[Boatanchors] Excessive Heat in Viking Valiant VFO Compartment

[Michael D. Harmon]

I have an old Valiant I transmitter and a Ranger I transmitter which 
were purchased (CHEAP) from a local CBer that I'm attempting to 
resurrect from the (almost) dead.  In addition, I have a couple of parts 
carcasses I've been cannibalizing components from.

I have a total of four VFOs for these rigs.  Two of them (in the 
carcasses) appear to be in excellent condition.  Both have the original 
18K 2-watt current limiting resistors with no signs of overheating.  The 
other two (in the transmitters) are badly overheated and charred around 
the hole in the phenolic sub-panel where the 0A2 tube comes through.  
The 18K 2-watt current limiting resistor in one of these "toasted" units 
has been replaced by two 10K 2-watt resistors in series.  In the other 
unit, it's been replaced by a 500 ohm 7 watt wirewound resistor (go figure).

I've heard all the stories about the 18K resistor in the VFO compartment 
and the need to increase the wattage and move it elsewhere.  Initially, 
the fact that the charring was just around the tube hole made me wonder 
if maybe the 6AU6 VFO tube was drawing more than the 30 mA that the 0A2 
was designed to handle.  After I looked up the 6AU6 in my tube manual, 
and found the screen current to be typically less than 5 mA, I realized 
that there's no way the 6AU6 could be overloading the 0A2.  I'm guessing 
that the fact that the VFO compartment is a closed box with no external 
air circulation must have caused enough heat buildup to create the 
charring. Obviously, the resistors in both overheated VFOs had been 
changed at some time in the past.

Is this charring around the hole the typical symptom of the overheating 
problem that everyone always talks about?  If, so, I should be able to 
deal with the problem.  I just wanted to make sure I didn't have a 
unique situation with my transmitter(s).

Thanks for any and all advice, tips and suggestions!

Mike Harmon, WB0LDJ