couple questions on an amp build

[9Lives]

Good morning every one. I just found this section of the forum so I'm stoked that I have new toilet material. I finished my 2x 2879 build and it works.. but I'm not sure if it's tuned properly as I'm learning still. I used pills I yanked out of an old amp I had laying around. I based out off the amplans file.

One question is that the white cap face on one of the pills popped off.. do I need to shut out down immediately or is it ok for now.

Also I have another amp. Boomer 250 deluxe that I picked up for really cheap and looking to experiment and learn. My soldering skills are great rf knowledge still lacking. Can I add rc feed back to ANY amp for improvement? Or do done models perform better with out? With guitar effects I use feedback on every single thing but then again I'm not dealing with near as many quirks as rf circuits.. very good section. I love it

 

[The Mad Scientist]

Hi there, What do mean popped off? sounds like it blew off. I have never heard of one comming apart like that.

Just curious as to why you have two transistors driving four? If you are looking for a clean amplifier that isn't the way to go. that amplifier will be well into overdrive before you even say "boo".

I would suggest that you do more reasearch befroe you get into building them.

Good luck.

 

[Shockwave]

As long as the transistor is still working, the cap popping off probably didn't hurt anything. The problem is the thin gold wires and silicon chip under that cap are very fragile. Dirt or debris here may cause problems in the future. I would very carefully crazy glue that cap back on to prevent this. Put some glue around the edge of the cap and place it perfectly over the center of the transistor without letting the cap touch any of the wires in the center when you do this.

Your question about RC feedback is a good one. RF amps use this negative feedback to stabilize the amp and prevent self oscillations. The values of these parts, especially the caps used are much different than the values used in AF applications. Most RF amps will stabilize with a 100 ohm 2 watt resistor from the base, through a .001 uf cap, to the collector of each transistor.

 

[Shockwave]

Hi there, What do mean popped off? sounds like it blew off. I have never heard of one comming apart like that.

Just curious as to why you have two transistors driving four? If you are looking for a clean amplifier that isn't the way to go. that amplifier will be well into overdrive before you even say "boo".

I would suggest that you do more reasearch befroe you get into building them.

Good luck.

Mad, 2 X 2879 means there are only a pair of 2SC2879's in the amp and no drivers. Having the cap pop off is rare but sometimes happens when reusing a transistor. If you're not careful desoldering the used part off the original board, lifting the solder tabs can put extreme stress on the bottom edge of this cap and pop it off.

 

[9Lives]

Sorry I didn't make that clear. It's just a 2 pill made of scraps Thai came out pretty nice. If I take any amp and add the rc feedback will it guarantee improvement? Or is it case by case? I know manufactured amps don't build for max potential, and I don't even want to push max watts at all times. I want it to do a good amount "well". With out over driving them and still getting what I can out of them.

 

[The Mad Scientist]

I am sorry, my apoligies gentelman. You did write it right, i just read it wrong.

damn, i really need to start getting more sleep. Only working on about 3 hours a day around here lately. Sorry Shockwave.

Yes he is correct. by adding those resistors and caps should help with stabilizing the amplifier.

When all is said and done, if you seem to have a bit of swr trouble on the input, check to see that the tx chain in the radio is aligned for the bandset you plan on using.

let me know if you do and i can better help you on tuning whatever radio you plan on running in front of that amplifier.

Good luck to you and good job

 

[bighammer]

the cap usually comes off due to excessive heat