Thanks alot. I saw one vendor that lists SD1446 as a sub for 454's. I
realize they may work for some applications and not others. Will they work
here?

Chris
wrote in message
...
| On Tue, 07 Sep 2004 17:14:00 GMT, "Chris"
| wrote:
|
| The driving radio is an RCI2950. The radio output goes directly into the
| input transformer. There is a VSWR of 1.2:1 between the radio and the
amp. I
| also tried adding various caps on the power lines to no avail. As
mentioned
| earlier, I have another amp with MRF455's with no problems. The only big
| difference I see in the circuits is thatr the good amp has an RC network
| between the input transformer and the bases.
|
| Chris,
|
| That amp was designed around MRF454 not 455 there is a difference
| and the 455 has a tad more gain and different match. I might add the
| MRF454 is a 80W device and the 455 is only 60w as well. The
| differences are not always apparent and the output match and reverse
| transfer are very different. Pull the datasheets for both off the
| 'net and look at them.
|
| The design requires a matched pair. The bias seems to be best
| at around 200-300ma per transistor. Any asymetery in grounding
| (that includes mounting the PCB to the heatsink) can also cause
| problems. The transformers MUST be wound as specified using
| materials as specified.
|
| If you going to try and tame the beast. Try this. From collector to
| base on each transistor: series connect, 12ohms (2w non inductive),
| .33uH molded choke and capacitor pair (10uf and .1uf in parallel).
| the leads for these must be as short as possible (none or nearly so).
| This may work. If not:
|
| Feedback version two: FT50-43 torid with 10t #26 from collector to
| collector, pass through the center a 4.7ohm 1/2watt resistor and
| connect the leads to the bases. If the leads are backward it will
| oscillate in that c ase exchange the leads for the 10turn winding
| between the collectors. If this doesnt work used MRF454s.
|
| If feedback works expect about 1-2db of gain reduction and a
| corosponding increase in required drive for the same output power.
| However, do NOT exceed 5W input power of the transistors will cook.
|
| Also if you scope the power leads at the junction of the output
| transformer and see anything going on there in the below 5mhz range
| you bypassing on the board is wrong or grounding ineffective.
|
| One OBTW: input and output connectors, case and grounds. I've seen
| one that when on the bench open worked as predicted. In the case
| built by the owner it would take off. Seems the case and the SO239s
| mounted to it were RF wise not at the same ground as the groundplane
| of the PCB and created a untidy feedback path.
|
| I've built "EB63", it was featured as an app note from Moto some 20+
| years ago and it was a stable design using the MRF454s. I had little
| trouble with it as a base design for a 18-35mhz commercial
| application.
|
|
| Allison