This is a radio I have NOT restored.

When I got it, it played well and I used it for 20+ hours over about a week. It got turned on and off multiple times.

One day as I was listening to it, it suddenly went silent across the band; no static, no hum, nothing. Unplugged it and checked it out later. All the tube filaments are good and the dial lamps light normally.

Fast forward to weeks later - today. I plugged it in to try it again, and whaddaya know, it works perfectly. It did, for about 10 seconds. It stopped the same way as last time - no 'fanfare', just cut to silence. Did the 'dummy' check again and all the tubes are still heating, lamps working.

I just want to know if this is an 'expensive' problem before I really recap this radio.

6 tubes and 2 AF stages
12SG7, 12J5, 12SK7, 12SQ7, 35L6 and 35Z5

Could be a intermittent connection. Clean controls, switches, and tube sockets with contact cleaner.

Wiggle, and tap individual components to see if you can isolate the location of the bad connection.

Since it has not been recapped, you could have a bad coupling capacitor somewhere as well.

Why not troubleshoot the radio? Measure voltages when the radio stops operating.

Not likely a cap as they usually don't recover and go bad again. Can't tell if it's an expensive problem until you find the fault. Could even be a poor weld in a tube?

There are no "expensive" parts in an AA5 or AA6 set like yours. No power transformer or big output trans. It may take a while to find an intermittent problem.
Don

Ok... finally noticed this.... when the chassis is completely cold, the 12J5 osc. tube lights up very bright as it is warming up, up to 'yellow' brightness.

It's not arcing, just bright. Hard to notice in bright shop lights though.

Not abnormal in some sets. First in the heater string?

http://oldtech.net/RCA/66x11-p2.gif

Confusing schematic... Reading L-R, it's #2, but then why are they numbered 1-5 differently?

It is the first one after the 35L6, by the diagram.

That's what I thought, but why the numbers?

You could addd a 100 ohm 5 watt resistor in the heater string between the 35Z5 pin 7 and 35L6 to limit the peak current during warm-up. This would reduce the bright glow you see in the 12J5. Of course you can't see the bright glows in the metal tubes.
Don

I'm going to site ohm's law. If the filament is abnormally bright, this would indicate excessive current due to decreased resistance. My best guess is that the 12J5 filament has a partial short and will probably burn out soon.

Current isn't controlled very well on some filaments. Others have a spot that isn't coated. This spot will start out being very bright. Very common on foreign made tubes. Tubes like 117Z6GT may flare up at turn on in one spot. A tube doesn't have to be worn out to do this, happens on new tubes. I don't like this but haven't had tubes burn out from flare up at turn on.

"It's not arcing, just bright. Hard to notice in bright shop lights though."

"I'm going to site ohm's law. If the filament is abnormally bright, this would indicate excessive current due to decreased resistance. My best guess is that the 12J5 filament has a partial short and will probably burn out soon."

Keep in mind, this is a series circuit, and it uses tungsten heaters. Tungsten has very low resistance when cold, allowing a much higher current. The 50L6 and 35Z5 have large heaters that take a longer time to heat up. The 12J5 and many foreign 12AX7 etc tubes have the ends of the small heaters outside of the cathode, so the ends heat quickly and their resistance increases. The spot with the highest resistance has the most voltage drop across it, the most wattage, and glows real bright until the rest of the heater(s) warm up and take their share of the voltage.
One method developed later was to design for an 11 second warm-up for series tubes. Sometimes there is an "A" suffix like 12AX7A for these tubes. It was said that transformer-powered tubes lasted twice as long as series-wired tubes, as even the transformer output voltage drops lower when the tubes are cold at power-on. I have measured this.
Don

Sorry to go off tangent here. I am trying to understand the physics of Don's statement that tubes in a radio with a transformer last longer. Is that due to back emf as the magnetic field builds around the transformer coil at startup? Is it limiting the flow of current at that point, thus not overloading a cold filament?
Vin

Re: Filament surge current.

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This is just an opinion but I view tube filament surge current a non issue. Have you ever noticed when an incandescent light bulb blows when you switch it on it's at the end of it's life cycle anyway. All filaments grow smaller and smaller over time and eventually go open. If a tube is at the end of it's useful life and cold start current causes it to blow, replace it with something new or a used tube that still has some remaining life span.

Measure the resistance of a cold tube heater with your meter, it is about a tenth of the hot resistance (calculated by Ohm's law). In a series set, the tubes have different thermal inertia, warm up at different rates, and some tubes will glow real bright until everything evens out. On a transformer set, you can measure that the heater voltage is lower than 6 volts at power-on, the transformer isn't big enough to provide a big current surge for the cold heaters. It also won't provide more voltage to some tubes during warm-up. Unfortunately, some European 12AX7 and similar tubes still "flash" at warm-up, but not as much in a transformer set.
As line voltage is 125 volts in many places, I add a 68 or 100 ohm 5 watt resistor in a 150 mA series heater string to reduce voltage and greatly reduce the power-on surge.
Don

Thank you guys for your time and effort. Your explanation makes it easier to understand the logic. Installing that resistor at the factory would have cost money.
Vin

Vin, Some (Philco and other) AA5 sets actually had small resistors or thermistors factory-installed to limit the power-on current. The early 300mA series sets had large ballast resistors or curtain-burner cord resistors which protected the primitive tube heaters (designed for transformer operation) until they could figure out how to make 150 mA heaters that would all warm up in 11 seconds reliably.
Don

When in doubt, install a resistor to somewhat tame higher mains power and conserve filaments. I like to see about 90% voltage on a typical day on these old sets. Easy fixes ar 50C5 for a 35C5, 50L6 for 35L6, and so on. Just make sure the current is the same, or do some fadiddling with Sir Ohm (when hot.)

Thanks guys, very easy for me to understand and prompted me to obtain a copy of an RCA tube manual to read. But, for the mean time. If radios were engineered to run at 117 volts or thereabouts, wouldn't it be logical to up the value of a resistor in place already to match the higher voltages of home power supplies? Also, to add a resistor to radios without the factory installed ones?
Vin