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screen current when off resonance question




 
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K8DI
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« on: May 20, 2022, 10:09:20 AM »

Quick question for anyone with a rig that has a screen current meter: what does screen current do/how does it change as you tune the output tank?  The plate dips, of course, but does screen current track/dip as well, or move the opposite direction, peaking when approaching resonance?  I ask because I am trying to track down some odd behavior, but don't have a rig with a  screen current meter.

Thanks,

Ed
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K1JJ
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« Reply #1 on: May 20, 2022, 10:37:38 AM »

What does screen current do/how does it change as you tune the output tank?  


Hi Ed,

I have five class C tetrode plate modulated rigs. (4-1000A, 813s, 4D32, (813 series modulated) and 6146s)    ALL of them have a sharp screen current  PEAK at resonance.  In fact, when the plate is loaded heavily, (less C2 loading) the screen is a clearer indicator of resonance due to the sharper peak compared to the broader, lazy plate current dip.

Also be aware that when monitoring cathode current, the screen current is included and may skew the plate current dip reading slightly.  Measuring plate current thru the plate xfmr CT will allow a more precise plate-current-only monitoring.


T
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K8DI
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« Reply #2 on: May 20, 2022, 10:52:28 AM »

Thanks, Tom.  That verifies it for me.  This rig is suddenly jumping in plate current with nothing obviously happening to cause it, but I noted screen current dropping/voltage increasing when it happens. I think I have a thermal failure in some part of the tank. If I shut it off, wait 20 or 30 seconds, then turn it on, it takes less time each time to jump; on a cold start, several minutes. Something is opening up or shorting when it gets to a certain temperature. Now I just have to find it!  Of course once I shut it off and start testing parts, they've cooled down and test fine...

Ed
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K1JJ
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« Reply #3 on: May 20, 2022, 11:17:17 AM »

Ed,

If it's thermal related, perhaps a cold spray and heat lamp / hair dryer will help to show the culprit.

Also, place a big mirror behind the rig with the lights off to spot any arcing or visual anomalies.

T

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« Reply #4 on: May 20, 2022, 11:28:22 AM »

A gassy tube can do that as well. Tongue

73DG
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K8DI
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« Reply #5 on: May 20, 2022, 06:55:40 PM »

A gassy tube can do that as well. Tongue

73DG
Any simple way to tell? The plate jump is not gradual, itís like a switch being thrown; current more than doubles instantly. 

Ed
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« Reply #6 on: May 21, 2022, 10:00:27 AM »

A gassy tube can do that as well. Tongue

73DG
Any simple way to tell? The plate jump is not gradual, itís like a switch being thrown; current more than doubles instantly. 

Ed

Usually a cap heating up will be gradual, as will the change in capacitance. The fact yours is instantaneous is telling.

I use an Led indicator in my screen circuits. The eye is pretty good about brightness changes, and an Led and resistor is cheap.

--Shane
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Chuck...K1KW
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« Reply #7 on: May 21, 2022, 08:48:46 PM »

What is the rig?  Sounds like either a parasitic or lack of neutralization is kicking in with the sudden change.
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K8DI
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« Reply #8 on: June 01, 2022, 09:49:51 AM »

I finally had some time to spend on the rig, and figured out what was happening. I had a little help from a friend (Dan/N8ZJV) who brought his spectrum analyzer and power meter over, so we could better visualize what was happening in real time. I gotta get me one of those...

For those who asked, the rid is my RCA BTA-1R1 conversion that I thought was more done than it is. Issue was on 40m; lower frequencies did not demonstrate the problem in testing.

The actual failure/problem that changed with temperature was the mechanical ground connection for the screen bypass capacitor.  But the solution is way more complicated than tightening up the mounting screw...


After multiple iterations, failed attempts, and testing, I had settled on driving the grids of the finals with a modern/solid state radio as an exciter. To match the radio's 50 ohm output to the grids' much higher impedance, I built a set of pi networks, using relays to select the one to match the band (I also switch the output network inductor tap with vacuum relays so it was easiest to do the same here). I found they were broad enough such that once tuned, they did not need adjustment to keep the exciter happy with the SWR across the phone bands on 80 and 40. This also meant that they are not tuned exactly to the input RF frequency most of the time...



I got the general schematic for the input to grids from a respected builder. I should have done my own analysis.  I've attached a partial schematic, I left off some junk like metering and the second tube, B+ bypass, overload relays, etc.

The thing is becoming a giant tri-tet oscillator. As it warmed up, the screen bypass MADE contact, not lost contact, with ground. As soon as it touched, the tuned resonant grid circuit, the grid resistor and swamping resistor along with the coupling cap and tuning cap made cathode to grid dividers, the cathode bypass (filament actually) and screen bypass connected the cathode to "plate" (for RF) via ground in the ECO/Tri-Tet method, and the thing (on night/low power/250w/1500v setting) made 500+ watts. I could slightly retune the output network and get the plate current under control, and the screen DC current remained reasonable, yet the output power was still WAY over what it should have been....and the 100 ohm screen buildout resistors popped after 60 seconds. They did not like passing that amount of RF....The screen grids are probably done/melted, but at least I did my testing with a pair of well-worn/weak tubes.

So, the pi network input goes out the window. I've looked at a number of big grid driven linear amp circuits, Tom/K1JJ's 813x813 circuit that's on here has grid drive, and whatever else I could find.  All of them use a transformer input so the "cold" end can be set up with a neutralization feed; the pi input did not employ any neutralization. I'll be adding that.  The pi input circuit needed 22-36 watts drive (band dependent) and was tossing half or more of that into heat. Although I had been able to do grid leak bias, I am going to change to a fixed or partial fixed bias as I redesign the input (since I am starting over, I may as well include it).  

As an aside, my other posting about bias hasn't become irrelevant, the bias voltage/current choices discussed there still make rational sense whether the electrons come from a bias supply or self-rectification.

I first got this thing out of the trailer last October. It is now June. Not what I expected, but it's been good learning and time spent with buddies poking around at it. It'll be on the air soon...

Ed



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w9jsw
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« Reply #9 on: June 04, 2022, 06:49:34 AM »

your swamping R may not be heavy enough.  I would try a non-inductive swamp closer to 2K. I am using the globar ones in my new 4-400 rig (not yet powered up), but also used 2K swamp in my 813 rig and it was stable on 40m without neutralization.
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K8DI
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« Reply #10 on: June 05, 2022, 08:51:19 AM »

your swamping R may not be heavy enough.  I would try a non-inductive swamp closer to 2K. I am using the globar ones in my new 4-400 rig (not yet powered up), but also used 2K swamp in my 813 rig and it was stable on 40m without neutralization.
I found a swamp resistor calculation for class AB amps on the W8JI site that would put it below 1K.  While not class C, it leads me to that same conclusion; that the swamp resistance is probably too high to do any real swamping.  I ordered a Bud box to put the matching/input network in for shielding (it sits out in the open now). When that gets here and I have time Iíll try a few things again.

Ed
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