Connections to beam mini-plates to triode an 813 ?

[W3RSW]

The ol' twin 813 rig construction is coming along fine but here's a proposed modulator question.  I feel I'll need more strap than the current 811A's (at 1500) volts are providing. -Well, headroom anyway.

In an audio amp using 813's as triodes do you just strap the beam forming plates together with the other grids or would it be more efficient to ground the mini beam forming plates? 

I'm aware that you can make a tetrode into either a hi mu triode or low mu triode by tying all the grids to either the 1st grid or all the grids except gr.#1 to the plate.  Also aware of the 20k jumper from screen to grid (signal to screen) lashup in triode service but wonder about the beam forming plates.   

Thanks.
Rick

[WBear2GCR]

The main trick is to not exceed the current ratings of the respective "grids".
Afaik.

But if you need swing, you'll get the most power out using it with a screen supply.

             _-_-bear

[KI4YAN]

The beam forming plates MUST be grounded if you want anything resembling the power of a beam tetrode. If you tie these plates to a positive supply, or to the other grids, you WILL nuke the tube at modest power levels. Tying the plates to the grid, you will get an uncontrollably high mu tube anyway.

To properly triode a beam tetrode, you'd ground the beam forming plates, and then tie the screen to either the plate, for a low mu triode, or tie the screen to the grid, for a high mu triode.

[k4kyv]

I forget where I first found the info on triode connecting the 813.  I recall strapping together the screen and control grids.  That way it would work like a zero-bias  triode.  I don't remember what I did with the BFP's.

Does anyone remember where this information first came out?  The tube data in ARRL handbooks from the 50's describe using the 807 as a triode by connecting the grids together with 20k resistors and feeding the audio to the screen grids.  The BFP's in the 807 are internally connected to the cathode.  But those handbooks do not contain any information about using the 813 as a triode.

I am thinking the original circuit with the 813 may have been published in GE Ham News.

[The Slab Bacon]

From what I have seen over the years, typically the screen grid was tied to the plate in smaller tubes for low power class A amps, but the screen and control grids were tied tobether in one fashion or another and driven for high power class B type applications.

I question whether it would be better to tie the BFPs to the grids or to ground them. I somehow think they would be better grounded or even kept at a slightly negative potential to keep the electron beam focused onto the plate properly.

                                              the Slab Bacon

[John K5PRO]

I saw the triode-connected modulator 813 in an old copy of Orr's Radio Handbook years ago. Don't know if it was in GE Ham News, have looked at a lot of them and not seen it.  But it could have been in RCA Ham Tips too, they really pushed the marketing of that tube. They had a whole article just on using 813s in modulator service. After they opened the Power Grid factory separate from Harrison, NJ, they built them in Lancaster, PA, same factory that they later made color CRTs in. I was there a year ago, as Burle Industries is now making large power tubes there, and we use these at work. Someone said, yes, this is the former area that 6146, 813, etc were made. Now its mostly empty dark warehouse space.

The beam forming electrode must be grounded to cathode. According to Schade's original paper on the theory (first done for the RCA 6L6), this electrode is used to help form a space charge enhanced region out between screen and plate, to try and flatten the upturn in the tube characteristic curves when the plate swings close to or below the screen. With these additions, and alignment of G1 and G2, the electrons form thin beams which shine out to the plate. Interception by the screen is lower, causing less secondary electrons. The beam forming plates are on the edges, to prevent stray beam from finding its way to plate, as well as back to screen. In other words, the working region is well defined, where the beamlets are all parallel and focused in a particular way.

In audio amplifiers, sometimes it is seen that the screen is connected to taps on the output transformer, but at the same DC potential as the plate. This pushes hard on screen dissipation, but for certain tubes, lowers distortion. Don't try connecting them together with an 813, or your tube will probably fail pronto. Most triode connected 813s either ground both G1 and G2 for audio service. If you jack up the screen voltage on it, you may need to apply some G1 negative bias (or positive on the cathode) to prevent plate overdissipation while idling. This only becomes a concern when you get above 2 kV on the plate. If both are grounded, I have seen characteristic curves online (and in ER) that show safe plate current with 2 kV of B+. Somewhere around 2200-2500 you start to cook them. BTW, I don't know of anyone running more than 2500 on an 813.

Connecting them this way, makes them more 810-like. Once I realized that, I decided to use 810s for modulators, on my HB construction project. Both have the same filament power, so its relatively easy to switch.

Go to the AM Window Tutorials, there is a RCA Ham Tip on Modulator design that uses an 813 as an example. While it isn't triode connnection, it does share insight into getting things right (for class AB2 service there).
http://www.amwindow.org/tech/htm/tutor.htm

Let us know what works for you.
John

[W3RSW]

Thanks all.  Really crystalized my thinking. Some stchufff just seems to bother me what with various explanations coupled with my fuzzier brain cells over the years.  Looks like solid good stuff, ideas and history. Man, the 813 is one classy tube.  All mine have graphite anodes, really tough. Run the fil's at 9.8 rms digitally meas. volts, the plates at 1500 and they ought to last forever.   (... have to the way I operate... not! )

Yes, I did have JJ's schematic already.  Now I notice he drew beam plates as a "grid" (perfectly acceptable practice, of course) and grounded them.  I like his cathode resistor on 'receive' or bypass to cut down current and be nice to adjacent receivers and his -75 fixed bias and the add'l -75 from a grid resistor. Should get improved linearity. 

So, back to 'grid' #3. - I'll definitely go with grounding the beam forming plates to ground. I see in the pix sent by '4KCY that the beams were tied to grids, but guess the pubes didn't melt down n that circuit. :)  Yes, from the 50C5 all the way up, focused beam forming tech. is the way to go.

I imagine if I did put 3500 on the plate and still no zorch, I'd get plenty of X-rays.  Um hmmm, the ultimate wobulator, Very Close Range, of course.  Have to invite a local SSB aficionado over for a look at my new rig.  "VCRX" at HF.  "Uh, Rick, why's ya puttin' on a lead vest?"  "Don'cha worry sport, thass why they call it 11 meters.  Now hold still, jus' look into this tube, one eye at a time."

[Steve - WB3HUZ]

The 813s would work well in Ultra-Linear service too. Some of the multi-tap mods trannies would allow you to run UL. I also recall seeing a circuit in an old QST that did something similar using smaller triodes in something like a pass-tube configuration on the screens of the modulators. With a pot on the pass tubes, you could set the operation of the modulators from full triode connection to full screen tube operations and anywhere in between, including UL.

As for running 813s with more than 2500 volts on them, yes, I've seen it done. IIRC, it was at least 3 kV and a pair modulated by a pair easily put out close to 1 kW fully modulated.

[w3jn]

The mudulator in my GPT-750 (homebrew but based upon a BT-500 design, IIRC) uses triode-connected 813s with 2800V on the plates.  Works FB and plenty of audio    I do not know how the beam forming plates are connected, however.

813s are cheap and plentiful compared to 810s.