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How much peak power can a class C 6146B put out?




 
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Author Topic: How much peak power can a class C 6146B put out?  (Read 46121 times)
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K1JJ
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« on: January 16, 2014, 08:39:42 PM »

NOTE:  To maintain continuity, here is the beginning thread of this project:

http://amfone.net/Amforum/index.php?topic=35443.0




Hola,

I'm wondering how much power can be expected out of a 6146B in class C when it is piss beat?

The data sheet shows with 600 volts, it will do 62 watts carrier out.  I assume they mean 248 watts peak with 100% modulation.

http://frank.pocnet.net/sheets/079/6/6146B.pdf


Has anyone tested 6146Bs really hard to see what they will put out maximum?  What maximum voltage and current parameters have you used?

I usually find the upper limits and then back the rig off to a conservative level.  The maximum power out is usually MUCH higher than the published datasheet. However, with four tubes, I cannot do better than about 70w carrier and 300 watts peak output per tube.  And this is after trying different plate voltages, screen currents, drive levels, loading , etc. Even to the point of abuse.

What is the limiting factor here, the filament emission or is it the plate voltage?  I am considering switching to 6DQ5 or 6LF6 sweep tubes which have twice the fil emission, and a higher plate voltage rating.


My goal with four tubes was to see at least 300 watts carrier and 1500w pep output. (X5)    Is this do-able with four 6146Bs with lots of air - without destroying them?  


T

Reference thread for more info about project:
http://amfone.net/Amforum/index.php?topic=35443.0


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« Reply #1 on: January 16, 2014, 09:03:42 PM »

What about 6293? It's a rugged 6146A.
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« Reply #2 on: January 16, 2014, 09:17:09 PM »


With a big Crown M600 as a modulator, and a negative cycle attenuator, I could get 800 watts PEP out of a single 4D32 with a B+ of about 700. A 4D32 is roughly equivalent to a pair of 6146A's except the huge cathode has enormous peak current emission; much more then a 6146.

Jim
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K1JJ
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« Reply #3 on: January 16, 2014, 09:29:49 PM »

Pete,

Interesting on the 6293.  The datasheet showed its pulse ratings at 3KV maximum plate voltage!  I wonder if that is the pulse rating on regular 6146Bs?   I was worried about flashing them over, since I have a series PDM modulator that can be variaced up to 2500V.   I was thinking of keeping the carrier at 250 watts and having big headroom with 2KV peak voltage.  Maybe the plate dissipation will handle it.

Jim, I will take a look at the 4D32.  


The good news:  I forgot that there are old, used tubes in the rig for testing.  I just replaced the four with NOS 6146Bs. With 1600V,  I get an easy 325 watts of carrier and JUST kiss 1500 watts pep.  HA!  I'll bet the old tubes had a dud in there.   Now I need to know how high I can run the plate voltage for peaks and still maintain proper plate dissipation on carrier conditions.   For example, 275 watts of carrier and 1800 w pep would be a great goal. (X 6.5)  Then back the plate voltage off for comfort.

T
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« Reply #4 on: January 16, 2014, 09:37:17 PM »

All the rigs that ran a pair did 100 watts, with 700 volts on the plate.
700 volts and 200 ma was typical on phone I think.

With four, you should get 200 watts at 700 volts and 400 ma.

No reason to push them harder than that, just use a bigger tube.

My 3x 4D32 rig runs 1250 volts and 300 ma, and does 300 watts carrier out.
Plate dissipation is the limiting factor there.
The 4D32 is rated at 600 volts on the plate with plate modulation, 700 for CW, but they work fine at much higher voltages, I would assume the 6146 does also.

If you can not get 400 ma of plate current at 700 volts, there is a problem.
If you get 400 ma at 700 volts and only get 70 watts out, you have an output tank circuit problem.
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K1JJ
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« Reply #5 on: January 16, 2014, 09:54:49 PM »


My 3x 4D32 rig runs 1250 volts and 300 ma, and does 300 watts carrier out.
Plate dissipation is the limiting factor there.
The 4D32 is rated at 600 volts on the plate with plate modulation, 700 for CW, but they work fine at much higher voltages, I would assume the 6146 does also.



Brett,

Three people have recommended the 4D32 so far.  They are even cheaper than the 6146Bs.   I would have probably used them if I had looked into it.   Too bad the socket pin-out is different than the 6146B....  Wink  It wud need some rewiring.

Well, with the new tubes, I am seeing much more than what the datasheet says, so I am now happy. When I can only duplicate the sheet, something is usually wrong.

I'll see how this works out and go from there.  Frank/GFZ  said that maybe the PDM filter won't handle any more power without saturating, so I might be at the limit now and should be satisfied.

T
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« Reply #6 on: January 16, 2014, 10:43:38 PM »

So what is the power in, power out?

You have 140 watts of plate dissipation (ICAS), 1600 volts on the plates, so you are only 1000 volts over the maximum ratings, what is the plate current at 325 watts out?

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K1JJ
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« Reply #7 on: January 17, 2014, 12:18:17 AM »

So what is the power in, power out?

You have 140 watts of plate dissipation (ICAS), 1600 volts on the plates, so you are only 1000 volts over the maximum ratings, what is the plate current at 325 watts out?


When I mention 1600V, it can be cornfusing since most of us think in terms of a standard plate modulated rig.  

This is a series modulator, so the PDM modulator tubes have the bulk of the voltage across them during resting carrier conditions. I have about 700 volts across the RF finals -  and during full modulation the modulator puts the additional 900 volts across the RF finals creating a 1600 V peak.  

Normally, with a conventional p-p modulator and iron, the same voltages would occur at ~ 120% positive modulation.

At 325 watts carrier,  700 volts, I am drawing about 575 mA.  This is 400 watts input.   325w/400 w = 81% efficiency.  This is about 75 watts of plate dissipation. The tubes are rated at 140 watts. (35w X4)    So well within ratings. As long as the tubes handle the HV, dissipation is not an issue.

I have actually run the HV up to 2000 V and set the PDM carrier back down to 300 watts to see upwards of 150%+ modulation. Still experimenting.  I think these tubes can take more plate voltage than we would expect, especially in pulse service, as long as the internal structures don't overheat from high average currents.

The tube plates show no signs of color and I have air blowing by them for good measure...

In the shakedown phase, my idea here is to push things to their limits until I start to break parts and then back off to a more conservative level. This way the rig becomes more reliable and the parts that are to break have already broken in testing, not on the air... :-)

T

 
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https://www.youtube.com/watch?v=IVqUecYGnoM
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« Reply #8 on: January 17, 2014, 07:35:32 AM »

Hey,

I just reread the RCA data sheet on the 6146B and noticed their comment that the screen must be modulated in AM Telephone service. WTF?

We seen screen taps on the PP primary side of modulation transformers.....for improved linearity....

Don't know if I ever saw class C rigs with the screen also modulated.

ever consider the 8122? I have a pile of them in matched pairs and even a matched quad.

r
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« Reply #9 on: January 17, 2014, 07:41:12 AM »

In the shakedown phase, my idea here is to push things to their limits until I start to break parts and then back off to a more conservative level. This way the rig becomes more reliable and the parts that are to break have already broken in testing, not on the air... :-)
T

Tom,

We did exactly that as part of new product development.   The recognized term for this is HALT  (Highly accelerated life testing).  Once you pushed the "corners of the design" and failures started to occur you would analyze the failure(s) and determine if it should be fixed.  In some cases design changes and or component changes occurred and in other case a given failure was not determined to be significant to the end user and things were left alone.  Once the first failure occurred and the analysis and fix took place you would continue to push it higher until the next failure occurred and just keep doing this until all the data was gathered.  Upon completion it provided a good demonstrated model of a given circuit.   

Joe, W3GMS         


http://en.wikipedia.org/wiki/Highly_accelerated_life_test   
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« Reply #10 on: January 17, 2014, 09:20:59 AM »

Well, I think you are safe with running higher voltages.
The 4D32 is just like two 6146's stuck together, and I have been running them at 1250 volts on the plates, over 2500 volts with modulation, for years with no issue at all.

I do 1250 volts and 300 ma with three 4D32's, with a total of 150 watts plate dissipation.
375 watts in, 300 out, 80% efficient, 75 watts plate dissipation out of 150....

1/4 inch copper tubing plug in tank coil, no switches or taps.

I never figured it out before, but it seems I am running the tubes very light.
All the old rigs seemed to get 100 watts out of a pair of 6146's, or under 100 watts for a 4D32, so that is what I run them at, but I wonder why the old rigs ran things so light.

On my rig, if I load it up more, the plate current tends to go up, but the power output does not go up much.
I tried different tank coil values, but did not see much change....

The 2x 4x150a rig has a total of 500 watts of plate dissipation, but runs the same power input and output as the 4D32 rf deck. The 4x150 wants 1500 to 2000 volts on the plates, but I only give them 1250...

At 1250V and 300ma, they only get warm to the touch, and cooling is with a low noise muffin fan in the bottom of the chassis, not moving much air.

Same deal though, if I increase the current, power does not go up as much as the current...
 
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« Reply #11 on: January 17, 2014, 10:16:22 AM »

Hey,

I just reread the RCA data sheet on the 6146B and noticed their comment that the screen must be modulated in AM Telephone service. WTF?

<snip>
r

Rob

That's the normal for plate modulating a tetrode amplifier.  There are several good articles in both the west coast (W6SAI) handbook and the east coast (ARRL) handbook of the 40s - 50's era.  

The most familiar approach is found in the ART13 for the 813 final.  Not only is there a secondary for the Plate but a secondary for the Screen.  Another approach is to just pick the correct screen dropping resistor from the plate supply - on top of the modulation secondary or to install a reactor in series with the dedicated screen supply which is called allowing the screen to "self-modulate."

Al

PS:  There is a fairly good article that describes plate modulating a tetrode

http://www.w8ji.com/amplitude_modulation.htm

Scroll down to the section "Plate modulated Tetrodes."
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« Reply #12 on: January 17, 2014, 12:48:24 PM »

Brett said:

"On my rig, if I load it up more, the plate current tends to go up, but the power output does not go up much.
I tried different tank coil values, but did not see much change....

Same deal though, if I increase the current, power does not go up as much as the current..."


Brett,

Initially, I had the same problem with my 6146B rig.   It turned out that I needed MORE screen voltage and current. Once I reduced the value of the plate-to-screen dropping resistor, the tubes came to life and the power output jumped.  The combination of grid drive, loading, screen current, plate current, tank impedance, etc. is important and you will find a sweet spot where it all comes to life and the plate tuning and power output is robust and sharp.

Carefully put a VOM probe on the screen and see how much voltage is there under full drive. It should agree with the current as speced in the datasheet. Actually, my screen voltage is slightly higher than specs, but the current is right on the nose.  Also, run your grid current a little harder than specs to be sure your peak power is available under modulation.

Soft tubes are another possibility, but check the screen first. Also, measure the grid DC voltage when driven harder. Some circuits charge up with too much grid leak bias and make the tube harder and harder to drive. Output power can actually decrease with more drive cuz the charging effect adds bias at an increasing rate.  A bias steering diode might solve this on some circuits.

Once loaded correctly, more drive should increase plate current, show an increase in screen current and saturate the output power at some point. (class C)

Let me know your test results.

Joe, that is a cool write up - and your thoughts on shakedown runs are pretty much my own beliefs.  I don't want to blow out tubes or mod transformers, but I have no mercy for most any other components that may fail. We should be especially careful when pushing mod iron, but hopefully other cheaper components will fail first. Disc caps, resistors, arcing, instabilities and the like should be rooted out like the weeds they are... Grin

T
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« Reply #13 on: January 17, 2014, 02:02:35 PM »

Everything is metered and adjustable on my rigs, so its easy to adjust things.
Turning the screen voltage up exceeds the screen current recomendations.

Grid drive is also above normal, with a very high bias voltage (fixed and grid leak).
Modulation is not a problem, at 300 watts carrier, if I crank things up, I can get close to 2000 watts pep, badly overmodulating.
So the tubes respond well to more voltage, but loading it harder seems to decrease the efficiency.

300 watts is a nice power level for 40 meters, so I do not worry about it much, if I got 400 watts out, I doubt anyone would notice any change.

And, I can not remember ever having a tube fail (in anything) for 30 years...

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« Reply #14 on: January 17, 2014, 02:50:11 PM »

Brett,

Sounds like you have a good handle on it.  If you can do 2KW pep, the rig must be FB as it is.  Your three 4D32s rig is like mine on steroids.  I might have done three or four 4D32s if I'd thought of it.


BTW, do you have any pics you could post of your two homebrew superhet recievers?

T

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« Reply #15 on: January 17, 2014, 03:44:15 PM »

Well, I should really look into picture web hosting.

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« Reply #16 on: January 17, 2014, 11:05:29 PM »

Well, I should really look into picture web hosting.




Try this.

http://www.flickr.com/photos/tags/flicker/


klc
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« Reply #17 on: January 18, 2014, 05:10:05 AM »

Hey,

I just reread the RCA data sheet on the 6146B and noticed their comment that the screen must be modulated in AM Telephone service. WTF?

We seen screen taps on the PP primary side of modulation transformers.....for improved linearity....

Don't know if I ever saw class C rigs with the screen also modulated.

ever consider the 8122? I have a pile of them in matched pairs and even a matched quad.

r

I never saw an AM xmtr where the screen wasn't modulated.
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« Reply #18 on: January 18, 2014, 10:13:58 AM »

Pictures (some very old) here:

http://n2dts.smugmug.com/Ham-radio
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« Reply #19 on: January 18, 2014, 12:32:06 PM »

Pictures (some very old) here:

http://n2dts.smugmug.com/Ham-radio


Hi Brett -

Wow, amazing.   Youse gots the sickness as bad as me.... Grin

Nice combination of old buzzard and newer rigs. I like the receivers.

The "dream" lives on.

Tnx for posting them.

T
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« Reply #20 on: January 18, 2014, 12:36:21 PM »

Darn, swept the Dual Quads rig.  I'm having a bad time with severe high frequency audio distortion above about 1700Hz.  The audio is perfect at 1 Hz, 100Hz, 1000 Hz.   Frank thinks it's the PDM filter design, so trying some different values.  

Blew out the MOSFET driver.  And I thought I had this in the bag.     Plate modulation with iron is so much easier... [sigh]

T
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« Reply #21 on: January 18, 2014, 04:13:35 PM »

Darn, swept the Dual Quads rig I'm having a bad time with severe high frequency audio distortion above about 1700Hz.  The audio is perfect at 1 Hz, 100Hz, 1000 Hz.   Frank thinks it's the PDM filter design, so trying some different values.  

Blew out the MOSFET driver.  And I thought I had this in the bag.     Plate modulation with iron is so much easier... [sigh]

T

Ahhh... don't get discouraged!   Wink  PWM with tubes is challenging, but the results are better when you finally get it working.

It could be the PWM filter, but there are other possible causes such as insufficient bypassing in the cathode of the modulator.

Did you do the pulse width range test?  This test checks the pulse width range, and is done with carrier only.  You start with a carrier, and then adjust the DC offset going into the pulse width modulation generator.  As you decrease the "on" pulse, there may be a point where the waveform may ring or otherwise not stay square.   

One of the jobs of the PWM filter is to be able to contain (store) the entire "off" time of the PWM waveform.  What can happen  is this: as you approach 0% duty cycle, the waveform can "collapse" before the modulator actually turns on again at the next PWM cycle.  The fix for this is to increase the inductance of the input inductor.  The input inductor's inductance should ideally be just big enough to allow the modulator to fully store the "off" pulse at 1% duty cycle.  Increasing the inductance beyond this point will both decrease the efficiency of the modulator somewhat, and also will contribute to mathematical integration of small on-time pulses.

Experimentation will definitely reveal what's going on.

Regards,

Steve
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« Reply #22 on: January 19, 2014, 03:17:27 AM »

Darn, swept the Dual Quads rig.  I'm having a bad time with severe high frequency audio distortion above about 1700Hz.  The audio is perfect at 1 Hz, 100Hz, 1000 Hz.   Frank thinks it's the PDM filter design, so trying some different values.  

Blew out the MOSFET driver.  And I thought I had this in the bag.     Plate modulation with iron is so much easier... [sigh]

T

Maybe you need more resistors Grin

Never built a PDM rig so I can't add much of anything
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« Reply #23 on: January 19, 2014, 08:29:48 PM »

I would think the problem has to be in the filter, no?
What else could change with frequency so much?
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« Reply #24 on: January 19, 2014, 09:44:22 PM »

Brett,

It may be the filter- I dunno at this point.

After a lot of testing and experiments, I still cannot find the problem.  Even with a resistive load across the PDM modulator tubes, I get harmonic distortion.   (no RF involved)    Even when they are running right, I don't think I will be totally happy with tubes in the modulator.

So I've given up on the PDM 6LF6 sweep tubes and tore them out. I'm replacing them with a single SS 4KV  IGBT device.

http://ixapps.ixys.com/DataSheet/DS99385B(IXEL40N400)_.pdf

It will sit on a copper spreader and heatsink with air blowing by it.  This will be a near perfect switch for PDM and the 6146B finals will make the rig the best of both whirls.  At $45 a pop, you know I will be careful with these IGBTs.

I just ordered an inductance meter, so will see if the cores got ruined or the caps are still good in the filter. The rig sounded pretty good until I had a flashover and then went to hell. Though, it always had that high frequency distortion from the beginning.

My goal is to get this thing sounding pristine and will not give up on the project til it's working right...

Frank / GFZ has been helping a lot via emails and Steve /QIX  suggested using the IGBT. Thanks for their help.    Dennis/ W7TFO has offered to send me some used 6146s for free, so no worry about running out.

Fred - Mo bigger resistors might solve it - 200 watters - ya never know...  Grin
T
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Favorite Song - Trololo thru the years:  
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