Do you want something JUST for headphones? If so, then why not look at inverted triode operation?
Inverted Tube Operation
Inverted Tube Operation
I liked it for the 2 days I had it running. I built something similar (different preamp stage) on a piece of plywood from parts in my junk box.
It didn't have enough power to drive my HE-500s as well as I would have liked, so I have shelved the project until I find tubes that can handle a bit more juice.
My suspicion is that a el84 strapped as a triode would probably work pretty darn well. Sadly I can't seem to find any data sheet that goes into detail about triode operation. I can't even find one that lists the amplification factor let alone the GM.
> el84 strapped as a triode... I can't seem to find any data sheet ....about triode operation. I can't even find one that lists the amplification factor let alone the GM.
Amp factor in triode is essentially the u(g2g1) shown on most datasheets.
Gm at a given current will be similar to pentode mode except higher by the amount of screen current (which now contributes to Gm). In Philips sheets Gm is spelled "S". 250V, 48mA, S=11,3mA/V in pentode. Ig2 is at least 10% of Ia, so S(triode) is likely to be at least 110%, so abour 12.4mA/V.
http://www.mif.pg.gda.pl/homepages/frank/sheets/030/e/EL84.pdf
I found triode curves (6BQ5 is EL84 sold in EIA markets):
http://www.4tubes.com/DATASHEETS/SCANS-Original/E/EL84-TRIODE-MODE.gif
EL84 is not a high current tube. As triode it can't go a lot over 80mA, so in SE not even 40mA peak. With LOW-impedance phones some listeners can ask for more than 100mA peak. That's why we use the fat bottles.
Amp factor in triode is essentially the u(g2g1) shown on most datasheets.
Gm at a given current will be similar to pentode mode except higher by the amount of screen current (which now contributes to Gm). In Philips sheets Gm is spelled "S". 250V, 48mA, S=11,3mA/V in pentode. Ig2 is at least 10% of Ia, so S(triode) is likely to be at least 110%, so abour 12.4mA/V.
http://www.mif.pg.gda.pl/homepages/frank/sheets/030/e/EL84.pdf
I found triode curves (6BQ5 is EL84 sold in EIA markets):
http://www.4tubes.com/DATASHEETS/SCANS-Original/E/EL84-TRIODE-MODE.gif
EL84 is not a high current tube. As triode it can't go a lot over 80mA, so in SE not even 40mA peak. With LOW-impedance phones some listeners can ask for more than 100mA peak. That's why we use the fat bottles.
All seriously helpful info, but I was talking about using it in inverted triode mode where you use the grid as a plate.
plate resistance (in this case) is shown to be 1/gm and the amplification factor is shown to be 1/mu.
Ideally I would want to find a small tube with low amplification factor, high transconductance, and a grid that is capable of handling maybe a watt or so of dissipation.
My thought is that maybe an el84 might be able to fill this roll nicely if it were to be strapped in triode mode, but I haven't looked too far into it just yet. Still cleaning up other projects
But hey, you probably know of a good tube to use in this application. I have looked at the 6as7g already, but its not very linear even in this operation. Any ideas for a decent inverted triode tube?
The 7233. It's designed to take a beating on the grid, it's linear, has the transconductance of a 6GK5 frame-grid tube, and they have the same open element look of a 6AS7. Only downside is they aren't very common and they aren't in production.
I don't see any information about them being used in inverted triode mode either but it wouldn't be hard to try it on one I have.
The 6S19P might work for you as well but it is closer to a 6080 than a 7233. Looks sweet though and the low cost means you can parallel a few of them cheaply.
I don't see any information about them being used in inverted triode mode either but it wouldn't be hard to try it on one I have.
The 6S19P might work for you as well but it is closer to a 6080 than a 7233. Looks sweet though and the low cost means you can parallel a few of them cheaply.
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> 7233. It's designed to take a beating on the grid, it's linear, has the transconductance of a 6GK5
7233 has no positive grid rating or G1 dissipation number.
The 7233's high Gm is at HIGH current, 10X the current a 6GK5 is tested at.
Even large transmitter tubes do not have G1 dissipation ratings remotely similar to their plate dissipation ratings. And most finger-size tubes, G1 is rated "no" power (maybe 50mW for the small ones, 500mW for power tubes, though if there's any splatter on the grid the heat may cause quick runaway.)
7233 has no positive grid rating or G1 dissipation number.
The 7233's high Gm is at HIGH current, 10X the current a 6GK5 is tested at.
Even large transmitter tubes do not have G1 dissipation ratings remotely similar to their plate dissipation ratings. And most finger-size tubes, G1 is rated "no" power (maybe 50mW for the small ones, 500mW for power tubes, though if there's any splatter on the grid the heat may cause quick runaway.)
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