6146B with separate screen and plate modulation

[WR6J]

In the last few weeks I found the time to return to experimenting with home brewing an AM transmitter. I had gutted a chassis for a Dentron 160M transverter and started to build up a 6146B transmitter. I had intended it to be screen modulated but decided to try a different approach. Having built some tube audio amps over the years I had a couple of Hammond 125ESE single ended output transformers (rated at 80ma and 15W). Having seen others do it, I decided to use one driven backwards by a SS amplifier as the plate modulation transformer with the intention of producing about 15W carrier to go into an SB200.
In the process of trying to get the rig up and running I was having difficulty getting screen voltage right via a dropping resistor. So I decided to take the 350V HV line from the HP-23A power supply I'm using and regulate it to 180V with a couple of VR tubes. Having got the tank circuit sorted and generating a good carrier I realized I could try modulating the screen with the second 125ESE, driven by the other channel on my power amplifier.

Long story short, it works very well. With separate volume controls for each channel, I can dial in the plate and screen modulation separately. My experiments reinforce the need to modulate the screen - with just the plate I can only achieve around 30-40% modulation. If I modulate only the screen I get a more distorted result. The combination works FB. I'm monitoring the output with an REA Modulation Monitor, which allows me to listen as well as look. At 15W output, I can get around 120-130% positive modulation without hitting 0% on the negative. If I drive the SB200 to about 100W carrier I have enough headroom to keep that headroom and not stress it too much.

Attached is a schematic for reference. I'm using a FT-817 at 0.5W as the exciter. So far I have it running on 80m and 40m. Further experiments include using a choke to get the screen to self modulate and adding some kind of negative peak limiter.

If you read this far - thank you!

73
Richard
WR6

[WR6J]

Here's an audio clip recording from a local SDR.

Richard

[KD1SH]

   That does sound very nice. I've used a larger Hammond output transformer, the 1628SEA, in an external modulator, and it's worked very well, with excellent audio reports. In my last homebrew rig, which I modulated with the 1628SEA, I used this approach to provide modulated screen voltage to a 6146B: http://www.amfone.net/AMPX/71.htm.  Scroll down the page to "Class C Optimization for Ultra Low Distortion."
   Others with more knowledge than my humble self may have different opinions, but to me, your idea of using a separate transformer and audio-amp channel to modulate the screen is quite ingenious. With your two separate volume controls, you can adjust the ratio of plate to screen modulation while leaving the actual DC voltage on the screen at a fixed value. Pretty cool!
   Have you taken any measurements to determine the best ratio of screen modulation to plate modulation?
   The only issue I might have worried about would have been ensuring that the modulated waveform on your screen was exactly in phase with that on the plate, but it seems that's not an issue with your setup, since it sounds great. Nice job!

[WR6J]

Thanks for the nice comments. I also have a 1628SEA that I have been using in some audio amplifier experiments. I've tried it with some external modulation of a Valiant too - more of that in another post.

I haven't tried a trapezoid modulation test yet, since I don't have an RF sampler. Though I have an idea I will try in the next few days. So far I've just been going by how it sounds. I'll post when I have some results.

Yes, phase hasn't been an issue. Both OPTs are identical as are the channels of the power amp. I took care to make sure they were connected up the same way of course!

[Steve - K4HX]

Neat set up and good exposition on the need to modulate the screen too.

Seems to align with this.

https://www.amwindow.org/tech/htm/scrnmod.htm

[W3SLK]

Nice grab Steve. This subject seems to come up often when using tetrodes in Class C. Simplistic as it may sound, the venerable ART-13 modulation transformer used a tertiary winding to modulate the screen. Some have advocated the use of an audio choke in an effort to self-modulate the screen. This was pointed out to me when I was contemplating on using a 4CX250B(s) in Class C. Woody, N4MQ, found by adding a cap between the screen and the modulated B+ of his Johnson 500, he could increase his % of modulation. https://www.youtube.com/watch?app=desktop&v=yXZBU3OPZdc This was a topic of discussion between me and Dave, W2VW on 160M one night and as he pointed out, he would like to have seen what it looks like on the aforementioned trapazoid test.

[WR6J]

Interesting stuff! Thanks for the comments.

I haven't been able to do a trapezoid test yet - I don't have an RF sampler (except the one that comes with the REA Modulation Monitor) and I'm not sure my PC based oscilloscope is up to the task.

I did try self-modulating the screen with a choke and it worked ok. I tried a 10H choke and also two in series for 20H. The extra choke didn't make any difference from what I could tell. The audio was lacking some bass compared with directly modulating the screen. I suspect that's because I required more audio power and exceeded the 15W rating of the ESE125 - leading to saturation at the lower frequencies. I could try again with my 1628SEA (30W).

What was interesting to me is how much modulating power was needed. With a choke in the screen supply I needed 45W audio to modulate an input power of 94W to the final - as expected. I then tested with the screen directly modulated, and found I needed 20W of audio to modulate the plate and 2W to modulate the screen. That means that one of the benefits of directly modulating the screen is that it reduces the audio power requirements by about half. This would be useful when building higher power rigs.

I found the video on using a capacitor to modulate the screen fascinating. It seems so simple - I will definitely give it a try on this test rig. I was surprised that the Johnson 500 doesn't have a dropping resistor to modulate the screen. How does it function? I also checked the Valiant schematic and couldn't see where the screen is modulated there either. Am I missing something?

I'll report back on the capacitor experiement.

Richard

[WR6J]

I have done some more experiments with this rig. Here's what I found:

I ran some trapezoid tests with a 1kHz sine wave. The best I could achieve was using mostly plate modulation with a small amount of screen modulation to get the trapezoid as close to ideal as possible. Attachment "Plate_Screen_Best" shows the result. As you can see there is still a little bit of distortion close to -100% modulation. Positive modulation is 100%.

I tried modulating only the screen with no plate modulation at all. The resulting trapezoid is in attachment "100%_Screen". In this test I am achieving about 125% positive modulation and close to -100% negative. As you can see I have some distortion (flattening) of the positive and negative peaks.

What is interesting is that in listening the audio samples recorded from the REA Modulation Monitor, I cannot tell the difference between the two - they sound identical to my ear. This means I can achieve 125% modulation using no plate modulation whatsoever. So far I have only done these tests with 9W output, since this rig is being fed to an SB-200 to produce about 110W of carrier. I will try the tests again with the rig putting out 40W to see if I get the same results. If this approach holds at higher outputs and on different tubes, it could greatly simplify modulation of higher power transmitters. In the next year I hope to build a dual 4D32 rig with about 250W of carrier. I'll be interested to see if screen only modulation scales up that far.

I also tried the experiment of connecting a capacitor from the modulated B+ to the regulated screen voltage. I bypassed the screen modulation transformer so that only plate modulation was being used. I tried capacitors ranging from 0.025uF to 0.33uF and got unacceptable distortion. I think I was over modulating the screen since I was only achieving about 50% positive modulation with 100% negative. I could try a resistor in series with the capacitor to reduce balance it out. More experiments to come, but I'm not optimistic given the distortion.

I'm sure all these experiments have been done before - I imagine that screen only modulation with a transformer has limitations that I have yet to (re-)discover. Nonetheless it's fun to have a rig to try these things out for myself.

Richard
WR6J

[w9jsw]

Have you tried any measurements using just a screen dropping resistor? That is how I run my 813 rig.

[WR6J]

Have you tried any measurements using just a screen dropping resistor? That is how I run my 813 rig.

I don't have any protection for loss of grid drive on this rig, so I'd have to make modifications to use a screen dropping resistor. I'm running a regulated screen supply and fixed bias and so it's safe when not driven. I might try it at some point, but my focus has been on modulating the screen directly and seeing how little plate modulation I can get away with.

Richard
WR6J

[Steve - K4HX]

Good stuff Richard. Thanks for sharing.

[AG5UM]

Very interesting and well done, thanks for sharing this.
Keep us posted on your results, very nice.

 

[WA4WAX]

Try a 4D32.