Drake L-4B resistor

Discussion in 'Amateur Radio Amplifiers' started by WD5ABC, Dec 1, 2018.

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  1. WD5ABC

    WD5ABC Ham Member QRZ Page

    I noticed last night that something was arcing a little bit in the L-4B power supply, it turned out to be one of the big 50k 50w wirewound resistors mounted to the top of the transformer. I'm attaching a couple of pictures, you can see the ring around the resistor where it was arcing. Do you guys know where to find one like this? I'm seeing wirewound resistors with the tabs on the ends that stick out 90 degrees but these have mounting screws on the end. I can probably figure out a way to hang the resistors with the tabs on them but I'd rather put the right one in there if I can find one. I don't see it at Mouser, just the ones with mounting tabs. On Ebay I also see the smaller ones in an aluminum case, I could probably suspend those by the wires if they are ok to use but I don't know how they are at high voltage.

    Kerry, WD5ABC
    20181201_085441a.jpg 20181201_085450a.jpg
  2. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    At 3kV, don't use metal encased resistors unless the manufacturer specifications state they are rated for the voltage -- most would not be.

    The big hollow-core WW resistors that are very common (I have lots of them) work fine with the V-shaped metal mounting clips at each end only if the +HV end clip is very well insulated from the chassis; to keep things "level" I usually mount those on ceramic pillar standoff insulators so both end clips are elevated above the chassis, and be sure the screws used to install the standoffs are fairly short so they don't come close together in the middle of the insulator.

    There is an advantage to using those little clips as opposed to a long threaded bolt with flat washers and stuff, because the little clips don't block all the air flow through the hollow core of the resistor; if you block both ends of a tubular power resistor, there is no air flow at all through the center, which makes the whole resistor hotter.

    I also generally install such power resistors where at least a trickle of an air stream (from a cooling fan) can blow across them to help remove the heat. It doesn't take much; if you can feel a slight bit of air moving on a wetted finger, that's usually enough if you operate the resistors within their power ratings.
    WD5ABC likes this.
  3. WD5ABC

    WD5ABC Ham Member QRZ Page

    Thanks for the info, I'd like to find the same resistor if I can, if not I'll have to build some kind of standoff like you're talking about. I suspected the metal resistors wouldn't work on HV, I've just never looked at them for that purpose. There's no fan in the L-4PS supply, I've thought about putting on on it but it does run cooler with the Harbach board in it, there's a lot more room for air flow than in the original configuration. If I need to get the resistors with the tabs I've found them at RF Parts and Mouser but I'd like to put it back with one like the original if I can. In fact, it's probably a good idea to replace both of them since they are approx 50 years old!

  4. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Yeah, I had an L-4B a long time ago.

    I added a fan to the power supply.:p I was kind of surprised the design didn't include one. The Swan MK-II HF amp, which is also a "separate RF deck and power supply" deal has a big cooling fan in the PS and the darn things never failed.:)
  5. WD5ABC

    WD5ABC Ham Member QRZ Page

    This one may have been at least partially self-inflicted. I let it sit on a shelf for 20 years and recently got it going again, recapped, new tubes, etc. I'm sure disuse, dust, TX humidity, etc. didn't do it any favors. You're right, it might not hurt to add a fan, maybe when I get it going again I'll do that. I might be able to find a small/quiet computer fan for it that wouldn't be too obvious. First I need to find that resistor!
  6. W1QJ

    W1QJ Ham Member QRZ Page

    Have you upgraded your power supply with a Harbach kit? DO you use ALC? Can you solder Depending on the answers to these questions, You may be able to do without those resistors entirely. Let us know. If you insist on having those heat generators I think I have one.
  7. WA7PRC

    WA7PRC Ham Member QRZ Page

    The L4B schematic I redrew shows this is part of the +120V source used to bias the amplifier off during standby, and supply the AGC (ALC) threshold voltage during TX. To do this, the PS uses (wastes) nearly 60W:

    P = E^2 / R
    P = 2500V^2 / R9 (50K) + R10 (50K) + R11 (5K)
    P = 59.5W :eek:

    If you don't mind scrapping this, a small 120V PS can be added probably cheaper/easier/faster than finding a suitable replacement for R9/R10. Worst-case load current is from 100K (R3)... a few mA. It can be placed in the amplifier or PS unit. :)
  8. WD5ABC

    WD5ABC Ham Member QRZ Page

    1. Yes, you can see the Harbach board in one of the pictures, above. 2. No, I don't use ALC, I'm driving it with a Flex 3000. 3. Yes, I can solder, I stuffed the Harbach board. I'm not a pro but I'm a fairly regular kit builder. Still, I'd rather keep this part original instead of coming up with a separate supply I think, that way the amp is compatible with other L-4PS supplies just in case I need to use a different one with it. I'm not completely against getting different resistors and mounting them differently if I can come up with a good way to do it.

    I found this one at RF Parts, the mounting tabs are different but maybe I can come up with a ceramic stand-off like Steve mentioned earlier. Here's the one they have:
    W1QJ, if you have one you're willing to part with, I'd be glad to talk to you. If not, maybe I'll figure out a way to mount this other one. If the one in that link is hollow I might be able to use the original mounting hardware (assuming the length is right). That's probably too many assumptions, my luck is never that good!
  9. WA7PRC

    WA7PRC Ham Member QRZ Page

    The datasheet for the Vishay pn FST05006E50K00JE (link) sold by RF Parts doesn't have a voltage rating. I wouldn't trust it to withstand 2500V. HV insulation is a good idea. Also as Steve @WB2WIK mentioned, it would be a good idea to ensure cooling by adding a small fan.

    The Harbach pn ES4L4-B board is contained in the amplifier unit, and doesn't provide the +120V used for cutoff bias and AGC (ALC) threshold. It's just a rectifier/step-start/relay-driver board (that costs a lot).

    A separate bias & AGC supply can be a simple xfmr-rectifier-filter that wastes virtually no power, for not much money. If mounted in the amplifier unit, it can be backwards-compatible w/ older HV PS units by not connecting to J1 pin 5.
  10. WD5ABC

    WD5ABC Ham Member QRZ Page

    Sorry, I do have both the Harbach PS board and also the softkey/step start board in the amp. The datasheet for that resistor says 1000v rms for a minute or something like that so yes, it's a little light. Rats, when I moved a couple of years ago I got rid of a TON of junk box parts including all kinds of transformers. So are you saying to get rid of R9, R10 and R11 and building a 120vdc supply between pin 5 and pin 6? If I'm going to break the compatibility there's probably more room in the PS since I got rid of the big old capacitors. I still think I'd like to keep it stock but it may be hard to find the right resistor. I'll keep looking but in the meantime I'll see if I can find something to steal a small transformer out of. I should only need less than 500 ma so it wouldn't take much. I can see how that would save a TON of heat. I did actually dig around and find a couple of different computer fans today so I may mount one in there while I'm fixing it.

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