[Hallicrafters] Transformer Temperatures

[WA1KBQ at aol.com]

If you like to run your equipment for extended periods of time it makes a big 
difference in power transformer operating temperature by reducing AC line 
voltage to 110. I have several variacs of various ratings here and I run 
everything I have through them. Some have recommended not using a variac because of 
the possibility of not paying attention and making a mistake with over voltage 
but if you are careful you should not have a problem. It is true a bucking 
transformer arrangement is a probably better plan but the variacs are widely 
available and convenient and have always worked well here. I guess the proper 
method would be to measure tube filament operating voltage at the pins and adjust 
the variac for exactly 6.3 volts or 12.6 volts or whatever. Hallicrafters have 
perforated lids which really help reduce operating temperatures too but some 
others such as Nationals and early Hammarlunds will greatly benefit by propping 
the lid open about an inch or so (this really helps). It doesn't hurt to 
check on them with your bare hand occasionally to be sure power transformer 
operating temperatures are within acceptable limits. You should be able to hold the 
palm of your hand on the transformer for at least a few seconds. If you can't 
it's running too hot and the old insulation might not last long. There are 
other things that can cause high power transformer temperatures as well such as 
leaky plate and screen bypass capacitors and leaky control grid coupling 
capacitors but these can be found by measuring tube pin operating voltages and 
comparing with the owners manual or an RCA tube handbook. Any voltages found to be 
off by 20% or more from the chart values should be investigated and caps on 
that particular circuit suspected. Resistors can creep upward in value also and 
absorbed humidity plays havoc sometimes. Ever get a receiver going again after 
not being used for many years and wonder why the more you use it the better 
it seems to work?

Regards, Greg Gore