You want a matched pair. Push-pull PA stages usually don't bias the
transistors independently, and setting the bias only sets the DC
operating point, not the gain which varies from differences in the
production process. Typically the current gain of bipolar transistors
can vary as much as 5:1 over a collection of transistors. To get the
push and the pull cycles the same size you need the same operating point
and the same current gain. Otherwise one half cycle is different sized
than the other creating even harmonics that usually are cancelled in the
push-pull stage and generally the output low pass filter is designed for
mostly odd harmonic rejection which makes for lower loss and a simpler
filter.
You need to add a protective fuse or circuit breaker to the DC supply.
73, Jerry, K0CQ
On 2/25/2011 9:32 PM, Scott Zastoupil wrote:
> Hi All. I just did something real stupid by forgetting that my antenna was
> grounded and transmit my Argo into a dead short. Now the power meter just
> slightly wiggles at a power setting what once was 5W. I'll open it up and
> try to trouble shoot the voltages up to the power amp and scope the driver
> input and output to see if that is blown. The ALC LED does not light
> either on TX with a CW key. Not sure of the criteria for that to light up.
> It is very likely I toasted the 2SC3133 output transistors. I went to
> RFParts.com and they call for a replacement of using a matched pair of
> 2SC1945. The price is a little more than I was expecting. My question is do
> they really have to be matched pairs? It seems just adjusting the bias
> voltage on each would be sufficient. I'm not exactly sure where they should
> be biased besides have an equal collector current at idle. I see some on
> ebay, but kind of leery of those being gray market parts since they're
> coming from Hong Kong.
>
> Any thought/suggestions on repair and alignment?
>
> Scott, KB0KFX
>
>
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