The AM Forum

I have been reading up on the 813 tube, to use it as a triode, do you connect the screen grid and plate together? Or connect the control grid to the cathode, and let the screen act as control grid? Thanks for info. Matthew

Matthew,

For class B audio service, connect the screen and control grid together and drive them. Add the bias to the fil CT if needed via some series diodes.  If under ~2400V, you will not normally need bias... just tie the fil CT to ground.

See this schematic. This is the 813's X 813's rig using triode connected 813 as modulators:

http://www.amwindow.org/tech/htm/813/813.htm

73,
Tom, K1JJ

Matt,
        There are many variations of triode connected tetrodes. I have seen a few strange ones over the years. There are 2 common approaches. For LOW POWER class A audio G2 is usually tied to the plate.
For HIGH POWERED class B audio or RF service G1 and G2 are tied together and driven as the control grid.
In Grounded grid RF service even though both grids are grounded and the drive is applied to the cathode, the circuit is still a triode connection. (as long as the incoming signal to be amplified is fed between the cathode and grid it doesnt matter if the grid id grounded or floating)

The most commom application for 813's is either grounded grid as a leenyour amp. Or as Class B modulators
As class B modders both grids are tied together and fed the incoming audio. Tom's "tesla 360" transmitter in the above post is a real good example if you want to use them for audio.

                                                                             The Slab Bacon

I suppose that the 813 would also work if the 813 control grids were at mid-filament potential, and the drive went to the screens.  Easier on the 'control' grid, but not as much oomph.  So if the conrol grid can take it, why not tie it to the screen and drive the two gids together.

What amazes me is that the 813 becomes a zero-bias tube that way, up to 2000+ volts.  That's perfect - the tube can take the voltage, and then the filament emission will permit some pretty high peak powers.  Certainly the tube will easily go to the point where plate dissipation is the limit.  I think that somebody - probably a ham - had to have designed it to work that way, because that's quite a coincidence, I think.

  Bacon, WA3WDR

Interesting thought Bacon. If someone did design it that way, they were pretty forward looking. The 813 came out in 1938. At that time, a zero-bias, grounded-grid amp was pretty rare, if not unheard of within amateur radio.

Hmmmm, yes interesting thought for sure...

The 4-1000A has similar idle characteristics when in GG.... 

The larger tetrodes like the 4X5, etc do the same thing more or less at their rated voltages.

I think it's just a rough coincidence when  grounding the screen of a once high gain tetrode. (or "beam power" tube in this case)

I'm betting the design engr was simply under the gun in 1938 to design a class C final that would work in many applications w/o regard for GG.

Did you know if you spin an 813 backwards, it says, "turn me on, grounded grid" ?  ;)

T

Grid and screen tied together + hi-Mu triode. Plate and screen tied together = low-Mu triode.

Well, does this mean one can use the 4-400a  as a triode in rf deck, or would it not be good idea???? Matthew

It's done all the time. Not as power sensitive as it's 3-400 brother but it does work.

Matt -

There was a time when I ran quad 4-400A's in GG. It was a nice amp back in the mid 70's.  Not as clean as running 3-500Z's however.

T

 i'm running single 4-400A in GG for 160 lately, it's an old amp i had as a spare. hard to drive, and a bit dirty on SSB. cleaner on AM for some reason. i get about 175 carrier/600 PEP out of it. works reasonably well but if i stick a 3-500Z in it cleans up and drives easier.