currents lowers distortion and gets the best out of the tube in terms of lowering rp and increasing gm and this is not only highly desirable but sonically pleasing. However apart from the obvious downside of tube life, in certain circumstances such as in SE driver applications, lowering the bias to increase the distortion of the driver stage can actually improve the overall result at the amp’s output both sonically and measured, through distortion cancellation.
So, the variables are many but to my mind the circuit type and the loading are the key parameters assuming of course that the circuit has been properly designed for output swing etc. After that bias can make a considerable difference and can be used to fine tune for a desired result. This is all about proper selection of the of operating point, which must be chosen for the application.
My 5 cents worth ... FWIW.
Best,
Naz
This seems to be the common advice/practice, but I often think it a bit wasteful to use something like 10 ma for just a preamp stage, compared to only 1 ma or so, as often seen in equipment using 12A*7 type tubes.What I wonder is whether it makes all THAT much difference to the final sound. I guess I should set up some kind of experiment to see for myself :-) Maybe compare a 6SN7 at 2 or 3 ma to one at 10 ma...
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or little, depending on other factors, particularly P-P voltage swing and plate load. Generally, higher currents tend to provide a fuller, smoother sound and this is true of pre's and power amps. However, there can be exceptions depending on the circuit position, ie which stage. I've found that grounded cathode driver stages in SE amps sound considerably smoother/sweeter at lower currents due to distortion cancellation with the OP stage ... the OP stage being the dominant contributor, particularly of 2ndH. Conversely, higher currents generally sound better in OP stages, but this depends on overall system balance, particularly speakers.Naz
Good info NAZ, thanks!
This could fill volumes! The two cases call for completely different plate loads. Feedback will virtually change every component value in the circuit. Ray Hughes
"I take you as you are
And make of you what I will,
Skunk-bear, carcajou, bloodthirsty
Non-survivor.
Lord, let me die but not die out." THE LAST WOLVERINE by James Dickey
Didn't realize with and without feedback made a difference where you want to run the tubes, humm!Learn something new everyday!
One way you'd want to run the current aproach and one the voltage aproach?
Ideally, you should shoot for very good performance without feedback with the idea that you might use feedback sparingly, to correct ringing or some fly in the ointment, preferable interstage rather than overall from output back to input. The other side of the token is that feedback is kind of like turbocharging applied to an engine. If the engine is not designed for it, the engine might blow up or suck the whole turbocharger into it. Global overall feedback to an amplifier won't cause it to blow up just make the sound SUCK! Feedback multiplies odd harmonics expecially as the amp approaches clipping. At clipping all hell breaks loose distortion wise! Learn to plot characteristic curves of triodes and choose the most linear point on the plot. Looking at them, all the grid curves should be equally spaced and climbing, if the tube is linear. The 0 grid voltage shouldn't erradickly verr off this climb but remain steep. Tubes can tolerate a tremendous range. Norman Crowhurst wrote alot about this and his reprints are avaliable from GLASS AUDIO if not online. I limit my use to two preamp tubes and I have a lifetime supply of them; 12ay7 and 6sn7. I can do everything I need to do with these two tubes. Good luck! Ray
"I take you as you are
And make of you what I will,
Skunk-bear, carcajou, bloodthirsty
Non-survivor.
Lord, let me die but not die out." THE LAST WOLVERINE by James Dickey
Thanks for the input.I understand the plots and how to use them, it's just you can plot many
different ways and still be in curves, I'm just wondering if theres one
place that's better then others and is it different for certain tubes.Curious the 2 tubes you mention, where do you normally like to run them?
12ay7/6072A, 6sn7GTB, 12B4A is a good output triode operated SEClassA (it's difficult to match them for push pull because they are all over the place like a damn transistor), 6AX5GT and a 6DA4A as rectifier. 866A for high voltage rectifier. The 6au6A/7543 & 6BH6 for tuner service and if I could pick power pentodes, the 807 and 813 transmitting pentodes. Also of course, the 211 and the 845 and 838 transmitting triodes. If I was looking for a high transconductance tube to play with I would choose the 6J4 (an old Navy radar tube with gold parts) Ray Hughes
"I take you as you are
And make of you what I will,
Skunk-bear, carcajou, bloodthirsty
Non-survivor.
Lord, let me die but not die out." THE LAST WOLVERINE by James Dickey
All good info, thanks.Tubes I generally like to use/run/find are 6SN7, 6J5, 12BH7 (One of my favorites)
And for output tubes I really really really like the 6BG6G, looks great, sounds great
and cheaper then dirt, nobody likes using top caps, that's fine by me though anyway! :)
The choice of cathode load affects equivalent input noise on the 12ay7. I generally pick cathode and anode loads which gives the lowest self noise with the voltage gain I like in my circuits. If you cascade two sections into one then all this changes noise wise. It's difficult to change anything without affecting something else. Your case may be different. 12ay7 is a voltage amplifier and the 6sn7 is more of a current sink or current amplifier, it just depend on what you are trying to do. The 6sn7s, like a lot of voltage on the plate, 425 volts is typical. Ray Hughes
"I take you as you are
And make of you what I will,
Skunk-bear, carcajou, bloodthirsty
Non-survivor.
Lord, let me die but not die out." THE LAST WOLVERINE by James Dickey
Thanks Ray!At the moment I'm in a bit of a quandry as I have a pair of organ mono
amps with no phase splitter but have 2 octel sockets to build with and
only a 315v B+. They are a 6v6 ppp output, and I'm thinking with this
low a voltage going to a 9 pin socket and using the 6DJ8 in the schmidt
type phase splitter as it should give me enough gain/drive with my active
pre's and the voltage of the amps about right for decent circuit with them.looking at around 37k plate resistors seems to give decent voltages and
currents to work with, even in grounded cathode schemes.
Why not use a 12AU7? It is used very commonly as a phase splitter to drive tubes like the 6V6.
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I had messed with building the circuit with either the 12AU7 or the :)
12BH7 but then thought the extra mu plus voltage needs of the 6DJ8
might be a better fit with only having a 315v supply to start with though.It's just that yes I can make the 12xx7's work decent but thought the 6DJ8
looked to be a better fit and run more in there desired range then the others, no?
The 6dj8 doesn't have nearly the gain that a 12ax7a has and if feedback is applied then I doubt the 6dj8 has enough spunk to accomplish the feedback task at hand. Your amp would have to have a phase splitter of some type if it's push pull. You could use a 12ax7a as a high gain first stage and as a phase splitter combo (kill two birds with one stone). That's probably what the organ company designed in. Ampex used a similar circuit in their recording amplifiers. Ray
"I take you as you are
And make of you what I will,
Skunk-bear, carcajou, bloodthirsty
Non-survivor.
Lord, let me die but not die out." THE LAST WOLVERINE by James Dickey
Yah I hear yu with the feedback, was going to use the 6DJ8 without any.
And yes I understand I need a phase splitter as that's why I'm building
a new front end, the organ had the phase splitter in the preamp on the organ.So I'm just trying feel out a good way of doing this with only having 315 volts
B+ at my disposal, and figure the 6DJ8 would be about right for no feedback and that
voltage and get away with just one tube might be allright.
Email me, i3components@att.net
"I take you as you are
And make of you what I will,
Skunk-bear, carcajou, bloodthirsty
Non-survivor.
Lord, let me die but not die out." THE LAST WOLVERINE by James Dickey
It's puzzling me as to why you see that as a limitation. Most 12a*7 circuits work around 250 vdc. By the time you add a filter stage or two between the power stage (running off 315 volts) and the preamp, you'll be getting down towards the 250 - seems ideal.
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I'm not saying it won't work or even work pretty good.But I wanted to try and keep the plate resistors of a fairly large value,
if I do with the the 12XX7 series compaired to there plate resistance I
loose a good amount of voltage and current not to mention the -grid volts.But with a 6DJ8 and it's 1/3rd the plate resistance I can get right in the
sweet spot plus I get a little extra gain. I want to try all the 12XX7's
in here once I do it and see how they perform which is no big deal, just
rewire the heaters to accomodate but, I'm just thinking the 6DJ8 will be
a better fit is all. Am I off in that logic?
How large do you want the plate resistors to be? A 12AX7 working off 250 volts, at a recomended operating point, has a 100k resistor IIRC. Large enough?
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The SOP seems to be at least 2x the plate resistance so at least 150K.
Going that route and a B+ of 315 to start with would leave us around
160v on the plates and a -grid of about 1 and 1ma current draw.Those don't sound like to stellar of operating points to me.
The 6DJ8 family of tubes have a much much lower plate resistance which
leaves it well suited for a multiplyer of the plate resistance and in
turn some decent specs to work with on a B+ of 315.I just don't see anything with a plate resistance higher then say 7500
having a realistic chance of some good numbers in this situation.
And the lower the better we can dial it in.
No, you won't be running them off 315. As I said, normal RC filtering will get you down towards the 250 I was talking about. Then you'll get about 100 volts on the plate using 1 ma current with 150k resistors (or 1.5ma with 100k resistors). Trust me, they sound fine, and many manufacturers' circuits use them this way.
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I'll check out the links you got there, thanks.This goes back to the original post and why I posted it in the first place though.
I was hoping for some discussion on how people like to run there tubes and why.But it seems most are content to say "Use the curves" and nothing else.
We can all use the curves and get a tube in there but the way/variance
in how you can is pretty large, we could run with large numbers or small
and still be "In the curves" but I gotta feel there are some extremely large
differences in sound when run these different ways and do some tubes react
better then others when ran a certain way.
I mean I've seen quite a few times where it's been stated you go at least
4X the plate resistance as for the plate resistors to give the tubes extra
room to breath and others where the tubes are run at near 80% voltage
rating and 70% plate dissipation which is quite different from most of
the vintage pieces of equipment and how they did it back then, to where
some will say that's how the old school tube sound is produced but if
you want more linearity and frequency extension you run the tubes hotter
and use at least 4X plate resistors.I believe from all the different types of designs I see and how some pieces
are highly respected that all these different ways do work and tubes are
very forgiving, but I do still feel that with different operating points these
tubes will react differently and would certain tubes be better at the lower
side of things while others really should be ran on the hotter side and or
larger plate resistors, what about neg grid voltage on some tubes as a rule.Take the 6SN7 for instance, there seems to be quite the consensous that
these should be run high on voltage/current/plate dissipation and at least
45k plate resistors for them to sound right, if that is the case we need a
good amount of B+ supply to start with to get that accomplished.So what is it, just as easy as saying low voltage/current/plate dissipation
and smaller plate resistors is old school tubey sound and the opposite is the
new train of thought for better linearity and extension with being less tubey?If there is something to this and we can use this to our advantage when
we want to inject say a little extra tube magic in our systems maybe we
can do a little of both to have our cake and eat it to, not sure on that one.Anyway's that's where I'm at on the subject and generally like to run my tubes
a little hotter then some and use larger plate resistors to get the sound as to where
I have described above and why I think it's so, but am wondering what others think.I am open to what others have to say/think and take all oppinions with a grain of salt
including my own and allways check/question myself to what I'm really hearing and why.Sorry for the long winded post, Again!
Like I say I'm not saying this kinda tube won't work, I just gotta feel
a different tube would be better suited here.Also when you make those plate resistors so large the balance is quite
off and you have to use quite a bit larger plate resistor on the grounded
grid side which never really sits well with me.
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