Need help from amp builders

Discussion in 'Amateur Radio Amplifiers' started by KN4CTD, Nov 16, 2018.

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  1. KN4CTD

    KN4CTD Ham Member QRZ Page

    Sorry, I’ve just been “winging” it based upon schematics of other grounded grid Hf amplifiers using similar Russian tubes and 12.6v for filaments. I’m using a cathode choke (150 uh) instead of a ct on the filament transformer. The filament choke I’m using is bi-filar wound. It came out of a dual 3-500z amplifier. It’s overkill for the filament current requirements of these Russian tubes, but I had it and used it.
  2. KN4CTD

    KN4CTD Ham Member QRZ Page

    The power supply is a voltage doubler and it puts out 3100vdc no load. It dropped to 2750-2800 under the conditions where the amp was keyed without rf drive. So, it seemed as if the tubes were pulling a lot of current. No metering has been installed yet as I am still unsure how to do this. I have a plate current meter that is scaled to 1 amp that came out of the same dual 3-500z chassis that I’m building this into. It’s a yokogawa p9306ul chassis. It is supposedly the medical version of a Tokyo hy power hL-2k. Someone had already butchered the control circuitry inside in an attempt to convert it to run a Russian gs-35b tube. I completely gutted it all except for the tank circuit and input tune board. I installed a multitech industries bandswitch made for the tl-922 and set it up for six bands 10-160. The original band switch from the yokogawa had been arced badly from the previous owner and was junk. It was also a much more complicated band switching arrangement also. I completely redid the tuned input board to set it up for 6 bands also. It was originally “missing” parts for the 40 80 and 160 meter positions. I wound toroids (on red Amidon powdered iron cores) for 40 80 160 and added silver micas along with a variable cap for the pi network on those bands. All of these values were based upon numbers given to me by a foreign ham who has successfully built several multiband amplifiers based upon the same tubes. I’m sure the input circuit will need tweaking once rf drive is applied but I’m hoping it will get me in the ballpark to start. The output tank pi circuit has already been tested using my antenna analyzer and if my calculations for output resistance at my target plate voltage is correct (about 1600 ohms), then it should be fine. I used a 1600 ohm resistor to ground looking into the tank circuit and was able to tune all bands close to 50 ohms and low swr. I wound my plate choke using Matt Erickson’s method of stuffing a powdered iron rod into a 6” Delrin rod. I think I wound it somewhere between 130-180uh. I completely rewired all the control circuitry to be simple. I’m using an electronic timer board I purchased that runs on 12vdc. When power is switched on, the filaments light and the board counts down 3 minutes before applying 12vdc to the main power relay (12vdc coil) that supplies the 240v mains voltage for the hv transformer. It also holds off the 12vdc that powers the 3pdt relay used for input/ output/bias switching. Sorry to write a book just trying to give some more info since I have no schematic. I’ve done a lot of work on this project and I really want to get this thing figured out
  3. W1TRY

    W1TRY XML Subscriber QRZ Page

    May I suggest you draw the schematic of your intended design at this point? You really should have circuit documentation for the future and it will also provide those offering help some visual guidance. Do it in pencil, it's gonna change...
  4. KN4CTD

    KN4CTD Ham Member QRZ Page

    I’ll attempt it. I’ve never drawn a schematic of anything this complicated before. Never tried either so I guess I’ll give it a go.
    W1TRY likes this.
  5. W1TRY

    W1TRY XML Subscriber QRZ Page

    Imagine telling the story of building something before you knew how to draw the schematic. It ain't bragging if you did it! OF COURSE you can do it.
  6. KM3F

    KM3F Ham Member QRZ Page

    Appears that you should take a couple months off to study amplifier circuits and vacuum tube theory before continuing.
    Hope your taking enough precaution dealing with high voltage or the time off could be permanent.
    I'm not trying to be hard nosed but this is the first help you need now at this point.
    Good luck.
  7. KN4CTD

    KN4CTD Ham Member QRZ Page

    I’ve been reading and studying about grounded grid amplifiers and tube theory for the better part of a year now. This stuff doesn’t come as natural to me as others. I tend to learn more with a mix of reading and hands on approach. I do need to study metering circuits more though. I’ve read W8JI tutorial on this but still can’t quite grasp it. If my plate current meter was wired safely and reading correctly, I may have more confidence in truly knowing what is going on. I take all precautions around HV. One hand in pocket rule and chicken stick everything. If I am trying to read my idle current wrong, then maybe I don’t have a problem as I didn’t actually check the bias voltage. But it sure acts as if the tubes are pulling a ton of current at idle just by the sheer heat coming from the tubes and the amount of B+ voltage drop when keyed.
  8. K9STH

    K9STH Platinum Subscriber Platinum Subscriber QRZ Page

    With the B- floating above ground, so that you can measure the grid current, if you put a 1-ohm resistor between the B- and the chassis / ground and another 1-ohm resistor from the negative end of the filter capacitors (with a voltage doubler circuit), you can measure the voltage drop across the resistors and then interpret the current from that reading.

    Using 50 uA meters (very common and very cheap especially from Chinese sources), to read plate current (actually cathode current but MUCH safer to meter), if you put a 40K resistor in series with the positive terminal of the meter, then full scale will be 2-amperes. If you put a 20K resistor in series with the positive terminal of the meter, then full scale will be 1-ampere. The resistor needs to be on the capacitor side of the 1-ohm resistor and the negative terminal connected to the B- connection.

    If you put a 10K resistor in series with another 50 uA meter's positive terminal and then connect the resistor to chassis / ground, and then connect the negative terminal to B-, then full scale will be 500 mA. This will read the grid current.

    With the "plate" meter, 50 uA will mean 2-amperes current if the 40K resistor is used or 1-amperes if the 20K resistor is used. You can make a chart showing that, if the resistor is 40K, then "5" on the meter is 200 mA, "10" is 400 mA, "15" is 600 mA, "20" is 800 mA, "25" is 1000 mA, "30" is 1200 mA, "35" is 1400 mA, "40" is 1600 mA, "45" is 1800 mA, and "50" is 2000 mA. If the resistor is 20K, then "5" is 100 mA, "10" is 200 mA, "15" is 300 mA, "20" is 400 mA, "25" is 500 mA, "30" is 600 mA, "35" is 700 mA, "40" is 800 mA, "45" is 900 mA, and "50" is 1000 mA.

    For the grid meter, "5" will be 50 mA, "10" will be 100 mA, "15" will be 150 mA, "20" will be 200 mA, "25" will be 250 mA, "30" will be 300 mA, "35" will be 350 mA, "40" will be 400 mA, "45" will be 450 mA, and "50" will be 500 mA.

    If you reduce the grid meter series resistance to 5K, then "5" will be 25 mA and so forth, being 1/2 of the current when the resistor is 10K.

    This conversion "works out" when a 1-ohm resistor is in series with the "load". Then, a current of 1 mA produces a voltage drop, across the resistor, of 1 mV (0.001-volts) so a 1-ampere current produces a voltage drop of 1-volt. A 2-ampere current produces a voltage drop of 2-volts, a 3-ampere current produces a voltage drop of 3-volts, and a 4-ampere current produces a voltage drop of 4-volts.

    With a 20K resistor in series with the meter, 1 volt produces a full scale meter reading. But, if the expected current is going to be higher than 1-ampere, then, with the same meter, the series resistor has to be increased. With a 50 uA meter, if the measured voltage will be 2-volts, then the series resistance has to be doubled making it 40K. If the measured voltage is 3-volts maximum, then the series resistance has to be tripled making it 30K, and if the measured voltage is 4-volts maximum, then the series resistance has to be times 4 making it 80K. With the tubes chosen, the current is going to be more than 1-ampere but less than 2-amperes. Therefore, a maximum of 2-volts is going to be dropped across the 1-ohm resistor so a 40K series resistor is appropriate.

    Since the grid current should not exceed 500 mA, then the maximum voltage drop across the 1-ohm resistor is going to be 0.5-volts, then the series resistor needs to be only 10K for 500 mA to be full scale. If, for some reason the grid current is going to be more, then a 20K resistor should be used and then the full scale reading will be 1000 mA (1-ampere).

    Glen, K9STH
    KN4CTD likes this.
  9. KN4CTD

    KN4CTD Ham Member QRZ Page

    Thank you for taking your time to educate me. I will sit down tomorrow and really try to understand this. Are there certain types of resistors to use to measure voltage drop across? Since I already have the original Ip meter from this chassis to use, how do I determine the uA of its movement?
  10. W1QJ

    W1QJ Ham Member QRZ Page

    Determining what the specs of a meter is pretty simple. Here is a simple list you can go by to determine all you need to know about a meter.

    1. Most meters that read low currents like 0-1ma do not need external shunts to read that current. Same with meters like 0-50ua or even 0-1 amp. SO if a meter says it is 0-1ma that means the FS (full scale) current is 1 ma. So I of the meter = .001A
    2. Determine the resistance of the meter, Most newer VOM meters can safely measure the internal resistance of a meter safely on the correct scale. Start on a high scale and drop down until you can measure the meter without pinning the meter movement. Measure the resistance. R= Measurement you get. Lets say it is 1000 ohms. Rmeter= 1000 ohms.
    3. What is the FS (full scale) voltage of the meter? Use ohms law. Vmeter=ImeterXRmeter Vmeter=.001AX1000ohms=Vmeter=1volt

    Now that you know all the parameters of the meter you can make the meter read anything you want by using parallel shunts and series resistors. All you need to know is ohms law to figure out any series resistors or shunts. You would want your shunt resistors to be precision resistors like 1% if possible to keep accuracy.

    Many old time hams usually used "straight" or "direct reading" meters to read amplifier currents. That means they would seek a meter that requires no external series or shunt resistors. When building amps many like to use a 0-1ma meter for everything and they series and shunt them according to what they want them to read. A meter function switch moves the meter plus and minus poles around switched shunts. Most commercial ham amps do this. The meters have multi scales on the face to indicate each function.
    WE4E, W1TRY and KN4CTD like this.

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