Quote:
Originally Posted by init4fun
Hi Nathan , Welcome to VK . That's a sweet looking radio / amp ya got there and bonus points for it coming with the schematic . You mention one of the EL84 outputs possibly red plating . As you know any of the resistors could and most likely have drifted in value and most if not all of the electrolytic and paper types of capacitors are suspect as well .
Something curious I noted on the schematic , the circuit calls for 415 volts on the plates of the EL84 outputs , and yet every tech sheet I have on those tubes lists a maximum plate voltage of 300 to 330 volts , depending on if it's RCA's specs (300V max) or Phillips or Mullard's (330V max) . Are the really standing THAT hard on those tubes , pushing them that far over their design rating , to get a maximum output level , tube life be damned ?
|
That doesn't seem right to me, either. If you are getting red plates on the EL84s, the tubes are definitely being overloaded. That Bogen system is likely old enough ('60s or perhaps late fifties, as evidenced by the Conelrad icons on the AM tuning scale) to have been designed with enough reserve as not to overdrive anything in the unit. I agree that the problem is almost certainly being caused by resistors drifting in value and/or weak or failing capacitors. I'd replace the wax-paper caps as a matter of routine, as they are almost certainly beginning to fail if they haven't done so already.
BTW, I would not expect a tuner/amp system such as your Bogen unit to have been purposely designed as to overdrive the output tubes; when this system was made, such cost-cutting measures, at the expense of tube life, simply were not employed, unlike many of today's radios, TVs, etc. in which every component in the set is being driven to within an inch of its life, if not severely overdriven to the point of early failure. Again, red plates on any tube means the tube is being severely overdriven and will fail in a very short time, so don't operate the chassis until the cause of the red-plating problem has been identified and the problem corrected.