Topics

#u3s Relocating R5 #u3s

Nik
 

Hi Hans,

is is the location of R5 critical to the tuning of the BS170 or can I move it to a different location for ease of access?

Thanks
Nik M1DOX

Hans Summers
 

Hi Nik

R5 provides only a DC bias. So you could relocate it. But still I would be inclined I think, to err on the side of caution, and to use a shielded cable and put a 0.1uF capacitor at the U3S end... so from the top of R6 to ground.

73 Hans G0UPL


On Mon, Jul 15, 2019 at 1:49 PM <nik@...> wrote:
Hi Hans,

is is the location of R5 critical to the tuning of the BS170 or can I move it to a different location for ease of access?

Thanks
Nik M1DOX

Nik
 

I’ll give that a go this weekend.  I’m considering trying the RD16HHF1 as an alternative PA transistor.  Do you have any thoughts or suggestions on this?

Hans Summers
 

Hi Nik

The 5W HF PA kit includes a pads for this transistor (it has a different pinout to the IRF510). But I have not tried it personally. I've never owned an RD16HHF1. Too stingy :-/

73 Hans G0UPL 

On Tue, Jul 16, 2019, 13:27 <nik@...> wrote:
I’ll give that a go this weekend.  I’m considering trying the RD16HHF1 as an alternative PA transistor.  Do you have any thoughts or suggestions on this?

Nik
 

What do you reckon to replacing the BS170s with one. Is there enough level from the XO to drive it?  Obviously the connecting to the U3S would be a faff. 

Hans Summers
 

Hi Nik...

I don't have a particular opinion on it... unless I tried it out myself... but that would involve having an RD-whatsit. And having some time :-//    If you do try it out, then let me know how it goes!

73 Hans G0UPL

On Thu, Jul 25, 2019 at 9:20 PM <nik@...> wrote:
What do you reckon to replacing the BS170s with one. Is there enough level from the XO to drive it?  Obviously the connecting to the U3S would be a faff. 

Nik
 

Well I’ve built a second u3s and used an RD-whatsit as a replacement for the BS170, and feeding it just 5V with a tiny bit of bias current (about 30mA measured on my Chinese budged volt and ammeter) giving about an 110mA total idle current, I’m measuring around 200mW RF output into the dummy load, across the 10M to 30M band and those in between.  

At around 180mA total idle current, it seems to go into self oscillation on certain frequencies with no measurable output but a lot of heat and around 450mA drain, which only ceases when the power feed is disconnected. 

Is this the same kind of problem you encountered in the 5W PA, if so I’ll try the same resistor mod that you recommended for that kit.

200 mW is fine, as I’ll use it to drive the 5W PA.

As I said, it’s only been connected to a dummy load so far, but I have used a radio and WSJT-X to validate the transmissions. 

ajparent1/KB1GMX
 

for a 5W amplifier the RD15HHF or RD16VHF is a over sized device.
There are also RD06 parts that might be a better match.  Or better
yet MRF136.

As to bias, this is to be a class C or maybe E amplifier.  Two things
have to exist for that enough drive to saturate the device and also 
drive it to cutoff.  The yabut is that Most mosfets have a gain vs
bias curve that forces some current flow to get reasonable operation.
So the gate bias voltage (NOT CURRENT) needs to be sufficient for
a small amount of drain current (under 50ma!) or you moving from
class C to class B or even class A.   That move impacts gain,
power out and output and input circuits for stability and power.

So dumping the supposedly better RD part usually is more illusion
that actual performance.  Those that claim improvement often had
other issues in the works or due to the significantly different
characteristics they changed the amp in ways they do not realize.
At RF there are few (nearly none) drop in improvements.

Bottom line is to get 5W out you need enough power in and about
200-500mw is enough depending on circuit and frequency.

FYI I use IRF510s and have RD15 and RD16 parts and generally I get
more out of the IRF510s as I can run higher voltages (up to 28V) that
would destroy the RD part (designed for 13.8V nominal, 16V max use).
As to the claim the IRF is not a good HF part the QRPlabs amp runs
to 70mhz at acceptably reduced gain (could be optimized) and solid
to 30mhz.   I have two amps of my design that are for 6M use one is
20W class and the other using 8 of them does 220W at 30V.  I also
have refitted a 2006 6M SSB radio that was then designed for 1W
based on junkbox final and that is now a nice 2.5W using the
IRF510 and of course the right circuit to extract that power.
Could it do more yes, but the existing driver is the limiting factor
not the output and I didn't want to change that.

The only place the RD parts have been better for me than IRF510
is at 144mhz  and up but I can get more power per $ using MRF
series parts.  But then we are talking 100++W levels and 30-50$
transistors.

Allison

J68HZ
 

Well let’s not kid ourselves here.  The IRF parts and the RD parts are very (very very) different animals.  The IRF510’s dies are hard-on switches, meaning they saturate/ switch on hard and abruptly…they are designed that way because of their actual intended service as motor switches and power supply parts…. and that, in an RF world can cause harmonics, ringing, and trash.  It means you need to work extra hard to filter that stuff out of your output signal when you push them… and it’s frequency dependent.  The RD devices are made to be RF switches with a smoother approach on the curve at full on.  Much less trash generated.  You already know about the capacitance issues.

 

 

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From: QRPLabs@groups.io [mailto:QRPLabs@groups.io] On Behalf Of ajparent1/KB1GMX
Sent: Saturday, August 10, 2019 5:59 PM
To: QRPLabs@groups.io
Subject: Re: [QRPLabs] #u3s Relocating R5

 

for a 5W amplifier the RD15HHF or RD16VHF is a over sized device.
There are also RD06 parts that might be a better match.  Or better
yet MRF136.

As to bias, this is to be a class C or maybe E amplifier.  Two things
have to exist for that enough drive to saturate the device and also 
drive it to cutoff.  The yabut is that Most mosfets have a gain vs
bias curve that forces some current flow to get reasonable operation.
So the gate bias voltage (NOT CURRENT) needs to be sufficient for
a small amount of drain current (under 50ma!) or you moving from
class C to class B or even class A.   That move impacts gain,
power out and output and input circuits for stability and power.

So dumping the supposedly better RD part usually is more illusion
that actual performance.  Those that claim improvement often had
other issues in the works or due to the significantly different
characteristics they changed the amp in ways they do not realize.
At RF there are few (nearly none) drop in improvements.

Bottom line is to get 5W out you need enough power in and about
200-500mw is enough depending on circuit and frequency.

FYI I use IRF510s and have RD15 and RD16 parts and generally I get
more out of the IRF510s as I can run higher voltages (up to 28V) that
would destroy the RD part (designed for 13.8V nominal, 16V max use).
As to the claim the IRF is not a good HF part the QRPlabs amp runs
to 70mhz at acceptably reduced gain (could be optimized) and solid
to 30mhz.   I have two amps of my design that are for 6M use one is
20W class and the other using 8 of them does 220W at 30V.  I also
have refitted a 2006 6M SSB radio that was then designed for 1W
based on junkbox final and that is now a nice 2.5W using the
IRF510 and of course the right circuit to extract that power.
Could it do more yes, but the existing driver is the limiting factor
not the output and I didn't want to change that.

The only place the RD parts have been better for me than IRF510
is at 144mhz  and up but I can get more power per $ using MRF
series parts.  But then we are talking 100++W levels and 30-50$
transistors.

Allison


Virus-free. www.avg.com

Nik
 

I’ve not tried the RD16HHF1 in a 5W PA kit yet, the one I have been talking about is fitted directly into the U3S, and under test at 5V. If all is successful then I will use this U3S to drive a 5W PA kit.  As the 5W PA kit has provision for the RD16HHF1 I may try this device in this kit too at some time. 

My goal is a system that outputs 5W fairly linearly across the HF bands. Then I will lower the power to 2W and use it as my home WSPR system.