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SB-220 HV and line voltage Concern

Discussion in 'Amateur Radio Amplifiers' started by N8PWM, Aug 19, 2016.

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  1. N2EY

    N2EY Premium Subscriber QRZ Page

    Sure - but remember, the OP is using the SB-220 on the CW/TUNE setting, so the output might be 600 watts on a good day. According to the chart, the Cantenna is good for at least 10 minutes at that level, with mineral oil. IMHO, tuneup shouldn't take more than 30 seconds or so.

    Fun fact: The Globar resistor in the Cantenna comes in two versions - one is coated to resist oil, and the other isn't. Homebrewers take note!

    IMHO, it all depends on what the dummy load will be used for. What amateur application requires dissipating large amounts of RF for more than a few minutes at a time?
  2. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    More likely my problem! :p
    If you're running CW or AM, then the carrier control is your power level control, and dialing that down is easy. In SSB, you can turn down the mic gain but some report (myself included, alas, with a different radio) that it doesn't sound as good as running the mic gain like normal and turning down the power with ALC.
    Others here who own your exact equipment are better suited to answer your questions, so now, I'll bow out.
    All the best, and your shack is beautiful!
  3. WA7PRC

    WA7PRC Ham Member QRZ Page

    Of course, you're absolutely correct (not). As hams, we're expected to assume the absolute best scenario. We should use parts that can BARELY (or almost barely) handle the load. Then, when it fails, whine about it. Or, we can do what the rest of the world does, and allow for enough margin in order to definitely NOT fail. That's what I did w/ my homebrew DL. Others have built similar loads.

    Fun fact: the 90W resistor used in the paint can DLs that I researched, Kanthal/Globar pn 886SP (datasheet), is rated to resist oil.
  4. N2EY

    N2EY Premium Subscriber QRZ Page


    But seriously - the Cantenna with mineral oil will be fine with the SB-220 IF it's used wisely.

    Still, as you say, there are those who figure that if it can take a certain power level for 5 minutes and not explode, twice that power level for a half hour should be fine.....

    That's the one!
  5. WA7PRC

    WA7PRC Ham Member QRZ Page

    Since the much better transformer oil is easily & cheaply available, there is NO reason to use mineral oil. Of course, since a much better DL can be built for not a lot, there's not much reason to use a marginal (at best) paint can DL.

    My homebrew load used parts that were scrapped by my then employer (cost = zero). However, suitable parts that don't cost a lot are available all over. For example, right now, Henry Radio (link) has 50Ω/800W flange-mount RF resistors for $50 shipped. Need more? They have two 100Ω/800W resistors for $125. Add a heatsink(s) and fan(s), and you're in bizness.

    Note that these resistors are typically rated full power to 100°C, and monitoring resistor temperature is simple/easy/cheap. I used a LM35DT 'Precision Centigrade Temperature Sensor' IC. In the HN-31 manual (page 10), Heathkit advises only that you should:

    "Touch the side of the pail near the bottom with your fingers; if you are unable to hold your fingers on the pail for more than a few seconds, the RF power device should be turned off until the oil cools."

    That is silly. Different people have differing sensitivity to heat. Using a cheap sensor, there's NO doubt about how hot the DL is.
  6. N8PWM

    N8PWM XML Subscriber QRZ Page

    Latest Status Update

    Anyway. I hooked it all up to my FT-101E and a seperate 120V circuit (house plug). Set the amp for CW/TUNE. I have an ammeter on the line so I can watch it. In receive it is pulling about 2A. In idle transmit its pulling about 5A and the plate current is ~100 ma. I tuned it using CW as Heathkit recommends with very low drive (~20 watts) and it tuned very very easily. I have the padders fully unmeshed. I slowly increased the drive from the transmitter while re-tuning with each small increase in drive until I am seeing about 500 ma on the plate. Grid is running about 110 ma. No ALC line being used. Drive power about 80 watts. At this level I see about 13 amps on the power line. I have not got my wattmeter yet and my old one is a 300 watt max but based on what I have read and what the amplifiers' meters are telling me, I should be running between 500 and 600 watts at this drive level. Does that sound about right? SWR showing 1.4:1 after tuneup and antenna tuner touch-up.
    FT-101E is just coasting along, happy as can be. SSB check shows line amperage bouncing around 7-8A at a normal voice level.

    Ok smoke test passed. On the air I go.
    QSO 1: 59 into Hawaii. Good contact
    QSO 2: 59+20 into Nova Scotia. Told the signal was very clean with no distortion at all. The general comment was that it sounds like I am sitting next to him. I would say that's a good report.

    I do see my HV varying but I have been seeing that since the first smoke test. Just for good measure, I am going to replace the capacitor bank. As was suggested earlier in the thread that a small variation in the line voltage can have a large effect on the HV readings. I do know that my house voltage can vary from 117V to 124V at any point during the day. DTE Energy tells me that this is within tolerance. OK, I don't like it but I have to live with it.

    Well that's the update for today. I have been recruited to help can the garden harvest so I will not be able to play until later.
    WA4SIX likes this.
  7. N2EY

    N2EY Premium Subscriber QRZ Page

    Great news! Congrats!

    However, you may wish to hold off on the filter capacitor replacement, unless there's a clear need to replace them.

    Here's why:

    Even with a perfect power source (no line voltage change at all), and brand-new, low-ESR capacitors, you will see some HV change with load. It's the nature of the circuit - the HV is NOT regulated in any way. How much it would vary under ideal conditions I don't know, but at least some variation is normal.

    Now add the varying supply voltage under load and you can imagine the overall effect. Replacing the capacitors may not have much effect if the line voltage is bouncing up and down several volts. Think in percentages and you'll see.

    If you DO decide to replace the filter caps, might as well go the full monty and do the bleeder resistors and rectifier diodes too.


    73 es congrats de Jim, N2EY
    Last edited: Aug 27, 2016
  8. W1QJ

    W1QJ Ham Member QRZ Page

    You are dealing with two transmitters now (the radio and amp) that both have adjustable pi net output networks. You have to initially tune up the radio first either into a dummy load or an antenna to resonate the tank in the radio. Here is where you really need a standby switch on the amp!!!!!!!! So you first have to load the radio up. Once you do that, now you have to load the amplifier up with the radio now driving the cathodes of the amp. The loading of the radio will change with the different 'load" it is now seeing. Now you have to go back and after you tune the amp you should go back and peak the load and tune of the radio for maximum output of the amp into the antenna. You will pick up extra power and the tank in the radio will now be exactly tuned for the cathodes. This is why a standby switch in the amp is imperative so that you do not have to turn the amp off all the time to load the exciter if you change bands.
  9. N2EY

    N2EY Premium Subscriber QRZ Page

    Fortunately, all that's needed to implement a standby switch in the SB-220 is to open the wire going to the coil of RL-1. This can be done in the amp itself, or externally, by opening the wire to the ANT-RELAY jack.
  10. N8PWM

    N8PWM XML Subscriber QRZ Page

    Yup that is in the works. Just got done at Lowes picking up a switch and box to mount to the shack desk. Going to put it inline on the relay line from the radio. I unplugged the relay line to tune the transmitter and antenna and then hooked it back up to tune the amp. It's amazing what coffee in the morning can do for the mind lol.
    KJ4NOO and N2EY like this.

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