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Multibanding the 50w amp question

 

Hi all,
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I have an unbuilt 50-watt amp kit and I'm trying to figure out how to best build it. I just completed a 5-band (40-15 meters) homebrew rig (it's not a QMX, but it works!). I'd like to use the amp on multiple bands for this radio (and possibly a QMX). I am thinking of just building the amp for the highest band I need and adding a switchable external low-pass filter.
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But I had a question: obviously the output of the amp won't be as large on higher bands. However, if I build the amp for 15 meters and transmit on say 40 meters (with a 40 meter external low-pass filter) should I still similar output as I would get if I built the amp for 40 meters, or would there be a significant sacrifice running though the 15 meter filter first? I'm sure some of you have experience trying this out. Thank you!
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72, Sam WN5C
 

Sam, I think others have found that without changing ferrite materials, output above 17m is pretty poor, if it works at all. If you look up some threads about 10m builds of this amp, the changes are discussed and laid out.?
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To answer your question, there would not be significant sacrifice. Some rigs are set up this way, where the highest LPF is always in line. I've had considerations of doing the same, making a 40-15m amp.? Currently though, I have a 50w amp built for 20m, that I have run on 40m with an external 40m LPF added in line. Output on both is as expected.?
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One LPF board to look at/consider:
The 3 band LPF version is only 5cm x 5cm! Problem is that I'm not sure that a 5 pole LPF would be enough with this Class C amp, especially if covering more than one band with that LPF.
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-Nate?
N8BTR
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Thanks Nate! Well, this looks promising. I was also considering just building a 20-meter amp and being happy with external filter for 30 and 40 meters, but of course a little extra on the higher bands would be nice. I'll just have to do some research on the highest band I can pull off then. Also, while that board might not work I appreciate you sending it along.. I like the idea of using relays to interface with my rig for automatic band switching so fortunately (unfortunately?) this project just got more complicated. best, Sam
 

Sam,?

One other thought ...

Another 50W AMP kit plus the nice enclosure is just under $46 plus shipping.?

I ordered three and the shipping was the same as a single kit. Built one of them for 15m using values from the QRP-Labs LPF kit information with? purchased capacitors that matched the V Max Specs of the 50W supplied components. I also researched turn windings for the kit supplied toroids to get close to the inductance suggested for 15m.? Made good contacts on FT8 and WSPR.?

And just in case,? the 50W kit is not a linear so SSB is likely not going to work.

GREG KI4NVX?
 

Thanks Gregg,
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I could totally go that route as well. This is just for CW, and it's good to hear the values for the LPF kit worked for you (I already bought the bigger toroids and higher-rated caps). Say, I'm sure I could look this up using the search function, but while you're here, what kind of output did you get on 15 meters? 73, Sam
 

You'll probably find this strange but ...

I was thinking about portable field use at 12V? power IN with a target of 10 Watts out, for just an extra 6dB (I know, just one S unit maybe, but a bit more none the less for crappier conditions).

So I experimented with adjusting the PA bias to get just that. Seems to work fine, though I limit the duty cycles in general to protect my Q's and the amp.?

If/when I want or need a LOT more power from the home shack,? I just fire up the ICOM 7300.?

Now having said that, I just recently bought a NanoSA but haven't gotten around to testing the actual outputs on my three 50W amps. That's a To-Do for later this spring. I self taught on the SA ( gotta love UTube)? checking the outputs of a couple of QDX's I assembled figuring the harmonics would likely be in spec. With 102dB of attenuation of course!? :)?

GREG


On Sat, Mar 29, 2025 at 10:02 PM, Sam WN5C via groups.io
<sgduwe@...> wrote:
Thanks Gregg,
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I could totally go that route as well. This is just for CW, and it's good to hear the values for the LPF kit worked for you (I already bought the bigger toroids and higher-rated caps). Say, I'm sure I could look this up using the search function, but while you're here, what kind of output did you get on 15 meters? 73, Sam

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