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Cathode driven tube - grid dissipation measurement.




 
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Author Topic: Cathode driven tube - grid dissipation measurement.  (Read 701 times)
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KD1SH
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« on: November 04, 2021, 12:47:31 PM »

   I know that in a conventional grid-driven configuration, a true characterization of grid dissipation is more complex than simply multiplying grid voltage by grid current, due to the time-varying nature of both and the fact that RF current is flowing in the grid circuit, but what about a cathode-driven configuration?
   My Dentron Clipperton V is cathode-driven, but not grounded grid - the grid is bypassed with a .01 to ground and DC biased with a conventional zener circuit. Would I be correct in assuming that in this configuration, simple grid current X grid voltage arithmetic would yield a more accurate representation of grid dissipation than it would with RF drive applied to the grid?
   Asking because the Dentron has no grid current meter - I'm monitoring it with an external meter, and driving it slightly into AB2 and wanting to stay clear of max grid-dissipation.
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K8DI
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« Reply #1 on: November 04, 2021, 02:12:21 PM »

I can't tell you how to come up with a grid dissipation number, but..

My Loudenboomer amp is the same way, grid bypassed to ground through capacitors, cathode driven.  It's different in that it has a grid current meter, and is a no-bias 3-400z tube. 

That said, it and your Dentron are both grounded grid designs. The bypass capacitor to ground provides DC isolation, but is an RF short circuit. From an amplifier topology perspective, the signal path describes the topology.

Ed
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KD1SH
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« Reply #2 on: November 04, 2021, 02:30:18 PM »

Good point: it's the RF path that matters. And when the RF voltage drives the cathode below the grid, RF current flows in the grid circuit just as if that same RF voltage had driven the grid above the cathode.

I can't tell you how to come up with a grid dissipation number, but..

My Loudenboomer amp is the same way, grid bypassed to ground through capacitors, cathode driven.  It's different in that it has a grid current meter, and is a no-bias 3-400z tube. 

That said, it and your Dentron are both grounded grid designs. The bypass capacitor to ground provides DC isolation, but is an RF short circuit. From an amplifier topology perspective, the signal path describes the topology.

Ed
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WBear2GCR
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« Reply #3 on: November 04, 2021, 04:05:06 PM »

put a very small resistor in series with the zener? measure the voltage, derive the current...
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KD1SH
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« Reply #4 on: November 04, 2021, 05:40:32 PM »

Already patched in an external current meter and voltmeter, so I guess I'll just assume those numbers get me in the ball-park and also assume that the *real* dissipation is actually a tad higher and allow a safety margin. I'm just barely tickling the grid current: 2 milliamps at -36V = .072 watts. Now assuming 2 watts grid dissipation max per Svetlana specs (4CX400A), I'd max out at 55 milliamps, and I'd never even think of driving the tube nearly that hard. I'd probably max out the screen dissipation before then, but there's another metering value that Dentron doesn't give me.

put a very small resistor in series with the zener? measure the voltage, derive the current...
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« Reply #5 on: November 04, 2021, 08:45:04 PM »

Assuming a 1V max scale on a cheap VOM (which is what I use), a 100 ohm 1W MF resistor in series with the K2A lead to FT3 will measure control grid current. A 0.005 uF on each side of the resistor to shunt RF should suffice. For example, 5 mA of control grid current will yield 0.5V across it.

Again assuming a 1V max scale on a cheap VOM, a 33 ohm 1W MF resistor in series with the wire going from D22 to FT2 will measure screen grid current. A 0.005 uF on each side of the resistor to shunt RF should suffice. For example, 15 mA of screen grid current will yield 0.5V across it.

However, you will see a lot of bouncy bouncy on the meters with voice.

Phil - AC0OB
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W4AMV
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« Reply #6 on: November 05, 2021, 02:30:57 PM »

Take a look at an article in ER Magazine.
Ray was kind enough to provide the pdf as a download.

You will find it at Electric Radio Magazine, News and first line item. The
pdf is there. The view attached here.

I think it may help.

73' Alan


* resource.jpg (12.04 KB, 604x284 - viewed 53 times.)
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KD1SH
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« Reply #7 on: November 05, 2021, 06:50:06 PM »

   Thanks - excellent article. "Dynamic" is the word; there's a whole lot more going on there than just simple DC.
   I love ER.
   

Take a look at an article in ER Magazine.
Ray was kind enough to provide the pdf as a download.

You will find it at Electric Radio Magazine, News and first line item. The
pdf is there. The view attached here.

I think it may help.

73' Alan
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KD6VXI
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Making AM GREAT Again!


« Reply #8 on: November 05, 2021, 07:39:36 PM »

I didn't read the article...  So if this is covered in the article, I apologize.

For grid and screen current in delicate tubes like those being discussed here I normally use an LED as an indicator in addition to a meter.

Throwing a carrier is nice and will give static numbers easily read, but the nature of the voice makes getting decent readings on a meter, well....  Not so accurate.

However, the eye can see the brightness flashing of an LED fairly easily. 

You just have to size the bypass resistor of the LED correctly so it will give full brightness when you are fully hitting the peak grid current.  Not so hard to do.

Also, the 250B/R, 4CX400, etc tubes with 2 watt grids are pretty robust.  The CBers just try and destroy them and the tubes keep coming back for more.  It's those 0 watt grid tubes that are the ones you really have to watch out for Smiley  Typically they can take a watt of dissipation for very short periods of time. 

FWIW, CBers are hitting the 250s and 4CX400s with 12 to 20 watts PEP input autotransformer coupled with no grid R and the tubes are lasting.  IMD is crap, but I wouldn't overthink the tubes fragility.

--Shane
WP2/KD6VXI  (My WP2 call is locked in, but not yet assigned)
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VE7RF
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« Reply #9 on: November 07, 2021, 08:57:39 PM »

I didn't read the article...  So if this is covered in the article, I apologize.

For grid and screen current in delicate tubes like those being discussed here I normally use an LED as an indicator in addition to a meter.

Throwing a carrier is nice and will give static numbers easily read, but the nature of the voice makes getting decent readings on a meter, well....  Not so accurate.

However, the eye can see the brightness flashing of an LED fairly easily. 

You just have to size the bypass resistor of the LED correctly so it will give full brightness when you are fully hitting the peak grid current.  Not so hard to do.

Also, the 250B/R, 4CX400, etc tubes with 2 watt grids are pretty robust.  The CBers just try and destroy them and the tubes keep coming back for more.  It's those 0 watt grid tubes that are the ones you really have to watch out for Smiley  Typically they can take a watt of dissipation for very short periods of time. 

FWIW, CBers are hitting the 250s and 4CX400s with 12 to 20 watts PEP input autotransformer coupled with no grid R and the tubes are lasting.  IMD is crap, but I wouldn't overthink the tubes fragility.

--Shane
WP2/KD6VXI  (My WP2 call is locked in, but not yet assigned)

Here's what would really be handy...a peak reading analog  plate + grid current meter.   Sorta like a bird 43 with pep kit added.   JI has already done just that..but won't divulge any of it...'trade secret'.

Jim  VE7RF
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KD6VXI
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« Reply #10 on: November 08, 2021, 05:42:31 AM »

That's his TOF board.  For 811.based amps.

Adding a switch to enable peak hold would be even better.  Give it a 1 or 1.5 second peak hold and once tuned up, you'd know exactly what your amp was doing.

And yes, it's basically an op amp peak board.  Not really any trade secrets to it.

--Shane
KP2 /KD6VXI
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