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Thoughts on building a 3CX3000A7 amplifier

Discussion in 'General Technical Questions and Answers' started by N5FOG, Dec 2, 2009.

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  1. N5FOG

    N5FOG Ham Member QRZ Page

    Before I get flamed about "illegal power" I'm looking to build a amp for running 160 and 75 meter amplitude modulation at legal limit (350-375 carrier and 1500watts at 100% modulation).

    I have no desire nor need to run illegal power I just want to build a bulletproof amplifier that won't break a sweat running a demanding mode like AM at legal limit.

    Also I cannot justify the $$$ for a 4KV at 2 amps CCS transformer capable of running the tube at its full abilities so it would only get a 4KV at .75 or 1 amp CCS xfmr in its power supply.

    So with all that said I was looking at all the tube specs and prices and came on the 3CX3000A7. It has a 4,000 watts of plate and 225 watts of grid dissipation so running at legal limit the tube would leave ALLOT of headroom so it would stay linear and keep distortion down.

    Now for the questions.

    With the cost of a import 3CX3000 being around $700 or a genuine Eimac being $1200 why are they not as popular as a the 8877/1500A7 ??

    I know the 8877 has about 3dB more gain than a 3CX3000 (15dB and 11dB respectively) but aside from that why don't more amp makers just use the 3CX3000 but with a power supply only big enough to run it at legal limit so they can get FCC approval?

    I do realize that the 3000 needs about 1 KV more than the 8877 and uses 400 watts for the filament. But is there any stability issues or other reason I've over looked as to why the 3000 isn't used more ??

  2. K7MH

    K7MH Ham Member QRZ Page

    Sounds like a great way to get to know your neighbors!!:D

    What kind of drive does the 3CX3000A7 need to get to 1500 out?
  3. N5FOG

    N5FOG Ham Member QRZ Page

    From Google-ing around the net I found that the 3CX3000A7 was used in the Henry Radio 8K ultra. The 8K would provide 1500 watts with 60-70 watts drive.

  4. KC8VWM

    KC8VWM Subscriber QRZ Page

    On the other hand, newer homebrew amp design techniques typically involve using far less parts, less heat involved, much lower (safer) voltages are used, PCB construction techniques are employed, cleaner output through more organized design, less rats nest wiring to get tangled up in, better harmonic suppression techniques are used, and the use of lower cost, yet high yield solid state parts are used by comparison.

    Cost of construction is usually another consideration. For example and HV transformer is expensive. (I know..duh! right? :) ) But since, newer solid state designs don't require HV to operate, you end up lowering your actual costs of construction in the process.

    For example, Google the $50 solid state # BLF177 RF transistor for example... It only requires 48 VDC, It has a 300-500 watt output rating with only 10 watt drive, (attenuator recomended) has an efficiency rating of 68% and it can even withstand a VSWR of 50:1 !!! etc.

    Extremely rugged and incredibly efficient when compared to (old) tube amplifier technology by comparison.

    $250 worth of FET's like Philips BLF177, a few PC boards and a weekend at the bench, will provide you with at "least" legal limit power (visit for more information) which operates from 160-10m at a fraction of the cost, about 1/4 of the physical size and without any heart stopping HV involved in the process.
    Last edited: Dec 2, 2009
  5. N5FOG

    N5FOG Ham Member QRZ Page

    I've built my fair share of amplifiers for 70cm and 1.2ghz back when I was big into ATV.

    For under 300 watts solid state is great. But when you go above 300-500 watts the vacuum tube is still and probably will be king for a long time.

    You can use a single tube were with solid state you would need multiple devices, power dividers, combiners, ETC. Also a tube is by fair more tolerant of abuse and "out of spec" operations than a RF transistor.

    To build a legal limit solid state amplifier than could handle the rigors of running AM would cost ALLOT more than building a tube amp. That's why a legal limit Ameritron is less than half the price of a legal limit ICOM or Yaseu solid state amplifier.

    Also as I found out twice a amplifier can become a doorstop really quickly when the manufacture discontinues them which happens ALL the time. Just look at the SG power cube. The earlier versions had a RF power transistor that is no longer made and what stock is left is VERY expensive.

    With a tube amp if the tube become unavailable, plenty of other tubes can be adapted with minimal changes.

    Last edited: Dec 2, 2009
  6. KC8VWM

    KC8VWM Subscriber QRZ Page

    These FET transistors are widely used by the commercial FM and TV industry. As such there is hardly a "supply issue."

    The $50.00 Phillips BLF177 and their equivalents are very commonly used in many Harris FM broadcasting transmitters for example.

    It just sounds to me you want a tube amplifier and not because it's any better, exhibits far less harmonics, FET's exhibit less distortion, are operationally more stable or even the fact FET's are even more available and cheaper in comparison. 'Cause any scope and anyone's wallet will tell you they aren't. :)

    You do realize an Eimac 3cx3000A7 costs $1,500...

    ...and the socket alone costs about $300.00

    How this could possibly end up costing less than an FET amplifier of equivalent output power and operational capability is beyond me.

    My best and do have fun with this project.
    Last edited: Dec 2, 2009
  7. W0GI

    W0GI Ham Member QRZ Page

    First of all, the FCC doesn't only approve HF amps that can only produce 1500W or less. There are many high dollar amps that can go way over 1500W. It is our responsibility to limit the power.

    As for the 3CX3000, it is a fine tube, but an 8877/3CX1500 has lots of headroom over our maximum power limit, so at 1500W the 3CX3000 doesn't really have any advantage over an 8877/3CX1500.

    On the other side, the 8877/3CX1500 has more gain. In practical terms, that means I can drive it with my Mark-V running Class-A, and have a very clean signal. It also means, as you mentioned, less transformer needed for the filiment and HV.

    The reality is that the 8877/3CX1500 will do the job.

    I am not a rich guy, but the Alpha 9500 is one hell of an amp using an 8877/3CX1500. A 3CX3000 can put out more power, but when you have a limit of 1500W, the gain of the 8877/3CX1500 alone makes it the right choice for a ham amp.

    If you are building, there is nothing wrong with using the 3CX3000. It doesn't cost more, but you aren't going to see any benefit when you are limited to 1500W.

    But I just have an old Henry with a pair of 3-500Z Eimacs. I would love to have an amp with a ceramic 8877. But then, the Henry does work ok. :)
  8. N5FOG

    N5FOG Ham Member QRZ Page

    Charles I never said there is a supply issue with the transistor you listed. I said that RF transistors get discontinued all the time and when they do they get very scarce and VERY expense QUICK.

    There is plenty of Harris and other solid state commercial broadcast gear that is useless today because of discontinued RF transistors.

    Just look at the earlier runs of the SGC power cube. They used a MRF-454 transistor which just a couple of years ago was very plentiful and cheap. But now they are just about impossible to find and when you do they are BIG MONEY.

    Also for me building something like a tube amp is allot easier because I don't have to deal with designing the PC board for the transistors to mount on.

    Also I don't care how you slice it a tube is far more resilient to abuse especially from ESD (electrostatic discharge). If you get a near by lighting strike with a solid state amp its toast. With a tube amp your chances of massive damage is much much lower.

    Its not about an fixation on building a tube amp. Its about building a amplifier that will be built like a battle tank and be fully working and serviceable 30-40 years from now. Not something that has to be massively re-engineered or scraped because a part has gone out of production.

  9. N5FOG

    N5FOG Ham Member QRZ Page

    I wouldn't say it doesn't have an advantage over the 8877. The 8877 has only 25 watt grid dissipation and a 1,500 plate dissipation.

    The 3CX3000 has a 250 watt grid dissipation and a 4 kilowatt plate dissipation.

    The plate isn't as much of a big deal as the grid dissipation is. With a 250 watt dissipation you couldn't burn up the grid under normal legal limit operation even if you tried to.

    But with a 8877 you can over current the grid really easy and destroy the tube really quick.

    That and you can get a 3CX3000 for less than a 8877.

  10. W0GI

    W0GI Ham Member QRZ Page

    That's all great, but with a properly designed system it doesn't really matter.

    If you are going to experiment, then I see the point, but at the same time, are you trying to convince me that an Alpha 9500 is running on the edge of failure?

    It isn't.

    A ladder that is rated at 900 lbs is stronger then a ladder rated at 400 lbs.

    Since I weigth 180 lbs, it really doesn't matter.

    I'm not sure what you are arguing about here. Yes the 3CX3000 is a tube that is rated for more output. AND????

    I wont get into an argument over this, as I don't sell amplifiers.

    Take it to RF Concepts/Alpha, as they are the people selling those inferior amps using the 8877. :)

    I have no problem putting out 1500W legal power with my Henry and two 3-500Z tubes. But then I just worry about communicating with other hams, and not what tube I am using.
    Last edited: Dec 2, 2009
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