KPA500 - 120 or 240?

Discussion in 'Amateur Radio Amplifiers' started by W4HWD, Aug 28, 2019.

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  1. W4HWD

    W4HWD Ham Member QRZ Page

    I switched my KPA500 over to 240v and have been using it this way for a little while now. This won't settle any debate (of course not, you seen some of these threads?) but that's not my intent. I just wanted to share some observations in case any of you are using this or a similar amplifier on a 120v circuit now and are wondering if it would be worth it to switch to a dedicated 240v circuit:

    1) Same on the air performance (duh) but with half the current draw of a 120v circuit (again - duh). The difference in the fuses used (120v/12A vs 240v/6A) is a good clue.

    2) On a 120v shared circuit, my HV under full load would sag as low as 53v depending on band - now, my voltage never sags below 63v at full load on any band (Elecraft says 60v or more for best linearity so there's that). Less stress on the finals and better linearity - this right here is probably the biggest benefit you will see switching to 240v, especially if your current 120v circuit is really "crowded".

    3) No more dimming lights on a shared 120v circuit. My 240v circuit is a straight shot to it's own breaker in the panel now. In case you're wondering, I used a NEMA 6-20 receptacle (with gang box) wired with Romex 12-2 (the yellow stuff) with about a 50' run back to a 240v/20A breaker in my panel.

    4) Having this 240v circuit gives me the option of going legal limit if I ever choose to.

    5) Materials totaled about $35, electrician inspection and sign-off $100 (this cost will vary depending on where you live - I did the install and had it inspected because I'm a good boy). Done.

    Anyhoo, that's what I found when I made the switch.

    Harry W4HWD
    Last edited: Aug 28, 2019
    AA4PB likes this.
  2. WA3GWK

    WA3GWK XML Subscriber QRZ Page

    I run my KPA500 on a dedicated 20 amp 110 line. Voltages are 72.5 at idle and 56.8 at full load. I use my amp almost exclusively on SSB so I don't have any issues with voltage, light dimming, etc. Running it on 240 is great if you have issues and, as you say, it gives options for higher power.

    I got the KPA500 so that I could run it on a 110 v line, weighs only 26 lbs, and it has such a small footprint.
  3. W4HWD

    W4HWD Ham Member QRZ Page

    SSB is probably the easiest mode on equipment, but I was running RTTY and FT8 so I was seeing less than ideal operating parameters from my setup. If you like your amp on 120v, don’t fix what ain’t broke.
    WA3GWK likes this.
  4. W1QJ

    W1QJ Ham Member QRZ Page

    All amps work better on 240v. If nothing else the dynamic regulation is much better and that equates to more power and linearity. Even on SSB, your peaks will be higher and not clipped in output. If at all possible run all amps on 240v.
    N4UP, W4HWD and N6UH like this.
  5. N8VIL

    N8VIL Ham Member QRZ Page

    Delivering a given wattage with less current is always a good thing. Less stress on the primary side of your power supply. Conductors, switches, etc. Whether it be radio related or any other appliance if the option is given.
  6. K6BRN

    K6BRN XML Subscriber QRZ Page

    Well... at QTH#2 I run my KPA-500 off of 120VAC and have zero dimming of lights and my finals voltage never sags as low as yours, either. I'm going to guess you either have a very heavily loaded line from the breaker or that the wiring is pretty old, long, thin guage and/or there is some bonding resistance (and heat) in your circuit. In that case, going to a newer, dedicated 240VAC run is a really good idea.

    I DO run my Quadra off of 240VAC at QTH#1, in a dedicated run to a breaker on the entrance panel. But that is a 1KW amp.

    Brian - K6BRN
  7. W5LZ

    W5LZ Ham Member QRZ Page

    It depends on how your shack is wired. If you have a 240 line, and if it isn't loaded very heavily, that is what I consider the best way of running any amplifier capable of using 240 vac. Most of the 120 vac line into my shack are at least moderately loaded (read that as too much on one line).
    So, adding to that load isn't the best way of going about it.
  8. K6BRN

    K6BRN XML Subscriber QRZ Page

    The nominal draw of a KPA-500 is about 8.5 Amps @ 120VAC, which is not too much of a load, even for an older 15 Amp mains circuit. It's FINE to run the amp from 240VAC, but since the amp draws less power than a toaster, running it AND your rig from ONE 120VAC line (about 12A max) is very practical. My homes have 20 Amp circuits so the load margin is on the order of 100% for just the KPA-500 and there should be NO visible light dimming at that load level. Even with my rig and the KPA-500 and some accessories and lights running in the same circuit, I do not experience noticable dimming and voltage sag.

    Might want to check your mains circuit if you are really seeing that much drop. It's unusual in a modern home. Or perhaps you are using a long 18-Ga. cord from the wall plug to the amp - that would account for excessive voltage droop in the amp.

    But then, I also spent time growing up in a very old house that had been converted from gas lighting to electric. The entire entrance panel consisted of two 15 amp fuses. When the toaster kicked in, the electric meter literally groaned.

    Brian - K6BRN
  9. N8VIL

    N8VIL Ham Member QRZ Page

    Just as a reminder, typical residential circuit breakers are not rated for constant current at the marked rating on the breaker. A 15 amp breaker has a constant current rating of 12 amps. A 20 amp breaker has a constant current rating of 16 amps. They do make breakers rated at constant current at their marked rating, but are not typically manufactured for residential load panels. At 120v 15A breaker you can safely consume a constant 1440 watts and at a 20A breaker 1920 watts. At 240v 15A breaker you can safely consume a constant 2880 watts and at 20A breaker 3480 watts. These values are for constant current handling. Of course the length of the line will have an effect on voltage drop for a given guage of wire and the quality of the feed to your home/business.
    Last edited: Aug 31, 2019
  10. N8VIL

    N8VIL Ham Member QRZ Page

    Another thing that can cause excessive voltage drop in a branch circuit, other than wiring connections and long runs, is the breaker itself. Most residential load panels are of the stab design where the breaker relies on tension of the terminals gripping the bus bar. Corrosion or poor clamping force there can add additional resistance causing a voltage drop or the contacts inside the breaker becoming oxidized and adding resistance causing a voltage drop. An easy way to check would be to heavily load the circuit (not exceeding the rating) and measuring from the buss bar feeding the breaker and the load side of the breaker to check for a voltage drop. I usually use an electric space heater with variable wattage settings. I have seen this occur many times. It is a good practice to periodically cycle circuit breakers on and off to "polish" the internal contacts.

    Don't do this if you are uncomfortable around line voltages. One slip and you can end up with a severe shock or arc flash. I have a tendency to assume everyone in ham radio has electrical/electronic experience.
    Last edited: Aug 31, 2019

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