Drive Current for EL509 Enhanced Triode

[wrenchone]

I'm planning on building a push-pull amp using Russian EL509s in enhanced triode (screen drive) mode, with an output stage B+ of about 400V. I have a lot of possible options for the preamp/drive stage. One is using a couple of the Russian 6BM8s just like in the Svetlana single ended design. I was also thinking of using a pair of vertical oscillator/amp dual dissimilar triodes like the 6CY7. My question is this - in the enhanced triode mode, what kind of screen current can I expect? Do I really need a husky 8W tube section like in the 6BM8, or can I get away with a smaller (~5W) section like in one of the dual dissimilar triodes? Also, what sort of output iron would be suitable for the 509s in enhanced triode mode?

[SY]

I use MOSFETs for mine- it's a lot to ask out of a tube to drive a load like that. It's more than just max current, it's also the nonlinearity of the load. You need super low source impedance.

I haven't driven 509s in that service, but 6LF6s that I've used want to see 20-30mA at full tilt.

(edit: 400V is too low unless you're happy with 30W- these tubes give better performance at 700-800V)

[wrenchone]

Interesting - I wanted to stay vacuum state with this project for the most part. An alternative to using MOSFETs might be to use a crude and hairy monster like the 6080 in the driver stage with 100-150V plate voltage. This would require a separate supply for the driver stage, but what the heck. I'm already in for a separate transformer to run the 509 filaments (10A total!) anyway.

I plan to use an old R-390A receiver power transformer to run the power stage. It has a 280-0-280 @200ma winding. I could float the center tap and use a full wave bridge to get 800V for the 509 plate supply. I'll have to check the isolation ratings on the plate winding to see if I can pull it off. The alternative would be to use a biphase doubler, which is a lot more parts. The transformer also has a 6.1V/8A winding I can use for the 6080 filaments.

[SY]

I'll be nice and not ask what happened to that R-390A. What a great piece of gear....

You may find that 150V of B+ on the driver isn't enough- you've got to swing very, very positive to tickle those grids. A 12B4 might be a good candidate. Note that you can actually run the CF closer to class B; you don't really care about the negative swings, the output tube is cut off.

Take a look through some of the schematics on David Berning's site- he uses both tubes and solid state in various amps for that follower.

[Tony]

way i understand it the 6080 likewise will need a lot of drive too with a mu of 2.

[wrenchone]

The 6080 will be used as a straight voltage follower, so it will pass on whatever voltage swing was developed in the previous stage. The tube has a high transconductance and is capable of high plate current at a very modest plate voltage, so it will make an excellent cathode follower. The 6080 also has a max dissipation of 13W/side, so I can use a high bias current without frying anything. The buffer makes life easy for the voltage gain stage in front of it, since it won't have to supply significant current. I'm toying with the idea of using a replacable standard size plate that fits in a hole in the chassis so I can easily sub in various front end circuits. I'm thinking of trying a standard voltage amplifier/concertina splitter (6CY7) vs a cascoded differential front end ( 5687 or 6SN7/6CG7).

[Fuling]

12B4 IS a good candidate for the job!
I used them as FC driver stages in my first PL519 screen drive prototype and it worked great.
In the mkII version I used mosfets instead, which worked just as fine though they didn´t look quite as good

[wrenchone]

If a 12B4A will make the grade as a driver for an EL509, then triode #2 from a 6CY7 should work as well. Check the specs - it's a hot little number, with the same dissipation as a 12B4A, and similar transconductance (not quite as much, but still pretty good). As a bonus, you get an extra pair of triodes for gain and splitter duties.

[Tony]

have you looked at 6CM7? this looks good although for a 150volt screen drive, looks to me gain may not be enough if global nfb is to be used.

[wrenchone]

I have a nice little pile of 6CM7s (another project in mind), but the 2nd triode on the 6CY7 is a bit beefier (higher transconductance), and the 1st triode has more mu. The two triodes in the 6CM7 are more nearly similar. There's another subset of these vertical deflection tubes with two low mu monsters. I haven't tried picking up any of those yet. Another obvious choice might be the 12BH7, but that tube is just a bit too trendy lately, and prices are starting to go through the roof.

BTW - SY, as far as I know, no R-390A was harmed to get the transformer I'm using for this amp. Fair Radio sells them as power supply spares.