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Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Today i discovered that my GU-81M (apparently) does indeed work, after thinking it was dead for a year and a half. I figured it was dead because the plate current did not seem to change with -50v on the control grid, but today i tried -100v, and the current dropped to almost zero. Now, i want to make a makeshift curve tracer as suggested by Myke, to ensure the tube is indeed in working order.
Here is my problem. This is the datasheet i am using: Looking at the picture of the pinout, it says that the top cap C3 (which i already know to be the control grid) is grid 3, and therefor the screen is grid 2 and suppressor is grid 1. But then, if you look at the limit operating values, it only shows grid 1 and grid 2. It seems like there should be a ton of information on the control grid, but not alot on the suppressor since in most cases it seems to be at cathode potential.
What confused me even further is that there is no information about grid 3 on the graphs near the bottom of the datasheet. However, the graphs show a lot about grid 1, such as comparing anode current to grid 1's voltage. Also, the values they show for grid 1 seem to be the values one would expect for the control grid, rather than the suppressor.
So, is it just me, or is the datasheet contradicting itself? Maybe this datasheet was actually made from 2 different datasheets, and each refer to the grids as something else? If this is all just one datasheet, and it is accurate, i must be loosing my mind!
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
The top cap C3 is the only output connection which could be the suppressor grid (g3) since it would be all but impossible to operate the valve if the control grid (g1) connection were to be parallel to the anode connection, also located at the top of the envelope. Uncontrollable oscillation will break out in an HF power amplifier if opportunities for regenerative feedback like this are not reduced as much as possible when the valve is designed.
The role of the suppressor grid is to repel unwanted secondary emission electrons back to the anode, and so iron out the 'tetrode kink.' Aside from use in a few specialist circuits seldom seen, (e.g. occasionally mixers, modulators, negative resistance oscillators) the pentode suppressor grid is almost invariably connected directly to cathode and/or Earth, so no data are given for this. In many pentodes, the connection between g3 and k is a permanent one made internally.
The data sheet looks sensible to me, so I'd advise you to set the valve up in line with the recommended connections and values if you want a circuit using this device to work properly.
Registered Member #540
Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969
Hmm... Now looking at the datasheet, you might want to use two parallel, rectified, unfiltered MOT outputs for the anode supply (with a PFC on the primary side) so that you can get the tube into an operating region that is more accurately described by the graphs. I say unfiltered so that you can view the tube's response across all voltages from ~0V to the peak voltage under the tube's load. 0-(-100)V is perfect for the control grid. ~24VAC into an MOT will give you ~400VAC so you could rectify, filter, and use it for a screen supply.
I agree with Proud Mary that the suppressor grid is connected to the cathode. I think any of the filament connections would do since an added ripple of the filament voltage in comparison to the plate voltage shouldn't have any effect. If you really wanted to have as little ripple as possible, you should probably connect the filament supply to 1 and 2 and the cathode connection to 5.
To set up the curve tracer, I set my o-scope into xy mode so that I could draw the plate characteristics and compare it to the datasheet.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
Damn! That belongs in that thread as the biggest mistake i have ever made! I just recall seeing the top cap used in some vttc as the feedback, i guess i was wrong about that too! Other tubes like the 810 and 833a/c (which i have used) have a cap for the control gird (well, the *only* grid) which i assumed meant the GU-81M was the same. Also, i know C3 is connected to the largest (in diameter) of the grids, which i assumed meant it would be the control grid. As my mom would say, assuming makes an ass out of u and me. I am such a fool.
So i have my control grid grounded, and my suppressor grid as the control grid. No wonder my makeshift curve tracer did not agree with the datasheet.
Thanks guys, you surely saved me from destroying my tube, and/or a lot more confusion!
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Arcstarter wrote ...
Damn! That belongs in that thread as the biggest mistake i have ever made! I just recall seeing the top cap used in some vttc as the feedback, i guess i was wrong about that too! Other tubes like the 810 and 833a/c (which i have used) have a cap for the control gird (well, the *only* grid) which i assumed meant the GU-81M was the same. Also, i know C3 is connected to the largest (in diameter) of the grids, which i assumed meant it would be the control grid. As my mom would say, assuming makes an ass out of u and me. I am such a fool.
So i have my control grid grounded, and my suppressor grid as the control grid. No wonder my makeshift curve tracer did not agree with the datasheet.
Thanks guys, you surely saved me from destroying my tube, and/or a lot more confusion!
The control grid exerts the most control because it is closest to the cathode. It is usually wound of fine wire closely wound compared with the screen grid, which must handle quite high currents in a power valve like yours. Suppressor grids are often quite coarse compared with both the control and screen grids, and are sometimes wound with flat wire, basically a metal tape.
To make the role of the suppressor grid clear: energetic electrons impacting the anode knock out other electrons, which motor back towards the screen grid, where they add themselves to the screen grid current. This unwanted effect creates a negative resistance slump in the characteristic curve of the valve, often called 'the 'tetrode kink.'
The addition of a suppressor grid at zero volts - making a tetrode into a pentode - blockades the screen grid from the anode side, and suppresses these wayward electrons, so they just have to go back to the anode and join in with the rest of the electron crowd and go into the anode current whether they like it or not.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
That actually makes sense. At first it made no sense having another grid at cathode potential *closest* to the anode. It seemed like that would act like another cathode, but oh yea, the cathode has to be hot to boil electrons off .
However, if the suppressor somehow got hot enough to make electrons boil off (say someone stupidly connected it as the control grid or something ) i guess it could act like a cathode? If so, I guess i'm indeed very lucky this did not happen! But, then i guess someone would really have to be trying to kill the tube to get it hot enough to start emitting electrons.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Arcstarter wrote ...
That actually makes sense. At first it made no sense having another grid at cathode potential *closest* to the anode. It seemed like that would act like another cathode, but oh yea, the cathode has to be hot to boil electrons off .
However, if the suppressor somehow got hot enough to make electrons boil off (say someone stupidly connected it as the control grid or something ) i guess it could act like a cathode? If so, I guess i'm indeed very lucky this did not happen! But, then i guess someone would really have to be trying to kill the tube to get it hot enough to start emitting electrons.
Surely yes, if g3 was hot enough thermal electron emission would occur, but it could well be time to send for a priest to give the poor old valve the Last Rights.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
So, it works as expected now, but i noticed a slight defect on the plate. Where the bit of flat metal that connects the top cap to the plate, and where the getters are, i noticed some light. It does not seem to be a full on arc, but it can't be good. Here is a short video showing the defect:
This is a bit worrisome. The plate wattage is lower than the filament wattage in this video, it is 40v into a MOT, which is probably about 800vdc after the loading. I can only imagine things getting much worse as power gets higher, and im afraid the little spark will eat the metal away, until it goes completely open circuit. I know the video is not the clearest out there, but you get the idea.
It would seem that the piece of metal is pressure fit onto the plate by putting it in between the plate and the vertical ceramic support (clearly seen in the video before i zoom in). Maybe the ceramic support cracked somewhere inside there, but it seems more likely that this was a manufacturing defect.
Any opinions? I'm wondering how long i can expect the tube to last (especially at higher powers).
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Arcstarter wrote ...
So, it works as expected now, but i noticed a slight defect on the plate. Where the bit of flat metal that connects the top cap to the plate, and where the getters are, i noticed some light. It does not seem to be a full on arc, but it can't be good. Here is a short video showing the defect:
This is a bit worrisome. The plate wattage is lower than the filament wattage in this video, it is 40v into a MOT, which is probably about 800vdc after the loading. I can only imagine things getting much worse as power gets higher, and im afraid the little spark will eat the metal away, until it goes completely open circuit. I know the video is not the clearest out there, but you get the idea.
It would seem that the piece of metal is pressure fit onto the plate by putting it in between the plate and the vertical ceramic support (clearly seen in the video before i zoom in). Maybe the ceramic support cracked somewhere inside there, but it seems more likely that this was a manufacturing defect.
Any opinions? I'm wondering how long i can expect the tube to last (especially at higher powers).
I can't see what's going on there with any clarity. It couldn't just be a hole in the anode through which light is shining, could it? An anchor point for spot welding made by a press punch? Can you examine it with a lens - with the device cold and safely disconnected - and draw an expanded sketch so we can have a clearer idea of the structure at that point?
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GU-81M datasheet.
I just recall seeing the top cap used in some vttc as the feedback, i guess i was wrong about that too! Other tubes like the 810 and 833a/c (which i have used) have a cap for the control gird (well, the *only* grid) which i assumed meant the GU-81M was the same. Also, i know C3 is connected to the largest (in diameter) of the grids, which i assumed meant it would be the control grid. As my mom would say, assuming makes an ass out of u and me. I am such a fool. 
.