G Cornelius wrote:

On 04/16/2016 04:00 AM, analogdial wrote:
Tubes last alot longer than paper and electrolytic capacitors.

Now there's a blanket statement if I ever heard one.

My experience with the tube era was that a power supply
would commonly have problems with either the rectifier
tube or the electrolytics.


Series string sets are harder on tubes than transformer sets. Open
heaters and HK shorts are the usual failures. A thermistor used as an
inrush current limiter helps.

The failure rate for paper caps must be close to 100% by now. That
doesn't mean that the circuit no longer works but that the leakage is
now out of spec and they will only get worse in the future. I doubt
there's been a decent paper wrapped elecrolytic seen for at least 20
years.

I seem to recall reading that some small signal tubes are rated for a
life of 20,000 hours. Or was it 50,000 hours? Anyway, 20,000 hours at
4 hours a day is about 14 years. Maybe twice that long before the radio
actually goes deaf. I like to fix up old radios and I very rarely find
tubes with low gain. As I mentioned, the occasional common failure
modes are open heaters and HK shorts.

I have a strong suspicion that most of the tubes replaced in the past
were still useable. A gently aging tube might work quite well at normal
voltages but be an underperformer in a circuit in which a leaky screen
bypass cap is pulling down the voltage. A new tube might perk up the
circuit but the real problem is the leaky cap.

And it was common repair shop practice to knowingly replace good parts
such as tubes.

Shocking, I know! And I understand why. I was a new tech at a shop,
and a customer called in about her microphone preamp all of the sudden
having distorted sound. I asked the obvious question "Did you put new
batteries in?" "Yesssss! Of course I put new batteries in!!!!!!!! Do
"you think I am an idiot??!!!"

Needless to say, her "new" batteries were just as dead as the first set.

Stupid me, I thought she'd be pleased that new batteries that actually
worked brought the preamp back to perfect operation.

"YOU CHARGED ME THAT MUCH (our minimum charge, actually pretty cheap)
JUST TO REPLACE THE BATTERIES!! YOU'RE A DAMN CROOK!!!!"

No, if I was a crook, I could have charged her five times as much for
replacing good parts and we both would have been so much happier that
day.

Long story short, tubes get an largely undeserved bad rap for
unreliability.