uBITX PA
I will be trying out the Mitsubishi parts after I get all of the other things sorted out with my uBITX, but it is a very pleasing rig.? My uBITX starts out at 12W on 80M, 7.5W on 40M, 2W on 20M, and is down at 0.4W on 10M. so I will be checking the driver stages as well.? I expect they are not flat over the 80-10M bands either.? ?I will take it step by step and I have a good spectrum analyzer to keep things clean as I work my way through that.?? I'll post the results when I get to this down the road...??
Ahead of the potential PA mods are: 1) I just received the neat little CW Keying Adapter PCBs from the QRPGuys.? Nice job, but I'll need some smaller standoffs (4-40 or smaller mounting it looks like). 2) AGC is up after that using the unused Audio AGC amplifier to control ether IF or RF or both for AGC. 3) Finally I have an annoying high frequency tone at medium volume leaking through somewhere on receive to locate and get rid of.?
Keep the soldering irons warm...
Dave WI6R
On Tue, Jan 16, 2018 at 12:55 AM, Gordon Gibby <ggibby@...> wrote:
Bill—. Thanks!!!
Sent from my iPhone
On Jan 16, 2018, at 00:12, K9HZ < bill@...> wrote:
Name is Bill…
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There are considerations in the amplification stages of the uBITx leading up to the PA too … the bandwidth of the output and interstage transformers T9, T10,
and T11.? I have not scanned them to know… or measured the drive from the earlier stages.? Theoretically if they were designed right, the PA with an RD15HVF1 would generate about 18 watts output from 1-54 MHz (60% efficiency).? The biasing is at a different
point but what is on the board should work. ??Maybe someone has already measured the output of the last driver stage before the PA on all bands with a scope.. and knows if there is change in available drive level with frequency?? You wouldn’t want to go through
the work of replacing the 510 with the RD device just to find out the other stages have a severe gain variance with frequency.
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I always replace the 510’s with RD parts whenever I can…
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Dr.
William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ
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Owner - Operator
Big Signal Ranch – K9ZC
Staunton, Illinois
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Soufriere, St. Lucia W.I.
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email:?
bill@...
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?So Dr. Schmidt --- are you saying that the severe dropoff in power output from the ubitx / bitx40 would be ameliorated?
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If so, I might be interested!? ? It would be quite nice to have a bit more power on 30 and 20 meters.....
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comment?
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gordon
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Of course the upside of using RD16HVF1 is that it is a real RF transistor... not a motor switch like the IRF510.? What you get for that is actually
hidden in your comment about gain at high frequency. In fact, the gate capacitance with frequency is far better behaved with the RD device so complicated compensation networks to keep gain constant is not needed.? Hans Summers has a second set of pads on his
5 watt PA just for the use of these parts.?
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Dr.?William J. Schmidt - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ PJ2/K9HZ
?
Owner - Operator
Big Signal Ranch – K9ZC
Staunton, Illinois
?
Owner – Operator
Villa Grand Piton - J68HZ
Soufriere, St. Lucia W.I.
Rent it:
?
email:??bill@...
?
I think I mentioned it before, but anyone seeking to drop in a Mitsubishi RDMos part should be aware that?
a) They're designed for 12V operation.? You won't want to run them at 18V/etc
b) They have a different pinout, you'll have to solder them in with the leads mangled/crossed.? IRF510/20/30's are Gate-Drain-Source.? The RDMos series are Gate-Source-Drain.?
This is a mod I still intend to try at some point, but have other projects on the plate first.
c) They have a different bias voltage.? Turn down the bias voltage before you key up, and then re-bias appropriately. If your IRF510's are biased at something like 4V when the
bias voltage for the RDmos's are 2V (I'm making total wild assumptions, it could be the other way around), you'll instantly turn the devices on full-current and potentially blow something up.??
d) The feedback network will want to be different.? These have much more gain at high frequencies.? You may need more negative feedback.
e) You can use an uninsulated grounded heatsink, as the tabs of the these devices is the source and not drain.
I'm looking forward to hearing from someone who's done the mod and tried it out.
?
On Mon, Jan 15, 2018 at 10:52 AM, Tim Gorman <tgorman2@...> wrote:
Dave,
Would this be a satisfactory drop-in replacement? I can't find any info
on input or output capacitance to compare it to an irf510.
From the Smith chart on the datasheet I'm guessing the input
capacitance is about 60pf (been a long time since I've used a Smith
Chart) at 30Mhz, much less than the irf510. Would that impact the
negative feedback used on the amp significantly (0.1uf in series with a
220ohm resistor)?
By the same token the output capacitance would appear to be much higher
than the irf510. I suspect that will impact the design of the bandpass
filters but I'm not sure by how much without doing a lot of
calculations.
I'm just looking for something that would provide more reliability
under adverse conditions (e.g. high swr) than the irf510. Don't really
care if it provides higher output or not - but that might come later
<grin>.
On Sun, 14 Jan 2018 22:51:06 -0800
"Dave Bottom" <ars.kd6az@...> wrote:
> Or the Mitsubishi RF RD16HHF1 MOSFET designed for 2-30MHz RF
> applications. Under $5 each.? Or the RD16HVF1 only pennies more and
> good through 2M.
>
> With a 12VDC to 24/28VDC DC to DC converters (about $4) and the Mitsu
> parts can be bolted directly to a big heatsink attached to the
> chassis, no insulator required.
> Probably good for 20W through 10M although run off batteries a bit
> shorter battery life.? At real 5W QRP they should be happy forever.
>
> Dave WI6R
>
> On Sun, Jan 14, 2018 at 4:33 PM, Tim Gorman <tgorman2@...> wrote:
>
> > All,
> >
> > Looking to order some spare output finals for the ubitx I have on
> > order.
> >
> > In looking around the IRF520 appears to have higher ratings than the
> > IRF510 with a lower Rds.
> >
> > The 510 does have much lower input and output capacitance.
> >
> > There is hardly any difference in price.
> >
> > Any recommendations on which to order?
> >
> > tim ab0wr
> >
> >
> >
> >
>
>
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