Grounded Grid, Class-C, Plate Modulated AM Transmitter Using Eimac 3-400Z High mu Triode 

Hi Jim, always good to hear from you. I understand having to store projects away for a while. It can happen for many reasons. I always thought, theoretically, if I was ever along, my hobbies could help sustain me. I guess they do help. As much as the hobbies help, sharing and communicating with you all on RU-vid is a true blessing.

Fans can be a noise problem. The other day I had an issue when looking at the oscilloscope display and I could see an alarming looking display with the modulation. Turned out to be the floor fan blowing across the back of the microphone. Whewwww..... Got to keep the mic gain down and speak directly into the microphone to keep the background noise down when transmitting. I do hear a few guys with the mic gain up too loud and it sounds like they are in an auditorium.

Hi Bill, Thank You. Crazy projects I have, doing things a little different. I am blessed with six cats now, four are four-month old kittens. What a blessing to have them around. I have some more simple projects and small pieces of equipment I want to put on YT. They are more audio oriented which most people like the most. Maybe even another one tonight.

@@ElPasoTubeAmps It is good to hear that you have found some good company to help out! Gives you something to look forward to and I bet they are a comfort for you. Take care and stay well! Your knowledge is invaluable and your willingness to share is a treasure!

Thank you for your kind comments. I commented a time back that when I was a young fellow, I met some really good people on ham radio where we would be talking about building something and it could be mentioned that that I, or anyone else, needed something like a filament transformer to finish a project. On several occasions, I have had people just up and volunteer to send that component to me. I have been able to do that for others. But RU-vid and the group of fine people I have met here tops even that kindness. I hope to post some more simple videos soon. I have purchased some inexpensive audio equipment, mostly for ham radio, but I think viewers that like audio equipment might like them also.

Hi Manuel, good to hear from you and that you are busy building. I think about detail videos like you mention. That would take some planning, with schematics, and last a couple of hours. Years ago I made a multi-part video on Williamson audio amplifiers. I will have to think about the commitment for another set of videos. This transmitter is rather unique. I was not aware that, class-C, grounded grid (GG) amplifiers were so common until I started this project. GG has a lot of advantages for class-C FM amplification but high-level plate modulating it was something I could find nothing on - but I had to try it anyway, and it works quite well. It is a little more difficult to get 100% positive modulation than the classic grid-driven, plate modulated, class-C, AM transmitter but it can be done. Efficiency is that of a class-B amplifier (around 64%) rather than the 85%+ for a classic high-level modulated class-C amplifier. Can't have everything 🙂 Almost by accident I discovered that if I put a high value inductor (20H) in series with the ground and meter connection to the grid meter, it improves the modulation significantly. It allows the grid to self-modulate much like how we do the same to the screen of a plate modulated tetrode. Improves both the level of modulation and quality of modulation. Also, a high mu triode like the family of 3-500Z class triodes, require no fixed bias supply and all the bias that is required, in this case -90 volts, is from a grid-leak resistor. It also does not require neutralization. I will have to think and prepare before getting into a detailed video on this amplifier but it is rather unique and very stable. I would love to tell all the details of such an amplifier - actually better described as a transmitter as it is modulated and not just an amplifier. 73, Please keep in touch - David, WA4QGA

thanks David, just by what you just told i can really see how little i know haha. unfortunately i am having some health related break from the lab, this is why i am playing around with software lately. anyway i really hope to be up and ready to build something really soon, i am not giving up to alternative ways to heat the home in the winter! if it is in your comfort zone of work to put up such video i would encourage you to do so, this kind of knowledge is vanishing fast from the world and all the manuals i can find have to be translated from theory and math to real world so it is a lot of work,on top of that being in Italy all the manuals are a poor translation form English and very expensive even if you can find some. very nice to hear from you, stay safe and keep in touch! 73, Manuel IU3IRR @@ElPasoTubeAmps

Thank you for the information. I did look up WBCQ on RU-vid. I have only recently become aware of grounded grid, class-C operation as being much more common that I realized. I have not seen any schematics of anyone plate modulating the GG, class-C amplifier. I did it and it works but it seems to take more audio power than is required for the classic grid-driven, class-C amplifier like my push-pull, James Millen amplifier that runs a pair of 812A. I have incessantly looked for class-C, GG, plate modulated transmitters for schematics or manuals or any other information and can find nothing on the Internet. If you have any links to such information, I would greatly appreciate a link so I can learn more about it. Thanks 73 WA4QGA El Paso, TX

@@ElPasoTubeAmps WBCQ uses a water cooled tetrode . 250KW SW transmitter. Plate mod threw fancy reg power supply. The station I keep on air is 2 water cooled tetrodes. digital audio to HV supply and also the screen supply as well. saves a lot of TX power on the input side. 125% mod max on pos side and no clipping on neg side. old rules which was on back up transmitter is 99% mod no clipping. draws way more power for less to antenna. the am station buy me is mosfets class D . much less power . that just air cooled. 73's

Still kicking and playing with high voltage... Interesting you ask about the 6H3N tube as that is what is in that preamp/headphone amplifier I show in this video. I am going to guess that some enterprising fellow found thousands of them and thought about it and came to the conclusion he could get them for pennies as no one wants them, and build a bunch of instruments with them and sell each for, let's say, an average of $100 each. Well, 10,000 tubes would make him $1M. I do not know this but it makes sense to me. Once they are gone - they are gone forever. Just a guess of how these tubes have shown up lately. Seems like I found some data on the 6HN3 and they were used as IF amplifiers in receivers once-up-on-a-time. They are probably OK and will last a long time. Here is something on Ebay. They say they are GE JAN 5670. I think they were used for a while in VHF/UHF equipment. www.ebay.com/itm/223590552581?hash=item340f091405:g:FcgAAOSwz21dLyUQ&amdata=enc%3AAQAIAAAA4PGSBbTrS19KzZxJrheRfDIUTjKvcg8%2BP2A0efGiBwVSqEvMZZJ%2FLrAmFZ8hPjuMVVfJglghxzvjE6IKpZ4xnGcI0EgKC%2BenSIV8gG3%2FWnI5CqvKLWFvSqthv68H%2FzkyqS0ZSz9eRWOr40%2Fz4rxT0fbF40RXG4ezfgHl%2Bg98ND7%2BCQpFn8w3vJ9l6h1t1JeOS9DBN4We7BpxUw5Nmh7sRfiwuISUd%2BGoicxbaRiwqtpnCC%2F3yr51P3adE8GcR1%2BgdwgQhXEM8NiIcaQbhf7nCvb59d6oUCfQaNRSpfmlKCF1%7Ctkp%3ABFBM8NzY-r5i www.iwistao.com/products/vacuum-tube-6h3n-inventory-new-for-hifi-tube-amplifier-replace-5670-6n3-2c51-396a-high-reliability-hifi-audio?variant=40176763076723&currency=USD&gclid=Cj0KCQjwoeemBhCfARIsADR2QCvr3XI7k5o3r4riLQ5KozgPgj_0jNDx4_TQTFY93auYpu7QN2ioj0IaAlG5EALw_wcB Here is the datasheet for the 5670: frank.pocnet.net/sheets/093/5/5670.pdf

@ElPasoTubeAmps I like how they make them out to be a HiFi Audio tube. Which they may do quite well.

Thank you. A bit of a novel design but during its build I found something that really surprised me. Take a look at the Eimac 8877 (link below) also known as 3CX1500A7 and notice on the third page the, Class-C - Cathode Driven - Plate Modulated, rating for the tube. frank.pocnet.net/sheets/140/8/8877.pdf Very much a surprise to me but also encouragement for me to continue. Had a contact with JH7MQD last night on 40M and was quite a surprise to him I was on AM. Even the 3-400Z has a class-C, plate modulated rating of 550 watts of carrier output. Pretty tough little tube. 73 WA4QGA

Yes, single ended, class-C, cathode driven and high-level plate modulated. I have learned that grounded grid RF amplifiers running class-c are actually very common. Even in ham radio class-C would be the preferred way to run CW because of efficiency - also FM amplification - Grounded grid, class-C requires no neutralization (with the high mu triodes). This has been quite a learning experience. I will attach a tube datasheet for the Eimac 8877 and you can see especially on page 3 where they suggest a cathode resistor value in series with a positive power supply voltage and then bypass that for audio. They also recommend 83% modulation of the driving signal -not sure how accomplish that - any thoughts ? What am I missing? I added a 10H choke in line with the grid before the ground connection and it helped the grid self-modulate and increased both the level and quality of modulation. This is a completely new and novel adventure for me in RF amplifiers. Biasing a tube for class-B linear operation for SSB is no challenge but this class-C, high level plate modulation is completely different thing. frank.pocnet.net/sheets/140/8/8877.pdf

No, I don't worry about it one bit. Hasn't hurt me in 60 years or anyone else I know. The screen is there for protection from the 3000 volts inside - not the RF although RF shielding is a good, and sometimes necessary step to help keep RF feedback out of the microphone and audio equipment in close proximity to the transmitting equipment. On the Collins transmitter, the front is made of glass so, as you say, we can see the beautiful tubes glowing. As long as you stay away from the high voltage, the RF at HF frequencies (3 MHz to 30 MHz) is not going to be a problem from an amateur level transmitter. If you remember a few years ago, maybe even today, people propagate fear that holding a cell phone to our ear will give us brain cancer from the RF... wow... also, I have read where those "microwaves" change the molecular structure of atoms. 2.4 GHz Microwave ovens have a wavelength of about 6 inches, a massive amount longer than the atomic structure of atoms to change them at the molecular level. From the Internet: (An atom is so small that its size is difficult to imagine. One of the smallest things we can see with our unaided eye is a single thread of a spider web: These strands are about 1/10,000 of a centimeter (one ten thousandth or 0.0001 cm) in diameter. Although the cross-section of one strand is almost impossible to see without a microscope, it is huge on an atomic scale. A single carbon atom in the web has a diameter of about 0.000000015 centimeter, and it would take about 7000 carbon atoms to span the diameter of the strand. ) There is some very good stuff on the Internet but there is also a lot of misinformation.