Hey guys I'm building a WA2EBY amp and want to put the 5W pad on it for 5w drive to 1w but am missing one part....

130 Ohm carbon comp 3w resistor

?Any help would be great ....I am happy to pay for shipping and the part....I have looked everywhere all I can find is 3x 390 Ohm 1w but I don't think I can get them to fit.....

?

Help!

73

? David

? N8DAH

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just stack them on top of each other vertically sky-scraper style with the leads straight and close.? ?

You looked at DigiKey,? right?

you might have to use metal film, but here's something that might be in the right ball park;

?

Almost done!

That is metal Oxide

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Yes I know but it may not have so much inductance that it is unusuable.? ?The one to avoid completely is WIREWOUND.

By the way I have the same circuit boards, arrived a few weeks ago.

I designed a PCB to try it myself but I'm swamped andmay not get to any of it for months.

If you have an antenna analyzer you can measure the inductance of resistors to determine whether they will present anissue at the frequency of interest.? ?

?

Cheers!!

Gordon

well I already have the other two for the PI already installed I will either make the 390's work or pray for a 130 3w as it will fit a lot easier

David,

? ?I used a metal oxide to make the pad when I built my WA2EBY linear earlier in the year and I have six extra.? They're Mouser 281-130-RC.? If that'll work for you, let me know where to send a pair and I'll put them in the mail.

? Andy, KG5RKP

Hmm I was told by a few elmers around my club I should avoid metal anything wire/oxide/film when dealing with RF for it may become an inductor..... I have 6x 390's 1w coming I will see if 3x will fit, its R10 and may be a hard squeeze but we will see.

If the metal oxide is working for you w/o fail I may just pick up what I need. Thanks for the heads up and the offer.

?

?73

? David

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?I do beliee that there can? be metal spirals in the metal films (not an expert on that) but we aren't talking UHF here.....so even so the inductance may not be huge.....? Maybe just build all of it just a bit (2-3 mm) above the board so you can clip it off and change it easily if you need to later.? ??

Measure the SWR "into" the device and if it isn't horrible....thenyou would probaby be good to go!

gorodn

I hear ya.? This from the reissued EMRFD, page 7.10:

Care must be exercised when picking resistors for attenuator applications.? Many power resistors use wire wound construction, often hidden in ceramic, making them too inductive for RF use.? Carbon composition and the various types of film resistors are generally suitable for RF through UHF.

There's a table below that paragraph the summarizes resistors tested with an HP-8714 network analyzer.? The two resistors with the lowest inductance at RF was a 47Ω carbon composition (0.0095 ?H) and a 47Ω metal oxide (0.0099 ?H).? A Tru-Ohm 100Ω 'non-inductive' measured 0.24 ?H and a 50Ω wirewound was 6.4 ?H.

For what it's worth.
Andy

On Sun, Dec 17, 2017 at 08:50 pm, N8DAH wrote:

Hmm I was told by a few elmers around my club I should avoid metal anything wire/oxide/film when dealing with RF for it may become an inductor..... I have 6x 390's 1w coming I will see if 3x will fit, its R10 and may be a hard squeeze but we will see.

If the metal oxide is working for you w/o fail I may just pick up what I need. Thanks for the heads up and the offer.

?

?73

? David

Well it fits !!!!!

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?There ya' go!!!

Tell us what the SWR *into* the device ends up.....

And I'd love to know the power output on? a few bands......I put together my own schematic for this, (without the filters, relays etc)? but not sure if I want to go ahead with it or just build the one you are doing.

Gordon

Yep I will put it to the test once I find a good case and heatsink for it. I will be happy to post the specs once its up and running.

If you're going to stay with SSB or CW and are looking for a heatsink very close to the discontinued one used in the WA2EBY build, this works well:

I used this case:


You may need a fan for digital modes.? I think that KC0WOX used a graphics card heatsink/fan combo.


I do use mine for digi modes, but drive it with my Ultimate3S - and 250mA in is easy work for the IRF510s.? I didn't install the pad on the amp board because of the Ultimate3S, but do add it in-line when I drive it with the BITX40.

Nice looking build - and I do love the look of those old carbon resistors!

72/73,? Andy, KG5RKP

Question do you like the COR keying for digi? or CW? I thought about a PTT jack for later. I have a nice Cast alum case with a full surface heatsink I just have to stop being lazy and finish it. Thanks for the pics and ideas. I do plan to add either one 24v fan or 2x 12v.

I am going to homebrew a wspr tx and use a Hardrock Driver set for 500mW in since I can't seem to find it a new home so with 10mW drive I can expect about 100-200mW out.

73

?David

Hey David,

? ?The RF sensing has functioned perfectly for SSB and for digital modes.? I'm not a CW op yet so can't speak to that.? I would think, though, that with the designed-in hysteresis that the code would have to be Really, Really Slow(tm) before the relay would disconnect.? Frankly, I was expecting to have to adjust things but this project has been basically 'plug and play'.

? ?73, Andy

Nice that was the plan to have a good solid amp not in the 300$+ price range. I had a Hardrock 50 w/o the atu, it was a 1st run version and needed a lot of mods to run any of the new parts. Plus it was a bit more "tech" than I felt was needed. Not to mention I know who built this WA2EBY and who to blame if it fails hihi as well as how it works.

?

? 73

??? David