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In Reply to: RE: 6gk5 users, what do you do with the internal shield? (nt) posted by Tre' on May 04, 2017 at 11:46:41
My preference for dealing with internal shields and pin 1 of some Octals is connection to the chassis as close as can be. Shielding is shielding.
Eli D.
Follow Ups:
Thanks Eli but I'm a little confused.
My reading leads me to believe that the internal shield in the 6gk5 is not just a shield, like the shield between sections in a 6cg7. It's a partial screen grid according to the Sylnavia data sheet.
"A partial shield between the grid and plate which lowers the capacitance between these two elements..."
So, I guess my real question is what happens if you leave it disconnected?
Thanks.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
FWIW, I'm partial to GE data sheets. There's nothing like what you said is on Sylvania's on GE's
Eli D.
~!
The Mind has No Firewall~ U.S. Army War College.
If you look at pin 6 you will see that it connects to a post that runs inside the plate to the top of the plate, then crosses over and runs down the other side inside the plate area. (BTW the getter holder is connected to this structure)
So at least in those areas (up one side and down the other) the grid is shielded from the plate.
Kind of like a 6as7g where the 'cathode to plate' current flows only from the wider sides of the cathode (not off the skinny ends of the cathode).
With the 6as7g there is no plate at all on the "skinny" sides of the cathode. I think those are called "planar tubes"?
The RCA 6gk5s I have do not have a plate at one end (adjacent to the skinny end) and large holes on the other end.
I found the image above of the guts of a 6gk5. At the link below #2 is labeled as "screening plates".
I think they have that wrong. I think #1 is the "screening plates" and #2 is the plate/anode.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I found this image on a Japanese page describing no only the 6HA5 but the 6gk5 as well.Translated, " In the literature *, in 6GK5 and 6HA5, there was explanation that Cpg was decreased by placing a plate of a small area far and focusing the electron flow by the beam plate."
The "beam plates" shown would be the internal shield if a 6gk5 were being depicted.
So, like a planar tube, the cathode to plate current (in the 6ha5 and the 6gk5) only flows from the wide part (top and bottom as shown) of the cathode to the portion of the plate that is closes to the cathode, each side and not off the ends (left and right).
The rest of the plate is "shielded" from not only the grid but also the cathode.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 05/04/17
This looks more like a tetrode/pentode. Does that page say what the screen grid is connected to?
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Buy Chinese. Bury freedom.
The page says "However, the figure is [labeled] 6HA5, but with a screen grid it is a 4-electrode tube, so it's not 6HA5. This is a 4-pole tube such as 6CY5, 6EA5, and 6AB9 one unit."The point is, the internal shield of a 6gk5 looks like and does the job of the "Beam Plate" as shown in the mis-labeled image in my previous post.
It forces the tube to behave as a planar tube, conducting only where the plate to cathode spacing is parallel and controlled.
From the book (Amplifier Handbook) "incorporates beam plates to direct the flow of electrons to the desired portion of the plate"
https://books.google.com/books?id=ieFEAAAAIAAJ&focus=searchwithinvolume&q=6ha5
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 05/05/17
.
Have Fun and Enjoy the Music
"Still Working the Problem"
That makes more sense. You know, there's a large range of tubes of this genre, many different types and many have Gm of 15K. I wonder if anyone here has performed any distortion measurements with these?
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Buy Chinese. Bury freedom.
Tung-Sol also refers to this as a "partial shield between grid and plate." Never noticed that before. A couple of the data sheets refer to the internal shield being connected to ground relative to the measurements of interelectrode capacitance. For that reason, I've always assumed that's how it's meant to be used.
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Buy Chinese. Bury freedom.
I'm have 1500 ohm CC in there now.
Thanks
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
I think 1.5K should work well. It's important to keep all the leads short, especially the cathode connections and the grid pin. The grid stop should next to the grid pin with as short a lead as possible. I noticed in one of the sheets, it states that the internal shield eases neutralization requirements. That tells us this tube will be unstable unless good practices are used.
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Buy Chinese. Bury freedom.
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