They're no good! Don't use them! 
My personal favourite is the 6au4. I have a huge box of super ugly coin-base sylvanias that I got increadibly CHEAP a few years ago. They are phenominally reliable and have such a lovely slow start-up. My 2a3 heaters have time to get nice and warm while the B+ gradually rises.
Those russians look excellent contenders - top plate, 9pin base, unique look too. Zero chance of me reading the specs sheet though.
My personal favourite is the 6au4. I have a huge box of super ugly coin-base sylvanias that I got increadibly CHEAP a few years ago. They are phenominally reliable and have such a lovely slow start-up. My 2a3 heaters have time to get nice and warm while the B+ gradually rises.
Those russians look excellent contenders - top plate, 9pin base, unique look too. Zero chance of me reading the specs sheet though.
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Ian,
i made a table for easy and quick comparison...
i have around 200 pcs of these tubes in my stash...
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Hi Tony
I also have many of those. I pulled a bit more than 400mA through a pair of coin base 6AU4's without problem (choke input) for many years and they still test NEW. The GE specs sheet lists the continuous DC output currrent at 190mA. I think this is a low-ball estimate.
6AX4's are also very good, but they have a higher internal resistance than the 6AU4. These can be useful to drop the B+ a bit vs. 6AU4, or if your heater winding is not up to 6AU4. I actually find that some 6AX4's (especially some RCA) can't handle quite as much current as 6AU4's. The best performing of all these damper diodes tend to be coin base ones în my experience, and I have wondered if that helps in heat dissipation.
6DE4's and 6DW4's are somewhere inbetween 6AX4's and 6AU4's in terms of internal resistance. Much of the ones I have are labled with both designations. They can indeed handle 250mA but I did not test them for long periods of time.
Ian
I also have many of those. I pulled a bit more than 400mA through a pair of coin base 6AU4's without problem (choke input) for many years and they still test NEW. The GE specs sheet lists the continuous DC output currrent at 190mA. I think this is a low-ball estimate.
6AX4's are also very good, but they have a higher internal resistance than the 6AU4. These can be useful to drop the B+ a bit vs. 6AU4, or if your heater winding is not up to 6AU4. I actually find that some 6AX4's (especially some RCA) can't handle quite as much current as 6AU4's. The best performing of all these damper diodes tend to be coin base ones în my experience, and I have wondered if that helps in heat dissipation.
6DE4's and 6DW4's are somewhere inbetween 6AX4's and 6AU4's in terms of internal resistance. Much of the ones I have are labled with both designations. They can indeed handle 250mA but I did not test them for long periods of time.
Ian
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Hi Ian, i am making a Tabor clone this year,
contemplating using the japanese 6gk17 and the 6ax5's....
looking at the plate sizes, then maybe underrated....
i am reserving the bigger guys for heavy lifting...
6gk17, 19au4, 6da4.6dm4, 12BR3, http://tubedata.milbert.com/sheets/201/6/6BR3.pdf
contemplating using the japanese 6gk17 and the 6ax5's....
looking at the plate sizes, then maybe underrated....
i am reserving the bigger guys for heavy lifting...
6gk17, 19au4, 6da4.6dm4, 12BR3, http://tubedata.milbert.com/sheets/201/6/6BR3.pdf
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