Hi Everyone,

I'm making a little headway on my BITX40 rebuild and have finally got enough output to drive the 10W QRP Labs HF PA kit. The PA expects to be powered full time, and then put into transmit when TX is low (ground). The BITX40 works by turning on the PA with a 12V relay. I am thinking I could just tie the TX to ground permanently since the whole PA won't be powered unless I'm transmitting. Does that sound about right? Thanks for any and all input. 

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Ryan Flowers - W7RLF
MiscDotGeek - QRP and More

QCX Mini "Mini Tip" Series 

QCX Mini Troubleshooting and Repair

Ryan

A few key interfaces convey the intent. We have the coax cables, the PA ground will be at same potential as the rig driving it. When using the PA it has to be prepared for 2 states, send and receive. Yes one unusual thing, no off switch. Turning off my qcx plus, the PA on light stays on. I do suggest powering off PA supply when and if troubleshooting, smoked my pll in my original qcx this way. 

Curt

Ryan,

No, do the switching as the PA on all thing time is wasted power and heat.
Also I may contribute RX noise depending how your switching the antenna.

Hint: if TX is +12 on TX what is RX doing on tx... not +12!

Or ground the TX line and apply TX+12 to the whole amplifier and hope
the relay can take at least 2A plus charging current of the caps.

Allison
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Please reply on list so we can share.
No private email, it goes to a bit bucket due to address harvesting

Thank you everyone. I believe I misrepresented myself. The idea was to ground the TX line and only power the PA when the whole radio was put into Transmit mode via PTT. But as you've pointed out Allison, that's asking quite a lot of the TX power relay! So that idea is out.

The next thing then is grounding TX on PTT. Can anyone suggest the best way to do this? I could use a relay of course, and perhaps that's the easiest thing to do. But I know that a transistor or mosfet can be used as a switch. Will that work in this scenario, since it's switching to ground rather than from positive voltage? An EE I'm not. Thanks. 
--
Ryan Flowers - W7RLF
MiscDotGeek - QRP and More

QCX Mini "Mini Tip" Series 

QCX Mini Troubleshooting and Repair

Hi Ron,

I do have some extra BS170's. That will work a treat. Thanks so much!
--
Ryan Flowers - W7RLF
MiscDotGeek - QRP and More

QCX Mini "Mini Tip" Series 

QCX Mini Troubleshooting and Repair

Hi Ryan,

This is my take on qrp labs 10W PA. 

https://antrak.org.tr/blog/projeler/linear-rf-power-amplifier-for-usdx/

73

Barb WB2CBA

Hi all,
I have ordered a 10W amp and LPF but I noticed it is meant for the qrp U3s, and is only rated at 5w, so what are you guys doing to the LPF to make it 10w capable please?

Also, where does the LPF mount on the PA, I can’t see any reference to it in the manual?

73 Ken g4apb 

Ken,

it says in the description of qrp labs LPF that it handles 10 watts safely. I have a 10w linear from same and there is no place to mount LPF on it, you would need to mount it adjacent, perhaps on the spare bit of heatsink?

Hi Guys.

I think you may be confusing two offerings from Hans. There is a 5 watt PA designed for the U3S, and a 10 watt PA designed to mate with the upcoming QSX.

73

Roger

8P6RX

On Sun, Jun 20, 2021 at 02:51 AM, Ian MM0GYX wrote:

it says in the description of qrp labs LPF that it handles 10 watts safely. I have a 10w linear from same and there is no place to mount LPF on it, you would need to mount it adjacent, perhaps on the spare bit of heatsink?

If you feed harmonics into the AMP, then filter them out at the output of the AMP, you're wasting a lot of energy amplifying harmonics...


Your transmitter should already have a clean signal at their Antenna Jack...   Right?

If you feed a clean signal INTO the Amp, the amp itself is not going to generate harmonics on its own, will it?

The QL 10 Watt Linear Amp doesn't need an LPF.  That's why there's no place to install one.


Or, maybe I'm wrong ??


73
km6wt

km6wt,

Some distortion is almost always added when amplifying a signal.  All lineares should have a corresponding low pass filter to remove any harmonics that could be generated by the amplification process.  I would not run the linear without a low pass filter on the output to suppress the 3rd and other odd harmonics.  The testing results that Hans provided on the QRP-Labs web page indicated only a -38dBc on the third harmonic.  US FCC requirements are for -43dBc.  Easy to obtain with an added LPF.

You still need to feed it a clean signal, so whatever filter is required to run the transmitter without the amp is still required when using the amp.  For most cases that means there is an LPF on the output of the transceiver and one on the amp.

The QRP 10watt linear was designed to have both off-board antenna switching and an off-board separate low pass filter.  It was intended as one of the modules that would be part of the QSX transceiver.  At some point, I hope we will see the QSX.

The above are my understandings of the amp.
73
Evan
AC9TU

Sama is correct that the QRP-Labs low pass filter kits have capacitors that are rated for up to 5watt input to the filter.  The option would be to find equivalent parts with higher voltage rating for the capacitors.  Just make sure they are NPO or equal.

From the web page for the low pass filter kits:
"Capacitors: the provided capacitors are low-loss Class-1 dielectric RF types (also known as CC4), with low temperature coefficient (also known as NP0 or C0G). All the capacitors supplied have a voltage rating which is adequate for up to 5W RF."

https://qrp-labs.com/lpfkit.html

73
Evan
AC9TU

On Sun, Jun 20, 2021 at 11:20 AM, Evan Hand wrote:

Sama is correct that the QRP-Labs low pass filter kits have capacitors that are rated for up to 5watt input to the filter. 

Sarma,

I apologize, I had a typo in your name.

73
Evan
AC9TU

On Sun, Jun 20, 2021 at 04:52 PM, MVS Sarma wrote:

Pl read the spec again. Imho, it cant take high power input, if I got the interpretation correct. 

Sarma vu3zmv

On Sun, 20 Jun 2021, 15:21 Ian MM0GYX, <ianmichaelwatson@...> wrote:

Ken,

it says in the description of qrp labs LPF that it handles 10 watts safely. I have a 10w linear from same and there is no place to mount LPF on it, you would need to mount it adjacent, perhaps on the spare bit of heatsink?

Hi Sarma,

I read it again, it still seems to say 'The Low Pass Filter kit can handle at least 10W safely.' Just after the first paragraph. So maybe it's a wee bit misleading I suppose.

I realise it also quotes 'Capacitors: the provided capacitors are low-loss Class-1 dielectric RF types (also known as CC4), with low temperature coefficient (also known as NP0 or C0G). All the capacitors supplied have a voltage rating which is adequate for up to 5W RF.'

Given the cost of the LPF, if it was me I would throw caution to the wind and use it with 10 watts. I know, I'm crazy, but that's the way I roll. Of course, one could source higher voltage capacitors and go with those, but provided the load the LPF looks in to isn't too mismatched, voltages should be manageable. I'm guessing these are 50 V capacitors, I may buy one and try some experimenting to see if I can break it.

Ian MM0GYX

On Sun, Jun 20, 2021 at 02:55 PM, Mont Pierce KM6WT wrote:

If you feed harmonics into the AMP, then filter them out at the output of the AMP, you're wasting a lot of energy amplifying harmonics...


Your transmitter should already have a clean signal at their Antenna Jack...   Right?

If you feed a clean signal INTO the Amp, the amp itself is not going to generate harmonics on its own, will it?

The QL 10 Watt Linear Amp doesn't need an LPF.  That's why there's no place to install one.


Or, maybe I'm wrong ??


73
km6wt

Hi Mont, you are right, you should feed the linear with a clean signal, you don't want to be amplifying harmonics and other spurious emissions. Unfortunately there are non-linearities to some extent in any amplifier, so there will be harmonics. In this case, class AB push pull, it's limited to odd order harmonics, the even order harmonics are much suppressed through design. A low pass filter will still be required though for those odd order harmonics.

Ian MM0GYX

On Sun, Jun 20, 2021 at 11:03 AM, Roger Hill wrote:

Hi Guys.

I think you may be confusing two offerings from Hans. There is a 5 watt PA designed for the U3S, and a 10 watt PA designed to mate with the upcoming QSX.

 

73

Roger

8P6RX

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***************************
Roger Hill
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Me, confused, how dare you Sir! (-;

On Sun, Jun 20, 2021 at 06:55 AM, Mont Pierce KM6WT wrote:

Or, maybe I'm wrong ??

Thanks for setting me straight guys.  :)  :)

Showing my ignorance here....  I've always ran barefoot.

Your comments are well appreciated, and educational (especially to me).  


TNX !!
km6wt