[Lowfer] PA transistor getting hot - need advise

[Bill Ashlock]

Andy,
 
If you use the correct drive levels to your output 2222/2907 transistors and you are using a correct load the transistors will not get hot at the 1w output level. My circuit, that JA mentioned, has limiting diodes that help reduce the criticality of the base drive. 
 
You didn't mention checking the waveforms on a scope. I highly recommend this. Are you using a filter at the output? If so, remove it, at least for this test. (You will also be amazed with how a filter messes up the squareness of the output waveform) The typical filter must have a resistive, wideband, load to function properly and this is not the case for a lowfer antenna which loads only one frequency. BTW, I have never experienced an interference problem in many years of filterless operation. 
 
If you are using a load resistor, only, for your bench test the resistor is seeing the upper harmonics of the square wave and will consume more power than a typical lowfer high-Q antenna having the same effective resistive load. For bench testing build a dummy antenna consisting of a resistor, a coil, and a capacitor that have approx the same values that your antenna has. The resistor includes the total AC resistance in your antenna's ground system and loading coil and is also adjusted to accommodate the AC resistance of the bench loading coil. You may have to do some guessing, here, but you should be able to zero in on the correct value once you reconnect your antenna and compare load currents.
 
Let us know if this works.
 
Bill> From: chejmw at buffalo.edu> To: ku4xr at yahoo.com; lowfer at mailman.qth.net> Subject: Re: [Lowfer] PA transistor getting hot - need advise> Date: Fri, 18 Jul 2008 12:15:24 -0400> CC: > > Whenever I design in the 2N2907A I always put a solid heatsink on the> thing. Not the flaky "thin fins jobby" but the solid aluminum, about the > same> height as the transistor, and multi-ribbed.> > I have found that they run warm in RF service as Xmit output devices. There> are several other low cost devices that will also give same power output at> these freqs. Most of them also run warm. FWIW.> > Jim> WB2FCN> > > ----- Original Message ----- > From: "Andy - KU4XR" <ku4xr at yahoo.com>> To: <lowfer at mailman.qth.net>> Sent: Friday, July 18, 2008 9:39 AM> Subject: [Lowfer] PA transistor getting hot - need advise> > > > Hello LOWfers:> >> > I have a question, that someone probably has the answer for. I built the > > LOWfer xmtr using the 2222 / 2907 pair in the PA. The issue is that the > > 2N2907A is getting very hot under keydown.> > The 2N2222A just barely gets warm. The xmtr is working ( apparently ). > > Running into a 50 ohm dummy load, with 18 volts on the PA, I'm drawing > > around 77mA of PA current. I can hear the signal about a mile away on a > > portable radio, while xmit-ing on my LOWfer vertical, so I know the xmtr > > is generating RF. I am concerned about that one transistor getting so hot. > > It's not blown out so far, even after days of continous keydown ( burn-in > > I'll call it ). I've checked all the parts layout, and the parts > > themselves for defects, all is fine. This may be a normal condition> > with this type of PA layout, I'm not sure since this is my first LOWfer > > xmtr built. I would assume though that each transistor should be about the > > same temperature. Anyone else experience this, or have any thoughts ?? > > Thanks for your time:> >> > Andy - KU4XR> >> >> >> > _______________________________________________> >>From the Lowfer mailing list> > Send messages to: Lowfer at mailman.qth.net> > To sub/unsub visit: http://mailman.qth.net/mailman/listinfo/lowfer> >> >> > > > > _______________________________________________> >From the Lowfer mailing list> Send messages to: Lowfer at mailman.qth.net> To sub/unsub visit: http://mailman.qth.net/mailman/listinfo/lowfer