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QRPLabs 10 Watt Linear with QMX+

 

Iam working on the 10 watt amp and I need a little better understanding of the power input it will need. I have a Smart Power Attenutator that will give me a? 0 to 15 db reduction. I am not sure about the rquirement of the level of rf in watts (mili watts ?) for a drive level. My QMX+ shows average of 3 to 3.8 watts across the bands and I beleive this is just a little more than this little amp needs. Any help, thoughts, or other welcome, but be kind to an old gyser.?
Thanks all
--
73 ChuckR
 

Hello ChuckR,
?
The amp has a gain of 26 dB and a power output of 10 watts.? That would then be a power input of .025 watts or 25 milliwatts.
?
db = 10 log (Pout/Pin)
26 = 10 log (10watts/Pin)
Pin = 10watts/(10^2.6)
Pin = .025 watts
?
3 watts is approximately 35 dBm.? ?.025 watts is approximately 14 dBm.? To reduce 3 watts to 0.025 watts, you need a 21 dB attenuator.
?
Here is a watt to dBm calculator:
?
73
Evan
AC9TU
?
 

Chuck
?
... meaning that 10 watt amplifier makes no sense to add to the QMX.?
?
It is meant for being fed by a few milliwatt transmitter.?
?
?
?
Save it for another project.?
?
Curt
?
 

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Kind of but maybe not. I have one and I use it with my QMX to get 20 watts out.

My setup is like this:

I combine a 10db? 10watt attenuator from amazon, a 50 ohm Allen Bradley type J pot (rated at 2 watts) for drive level, a filter array from an old Icom IC-765 (ebay), and a small buck/boost module. The input from the QMX passes through the 10db attenuator, through the 50 ohm pot to the amplifier input. The power is from an external 20Volt supply. The buck/boost module is used to bring the 20 volts down to 12 for the filter relays and the TR relays. The amplifier output passes through the filter array to the output.

I get 20 watts on all bands. I have tested this with my thermal logger and the case of the finals never go above 63C running FT8 (1 hour runs).

So, 20Watts is 2 S-units. It is worth it to me sometimes.?

The QMX always sees a nice 50 ohms.

Picture attached.

Tony
AD0VC




From:[email protected] <[email protected]> on behalf of Curt wb8yyy via groups.io <wb8yyy@...>
Sent:?Friday, May 30, 2025 7:36 PM
To:[email protected] <[email protected]>
Subject:?Re: [QRPLabs] QRPLabs 10 Watt Linear with QMX+
?
Chuck
?
... meaning that 10 watt amplifier makes no sense to add to the QMX.?
?
It is meant for being fed by a few milliwatt transmitter.?
?
?
?
Save it for another project.?
?
Curt
?

IMG_20250511_144551018.jpg

 

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Make that 1 s unit but still matters sometimes.

Tony

From: [email protected] <[email protected]> on behalf of mux_folder2001 via groups.io <canthony15@...>
Sent: Friday, May 30, 2025 8:09 PM
To: [email protected] <[email protected]>
Subject: Re: [QRPLabs] QRPLabs 10 Watt Linear with QMX+
?
Kind of but maybe not. I have one and I use it with my QMX to get 20 watts out.

My setup is like this:

I combine a 10db? 10watt attenuator from amazon, a 50 ohm Allen Bradley type J pot (rated at 2 watts) for drive level, a filter array from an old Icom IC-765 (ebay), and a small buck/boost module. The input from the QMX passes through the 10db attenuator, through the 50 ohm pot to the amplifier input. The power is from an external 20Volt supply. The buck/boost module is used to bring the 20 volts down to 12 for the filter relays and the TR relays. The amplifier output passes through the filter array to the output.

I get 20 watts on all bands. I have tested this with my thermal logger and the case of the finals never go above 63C running FT8 (1 hour runs).

So, 20Watts is 2 S-units. It is worth it to me sometimes.?

The QMX always sees a nice 50 ohms.

Picture attached.

Tony
AD0VC




From:[email protected] <[email protected]> on behalf of Curt wb8yyy via groups.io <wb8yyy@...>
Sent:?Friday, May 30, 2025 7:36 PM
To:[email protected] <[email protected]>
Subject:?Re: [QRPLabs] QRPLabs 10 Watt Linear with QMX+
?
Chuck
?
... meaning that 10 watt amplifier makes no sense to add to the QMX.?
?
It is meant for being fed by a few milliwatt transmitter.?
?
?
?
Save it for another project.?
?
Curt
?
 

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Besides, why is 3db "not worth it"? A 2 element quad vs a dipole is 3db. I think a 3 element beam is about 6db over my vertical. People make big bucks adding 1db to an antenna design (lengthen the boom or what have you).

Tony
AD0VC


From: [email protected] <[email protected]> on behalf of mux_folder2001 via groups.io <canthony15@...>
Sent: Friday, May 30, 2025 8:14 PM
To: [email protected] <[email protected]>
Subject: Re: [QRPLabs] QRPLabs 10 Watt Linear with QMX+
?
Make that 1 s unit but still matters sometimes.

Tony
From: [email protected] <[email protected]> on behalf of mux_folder2001 via groups.io <canthony15@...>
Sent: Friday, May 30, 2025 8:09 PM
To: [email protected] <[email protected]>
Subject: Re: [QRPLabs] QRPLabs 10 Watt Linear with QMX+
?
Kind of but maybe not. I have one and I use it with my QMX to get 20 watts out.

My setup is like this:

I combine a 10db? 10watt attenuator from amazon, a 50 ohm Allen Bradley type J pot (rated at 2 watts) for drive level, a filter array from an old Icom IC-765 (ebay), and a small buck/boost module. The input from the QMX passes through the 10db attenuator, through the 50 ohm pot to the amplifier input. The power is from an external 20Volt supply. The buck/boost module is used to bring the 20 volts down to 12 for the filter relays and the TR relays. The amplifier output passes through the filter array to the output.

I get 20 watts on all bands. I have tested this with my thermal logger and the case of the finals never go above 63C running FT8 (1 hour runs).

So, 20Watts is 2 S-units. It is worth it to me sometimes.?

The QMX always sees a nice 50 ohms.

Picture attached.

Tony
AD0VC




From:[email protected] <[email protected]> on behalf of Curt wb8yyy via groups.io <wb8yyy@...>
Sent:?Friday, May 30, 2025 7:36 PM
To:[email protected] <[email protected]>
Subject:?Re: [QRPLabs] QRPLabs 10 Watt Linear with QMX+
?
Chuck
?
... meaning that 10 watt amplifier makes no sense to add to the QMX.?
?
It is meant for being fed by a few milliwatt transmitter.?
?
?
?
Save it for another project.?
?
Curt
?
 

Thanks all, Curt the 10 watts is a pre driver and with the second amp producing 300 wats, so the project continues...? thank all who have, are and will be helping ...
--
73 ChuckR
 

Tony, I like what you have done. I have a 10w linear amp too that I intend to add to my QMX+, partly for the power boost but mainly because of the alarming number of posts where the QMX+ finals have failed. This 10w amp was originally described as practically indestructible. I wondered what Low Pass filters you built, from the photo, you seemed to have put a lot of work into this!
73 Ken g4apb?
 

On 31/05/2025 06:44, KEN G4APB via groups.io wrote:
mainly because of the alarming number of posts where the QMX+ finals have failed
Ken,

It has often been said that most do not fail. Why should yours be one that does?

In any case I dislike the idea of producing several watts and throwing the power away into an attenuator.

The 10 watt linear is so sensitive it does not need the BS170s to drive it.

73 Alan G4ZFQ
 

On Sat, May 31, 2025 at 08:14 AM, Alan G4ZFQ wrote:
It has often been said that most do not fail. Why should yours be one that does?
Alan,
a) because I often run into poor match antennae, I am always experimenting....
b) Have you ever tried to replace the BS170 quad set? I find it a nightmare trying to clear the holes....
c) Despite all the warnings, and asking for trouble, all my QRPLabs kits (and other gear) run from 13.6v supplies....
?
In any case I dislike the idea of producing several watts and throwing the power away into an attenuator.
?
d) I agree, inefficient but it works. However, I am wondering why I can't just remove some of the intermediate stages in the 10w amp to reduce the attenuation requirements. Although a nice 50ohm load for the QMX is a good addition.
e) A very BIG reason...I bought this 10W amp for the intended QSX, let it gain dust for a long time and I am determined to use it....
f) got shot down in flames before for saying this but I have bought the higher frequency equivalent of the IRF510 to put in my 10w amp to try and extend the frequency range beyond the 6m band. (Look out QMX+ I'm going to push yer limits..)
?
Need any more reasons??? :)
73 Ken G4APB
?
 

On 31/05/2025 07:55, KEN G4APB via groups.io wrote:
Need any more reasons???
No Ken, it's up to you.
So many, including Hans, say that they mistreat their QDX/QDMs and fail to blow the BS170s.
I just wondered if you'd give them a chance:-)
In the event of failure clearing holes is not necessary, cut the leads and solder to them. But that's what others say, I've never had to, yet..

Best of luck with your endeavours.

73 Alan G4ZFQ

.
 

Tony, thanks for sharing that, pretty neat!
?
Can you share what buck/boost module you used? I assume it's RF quiet enough to not be a problem in your operating. I'm thinking of one as a possibility for a QDX+Amplifier project, to bring the supply voltage down for the QDX and for some accessories.
?
-Nate
N8BTR
 

It's me AGAIN! What are the pros and cons (other than a? aprox 1 watt drop of rf) of running a 12 volt QDX+ on 9 volts?
--
73 ChuckR
 

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Hi Ken,

Well,? I did not build it to protect the QMX. It is just a side benefit. As for the low pass array, it came from eBay and is from an old IC-765. All of the relays on the board were in need of cleaning. Fortunately, unsoldeing the relays with my old solder sucker was pretty easy. The relay tops can be popped off (took a while to figure out how) and the contacts were cleaned with thin strips of post-it note paper dipped in deoxit. That was the most labor intensive part.

The toggle switch kills the 20 volts and drops the relays into bypass mode so I can run the QMX barefoot.

Tony
AD0VC



From:[email protected] <[email protected]> on behalf of KEN G4APB via groups.io <lfoofui.nbz42@...>
Sent:?Saturday, May 31, 2025 12:44 AM
To:[email protected] <[email protected]>
Subject:?Re: [QRPLabs] QRPLabs 10 Watt Linear with QMX+
?
Tony, I like what you have done. I have a 10w linear amp too that I intend to add to my QMX+, partly for the power boost but mainly because of the alarming number of posts where the QMX+ finals have failed. This 10w amp was originally described as practically indestructible. I wondered what Low Pass filters you built, from the photo, you seemed to have put a lot of work into this!
73 Ken g4apb?
 

Hi ChuckR,
?
There are no receiver impacts as long as the voltage is above 7 volts.? The PA will run cooler, and there is more voltage overhead to protect against high SWR.
?
I run my QMX and QDX on 8 to 9 volts to drive a WA2EBY linear.
?
73
Evan
AC9TU
 

The 10W linear is at max 50milliwatt drive, typical is more like 20-25milliwatts.
?
So attenuation of the SMX will be in the range of 18 db and need to be
switched out for RX.
?
All that for maybe 10W, no advantnage.
?
--
Allison
------------------
Post online only, direct email will be deleted.
 

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Hi Nate,

Actually I did not worry about that. I use a separate receive antenna for the QMX. I have a relay antenna switch between the QMX and the amplifier to connect the RX antenna when receiving. Since I mounted my vertical in the yard about 10 years ago, the QRM from the underground utilities has grown to the point where I needed to move it. But moving it would have been very difficult since it is set in concrete and has all those radials. So, I have an active whip on the far side of the yard that I use for my NDB hobby and I brought that into service for ham radio RX. Works very well.

Tony
AD0VC

From: [email protected] <[email protected]> on behalf of Nate Reik N8BTR via groups.io <natereik@...>
Sent: Saturday, May 31, 2025 7:38 AM
To: [email protected] <[email protected]>
Subject: Re: [QRPLabs] QRPLabs 10 Watt Linear with QMX+
?
Tony, thanks for sharing that, pretty neat!
?
Can you share what buck/boost module you used? I assume it's RF quiet enough to not be a problem in your operating. I'm thinking of one as a possibility for a QDX+Amplifier project, to bring the supply voltage down for the QDX and for some accessories.
?
-Nate
N8BTR
 

To use 10W linear amp from QRPLABS with QMX or QMX+ , power PA mosfets with 5V instead of 12V. This will reduce power to 300 to 500 mW which is easier to tame with attenuators.?

Here is my take for usdx trx:
?
--
?

73, Barb
 

Guys,
if it is not a silly question, why did you not leave out the 2xBS170 pre-driver stage on the 10w linear amp to reduce the gain and not need such a large (value) input attenuator? The 50w amp (I know it is not a Linear) does not have this stage and is stated has needing 2-4 watts of drive. So It seems closer to the QMX output power and a much smaller (2-3) db attenuator could be used.
?
73 Ken G4APB
 

So..
?
I also have the 10 Watt amp that was designed for the QSX (head hanging in grief). I intend to put mine in ?uBitX in place of that amplifier. Or, just build something based on the same technology. If I had a QMX with 3 Watts of output I would not waste time with a 10 watt amp. I would want at least 10 times (30 Watts) and getting 50 Watts from that 3 watts would be even better If no intervening amp needed).
?
Wasting power? just the effort to double the power output has been shown over and over and over and.....ad infinitum... useless on the receiving end it as will be undectable and not measurable. Having said that, I have used double barrel 6146 tube amps throttled back to five watts (or less) so as to not be "cheating" in QRP events. The heater in just one tube draws more power than the output. And how many tubes in a Heathkit DX100 or Kenwood hybrid? The Johnson Ranger has only one 6146 so not quite as bad. Sometimes those rigs have bands that are not covered by my cute lil QRP radios. Sometimes my antenna for the band is *crude* and tubes just do not care about that.
?
It is your amplifier so you do whatever you like with it. It can be fun even if it is wasteful (like my DX-100 at 3 watts out). Best of luck with your project. Melt some solder. Fry some parts. Make some smoke. Have fun
?
73,?
?
Bill ?KU8H

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