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[AC5RY]

Hello all. Complete newbie here.
I have just replaced the output transistors on this amp with MRF247 from the original SRF3883. It puts out 170 watts but one of the transistors ran hotter. Do I need to adjust the bias? If so, how do I do it?
Thank you.

[N0GV]

First, the transistors need to be matched - if the ones you put in are not then pain will result as one will get hotter than the other. Second, the transistors need to be properly heatsunk - if the heatsinking is inadequate on one then it will get hotter than the other. The biases are set independently with a 10 Ohm and a 22 Ohm resistor on the base of each. Diagram is at: https://n6pet.com/wp-content/uploads/2015/12/RF_Concepts_2-417.pdf

The transistor that is getting hot is having too much base current and as a result the 22 Ohm resistor can be dropped by shunting it with a resistor until the collector currents for each match with no input.

Grover

[KA4DPO]

Unfortunately, N0GV is right, that circuit requires matched transistors.  Changing the bias can result in very poor linearity.   Just matching emitter current does not guarantee that each device will have the same operating curve.  I would recommend that you find a matched pair of transistors.

[N0GV]

Indeed... matching using biasing is a band aid. Sometimes it works ok, I'd give it about a 60-40 shot....

Grover

[N4ATS]

Two things I  will ask...

1) Are the Transistors OEM or from China? Its VERY hard to tell and China parts don't use the same die and do not function the same (counterfeit). Most counterfeit parts are located on eBay.

2) Did you put fresh heat sink compound under them and assure there was no FOD under them as well? Fod will jack up the transistor from the heat sink enough that the dissipation may be different thus they will heat up FAST