Discussion in 'Amateur Radio Amplifiers' started by WA6MHZ, Jan 11, 2019.

ad: L-HROutlet
ad: l-rl
ad: abrind-2
ad: Subscribe
ad: Left-3
ad: L-MFJ
ad: Left-2
  1. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    One thing it has is a Medival Soft Start. A BIG Brown resistor (of unkown Value) is in the AC line until the contacter shorts it out. Think the kind found in the Harbach or W7RY would be a better idea.

    So I am starting to design the control circuitry. It could be done with an ARDUINO, but it would require a MEGA or an I/O Port extender! An LCD readout would be pretty, but I am thinking of a Remote Readout and control panel similar to what I did with the SB-226, which was a Masterpiece in Over-Engineering! So far I Have about 12 LEDs that have to be lit for various functions of an Amplifier Cycle.

    AC ON Led, HV Led, SCREEN Led, Bias Led, Filament Led, FAN Led, WARMUP Led, COOLDOWN Led, RX, Led, TX Led, READY Led and maybe some more!

    He who has the most LEDs on their Project WINS!!!

    The Warmup Led will run while the Filaments are lit before the HV and SCREENS are enabled.
    The Cooldown Led will run for a few minutes with the fans on after the amp is shutdown from operating.
    Ready Led lights at the end of the Warmup Cylcle showing the Amp is ready for Transmit.
    IF an LED is NOT LIT in the particular Cycle Step: "HOUSTON WE HAVE A PROBLEM!!!!' (or there is a problem with the LED as happens frequently in the SB-226!

    As said, it could be done with and Arduino, but the programming on those is really tough (I sure have torn my hair out trying) and I am an OLD SCHOOL Guy who likes 4011 Nand Gates and RS FlipFlops!

    In the famous (and still in DAILY SERVICE) is the Famous Rotator Controller I built for the Twin Roof tower rotors.
    It has somewhere around 40 CD4000 series ICs in it. That also could be replaced with an Arduino, but the programming would require a Computer Genius!!

    So the Amp, Once tuned for the SSB Section of the band, shouldn't need to be Retuned unless I decide to run a KW to Access my local Repeater. Not sure where in the already Full Radio shack I am going to put this amp, but it will Go Somewhere. One location that would have worked good was under the bench. The problem came when it turned out the blower interfered with cabinet enclosure. I have a perfect 16 inch high cabinet sitting on the front porch (where it is been in the rain and sun for the past 20 years!) Probably full of Black Widow Spiders!

    Here at Cat Ranch Electronics, we are Masters at OVERKILL!
  2. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    OH one more thing I have to install! It is a CLASS C/CLASS AB1 switch so I can select between FM/CW and SSB. So there are 2 more Leds!
  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I did that on my amp, as you could see in the video.

    For Class C, you can reduce screen voltage from 350 to 250V, and increase bias from -55 to -90V. I have a front-panel rotary switch that does both of these in one "click."
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    The cooldown isn't necessary. As soon as you stop drawing plate current, the tubes cool off very fast if the blower is good enough to keep them cool when transmitting. 4CX250s have very little thermal mass. Mine cool down from 200C to 50C in just seconds, using an IR thermometer aimed at them.

    I used an old Amperite 120-second time delay relay to prohibit keying the amp until it was warmed up. Those used to be cheap, I have no clue what they go for now, but they're very reliable and do the job. You can have B+, screen voltage and bias all applied to the tubes the second you turn it on, that doesn't hurt anything as long as you cannot key the amp and draw any current.
    You'll need to re-tune if you QSY from 144.200 to 144.300; it's a very high-Q tank circuit. But unless you QSY up to the "FM" band, almost all activity is around 144.200 so you won't need to touch the tuning much.
    K2OS likes this.
  5. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    So here is the Question for QRZeders:

    What are the values of Screen Voltage and Bias for AB1 vs CLASS C? Sue said some thing about measuring the plate current and finding a value and then doubling that voltage. It went soaring over my head and I didn't get it written down.
    So it looks like I am going to need some Pots on the Screen side and Bias side, selected by logic and relays when going between CW/FM and SSB.
  6. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    Thanks, that is EXACTLY what I needed to know!!!!
  7. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    More excellent info that greatly influences my design!! I am wondering if the existing tuning has the range to extend from 144 to 147?
  8. WA6MHZ

    WA6MHZ Premium Subscriber QRZ Page

    This is why I bring my issues to QRZed. Cuz folks have already encountered these questions before and know what they are talking about from successful experiences. Thanks for all the help. Now, on with the redesign of the redesign.
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'm almost sure it does.

    My good friend Bill K2OWR built exactly that same amp (RF part, not the power supply) back in the 1960s and I've used it many times.

    We could easily tune it to 146.52 to make lots of contacts in VHF contests back then, with 600W output forever.

    Tuning up a tetrode amp is done almost entirely with a wattmeter and a screen current meter. Plate current is fairly irrelevant with these tubes (provided you have enough cooling) and nobody cares about a "plate current dip" or anything like that. You tune for max output consistent with near-zero screen current (which is largely controlled by the LOADing control). That amp can achieve 600+W output on FM, key down, for a very long time if you tune it this way.

    To keep the amp "quiet" in the shack, Bill used a remote blower in a closet on the other side of a wall, and used 3" dryer hose to connect the "blower box" to the RF deck. Quiet as a mouse, with a whole lot of air flowing. We used that a lot. Exactly the same "plumber's delight" RF deck design with the big pipes.
  10. AI3V

    AI3V Ham Member QRZ Page

    Re screen current:

    Some points, first, never apply screen voltage without plate voltage being applied.

    Reason is the screen will act as a plate, being the most positive element in the tube. If the cathode is hot (usually because the filament is on :)) excess current will flow in the screen, and may damage it.

    For best imd, you want to keep the screen voltage steady (read regulated), as tube gain varies with screen voltage.

    Screen current will sometimes be positive, and sometimes negative, so the usual practice is to use a zener diode or a gas discharge tube as a voltage regulator, you want to have extra current flowing thru the regulator zener or tube ,above what is needed for just the zener or gas tube.

    In my single 4cx250 I had -10 to +40 ma screen current available, and measured on the meter . A 10 ma bleeder resistor, on the tube side of the screen voltage regulator, and relabeling the 10 ma tic on a 0-50 ma meter was the method I used.

    Tune up, the fact that when the plate voltage is less than the screen voltage you get a sudden increase in screen current is a excellent tuning indicator, as it shows the tube is being "turned on" enough so that the instantaneous plate voltage is less than that of the screen, or, in other words you have full power being switched by the tube!

    So a combination of drive, to show screen current, and tank circuit coupling (here is where knowing your plate current is Handy :))to the antenna (obviously for maximum of power) and you know you are getting everything your tube can give.


    P.S. search "care and feeding of power grid tubes" and read it, before you build (or operate :)) a vacuum tube amplifier.

    You'll be glad you did.

Share This Page