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Preferred operating points of small signal triodes.

Posted by KEGGER (A) on August 17, 2006 at 09:37:12

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 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.

This post is made possible by the generous support of people like you and our sponsors:

Kimber Kable

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