[Lowfer] PA transistor getting hot - need advise

[James M. Walker]

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
>
>
>
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