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Old 6th June 2003, 02:23 AM   #11
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Many dual tubes have two identical sections, like 6SN7, 6SL7, 6BX7. The 6EM7 has a high gain triode (like half of a 6SL7) and a low gain, high power one (like half of a 6BX7). The latter will make a nice low impedance cathode follower. You CAN'T get 7-10W out of it - that's the input rating, and efficiency is quite low.

6DN7 is more like half a 6SN7, half a 6BX7. 6EW7 is similar in 9 pin.

Dynamic is short for electrodynamic, I suppose - magnetic as opposed to piezoelectric (crystal).
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Old 6th June 2003, 02:26 AM   #12
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Yeah you can do away with all that gain, and a 'SL won't do enough current for real good operation...get that 6SN7 on the cans.

Lesse... let's say you want 10mW (far more than your ears could ever handle I'm sure), at 300 ohms that's 5.8mA RMS, or 8.2mA peak. So for class A, it needs at least that much bias... maybe 10mA at 200V. Below that you could use a 10k 2W resistor for the cathode load, and use 300V for +V.
Grid bias voltage will be -5V, which can be set with cathode bias or DC coupling to a preamp stage biased at 100V plate..

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Old 6th June 2003, 05:41 AM   #13
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So, you think a 6DN7 would be good enough to drive 300-600-ohm cans to sufficient levels? With headphones, I am not a loud listener, I mainly care about sound quality with my cans, which is why I bought these particular ones to begin with. My current amp is decent, but I feel I am missing something... somethign I may find in tubes, or at very least, a Class A solid-state amp. I may throw together an SS amp out of spare junk and see what goes.... but I am torn. I NEED a sub amp, but tubes are driving me much harder than getting my sub up and running... and I don't know if I want a tube power amp or headphone amp. Odds are, if I used 6DN7 (NOS $8 a piece from The Tube Store) I could get a tube headphone amp up and running for under $100 EASILY. Right now, that is looking best...

I am finding minimal info on this 6DN7 also- TDSL has something about it, but I am a tad rusty on some of the abbreviations, and others I just don't know... It seems like everyone recommends 6SN7 over 6SL7, so I would have to go with 6DN7 over the others to accomodate for this. So, when creating the schem, I should just treat it as a 6SN7 and 6BX7? And this will provide adequate gain and SPL for my cans? Hmm.... I shall go to work on this... Thanks in advance for answering any of the questions in there (there are lots, I know... just trying to grasp this whole concept) Thanks
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Old 6th June 2003, 05:48 AM   #14
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When I look at the Mu's and Ra's of these tubes... it almost seems like the 6EM7 is better suited... Sure, the first stage has a mu of 64, but the second is only 5 and has an Ra of 750, as opposed to the 2000 of the 6DN7. I am guessing Ra is output impedence? Am I remotely correct? Most of this is new to me... If I am thinking corectly there, then 6EM7 is looking better...
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Old 6th June 2003, 05:56 AM   #15
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You can download a full data sheet for 6DN7 from the usual site:

http://home.planet.nl/~frank.philipse/frank/frank.html

The medium-mu section isn't quite a 6SN7 type (it's mu is slightly higher and anode dissipation is only 1W), but it's quite close. You can use the medium-mu section as a gain stage, DC couple it to the low-mu section (passing 20mA, or more, if you can afford it), and capacitor couple to your cans. You could put some global feedback around the whole thing if you wanted to pull the output resistance further down, but I expect I'll be howled at for that suggestion!

6EW7 is even nicer, and it's rather pretty because it's the diameter of an Octal on a B9A base. I've measured them, and the Sylvania ones are very good.
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Old 6th June 2003, 06:17 AM   #16
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So, as a bare bones schem, we are looking at something like the attached, right? Just a standard cathode follower... I notice the output impedence of the 6DN7 is way too high, and that of the 6EM7 is higher than needed (750-ohm), but that could easily be fixed with NFB... as bad as that may seem to some... as long as it sounds "tubey" then it will be fine with me... I haven't really heard a good tube sound playing hi-fi stuff.

Quote: 6EW7 is even nicer, and it's rather pretty because it's the diameter of an Octal on a B9A base. I've measured them, and the Sylvania ones are very good.

Hmm... what exactly do they look like? I will search for this- it could be pretty... I also like aesthetically pleasing tube designs.

Thanks all for the help
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File Type: jpg distriode1.jpg (25.6 KB, 367 views)
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Old 6th June 2003, 06:31 AM   #17
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Here's a 6EW7 flanked by a 6J5GT (half a 6SN7) and an ECC88 to give you a feel for size.
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File Type: jpg 6ew7.jpg (43.9 KB, 341 views)
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Old 6th June 2003, 07:35 AM   #18
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Your topology looks good.

Yes, Zo is typically around the tube's Rp. Don't worry about Zo or NFB, since a cathode follower has inherent NFB in it, and ought to bring the Zo below that of the cans.
I'll try running some numbers, humm.

For the 2nd half of the 6DN7:

Click the image to open in full size.

(Feel free to doubt this information and recalculate it yourself, as doubt and independent and repeatable experiments are what hard science is based on )
The figure of 3.9% THD (actually that's 2nd harmonic only) is at maximum signal, 140mW (probably more than enough to smoke your headphones, and ears!). At more sane levels such as 1mW, distortion will be slim to nil.

I'll leave the preamp section to someone else; post results please!

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Old 6th June 2003, 04:03 PM   #19
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TDSL says that the Ra of the second half of a 6DN7 is 2000. Isn't this really too high? Can just the topology reduce the Zo that much?

Another question... should I have a cathode bypass cap on the preamp half of the tube?

About what plate voltages would be best? 250V and 150? Something like that?

Thanks, Tim, for the graph and thanks to all for the help.
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Old 6th June 2003, 08:37 PM   #20
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Well it'll reduce it by the same amount as it does THD, which is 5.1 times (whoops, that should show 2.2% THD then). That brings Zo to 390 ohms, pretty close (damping factor of .77). I say, try it and see if you like it. If you don't you can try a different tube (rewiring the circuit as necessary!) with lower Rp (typical for the big triode on tubes like these is 800 ohms).

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