Mods to the Amp supply Lk-500 series

Discussion in 'Amateur Radio Amplifiers' started by KC3HUM, Jun 14, 2020.

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  1. KC3HUM

    KC3HUM Ham Member QRZ Page

    Hi everyone. I just aquired two amp supply lk-500 amplifiers. One is a zb and the other is a za model. Ive done a good amount of reading so far from what I can find out about them online and the general concensus is very positive. I feel like its a solid amplifier and looks really well built.

    I really would like to have a few things answered for me I have questions about though... I hope W1QJ and KM1H chime in, as they come to mind in the reaseach Ive done so far but I know there are others too very knowledgeable that Id love to chime in.

    1. Tube pin cooling seems to be a issue with these amps. Apperently there is a mod/new tube socket that solves this issue and Id like to know if I have the stock tube socket or if I have a modified one? And where do I source info on completing this mod if I don't have it done here?

    2. Band select switch had two different types used. I believe I have the good swithch as everything looks great and no arcing at the 160m taps but tell me which one I have in case I'm wrong. The swithc I have in both of these amps are the same and they feel pretty awesome with super positive clicks and looks rugged.

    3. Does it look like the 10m mod is completed on this za model I have photoed?

    4. Can anyone tell me if they see a glitch resistor anywhere on this unit. So far I have read that it should go in line after the supply / B plus. Idk what b plus is though...

    5. The za model has a mechanical relay for the ptt/amp keying , but the zb model has a vaccume relay. Should I modify either with a upgraded qsk kit? Leave the zb model alone and add vaccume relays to the za model replacing the mechanical relay?

    6. These probably haven't been ran in over a year untill I fired both up just a couple days ago. So far the voltage is 3000 and 3200 volts for both. They both seem to put out about the same power with about 700-900 watts, so I guess the tubes are a little weak. The zb model has plate current that seems a bit high (800-850 ma and 900 is max acording to the manual). I'm driving with 100 watts.

    Running on a 240v line.

    I ran a small pile up with the ZA model one evening and the next moring and I got GLOWING REPORTS! It seemed almost every contact was complimenting my signal and a couple guys couldn't get over how great my signal was lol. Ive never got reports like that with my al-1500 putting out legal limit, so idk maybe coincidence or great conditions. Also these things are quiet! Its about half as quiet as the al-1500 and this leads me to believe possibly a aftermarket internal fan was installed as Ive read in a couple places people changing the fans out because they were too noisy on "high". So far i'm only experiencing one fan speed on these units, and the fan installed is a "nidec torin". They also have additional installed fans on the outside of the cover on the transformer side.

    Thanks for taking your valueable time to help me out here and get some questions answered. I'll provide additional photos if needed.

    Attached Files:

  2. KC3HUM

    KC3HUM Ham Member QRZ Page

    more photos. All the photos posted so far in this thread are of the "za" model only.

    Attached Files:

  3. KC3HUM

    KC3HUM Ham Member QRZ Page

    These are photos of the lk-500zb

    Attached Files:

  4. KC3HUM

    KC3HUM Ham Member QRZ Page

    more photos of the "ZB" model

    Attached Files:

  5. W1QJ

    W1QJ Ham Member QRZ Page

    Both amps appear to have the good heavy duty band switch. Both look have 10 meter input components either factory or added, they are there. It seems both have that funky nichrome wire voodoo magic loodiloop from the blocking cap to the porcelain stand off, so you’ll want to get rid of that and change it for a large solid wire or wide strap. The amp is designed so that air flows underneath the chassis to cool the pins so no need for any change there. Just don’t slow the fan down any in transmit. Amps with vacuum relays are already QSK . Units with open frame relays are fine. No need to install vacuum relays in them unless you absolutely need QSK. If you don’t operate CW no need for QSK so don’t mess. Some of these units had quasi glitch resistors located at the base of the plate choke. They may be there you’ll have to see. There were usually two parallel 2watt carbon resistors down there that acted as glitch resistors. These amps very rarely if ever have catastrophic events because the power supply capacitance was held to are logical effective value!! Most amps that blow up components in “ events” like Ameritron 811 series amps is due to absolutely crazy outrageous power supply capacitance. Any capacitance above the effective value above the effective minimum just supports destructive energy in an event. As a result events usually go unnoticed as the energy in events are well tolerated because they are not excessive due to a energy storehouse in the power supply. These amps barely have the 30uf total compared to the AL811 amps with a whooping 67.5uf !! AL-811 amps should have no more than 4 150uf caps which is more than enough. They have 4 270uf from the factory. That is a recipe for disaster in tube events. Amp Supply had a sound design and there is no need to mess with them. They did goof when they installed that lesser band switch. Other than their very loud fans, they are great amps. Don’t go messing around.
  6. KC3HUM

    KC3HUM Ham Member QRZ Page

    At the base of the plate choke, all I have is some orange capacitors down there. No resistors I can see. Wondering where to add a glitch resistor... Reference 4th post and jpg 5519 photo for this area

    So that Nichrome wire deal. Your talking the bare wire in jpg 5528, correct? One wire is straight, and the other forms a coil in the middle. Okay I'll change that out with some copper strap. Thanks!

    So you wouldn't recommend the pm-800 Harbach board? I was going to change that out as I was reading that replacing the filter caps can help prevent a catastrophic event.

    Here is the link to the Harbach board.
  7. W1QJ

    W1QJ Ham Member QRZ Page

    Yes, That straight wire and lopped wire. Get rid of it. A glitch resistor goes in line with B+ at the bottom of the plate choke. The Harbach kit is fine.
  8. AA1XV

    AA1XV Ham Member QRZ Page

    FWIW, I installed a 15R 25W glitch resistor using two of the holes on the dividing wall in my LK500-ZC. I see your LK500-ZA has them too.

    Attached Files:

    K2XT likes this.
  9. AA1XV

    AA1XV Ham Member QRZ Page

    The picture doesn't show very clearly but the resistor is supported by two 1/2" ceramic standoffs.
  10. KC3HUM

    KC3HUM Ham Member QRZ Page

    Okay that brown resistor is supported by ceramic standoffs? Nice. Looks like a good spot for it especially if I'm already in there replacing the filter cap board with the harboch modded board.

    By the way what is the small harboch board on the left? I only see the filter cap board on their website for this model amplifier...

    Nice looking amp by the way

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