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807 Amp again




 
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Author Topic: 807 Amp again  (Read 10477 times)
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WA4WAX
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« on: September 02, 2019, 09:05:07 PM »

Hello all!

I have decided that running a single tube in AB1 might be best.

That should give me plenty of power to drive a pair of 809's, even with 400 VDC on the 807.

The output should be clean enough for what I am doing. 

Thoughts?

BTW, anyone going to Boxboro?
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« Reply #1 on: September 03, 2019, 10:17:36 AM »

Datasheet says it should make 9 Watts, should be plenty if coupling is efficient.
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WA4WAX
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« Reply #2 on: September 03, 2019, 01:38:11 PM »

That would be plenty.

I am thinking about - 32 VDC on g1, and +250 to 300 VDC on the screen, 400 VDC on the anode.

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DMOD
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« Reply #3 on: September 03, 2019, 02:49:02 PM »

Ok, here we go again.   Smiley

http://amfone.net/Amforum/index.php?topic=45062.0

Why not just cathode modulate the dual 809's and dispense with the 807?   Cool

Audio is excellent because there are no large audio inductances (coupling transformers) in the modulation chain.


Phil - AC0OB


* Cathode Modulated Dual 809.pdf (117.16 KB - downloaded 348 times.)
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WA4WAX
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« Reply #4 on: September 03, 2019, 05:23:02 PM »

Not a bad idea, but the 809's need RF drive irrespective of modulation.......or am I missing something?

:-)
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« Reply #5 on: September 03, 2019, 06:48:34 PM »

Not a bad idea, but the 809's need RF drive irrespective of modulation.......or am I missing something?

:-)

...Hello all!
I have decided that running a single tube in AB1 might be best.
That should give me plenty of power to drive a pair of 809's, even with 400 VDC on the 807.
The output should be clean enough for what I am doing.  
Thoughts?

Again, your language left many to understand you were going to Modulate with a single 807.

Running an RF driver in Class A1 or AB1 is wasteful of power and is not a good stage, as discussed earlier.

You must have missed the design I proposed earlier. See PDF files.

Earlier post stated:

Quote
RF input to 807 grid > 20V p-p across 100k
Power Input to Dual 809 150W maximum at 750V
Maximum Output Power expected = 105 Watts

If you insist on 400V plate voltage, you'll only get about 45 watts out.  


Phil - AC0OB

* Cathode Modulated Dual 809.pdf (118.16 KB - downloaded 369 times.)
* Dual 809 RF stage.pdf (115.54 KB - downloaded 360 times.)
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WA4WAX
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« Reply #6 on: September 03, 2019, 07:42:54 PM »

I am not going to modulate the 809's with an 807, but drive them with an 807.

So, most efficient way to do it.  I was thinking an 807 in class AB1.

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DMOD
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« Reply #7 on: September 04, 2019, 12:24:19 AM »

I am not going to modulate the 809's with an 807, but drive them with an 807.

So, most efficient way to do it.  I was thinking an 807 in class AB1.


The most efficient way to drive them is with a ClassC driver as pointed out in the previous message but repeated here:

* Dual 809 RF stage.pdf (115.54 KB - downloaded 338 times.)
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WA4WAX
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« Reply #8 on: September 04, 2019, 03:35:19 PM »

Pardon my oversight!

With my SSB mentality, I tend to think of driver tubes as being run in a linear class, and not class C.

For what I want to do, a class C driver is fine.

Thanks OM!

So, prolly about -45 VDC on 807 grid.

:-)
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DMOD
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« Reply #9 on: September 04, 2019, 06:40:02 PM »

Pardon my oversight!

With my SSB mentality, I tend to think of driver tubes as being run in a linear class, and not class C.

For what I want to do, a class C driver is fine.

Thanks OM!

So, prolly about -45 VDC on 807 grid.

:-)

There are three approaches to the 807 grid bias:

1) Fixed voltage source to the grid resistor,

2) Grid leak bias without a fixed voltage source,

3) Cathode bias.

I chose cathode bias for the 807 RF amplifier/driver because it is more stable and tends to be self-correcting. With 390 ohms selected as Rk and according to the tube curves with a - 43V grid bias, it takes a 61 volt positive excursion on the control grid to get the plate current to 100 mA.

The 22k grid resistor limits the control grid current to about 3 mA with a 61 volts positive peak.

The cathode modulation for the dual 809's is accomplished with a power MOSFET heatsinked to the chassis. A coax cable brings the audio over to the 809's filament transformer's center-tapped secondary. The FET was chosen because it has a lower Drain-to-Source resistance since you're running at such a low voltage. A tube triode would have required at least 800 volts supply voltage to accomplish an RF output of about 45 watts.

The .005, 100uH, .005 pi circuit is there to block any rf from feeding back into the FET current source.

With the 2.5Meg variable resistor, you can vary the FET's drain current to control output power.


Phil - AC0OB


* Dual 809 RF stage with Cathode Modulation.pdf (155.61 KB - downloaded 355 times.)
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« Reply #10 on: September 07, 2019, 08:13:16 PM »

See Page 4 for an exciter I used with the 813, 175 Watt Screen Modulated AM transmitter, but modified for use with an 807 and an LV B+ of 400V.

It has a Wideband Crystal Oscillator/VFO amplifier, a Tuned Buffer, and a Tuned Driver/IPA stage, and is continuously tuneable from 3.2 MHz to 28 MHz.


Phil - AC0OB

* Dual 809 RF stage with Cathode Modulation.pdf (245.77 KB - downloaded 378 times.)
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WA4WAX
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« Reply #11 on: September 17, 2019, 04:44:15 PM »

Phil:

I was thinking about a dedicated CW/RTTY amateur transmitter bulit around a pair of 6KM6 sweep tubes.  Word has it you have a scheme for cathode modulation of the 6BQ6.

How about the bigger bottle mentioned?

Thanks!

BTW, have you thought about borrowing Art Fong's technique fro low distortion cathode modulation of pentodes?
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WA4WAX
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« Reply #12 on: September 18, 2019, 02:38:23 PM »

In other words, do what Art did with the two 6CL6. 

I was thinking that some chip might sub for the tube circuit he used.  That would be great for a couple of cathode modulated 6KM6.

:-)


http://hpmemoryproject.org/timeline/art_fong/hp606_01.htm
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« Reply #13 on: September 18, 2019, 03:16:29 PM »

In other words, do what Art did with the two 6CL6.  

I was thinking that some chip might sub for the tube circuit he used.  That would be great for a couple of cathode modulated 6KM6.

:-)


http://hpmemoryproject.org/timeline/art_fong/hp606_01.htm

Matt, that is truly an innovative design.  But I do not believe a 12B4 could cathode modulate the push-pull 6CL6s with that level of performance.

If you look carefully at the grid biasing of the 6CL6s, you can see that the 12B4 in the cathode circuit is a series modulator, not a simple cathode modulator.  In addition, the circuitry not only provides feedback with detected audio, it also provides feedback for carrier level and thus sets the bias for the series modulator, which sets the operating point for the final amplifier.  Kinda like a series regulator, which makes a 12B4 feel right at home.  (The 606A uses five in parallel in the power supply!)

In summary, series modulation is actually high level plate modulation, whether the series tube is placed below the final (in the cathode circuit) or above the final (in the plate circuit).  

Having messed with class-B plate modulators (with the associated modulator and reactor iron) for years, I wanted to try something with more fidelity and frequency response.  The HP-606A generator was the impetus that got me thinking about series-modulating a legal-limit HF rig for 75 and 40 meters.  The lower-powered prototype is under construction using a couple RCA 810s modulating a couple RCA 8000s, with a total plate supply of 2000 volts (about 900 to 1000 on the final).  The legal limit rig will have a single 3CX3000F3 modulating another 3CX3000F3, or possibly a pair of Eimac 304-TLs in push-pull.  This rig will have a 5000 volt one amp CCS power supply (already constructed), running the final at 2000 volts and allowing for greater than 100% positive peaks.  (I will play with Class-E and PDM another day.)

The modulator test load will be a string of 20 each 60 watt 120 volt incandescent bulbs in series (rare these days, but I stocked up when WallyMart dumped their inventory).  This will enable testing of the modulator driver circuits, DC voltage control, linearity, etc, without the need for the RF stage.  This testing will reveal whether the series modulator will be above or below the final.  Each alternative has different trade-offs to consider.

I know this rig will probably be the most inefficient in the land, but it will be a great shack heater.  When you think about it, screen modulation is also inefficient.  High-level Class-B has lots of ancillary tubes to heat, power supplies to run, so overall,the efficiency does not seem that bad.  And it makes a great shack heater in January, although not an issue in DeLand.

So why mess with the puny 807?  Now back to your regularly scheduled program.

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« Reply #14 on: September 18, 2019, 03:26:54 PM »



I did try this  and published in feb 2016 in the subject "constant magnetisation modulation. I copied and pasted, but the pics did not copy So for the schematic diagram end the scope pics look for that article
It gave a very nice linear modulation upto more than 10 kHz.
   
Re: Constant magnetisation modulation
Reply #11 on: February 22, 2016, 08:41:29 AM
Reply with quoteQuote Modify messageModify
Hello all
I did try a combination of series modulation in the cathode and grid modulation in order to avoid high DC at the series modulator. The grid part is done by connecting the grid resistor to ground, not to the cathode circuit. The trick to get low distortion is partly in the grid drive impedance. I drive at approx 5 kOhm and 35 Vtt for an EL 84, Not too far from normal except the 5 kOhm. A higher impedance / higher drive will result in some compression of the positive modulation. I did not investigate yet why, but I assume the RF at the grid will start to modulate as well due to the grid current.
The results are very promising, and I am very positive in my assumption that it may be done as well at much higher power levels. I have 2 Watts carrier and 8 Watts PEP at 350 VDC at the anode.
The voltages at the modulator are very low, the drain has a voltage of approx 25 Vtt, That will not hurt me too much... .


* cathode modulation schematic.jpg (28.3 KB, 721x588 - viewed 271 times.)

* Cathode modulation 002.jpg (71.47 KB, 800x600 - viewed 255 times.)
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Re: Constant magnetisation modulation
Reply #12 on: February 23, 2016, 06:39:53 AM
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I did a few more tests, and indeed the distortion is due to the modulation of the RF drive signal. If the cathode voltage rises, the grid current becomes less and the loading of the driver becomes less. That results in an INcrease of the RF at the grid while the output should DEcrease. When I lower the dive impedance with a low G1 resistor to ground, this effect is much less and the modulation distortion disappears. Than it seems that the curve of the FET is quite nicely compensated with the modulation characteristic of  G1 plus a part series modulation plus a small part G2 modulation. (G2 is also referred to ground not to the cathode circuit).
But you need ti fiddel around a bit with the source resistor, the G1 resistor and the bias to get a very nice modulation. Seems worth the trouble, with a FET of approx 800 Volts and a few amps you can modulate kilowatts with a low FET dissipation.
WHO IS GONNA TRY THIS?
It results is a safe circuit, because, when the FET is pinched off or open, the cathode voltage is very limited. Both the drive and the tube are cutt-off. In my little circuit the cathode voltage will not rise more than approx 40 Volts with the cathode open.

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« Reply #15 on: September 18, 2019, 03:53:57 PM »

Nico, If you reference the grid circuit to ground, I believe what you have is the classic cathode modulation circuit, which is a form of efficiency modulation.  This mode is a bit tricky to balance all the parameters for clean modulation, especially when employing tetrodes.  You have accomplished that extremely well.

However, with series modulation, the series regulator tube varies the DC potential applied to the final amplifier, whether the series tube is above or below the final.  Referencing the grid circuit to the cathode or filament is necessary, such that the entire final circuit sees a the supply voltage vary with modulation.  This is high-level plate modulation, albeit the modulator is inefficient.

It is easy to see this with the series regulator between the plate supply and the tube plate circuit, just like a class-B plate modulation transformer.  Whether you modulate the negative or the positive side of the supply makes no difference, as the final sees the same power supply variation.  

The challenge of placing the series tube below the cathode involves the high peak voltage on the cathode and filament during the modulation trough.  This requires good insulation of all the circuitry, and a filament transformer with high voltage isolation for the final tubes.  The grid circuit could be the standard link-coupled parallel-tuned tank, using grid-leak bias.  If fixed or protective bias is desired, it must be provided by a floating supply that can also withstand high peak voltage excursions during modulation.  Under no modulation, the cathode of the final rests at somewhat above half of the supply voltage, providing headroom for greater than 100 percent positive peaks.  Under idle conditions, ant 100 percent carrier, the series modulator tube would be dissipating more than the total input power to the final.  For example, if the final sees 2000 volts at 500 mA, with a 5000 volt supply the series modulator will see the remaining 3000 volts at 500 mA, thus dissipating 1500 watts.  This only occurs during transmit periods of course, making it necessary to have a long old-buzzard transmission in January to keep the shack toasty warm.

The challenge of placing the series tube above the final relates to the audio drive and biasing of the series tube, which, in effect, is running as a cathode follower.  A drive tube, running grounded cathode, with a large plate resistor and high plate voltage, could be used to drive the cathode follower.  Typically, a large transmitting tube would be required here due to the high plate voltage excursion, however the plate current and dissipation requirements remain miniscule.

Apologies to Matt for the apparent thread hijack, but he did open the door with reference to Art's creation.  We can take this discussion elsewhere if you wish.  73, Rick
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Rick / W8KHK  ex WB2HKX, WB4GNR
"Both politicians and diapers need to be changed often and for the same reason.   Ronald Reagan

My smart?phone voicetext screws up homophones, but they are crystal clear from my 75 meter plate-modulated AM transmitter
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« Reply #16 on: September 18, 2019, 05:26:22 PM »

Rick:

Feel free.

I am thinking about applying Art's trick to a pair of 6KM6 sweep tubes, not an 807.

My interest in the 807 is to use it as a driver tube for a pair of 809's......another project.

Series modulation of those big sweep tubes, with feedback, would yield a nice AM signal.

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« Reply #17 on: September 18, 2019, 05:40:55 PM »

I used series modulation in the low end Rick, I am well aware of the system. . But in practice a little problematic due to the high voltage at the cathode and drive of the final. So if you take the overall efficiency, the efficiency modulation (actually 80 % grid 1 modulation)  gives very acceptable results without a big dissipation of the modulator and without high voltage at the filament / grid circuit. A big advantage in simplicity and safety And you don't get less power per watt dissipation than a series modulator
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