Demon 250 amp low output

[klj2497]

Hello, I recently i replaced the can capacitors the black diodes, and a couple of big resistors. on the demon 250. It worked great for about a month. Then it started to surge, and dim the lights in the room. then it stopped putting out full power. it only puts out about 70watts am down from 220watts dead key. The middle tune knob goes all the way to the left to increase power now. I used an extra tube to swap out with the ones in there but that wasnt much difference Im looking for a schematic with voltages, and ohms but so far no luck does anyone have any ideas how i can track down the problem Thanks Robert

 

[nomadradio]

The Demon is a live-fast and die young design. There is no grid bias on the tubes, so they tend to run really hot. A peek inside when it seemed to be okay would have revealed tubes running hot enough for the gray plate structure inside to be glowing red. This type tube is not meant to run that hot, and this causes premature failure. The "surge" and "dim the lights" behavior was caused by a tube breaking down internally, probably one each time.

It's built with one tube driving three. Changing just one tube at a time won't fix the problem if more than one of them has failed internally.

The 220 Watt carrier is about twice what it should have been running. If it lasted a month this way the original tubes were pretty stout.

Were.

A long-term strategy to run it would include adding about 8 Volts of negative grid bias to the tubes. Goes a long way towards reducing their operating temperature. The factory left it out because it raised the production cost without making it sell any better.

Or you could do like they did when the Demon was new and tubes were under ten bucks each. Replace tubes once a month. Not what I recommend.

73

 

[klj2497]

Bummer,