The Defpom Forum, for RadioMods, S&M Software and Music by The Defpom

Hey all, thanks for the membership, looking forward to this forum.

Anyhow, last night I built up an amplifier 4 2879's, class c. It seems to oscillating and i'm not sure why. I have put many of these amps together before but this time I used a very small case which caused me to do some minor stuff differently.
When i key the amp and modulate it, I get close to 500 watts so i dont think I have lost a device yet, but when I unkey the balancing resistor across the combiner starts heating up. I yanked the power leads quickly. I placed a finger on/around each transistor, all were starting to get warm with one pair warmer than the other.
So what is the greatest cause for oscillation in a 4 device amp? I wish I could share a picture but I'm thinking it's has to do with in/out tuning.
I have built off this design a lot but never in such a small case. I wonder if it's at all possible to put 4 transistors in such a small space? I will try to post a picture later.
Fill me in on oscillation, what else can causes it.

I isolated the problem so my amp should be ok after a few changes. For inquiring minds, the tuning cap for the output was maybe 3/8's of an inch away from the chasis but worse yet one turn on the combiner was all but touching the chasis throwing my tuning way out. I shifted the board forward giving at least an inch between the combiner and back panel. 500 watts, no heating of the balancing resistor and good in/out swr. So it's close to being done.
Also, for what it's worth. the case I used is the AB-4 that Westgate sells with a 4 transistor board. Good for a 2 or 3 transistor amp. Dont attempt what I have and put 4 2879's in one, it's just too small. I put a perfect fit high quality heatsink with a lot of surface area in this case which will cool just fine but if the fan failed it would grossly over heat. And to think, I had planned to put a regulated bias circuit in this little ole box. Pays to due ones homework which i did not.
The really sick part is that I'm only building this amp to make a little profit to fund a nice aluminum case for a tube amp i'm building.
yikes.


Best Regards,
Cincinnati Supa chicken