This problem is very common too. In fact it recently got me when I was hooking up a 2950 on my bench to monitor 10 meter activity while I'm working. I decided to chuck a typical 2 transistor amp behind it and boy was I surprised. Is was wiping out everything from the TV to the FM stereo. The SWR was poor after the amp and driving the amp. It has a 4 stage power switch and in one "gear" it worked OK while it oscillated in every other one.
It took 3 hours to correct this bad design. Adding a tuned input circuit, a 5 pole Chebyshev filter on the output and four more screws added to reinforce the ground path to the emitters of the transistors. I even found high inductance ferrite chokes placed inline with the existing RF feedback loop. The inductors were dropping nearly all of the feedback voltage across them before applying any of it back to the base of the transistors. One thing it had going for it was bias and today it runs super clean and unconditionally stable although it shows about 25% less on the Bird.
I see many folks taking a poor amp like this and using it to drive something bigger in the base like a SB-220. Now we have just made a bad situation ten times worse by adding 10 to 13 db of amplification to a signal that was filthy in the first place. These are important things to consider in the base since it can make or break your ability to use the station.
It took 3 hours to correct this bad design. Adding a tuned input circuit, a 5 pole Chebyshev filter on the output and four more screws added to reinforce the ground path to the emitters of the transistors. I even found high inductance ferrite chokes placed inline with the existing RF feedback loop. The inductors were dropping nearly all of the feedback voltage across them before applying any of it back to the base of the transistors. One thing it had going for it was bias and today it runs super clean and unconditionally stable although it shows about 25% less on the Bird.
I see many folks taking a poor amp like this and using it to drive something bigger in the base like a SB-220. Now we have just made a bad situation ten times worse by adding 10 to 13 db of amplification to a signal that was filthy in the first place. These are important things to consider in the base since it can make or break your ability to use the station.
