RF Amp for HT-1
|
The QRP-Labs 50W amplifier makes a great (and inexpensive) single-band amp for the HT-1A.
To key the amp, a 5v keying line is needed from the HT-1. ?On the HT-1 display board I tapped into the line supplying voltage to the red transmit LED and used that to feed a small reed relay mounted inside the rig's case (with a diode across it), connected to a 1/8" phone jack on the back. I'll attach the interface circuit I used to provide 5v to key the amp. 73 Howard K4LXY |
|
Hi all I don't like unnecessary use of relays compared to solid-state because they are relatively: 1) Expensive 2) Slow 3) Noisy 4) Large 5) Heavy 6) Unreliable There are applications where use of relays is necessary; but I have recently enjoyed avoiding them in recent QRP Labs kits for these 6 reasons.? My other suggestion is the attached circuit for driving the QRP Labs 50W PA - this uses a P-channel MOSFET and should work on any QRP rig with active-low keying/PTT output, even if the rig output is not open source / open drain.? 73 Hans G0UPL On Sun, Apr 19, 2020 at 10:51 PM K4LXY <k4lxy.cw@...> wrote: I'm guessing that a MOSFET solid state relay will be the way to go.? The DC line to the transmit LED provides a little over 4v.? I've not used MOSFET relays before but I plan to experiment with one. |
|
This is a question on the output transformer on the 50W PA from Hans (who appears to read these posts).? This uses a phasing transformer as the power feed.? It seems that the Phasing choke adds an AC? copy of the supply voltage to the supply voltage? on the currently supply leg of the output transformer's primary.? But this suggests that an amplifier using a phasing choke can't be more than 66 percent efficient? just on the basis of that topology regardless of the basic amplifier (semiconductor ) efficiency.? ?Is that correct or am I missing some vital point ?? ?I realise that the other main RF power transformer topology with a split winding transformer has issues with efficient use of the magnetic core? and? with getting a true RF centre tap (with an assymetric waveform if it is not dead on centre ) so the the phasing choke may be the best of a bad bunch.? ?Comment invited on the efficiency question. |
|
I'll wait for a while.? - You? might like to mention the query to any RF designers you know.? It's a very common topology , this phasing choke.? H. Granberg of Motorola used it a lot but he never described how it worked accurately (he says in one appn note that it provides a cntr tap equivalent to the output transformer -that's horse dung).? I have a few suspicions about how reproducable the (OPT)? transformer of this fifty watt class C amplifier is.? Virtually no RF transformers in that power class use a wound primary transformer.? Reasons being - not enough primary inductance compared with a copper tube type and the capacitive shunting that exists in the primary when turns are overwound that simply is not there in a copper tube primary.? In addition if you overwind a secondary on an RF primary (I had a look at the kit instructions and that's what it instructs) then you will get zero net flux in the secondary's flux window at the time the flux is being established (every half cycle) .. until the flux is fully established in the? outer part of the ferrite core - the problem disappears when you use copper tube primaries.? I decided to have look at Motorola AN 762 and An749? as below (the photos for Figure 3 which show transformer construction are very poor but they show braid used for the primary and it is sitting right up high, not in the web of the transformer core.)? The only reproducible design of OPT? I know of in this power class is the Wa2eby amplifier and he used Teflon insulated wire which spreads the windings and reduces the capacitive shunt effect in the primary. He used a pig-nose balun core which for a phasing choke fed application should be basically identical to a dual tubular coil transformer design. (But it is not identical magnetically if the transformer is centre tapped).
I attach a ref to the Motorola PDF on RF transformer design |
|
Hi Tim You raise many interesting points. I am sorry but I don't have answers for you. My level of knowledge on power amplifier and transformer topologies insufficient to grapple with such questions - I'm learning as I go, as many of us are.? What I can say is that this 50W amplifier certainly is reproducible, it works well as documented. To date 508 have been sold and (though doubtless, not all have been assembled!) customer feedback?is very good. As supplied the amplifier can be built for any single band 40, 30 or 20m. Of course modifications are possible.? Here I have operated mostly?with a 40m version, I have a 40m QCX CW transceiver kit assembled in an enclosure with a 50W amplifier pre-production prototype. I had over 500 QSOs with this set-up, all around Europe from my QTH (S.W. Turkey), East to Japan and New Zealand, and West to US. Some photos attached.? 73 Hans G0UPL On Wed, Apr 22, 2020 at 2:35 AM Timothy Fidler <engstr@...> wrote: I'll wait for a while.? - You? might like to mention the query to any RF designers you know.? It's a very common topology , this phasing choke.? H. Granberg of Motorola used it a lot but he never described how it worked accurately (he says in one appn note that it provides a cntr tap equivalent to the output transformer -that's horse dung).? I have a few suspicions about how reproducable the (OPT)? transformer of this fifty watt class C amplifier is.? Virtually no RF transformers in that power class use a wound primary transformer.? Reasons being - not enough primary inductance compared with a copper tube type and the capacitive shunting that exists in the primary when turns are overwound that simply is not there in a copper tube primary.? In addition if you overwind a secondary on an RF primary (I had a look at the kit instructions and that's what it instructs) then you will get zero net flux in the secondary's flux window at the time the flux is being established (every half cycle) .. until the flux is fully established in the? outer part of the ferrite core - the problem disappears when you use copper tube primaries.? I decided to have look at Motorola AN 762 and An749? as below (the photos for Figure 3 which show transformer construction are very poor but they show braid used for the primary and it is sitting right up high, not in the web of the transformer core.)? The only reproducible design of OPT? I know of in this power class is the Wa2eby amplifier and he used Teflon insulated wire which spreads the windings and reduces the capacitive shunt effect in the primary. He used a pig-nose balun core which for a phasing choke fed application should be basically identical to a dual tubular coil transformer design. (But it is not identical magnetically if the transformer is centre tapped). |
Timothy Fidler
