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Is there any way to build the 50w amp to work on more than one band. I have a QCX 20m and I am building the amp for 20m. It seems like such a waste not to be able the get the other bands active. Back in the day, the Wilderness Sierra had band modules. Pull one out and push in another. Hans has a LPF kit with all the other filters.

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Any ideas or maybe has this already been done.

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72, Dean

There are several ways to make your 50w amp multiband. My approach is to jumper out the low pass filter internal to the amp and build external low pass modules for each band of interest. I used low pass module boards available from QRPLabs and modified them with larger toroids, higher voltage capacitors, and heavier wire to handle the high power. Others have purchased multi-band low-pass filters. Searching "low pass filters" on this group will yield lots of information.

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Dennis

KK5DB

Well the simple answer is yes you can

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but not simple?

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you can’t use standard QRP labs low pass kits - at least not without thicker wire and bigger cores and higher voltage capacitors ? I.e. the PCBs are usable?

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But you could build a switch box with several suitable LPFs?

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Next you may find the Amp gain varies with frequency even if you have several switched inductors in play?

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So you need switched attenuators?

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.. and if your source is eg an Ultimate 3S its power output also varies by band and may be too small at times
so add a driver stage after the attenuators or make the driver gain switchable?

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but you can do it ..?

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Hi Dennis, Thanks for the reply.

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So reading the assembly manual to wind toroid L6 I need on 20m 14 turns on the T50-6 toroids (yellow) core. On L4 and L7 I need 20m 13 turns.

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Next Install Low Pass Filter capacitors C11 and C12
These two capacitors are 250V NP0/C0G types. The capacitor value depends on the band the
50W PA kit is being built for, as follows:
Band Value Code
80m 680pF “681”
40m 680pF “681”
30m 390pF “391”
20m 390pF “391”.?? So again if I am building for 20m that leaves 6 capacitors for the junk box.

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Next Install Low Pass Filter capacitors C19 and C20
These two capacitors are 250V NP0/C0G types. The capacitor value depends on the band the
50W PA kit is being built for, as follows:
Band Value Code
80m 390pF “391”
40m Not installed
30m 180pF “181”
20m Not installed?? That leaves 4 more capacitors for the junk box.

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So really, if I had two more cores I would be able to build two more filters. No need to upgrade anything because Hans has the right size included. Amidon has the cores for $2.00

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Now how to figure out how to install header pins to drop a filter on to.?? hmmmmmm.......

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Dean

Dean,?

? ? I have a 50W camp built for 20m. Since it's already built up, my approach/thought (that I have not yet implemented) is to switch LPF kits from Pacific inline when I want to operate on a lower band with the amp: ? ?To me, the nice thing there is there's no real modification or adaptation needed within the amp.

I have a 20m dipole in my attic currently, but have a Pacific 40/20m trapped dipole assembled, waiting to be installed.?

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-Nate

N8BTR

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Here are my old measurements with the amplifier supply voltage of 24 volts. Without the ability to regulate the input power, this idea will not work.

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73, Oleh, UY2ZA

On Wed, Sep 11, 2024 at 07:16 AM, Oleh I2/UY2ZA wrote:

Without the ability to regulate the input power, this idea will not work.

Yep,? tried to suggest he think it all through? before getting too committed.? ?I know I'm no fun ?
The gain variation is one challenge and finding a QRP-Labs Transceiver with the same output on all those bands is another?

As one ploughs through all this the reasons Large amps are expensive gets clearer

I don't have to carry a transceiver QDX so I use an external regulated voltage source to power the output stage. For example, for the 80 meter band I supply about 4 Volts to the output stage.

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73, Oleh, UY2ZA

I have a 20m 50w amp in which I changed the LPF to 17m. With a 24v supply (modified laptop supply) I get 50w out on 20m and 35w out on 17m.

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Sharing LPFs between adjacent bands is common on most HF rigs, including the QMX.?

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Running this amp on 17m is probably at its upper limit.

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73 Steve G4EDG

If you run the amp at a lower supply voltage, giving up some output, I'd think it'd work.?

Mine using 19V on 20m gives me 25W with 3w in. On 40m, I get about 30w out from that amp at 19V.?

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A good reminder I need to datalog heatsink temperatures.... I run my amp at 25w on FT8, and in a 100 degree F attic, no issues. Would be good to know if I need to be more cautious at 30w in those conditions.?

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-Nate

N8BTR

Nate, I change the supply voltage of the QDX power amplifier in order to change the power at the input of the external power amplifier.?

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73, Oleh, UY2ZA