KRIS "Big Boomer" Linear Amp.

Discussion in 'Amateur Radio Amplifiers' started by AC2YL, Dec 31, 2018.

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  1. W1QJ

    W1QJ Ham Member QRZ Page

    The NCX-5 was rated at 200 watts INPUT continuous. That relates to about 120-130 watts output at best with those tubes.
    N2EY likes this.
  2. KN4CTD

    KN4CTD Ham Member QRZ Page

    Thanks for the good information. Like I said, I do need to do more reading and studying in regards to IMD, harmonics, etc. I hope your bashing everyone because of s few comment wasn’t directed towards me. I simply stated my opinion about 11 meter operation. I personally do not care how any of them operate or talk on the air, so long as I don’t have to hear any of it on my legal station. To each their own. One thing I will say is that there’s also a lot of vulgarity and ignorance that goes on at night time on some of the Hf bands that is just as bad as anything I’ve ever heard on channel 19. Which surprised me after getting licensed. I guess there’s good and bad aspects to everything in life.
  3. AF7XT

    AF7XT Ham Member QRZ Page

    I'll bypass all the emotion and misinformation.

    I recommend putting negative bias only on the control grid. That requires pulling pins 2 and 6 loose from ground on each socket. Leave the other two grids grounded.

    A wire is added to join the control grid on one tube to the other. A .01uf disc cap goes from one grid pin on each tube to ground. We use a 1k 5 Watt resistor to ground from one socket's grid pin. The type of this resistor is not critical. A wirewound type is perfectly suitable, since this is a DC-only circuit with no RF voltage on it.

    I use negative 8 Volts bias for this type tube. The two tubes' heaters are in series. Where the two heaters are connected together, you get six Volts AC. If you connect the cathode (banded) end of a rectifier diode like a 1N4002 to this point, you'll get negative 8 Volts DC by adding a 1000uf 25-Volt filter cap to the not-banded (anode) end, positive side of the cap to ground, negative side to the rectifier.

    One additional rectifier gets connected, the cathode end to the filter cap negative and the anode end to the grid pins on the tube sockets.

    But that's the easy part. If you put a SWR meter and a second coax jumper between the radio and the Afterburner * BEFORE* modifying it, key the radio with the amplifier tuned up normally and check the SWR feeding into the amplifier while the Amp is keyed.

    You'll probably see a reading of 3-to-one, maybe more.


    Adding bias will usually just make this reading worse.

    The fix is to add a coil between the relay's input side and the cathode pin (3) of the tube sockets. The coil just goes in place of the wire that's there now. I don't have a record of the best-size coil, but 6 turns of insulated solid hookup wire wound on a half-inch form is a good starting point. The coil alone usually won't bring down the SWR enough. You'll also need a capacitor around 100 pf from the relay side of the coil to ground. A mica compression trimmer cap like a Arco 464 will let you adjust for lowest input-side SWR. The coil might need to get squeezed or stretched for lowest input-side SWR reading.

    But this is the non-advertised side effect of adding grid bias to this kind of amplifier. It will change the input impedance. When this amplifier was sold, most base radios had a tube in the final stage. Tubes are more forgiving about SWR than solid-state transmitters. Probaby didn't matter as much using a tube-type radio.

    Biggest advantage to adding fixed bias to this kind of amplifier is that it prevents the tubes from overheating and blowing up when the radio's carrier is turned down to a reasonable level. The term "zero bias" means that the grids are connected to the same DC voltage as the tubes' cathodes.

    A true zero-bias amplifier like this one depends on a high (4-Watt or so) carrier to control the tube current. Sounds backwards, but the lower you turn the radio's carrier, the higher the current drain the tube takes from the high voltage supply. This makes the tubes overheat if the radio's carrier is turned down. When this amplifier was sold, there was no such thing as a carrier-power knob on a base radio.

    Adding fixed bias holds the tube current to a safe value when the radio's carrier is turned down to a level that makes it sound good.

    You could feed the bias rectifier from 12 Volts instead of using the tubes' two heaters as a voltage divider down to 6 Volts. This would double the DC-bias to negative 16 Volts DC or so. My experience has been that this tends to be overkill.

    Your mileage may vary, but adding bias will nearly always make the tubes last longer.
  4. KE0ZU

    KE0ZU Ham Member QRZ Page

    Thats 500 Watts power input, of course PEP output is roughly half to 70% of that depending on efficiency. Here it is, rite from the manufacturer;

    The B+ on the NCX-500's PA was 1100V (upper pic) as opposed to the 700V in the NCX-5(bottom pic). The power transformer in the two supplies was the same, but the 500 used a Cap input filter, while the NCX-5 used an choke input filter.

    So, my NCX-5's power supply is now a cap input filter, so....


    Last edited: Jan 11, 2019
  5. AC2YL

    AC2YL Ham Member QRZ Page

    My original intent was to get opinions on if the "Boomer" amp was a viable piece or of any worth.
    The thread then just took off with SO MUCH knowledgeable amplifier and tube based opinions and information!
    I LOVE IT!!!
    Obviously I posted this question in the right place.
    It appears that I have a LOT more education in store for myself compared to all the good folks that chimed in here and I'm looking forward to it.
    (Besides, I have always had a soft spot for "fire and glass" equipment; even more incentive)

    For what it's worth and to put some at ease regarding it's use, I'm picking up a Heathkit SB-200 tomorrow to add to my station setup. The "Boomer" will sit on a bottom shelf looking somewhat attractive and literally holding the bench down.
    THANK YOU ALL for your advise, opinions and DEEP knowledge of tube amplifiers!!
    Please keep it coming!!
    73's all
  6. W1QJ

    W1QJ Ham Member QRZ Page

    We are NOT talking about the NCX500 he said NCX5 BIG DIFFERENCE
    N2EY likes this.
  7. KF5FEI

    KF5FEI Ham Member QRZ Page

    Yep. That is when I got into it as a teen. There were a few flakes, just like amateur radio, but many knew their stuff.
  8. AF7XT

    AF7XT Ham Member QRZ Page

    it's all RF to me
    AA7QQ likes this.
  9. AI3V

    AI3V Ham Member QRZ Page

    A classic read:


  10. KE0ZU

    KE0ZU Ham Member QRZ Page

    So, you and your mouse "WE", are in charge of the conversation, is that correct?

    I was the first to mention National, as in the model NCX-500 in post #19. Rege, refering to post #19, made a comment about 6LQ6 specs in #30, which is correct by the way, with NO other mention of a specific radio. Then, you mention the NCX-5 for the first time, in #31.

    I responded to Rege's post in #34. Up until that post, neither He, nor I, had made mention of anything other than the NCX-500 and 6LQ6's, until I included the comparison of the two models in an attempt to address your NCX-5 comment in #31. Further to the conversation, the NCX-5 didn't use 6LQ6s.

    I apologize for the information overload. Have a nice day.
    Last edited: Jan 11, 2019

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