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Old 5th May 2010, 01:18 AM   #1
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Default GM70 Datasheet Error!!

Hi,
I found three GM70 datasheet on the web, this on Image 1 seems incorrect on the tube pinage. On this GM70 datasheet PDF file(image 1) there is a pinout drawer totally wrong or Iam missing anything??
Click the image to open in full size.
Image 2 show the same pinout drawer with zoom, Image 3 is a real GM70 pinage photo, it is visible there is some differences in pins positions and mainly in the thicker pin tradicionally reserved to the plate.
Click the image to open in full size.
IF I identify correctly the tube elements in Image 4 and looking into the glass at tubes I have on hand, it show the four pins are connected as see on Image 5.
Click the image to open in full size.
Click the image to open in full size.
This Image 5 pinout is usual for GM70 graphite plate??
There is two pins connected to the grid, this is normal??
Thanks for any help, Gustavo
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Old 5th May 2010, 01:50 AM   #2
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Some major confusion on your part.
In "image 4" - what you've labeled as the "cathode" and "unconnected cathode" are the grid.

This is a Directly Heated cathode tube - there is no separate cathode - the filament is the cathode.
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Old 5th May 2010, 03:26 AM   #3
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I don't have a GM-70 to physically look at, but here is a datasheet in Russian that shows a base which matches your tube.

The drawing of the base on the datasheet you posted does not match your tube; the location of the large pin is different. Where did that datasheet come from?

Michael
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File Type: jpg GM70-base.JPG (49.9 KB, 480 views)
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Old 5th May 2010, 05:17 AM   #4
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Thanks for your valuable support boys, much appreciated.
Gustavo
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Old 5th May 2010, 05:26 AM   #5
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Quote:
Originally Posted by Michael Koster View Post
I don't have a GM-70 to physically look at, but here is a datasheet in Russian that shows a base which matches your tube.

The drawing of the base on the datasheet you posted does not match your tube; the location of the large pin is different. Where did that datasheet come from?

Michael
I find this datasheet at Frank's Tube Datasheets http://www.shinjo.info/frank/sheets/084/g/GM70.pdf there is other PDF file in this site, that show this drawer you show, looks this is the correct.
Many thanks, Gustavo
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Old 5th May 2010, 06:15 AM   #6
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Michael's drawing seems to be correct.

It's very easy to look into an actual GM70 and identify the pins on sight.

You'll find that the two pins closest together are the filament. One of those is thicker than the rest.

Then you'll see that of the remaining two pins, one is hooked up to the plate and the other is hooked up to one of the control grid support rods.

What I do is I put a GM70 loosely in a ceramic socket, then identify the pins in this way, and I write them on the bottom of the ceramic socket with a pencil. Makes it very easy while building the amplifier.

Kenneth
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Old 5th May 2010, 06:30 AM   #7
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So IF I understand right that thin wire on IMAGE 4 are both cathode and grid??
This wire are connected to two external pins, maybe the thicker pin is the +20V DC heater and the thin pin are the -20V DC heater??
Gustavo
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Old 5th May 2010, 06:54 AM   #8
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No, that thin wire in Image 4 is the grid. The two rods are the support rods for the grid. The heater/cathode is NOT shown in Image 4, it is a VVV shaped filament inside the grid wire loop. If you look at a real GM70, you can easily see it from above.

Kenneth
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Old 5th May 2010, 07:12 AM   #9
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Quote:
Originally Posted by kavermei View Post
No, that thin wire in Image 4 is the grid. The two rods are the support rods for the grid. The heater/cathode is NOT shown in Image 4, it is a VVV shaped filament inside the grid wire loop. If you look at a real GM70, you can easily see it from above.

Kenneth
OH YESSS, now this is clear. Thanks alot Kenneth. Taking one more image from JacMusic, the four shorts pins in this image should be the cathode.
Looks to me the support rod that are connected receive the Bias.
Thanks for your help, Gustavo
Click the image to open in full size.
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Old 5th May 2010, 07:35 AM   #10
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Yes. The 4 short rods are where the VVV shaped filament/cathode is held in place. You can still see some remnants of it in the photo. The three "tops" of the VVV are held in place with three springs at the top of the tube, as you can see in this photo:

http://www.positive-feedback.com/Iss...p_top_down.jpg

Kenneth
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