3CX3000 Decade amplifier

Discussion in 'Technical' started by KD6VXI, Mar 12, 2017.

  1. KD6VXI

    KD6VXI Member

    Newest project. 3cx3000 powered, 4kv at 3A CCS Pete Dahl iron.

    425 watts pep of drive in these pics. 100 Watt carrier.

    No, it won't be ridden this hard in day to day use...... But, my dummy load is 12 x 600 ohm / 600 Watt resistors in mineral oil, so I needed something to boil it every once in a while.

    This is actually a rebuild for a friend. He has a Bauer, a 4x10 and this is just another toy in his box lol.

    Covers a decade of bandwidth, no taps.

    IMG_20170312_160230.jpg IMG_20170312_160243.jpg
     
  2. KD6VXI

    KD6VXI Member

    First pic, carrier. Second, fully modulated.

    I have no want or need to know what it will do in ssb. Lol
     
  3. WQ5Q

    WQ5Q Member

    Very nice project. I had thought about doing this. I've also thought about using other tubes as well. The 3CX3000 is supposed to be very linear and low IMD in linear amp service.

    Here's something I've been wondering about - it's not meant as a critique of what you're doing - nothing of the kind, just trying to understand the merits of using a linear amplifier for AM in case I want to try it:

    At 4 kv a 3cx3000 is rated for 6000 w output in AB2 cathode driven. If you're using it as a linear and driving it with an AM signal, I read somewhere that you have to cut back to about 25% of full output for the carrier. In this case, the carrier output should not be more than about 6000/4 = 1500 w max. Since each of the sidebands is 1/2 the carrier power, about 1500/2 = 750 w or less is outputted for each sideband. 750 w would be a strapping signal for AM. Compared strictly on a sideband to sideband power basis with a SSB signal, the AM signal would be about 1/2 of a legal limit SSB signal at 1500w PEP or about 3db or 1/2 S unit less.

    upload_2017-3-12_21-58-22.png
     
  4. WD5JKO

    WD5JKO Member

    I am attaching a spreadsheet from fellow member Al W1VTP.

    It shows that at 100% AM modulation (a tone), each sideband is -6dbc or 25% of the carrier power. That is a disadvantage of AM over SSB.

    I lessen this situation with my Gonset station (G-76 into a GSB-201) by a combination of methods:

    1.) Use a D104 with K7DYY (tweaked) processor in the UG-8 stand. Gets average up, not too much bass, and added clipper to limit transient peaks during processor attack time. My asymmetrical speech requires me to get the MC-320 Astatic element properly phased.
    2.) G-76 changes that allow 40 watts carrier out @ > 125% positive peaks on voice. I reduce the B+ to the final, and add a negative peak attenuator (soft clip).
    3.) GSB-201 operates the four 811A's in class BC (12v zener bias). The bias at cutoff results in a transfer function that on AM, the peak modulation on the output is at a greater depth than with the excitation. Operating a AM linear in class BC lowers the gain (my case gain drops from about 10 db to 7 db), and also increases the efficiency. (> 33%)

    The end result of these changes is that this station produces 180 watts carrier and modulates to > 140% positive peaks. Using Al's calculator, 141% modulation results in the sidebands at -3dbc or 1/2 the carrier level.

    Jim
    Wd5JKO
     

    Attached Files:

  5. KD6VXI

    KD6VXI Member

    I wrote a spreadsheet similar a while back, while working on my assymetry board.

    It calculates voltages involved, etc.

    To the person above you. At 6kw pep and 100 pct symmetrical modulation, your looking at 4 times carrier power, so a 3cx3000 will do a legal limit carrier. Forever.

    The 3000 is rated for 4.8kv in those guidelines above. I'm running almost 6 kv unloaded. I have completely variable bias from 0 to 40 volts, plus an additional 7.5 volts I can switch in if need be... It will cut off completely, which is what I wanted. Dreams of outboard pulse width modulators (my current pwm will reliably switch 1.5 kv, need more hv standoff) are on the horizon.

    The pics above are with the 15 diodes in series with the cathode return, so about 7.5 volts of bias.

    --Shane
    KD6VXI

    Added. Don't know why, but it won't let me add the spreadsheet.
     
  6. WQ5Q

    WQ5Q Member

    The reason I started thinking about this was watching night time west coast AM and SSB signals on a friend's Flex 3000 panadapter (I'm in the Phoenix area.) The SSB signals were mostly huge compared to the AM signals that night. The AM signals that were the most "competitive" were guys running broadcast transmitters.

    Jim, so taking your enhancements to the Gonset station and scaling it up to published 3cx3000 levels, 33% carrier on 6000w is 2000w. If modulating at around 140% positive peaks, then each sideband would be 2000w x .5 = 1000w. So this ups my earlier estimate from 750w to 1000w not taking into account the compression you're doing.

    Thanks too for the spreadsheet. Very handy.

    Shane, that is one heck of a transmitter. Hope you can share some pics. Thanks for the comments and further description.

    Kelly
    WQ5Q
     
  7. KD6VXI

    KD6VXI Member

    Kelly,

    I popped a tuned input cap, so have some Russians coming. And as such, I'll be having to yank the rf deck.

    I've tried posting a couple pictures, but I can't post anything here? Do you have to upgrade to post and download?

    This is for a relatively unknown contester, who is setting up. He has a 4x10 for 160 to 20 with servos on CTune and CLoad as well as vac relays for each plate tap. Yes, it's truly remoted. This was built for the upper bands for him (it will row through just below 20 mhz to just above 30 now) It's going into a C Train with an ANAN for an exciter. Out in the middle of Nowhere!, on protected land. No Internet, that will be brought in with a Hotspot and a beam for the cell phone repeater. Truly a VERY low noise spot.

    He's a good AM guy as well, has a Bauer 707 sitting next to the 4x10 and this goes next to that.

    Once I get up to speed on the remote side of things, this will get servos on tune and load as well as the input. I just built a homemade CNC machine, so getting up to speed on steppers, etc now.

    Because it's a remote amp, I really wanted to go tetrode. But with the rf generators so cheap, it was a no brainer to buy a Henry and convert.

    Anywho, that's the point of this amp. It's going into a semi hostile environment, where you honestly will need a 4x4 to get to a month or so out of the year. Building an amp capable of 10 kw and running it at 1.5 means we won't be worrying about pesky swr, etc as he rows through upper hf.

    Myself, I'm going slightly smaller. I bought a 3000 for 160 to 40. 8877 above that. My 8877 has a power supply from a 3000 Watt generator though, so it's pretty beefy, 4kv 1a ccs. For the 3000 low band amp, I have here a Pete Dahl 4kv 3a ccs, identical to the power supply in this amp.

    --Shane
    KD6VXI
     
  8. WD5JKO

    WD5JKO Member

    I use Seamonkey at home, and Firefox at work to upload files and images. No issues except I was once denied uploading a PowerPoint file.

    just uploaded a file from my desktop, a few clicks, and done.

    Jim
    Wd5JKO

    400W Class E Overload Ckt.jpg
     
  9. KD6VXI

    KD6VXI Member

    OK. Figured out the issue. Was trying to link from my Google Drive. Downloaded directly to my phone and chrome uploaded correctly. Problem solved.

    I'll grab some pics over the next couple days and upload.

    --Shane
    KD6VXI

    IMG_20170312_155911.jpg
     

    Attached Files:

  10. KD6VXI

    KD6VXI Member

    Now that I look at it, that may not be the one I wrote from scratch and the one I posted may be the one I derived it from.

    Been so long, I can't remember now.

    Anyway.....

    --Shane
    KD6VXI
     
  11. WQ5Q

    WQ5Q Member

    Shane, I would love to be out in the country in a relatively quieter area but maybe not that far out. He probably would benefit also from some directive receiving antennas on the lower bands. Is he planning to do that also?

    I'm working on a 8877 amp right now as time permits. The 4kv power supply is done. The transformer weighs about 30 lbs. and was used in a Henry 750w RF generator of some kind.

    Here's a picture before I settled on the final layout and started wiring it. I etched the circuit boards myself. The primary circuits are on the other side of the cabinet. This transformer would be fine for 1500w CW or SSB duty cycles but not AM or RTTY at legal limit.

    upload_2017-3-16_19-43-21.png


    I have an old RCA broadcast transformer that weighs about 120 lbs. and puts out 2900 vac. To get up around 4kv I'd have to use a capacitor filter (no choke.) I know it was designed for a choke input, so I don't know if this is a good idea.

    Kelly
    WQ5Q
     
  12. W5HRO

    W5HRO Administrator

    Stop it! you are making me think about my single 4-1000A 6kV linear supply and project again. If I build it I might be tempted to use it on 60-meter USB and it will be your fault :mrgreen:
    .
     
  13. KD6VXI

    KD6VXI Member

    I did a 4-1k at up to 6.5kv before. I mean, their are some impressive numbers the 4-1k will attain. It's been stated that tube is more like 1.5 to 2kw PDiss in amateur use. From what that old amp did, I believe it.

    Nice looking power supply. I have an 8877 project here, it's another generator I bought with a 3000 based generator. Both had a single phase breaker, so yes please!

    When I got the units here and opened up, the 8877 has the bigger 3000 based plate supply in it! But, I will say this about the ECA transformers.

    They will take cap input fine! And their icas / amateur / whatever you want to call it is very good. In the Henry 2000D you can convert it to a cap input filter and the transformer will deliver 2A on mod peaks! For their '1 A rated' choke input xformer! I think you'll be pleasantly surprised on yours.

    I really went on a tube buying spree in the last decade. I need to build some stuff!

    --Shane
    KD6VXI
     
  14. WQ5Q

    WQ5Q Member

    So many projects, so little time. :mrgreen:

    I plan to use the 8877 amp as an "afterburner" for the Viking II or maybe later a Flex 3000 running AM, when needed, as well as other modes.

    Shane, in the late 80's I had a broadcast pull 4-1000A that I paid $20 for (I could have bought a box of them) along with all the other bits needed but never got around to building the amp. I still have the Peter Dahl filament transformer and Eimac chimney. I may still build it if I run across a deal on the tube.

    Thanks for the feedback on the RCA transformer. I have a smaller one that's about 90 lbs. and a big RCA choke to go with it. The secondary on that one is center-tapped. It may go in a 600w AM transmitter build someday. Yet another project.
     
  15. W5HRO

    W5HRO Administrator

    There is also a trick you can use when you need a higher voltage supply as long as the transformer has the extra current needed to do it.

    I have a 3000V @ 1A CCS transformer that was designed for a full-wave bridge. It has no center tap. I figured out I could use the voltage doubler trick to run one 4-1000A at 6kV. One tube at that voltage in grounded grid draws around 500mA max.

    The circuit below will take the 3000V transformer up to 6000V with at least 500mA of current and the output is super clean. I found a source for 10uF and 20uF oil caps that I could use on the output side so I wouldn't need another big bank of series caps like on the input side. The input side uses fourty 200uF 500V caps which I found as well. That equals 20uF on the input side so each side only needs 20uF. The whole thing just uses a high wattage 200 to 300 ohm series resistor to make it work like a choke supply.

    supply.png

    Anyway, the trick above will work when you need more voltage and this was in my 4-1000A amp topic until Clark hijacked the thread :icon_yawn:
    .
     
  16. KD6VXI

    KD6VXI Member

    Clarification.. I wasn't speaking of the RCA transformer. I was speaking of ECA. That's who made most of the transformers for Henry.

    Your going to require a dB pad between the 8877 and viking.. Or a variable power mod in the viking. That 8877 will cook quickly.

    Maybe you really need two rf decks. The 4-1000 for the Viking and the 8877 for the flex.

    Yes, that's what you need, another project lol.

    --Shane
    KD6VXI
     
  17. WD5JKO

    WD5JKO Member

    I once ran the numbers on an amplifier for a 100-watt plate modulated transmitter. In stock form, figure on 400w PEP of output. An 8877 can put out full output with 40-60 watts depending on the plate voltage. Think of a low mu triode amplifier that has a ton of feed through power when in grounded grid service. A pair of 304TL's, or better yet, three of them in G-G. The stage gain will be around 6DB, and each tube takes around 200 watts of drive, and uses 25 watts of that. The rest goes straight through to the output! In the DC input days, you could use this concept to run < 1KW DC input with an output of > 1KW!

    So that is the ticket 2 or 3 304TL's in G-G running legal output from a Viking II without any attenuator needed. :-)

    Jim
    Wd5JKO
     
  18. KD6VXI

    KD6VXI Member

    Yeah, increasing the output of the bigger plate mod rigs isn't the easiest thing in the world.

    That's why God gave us the 3cx3000A7 and its 400 Watt pep drive need :-)

    --Shane
    KD6VXI
     
  19. WQ5Q

    WQ5Q Member

    We could call it the Viking War Hammer...:biggrin:

    upload_2017-3-19_7-59-3.png
     
  20. W5HRO

    W5HRO Administrator

    Just call it the Johnson Thor amplifier :lol:
    .
     
  21. W1VTP

    W1VTP Member

    Where the he!! does one get new old stock 305TLs these days. I gave up after my second tube

    Al
     
  22. WD5JKO

    WD5JKO Member

    In my Shed Al.....

    Jim
     
  23. WQ5Q

    WQ5Q Member

    I saw one of these looked very good at recent local hamfest for around $30...maybe I should buy it if I see it again. Also a couple of the smaller ???-TL ones for $25 each. The guy is at every hamfest. A few years ago I wouldn't have expected them to last but now seems few people are building anything like that. I see all kinds of QRO stuff that no one is buying.
     
  24. W1VTP

    W1VTP Member

    Jim: please excuse the drool! :smile:
     
  25. WQ5Q

    WQ5Q Member

    LOL! Al, I was thinking the same thing.