Tube Sets & single failing tubes
 

How many folks collecting tube radios have found that a large number of old tube sets tend to fade out signals, have volume issues or similar problems?

You hear a lot about capacitor hum and that's definitely a problem but after twenty tube sets, I'd say that most of my tube sets benefit from a new primary detector/amplifier/AVC type tube. When that's changed out it seems most perform fairly good without much further tinkering.

I think non-tube folks just know the radio doesn't play or tune right and so pass them on for next to nothing even if the repair is a 6 dollar tube.

Am I alone in this "single tube replacement" phenomenon?

On another note, does anyone know of a good site which lists compatible/workable twin diode-triode to triode heptode exchanges?

Thanks for listening.

Yes I have seen cases of weak tubes in some radios...we first replace old capacitors and wiring and then check performance...have found really weak tubes in probably 30-40 percent of tube sets we have worked on.
Really have not seen many cases of just one bad tube except for a 6HA5 in Fisher tuner...usually if one tube is weak the set has a lot of hours on the tubes and they all are likely pretty weak.

Distortion/hum problems, usually caps...weak reception is usually weak tubes.
Power but no audio...usually burned resistors due to bad caps, or leaky caps inside IF cans.

Twin diode triode (such as 12AV6) and triode-heptode (such as 6A8) are usually not interchanged...the former is detector/first audio and the latter is mixer and oscillator...these are pretty different tubes to sub for one another...maybe you meant something else?

Put up a longwire antenna if you don't have one already. You'll be amazed how it lessens fading and pulls in more and stronger signals. Like turning the volume from nearly off to halfway up!

-Ed

Thanks Chad,

I meant to say triode heptode to triode hexode. Just that background hum in the head causing interference again.... :scratch2:

Old tube radios all need the wax caps replaced. I don't bother checking the caps, I just change all of them. All are or will soon be bad. The electroytics in metal cans tend to be okay, the ones in cardborad tubes always bad. After doing this, then I start looking for bad tubes or other devices. Tubes are more reliable than you'd think. That is, more often than not the trouble in a given set is not a tube, but some other part.

Is this akin to the "lone gunman" theory? Seriously, I have a Zenith table top radio that started to have trouble "locking into" a station. I'd tune in a station, then the sound would fade out--until I turned the tuner dial to get it back. Replacing the 12AT7 tube with a new one solved the problem. The old 12AT7 tube tested good but marginally good, so I just replaced it on a hunch. The single tube theory worked for me.

Incidentally, except for the 19T8 and now the 12AT7 tubes, all of the tubes in this are original Zenith marked tubes. I agree that tubes last longer than people give them credit for. From a vintage tube manual I've read (from the early 1960s when tubes were current technology), it was documented that many tube failures were secondary to other electronic failures or poor design of the circuit in general (such as insufficient ventilation being provided for in the chassis).

The 12DT8 or 12AT7 FM convertor tube seems to be a common failure point on AM/FM sets...somewhere I read that because many sets switch off the B+ to the FM tube but leave the filament on when the set is on AM, that is bad for the tube...people probably listened to more AM on the sets in the 50's/60's and the FM tube deteriorated.

I've heard of tubes failing that way--it's called "sleeping sickness" according to one technical book I read on the subject. Supposedly it even happens with oscillator tubes that are used but don't see a high enough duty cycle. Sometimes they'll fail for certain frequencies but be fine in others.

In my set, the situation is now opposite of the 50s/60s. I mostly use it in FM mode and only rarely tune in the AM stations. Mostly because I work in an office with lots of computers around, so there's a little more static interference on AM than I prefer to hear.

Have heard that you only need to draw a few microamps of current from an otherwise heated but unused tube to keep it from getting sleeping sickness. In an AM/Fm radio, I've connected a 470K resistor between the AM B+ and the FM B+ (at the switch is a convenient point) to draw small amounts of current from the unused tubes. This wouldn't be enough current for local oscillators to run so no am<->FM interference should occur.