Help with my first amplifier

Discussion in 'Amateur Radio Amplifiers' started by W5KV, Oct 14, 2018.

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  1. W5KV

    W5KV Ham Member QRZ Page

    Good day Internet Elmers,

    I just happened to purchase my first amplifier recently. A Heathkit SB-220 in pretty good shape (I think!).

    So far it looks like I need to replace the parasitic suppressor chokes with a kit from AG6K? They look a little scorched.

    The amplifier is wired for 110v. I'll have it on a dedicated 20a circuit that nothing else will be on. (Maybe one day i'll wire for 220). I'm not too sure what other mods have been completed on the amp. There are quite a few that i've been researching thus far. Just wanted to post some pics to get some eyes on the amp and some suggestions as to what steps I should take to ensure many more years of use :)

    Thanks & 73

  2. AC0OB

    AC0OB Platinum Subscriber Platinum Subscriber QRZ Page

    I looks nice and clean Johnny.

    Was it operational when you purchased it and how much experience and test equipment do you have working around lethal voltages?

    The first thing I would recommend is to spray Deoxit D5 on any ceramic switches.

    Secondly, remove that parasitic mess on the tube caps and replace the parasitic suppressors with these resistors with 5 turns of #14 copper wire wound over those resistors.

    Inspect the filter capacitors and the equalization/bleeder resistors for any signs of leaking and cracking, respectively. If the filter capacitors are original, replace them.

    And after that, replace the grid circuit components with 1/4" copper straps to ground.


    Last edited: Oct 14, 2018
    W5KV likes this.
  3. WA7PRC

    WA7PRC Ham Member QRZ Page

    The late Richard Measures AG6K developed the nichrome wire parasitic "suppressors". Unfortunately, they were problematic on higher frequencies (they typically run hot). OTOH, the OE design worked. The concept is to introduce enough series reactance to prevent the tube(s) from having enough gain to oscillate. You want that to be above the highest operating frequency. The resistor is used to broaden the response by lowering the Q. The resistor needs to have very low reactance. Carbon composition or Globar resistors are specified to have low reactance. Generally, about 25Ω to 100Ω is acceptable.

    After almost 40+ years, the OE suppressors in the SB-220 I purchased in 1975 finally needed attention. I rewound the 3-1/2T 14ga inductors (L ≈ 80nH). One of the 47Ω/2W carbon composition resistors had increased to about 100Ω. I found eBay seller 'bigsmythe74' (link) had NOS Allen-Bradley CC resistors cheaper than I could find anything anywhere else. I see that he currently offers 56Ω/2W for $1 each, plus $3.50 shipping (see That's close enough.

    The two turn inductor in series with your output network is puzzling. Typically, that is done in order to make a CTUNE capacitor function when there's too much tube COUT to work on higher frequencies. I've never heard of a 2 x 3-500Z amplifier having that problem below 30 MHz.

    I too operated my SB-220 on 120V... at first. However, even w/ a short run from the source, voltage regulation was gawd awful. Changing to 240V fixed it.

    There are several other changes that could/should be made. All can be done at about half the cost of ordering from a certain well-known aftermarket supplier.
    WQ4G and W5KV like this.
  4. WA7PRC

    WA7PRC Ham Member QRZ Page

    I used soft copper strip + #4-40 hardware:
    (click for yuuge image)
    The terminal is bent upward, and the copper strip is fitted & soldered.
    Note that it's not possible to have a shorter (and lower inductance) path to ground.
    The socket terminal is bent upward, and the strip is fitted. Hardware is #4-40.
    W5KV likes this.
  5. W1QJ

    W1QJ Ham Member QRZ Page

    Johnny, you have a nice complete SB-220 there. HOWEVER: You need to make some changes to it. Let me tell you the good and bad with your amp the way it sits right at this moment. First the good:

    Good: You already have a soft key. You already have an upgraded rectifier board. You already have a soft start.

    Bad: Those are the wrong parasitic suppressors, (voodoo magic) they work but an original is better. You have more voodoo magic on the grids. And more voodoo magic on the input to the tube. You have original caps and bleeder resistors.

    Suggestions: Get a good set of original type parasitic suppressors, remove everything on the grids now and ground them like the photo shown by WA7PRC. Remove the resistor on the blocking cap to the cathode of the tube, go right from coax thru cap, no resistor. Install a new set of caps and bleeder resistors. CHANGE YOUR 120 V LINE TO 240V Put that in caps because that will be the single most improvement you can make to the amp externally. TRUST ME!!

    If you are handy with soldering and can manage assembling your own original type parasitic suppressors and assembling a new "drop in ready" capacitor bank go ahead and proceed with that. Other than a good clean up that is all you really need to do. You may want to remove the little series choke going to the plate choke and replace with a glitch resistor also. NO NEED TO DO MUCH MORE THAN THAT.

    If you are a one stop shopper and would like to get parasitic suppressors and a capacitor bank read to drop in, I have that stuff available. You can PM me if interested. One days work is all it will take for you to get that amp in tip top shape.
    K1SZO, K2XT, WQ4G and 1 other person like this.
  6. WA7PRC

    WA7PRC Ham Member QRZ Page

    The the best choice is CDE 381LR-series "105 °C Highest Ripple, Snap-In Aluminum" (datasheet). They're available in small quantity for about 6 bucks each. To fit the Heathkit plastic OE holders, you want 35mm OD. There's no value in going nutz with capacitance... 270 μF is plenty... pn 381LR271M450A032.

    If you do that, you can rip the eight 30KΩ/7W wirewound heater (bleeder/equalizer) resistors out, and replace them w/ up to 100KΩ Metal Oxide resistors. A certain aftermarket supplier has all this... for at least double the cost.
    WQ4G and W5KV like this.
  7. G0HZU

    G0HZU QRZ Member

    It's worth making sure you wind a new suppressor coil correctly so you get the ideal inductance. It won't take much in the way of a subtle change in coil diameter to change the inductance away from the ideal design value. Because of the way the tube is made it has inductance in the grid connection and so the tube doesn't have a perfectly grounded grid. Any net inductance in the grid will tend to make each tube look like a negative resistance generator up at VHF at the anode. It will look like a series negative resistor in series with a few pF of capacitance. The idea of the suppressor is to add equivalent series resistance or ESR at the (potential) oscillation frequency and there has to be enough equivalent series resistance in the suppressor and the rest of the tank circuit to offset the negative resistance in the tube(s) at VHF. Once this is achieved the tube(s) should be stable.

    The inductor across the 50R? resistor will affect the amount of equivalent series resistance at VHF and it also affects the series resistance down at the operating frequency at HF. So there is a balancing act between having resistors that don't overheat on HF (prefer less inductance across the resistor here) and having adequate equivalent series resistance up at VHF to prevent oscillation (prefer more inductance across the resistor here).
    Last edited: Oct 14, 2018
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  8. W5KV

    W5KV Ham Member QRZ Page

    Thank you folks for the tips & suggestions. I will be purchasing some items this week it looks like!

    The amp is in working condition. I just fired it up last night after dusting the internal cabinet out a bit. I'm getting just shy over 500w out with about 50w in with my Kenwood 590SG.

    I'm ok with a soldering iron, but I think I can get some help from some more experienced folks around here for the modifications. As far as I know I am the third owner of this amp, it was purchased in 1977 it looks like on the paperwork.

    Very neat to own a piece of equipment from this era. The unit has a certain smell about it, can't really explain it, not bad, actually its quite pleasant, LOL. I think i'm hooked on 'Tube' equipment.

    Here's some additional pics, I was able to find a new faceplate on eBay for just $26 that i'll be installing when it comes in.



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  9. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    That's a great amp. It's easy to work on and a good performer.
    We love the smell of hot chassis dust in the morning... :D
    W7RY, WQ4G and W5KV like this.
  10. NQ1B

    NQ1B Ham Member QRZ Page

    You can probably get 800+ watts out with 50 watts drive just by switching to the high-voltage SSB setting. I find that I get about 10 dB of gain on CW voltage and 12 dB on SSB. 60 watts drive gives me 1 kw.
    W5KV likes this.

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