01-03-2010, 03:26 PM
I had a similar-sounding problem with a Panasonic tube radio. The problem turned out to be an open grid bias resistor for the 50C5 audio amplifier tube. After warming up a few minutes it would start to work fine. If I turned it off and then back on, it would take another few minutes before it started working.
I measured the bias voltages and discovered it always worked fine with the voltmeter attached from the control grid pin of the tube to ground. Some bias current must have been getting through the meter, which clued me to check the resistor, which was open.
Your problem is likely something different, but the symptoms sound similar to my Panasonic.
Oh, yes, and I just remembered I had a similar problem in a Philco 41-280, which suddenly started taking minutes to warm up. It turned out to be the XXL oscillator tube. If I remember correctly I swapped the oscillator and mixer tubes (both XXLs) and the problem went away, but I may have replaced the oscillator with a spare.
I measured the bias voltages and discovered it always worked fine with the voltmeter attached from the control grid pin of the tube to ground. Some bias current must have been getting through the meter, which clued me to check the resistor, which was open.
Your problem is likely something different, but the symptoms sound similar to my Panasonic.
Oh, yes, and I just remembered I had a similar problem in a Philco 41-280, which suddenly started taking minutes to warm up. It turned out to be the XXL oscillator tube. If I remember correctly I swapped the oscillator and mixer tubes (both XXLs) and the problem went away, but I may have replaced the oscillator with a spare.
John Honeycutt
