Using a toroidal transformer as a mod transformer Part II

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AB2EZ:
It works  :D

I was able to get a hold of a Samson Servo 600 (600 watts into 8 ohms, bridged) audio amplifier to complete these experiments.

With that amplifier, I could easily obtain 100% modulation, with a nice clean sine wave, at full legal limit power ... with a flat modulation frequency response and no visible distortion from 30 Hz to 9.5 kHz (as far as I measured at each end).

As a "bonus", the pair of GS-35b's run in a Class C plate modulated configuration with very high efficiency:

Input electrical power: [1700 volts of plate B+ x 220 mA of average plate current] + 30 watts of drive power + 50 volts of peak grid-to-cathode voltage* x 225 mA of average grid current]  = 415 watts of electrical input power

Output power (Bird power meter: 360 watts, "Powermaster" power meter: 314 watts ): 337 watts

Efficiency = 337/415 x 100% = 81%**

*The grid current flows when the grid is positive with respect to the cathode. The negative bias is 40 volts, and I am estimating that the average input power contributed by the grid current is ~50 volts x the average grid current ~ 11 watts.

** This number seems high, but the amplifier is biased heavily into Class C at carrier... with -40 volts of grid-to-cathode bias and only 1700 volts on the plates of the GS-35b's ... which would produce very high efficiency. I.e., the plate current flows during a small percentage of the rf cycle... and during that part of the cycle, the plate voltage is at its minimum value.

I am using a Johnson Ranger (running at 30 watts of carrier) to drive the plate modulated rf power tubes (a pair of GS-35b's) in grounded grid configuration***. I found that I could obtain significantly improved linearity by also applying some plate modulation to the Ranger. I.e. about 20% plate modulation of the Ranger.... just enough to get equal positive and negative peaks on the output of the rf power amplifier, with a nice, symmetrical sine wave on the output of my off-air envelope monitor.

*** In grounded grid configuration, the load impedance seen by the Ranger varies (as the modulation of the GS-35b rf power amplfier goes from 100% negative to 100% positive) from infinite -to- one half the load impedance seen by the Ranger at carrier. On positive modulation peaks of the GS-35b rf power amplfier... the reduced rf load impedance seen by the Ranger is translated by the Ranger's pi output network into a doubling of the plate load resistance on the Ranger's 6146. I.e. the load resistance on the Ranger's 6146 r.f. output tube is proportional to Q**2 x r, which is proportional to L**2/r, where Q is the Q of the tank circuit, r is the load resistance at the output of the tank circuit, and L is the inductance of the tank coil.  In principle, if the Ranger had more power output capability (at carrier), and was lightly loaded (rf output voltage swing << B+)... then the Ranger's r.f. plate current waveform would remain approximately the same... and the output power of the Ranger (looking into a higher load resistance) would double. However, with the Ranger fully loaded, the opposite is true... i.e., the Ranger's output power actually decreases (or, at best, remains approximately the same) on positive peaks of the modulation of the rf power amplifier. This results in a reduction, by a factor of more than 40%, of the peak rf drive voltage the Ranger delivers to the input of the GS-35b power rf amplifier (i.e. less power into a load resistance that is roughly twice as large).

Needless to say... I am pleased with the results.

This completes my experiments with the use of toroidal power transformers as modulation transformers.

Best regards
Stu

w4bfs:
looks really good Stu .... I am still considering winding to winding voltages ... if some way could be figured out how to establish a equal common mode gradient between the transformers, it might be possible to have some 'comfort zone' ...would you mind to measure winding to winding capacity on both cores and see what you get? thanks ...John

w1vtp:
Quote from: AB2EZ on August 14, 2008, 04:57:10 AM

It works  :D

I was able to get a hold of a Samson Servo 600 (600 watts into 8 ohms, bridged) audio amplifier to complete these experiments.

With that amplifier, I could easily obtain 100% modulation, with a nice clean sine wave, at full legal limit power ... with a flat modulation frequency response and no visible distortion from 30 Hz to 9.5 kHz (as far as I measured at each end)...... [text deleted]

.....Needless to say... I am pleased with the results.

This completes my experiments with the use of toroidal power transformers as modulation transformers.

Best regards
Stu




Excellent!  This certainly has changed my approach to my future legal limit HB AM rig.  I will be scrapping the idea of using tubes as a high power modulation source.  My plan would be to connect a solid state high power AF amp to an intermediate transformer to get the 8 ohms to ~ 500 ohms.  From there my UTC CVM-4 will take over and match just about anything I build by way of RF deck.  Another nice outcome of your experiment is the demonstrated use of a GG triode amp.  No more neutralization issues.

Are you going to have this setup on this weekend?  I wanna here it

73, Al

Steve - WB3HUZ:
Quote

I am using a Johnson Ranger (running at 30 watts of carrier) to drive the plate modulated rf power tubes (a pair of GS-35b's) in grounded grid configuration***. I found that I could obtain significantly improved linearity by also applying some plate modulation to the Ranger. I.e. about 20% plate modulation of the Ranger.... just enough to get equal positive and negative peaks on the output of the rf power amplifier, with a nice, symmetrical sine wave on the output of my off-air envelope monitor.


I would think this would be required since the drive power feeds through to the output. With no modulation of the drive power you could never achieve 100% modulation (there would always be unmodulated power in the output).

WA1GFZ:
Very cool Stu and I like the test set up on the floor.
my old 4-1000A had a stack of Zeners for bias and I could bias it well into cutoff. I also measured high efficiency in class C but it takes more drive.
I wonder if a 2  or 3 dB pad between them would help the Ranger. 

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