Power tube life

[smolder]

From what I can find, most folks test power tubes for a power rating, meaning that they work or don't work. Is there any way to test for how much they have been used? My assumption here being that it is extremely hard to tell by any sort of measurement if a tube from the 50's, 60's or 70's is NOS, ANOS or about to die. Is that correct?


.

[VacuumVoodoo]

Originally Posted by smolder

From what I can find, most folks test power tubes for a power rating, meaning that they work or don't work. Is there any way to test for how much they have been used? My assumption here being that it is extremely hard to tell by any sort of measurement if a tube from the 50's, 60's or 70's is NOS, ANOS or about to die. Is that correct?


.

Electron emission test will give you a rough idea on tubes condition. This tests how efficiently the cathode is releasing electrons from its coating. Cathode coating is about the only part of tube that deteriorates with age and use. Basically you want to measure plate current under standard operating conditions as given in the tubes data sheet.

It is not easy to asses how much useful operating time a tube has left since data sheets for civilian tubes don't specify "end of life" parameters. Looking at military data sheets that specify half life and end of life parameters is of not much help either since these tube version often had special long life cathode coating.

As a rule of thumb, I discard power tubes whose emission and transconductance are lower than around 80% of normal values.

Old production tubes were pretty consistent in their electrical parameters so emission test can be used as a guidance. New production are often all over the chart, hence some OEM suppliers grade their power tubes either by a numbering system or color codes on the screen print. For those you will need emission & transconductance data for each grade to make a valid assesment of tubes condition.

[mooreamps]

very easy to to determine condition of power tubes in an amplifier, if that amplifier has built in test lamps.



-g

[RussB]

Originally Posted by mooreamps

very easy to to determine condition of power tubes in an amplifier, if that amplifier has built in test lamps.



-g

say what?

[mooreamps]

Originally Posted by RussB

say what?

I've been talking about for several years now, and it's one of the pieces of technology I use, which was also one of the root sources of me being flamed by other builders, and one result being punished by this board.




-g

[RussB]

Originally Posted by mooreamps

I've been talking about for several years now, and it's one of the pieces of technology I use, which was also one of the root sources of me being flamed by other builders, and one result being punished by this board.




-g

I am even further away from understanding what you're talking about after that comment?

[TweeDLX]

Originally Posted by RussB

I am even further away from understanding what you're talking about after that comment?

I promise not to flame anyone, but I really want to hear Moore about this. Sounds kind of interesting to me. Are we talking something like the idiot lights on a cars dashboard that tell you your engine needs servicing?

Mike

[Wilbur]

Originally Posted by mooreamps

I've been talking about for several years now, and it's one of the pieces of technology I use, which was also one of the root sources of me being flamed by other builders, and one result being punished by this board.




-g

I think Russ was honestly asking a question. Me too! No flaming. I've not heard of this.

[strat a various]

I wish we could find all those old tube testers from all the supermarkets, 7-11s and Radio Shacks, calibrate them properly, make sure they work well, and set them up at Guitar Centers.

[SatelliteAmps]

Originally Posted by strat a various

I wish we could find all those old tube testers from all the supermarkets, 7-11s and Radio Shacks and set them up at Guitar Centers.

No you don't. Those things were usually WAY out of range. A great tube would register around 80%, and a good tube would usually read FAIL.

[strat a various]

Notice my corrected edited post. I just miss the good old days.

[mooreamps]

Originally Posted by Wilbur

I think Russ was honestly asking a question. Me too! No flaming. I've not heard of this.

I posted a schematic on other forums, which I am now forbidden to say where, and suffered such a back lash it caused me to get kicked off of them, and punished by Brian on this one. I'm not allowed to post on this board, but I will put something up on

http://music-electronics-forum.com/f43/


Just as a side note : it is "really" helpful to see how the lamps react to the changes in plate current, especially when bringing up and trouble-shooting a new build. I even use one in my cathode follower circuit. It's really handy to "see" all the gain stages working when you power up, yes including the power tubes.



-g

[strat a various]

OK, I'm not gonna build an amp with extra lights on it, but I like conspiracy theories ... why would anyone care if you design a circuit with test lamps? Is the problem that you're publishing protected or copywritten schematics without permission of the owner of that property? Are there pertinent rules regarding schematics that limit the plans that can be freely posted?

[Prairie Dawg]

Originally Posted by VacuumVoodoo

Electron emission test will give you a rough idea on tubes condition. This tests how efficiently the cathode is releasing electrons from its coating. Cathode coating is about the only part of tube that deteriorates with age and use. Basically you want to measure plate current under standard operating conditions as given in the tubes data sheet.

It is not easy to asses how much useful operating time a tube has left since data sheets for civilian tubes don't specify "end of life" parameters. Looking at military data sheets that specify half life and end of life parameters is of not much help either since these tube version often had special long life cathode coating.

As a rule of thumb, I discard power tubes whose emission and transconductance are lower than around 80% of normal values.

Old production tubes were pretty consistent in their electrical parameters so emission test can be used as a guidance. New production are often all over the chart, hence some OEM suppliers grade their power tubes either by a numbering system or color codes on the screen print. For those you will need emission & transconductance data for each grade to make a valid assesment of tubes condition.

The only process I know that even purports to address this is the Hickok life test. Here's how it works. You press the test button and adjust the bias control until your tube reads at 4000/6000. Then, holding down the button you reduce the filament voltage to the next lower setting. If the needle falls off rapidly your tube is nearing the end of its useful life.

There may be a similar test on some B&Ks too.

Having said that, I do not know what value that test is so I don't bother with it. What I do is simpler. If they're tubes that I keep around for my own purposes I don't worry about them very much because I've got more. If I'm going to sell them, they have to test at or above nominal value for new, or they get tossed.

The only variation I make to this policy is the following.

Some tubes (particularly the 2.5v tubes like the 45) have a heavy filament draw that will load down the power transformer in your tube tester. So what I do is I monitor filament voltage from an adjacent socket. If it falls off badly I raise the voltage selector to the next highest setting (3v) and the voltage usually sets right close to where I want it-2.5v. It is not a good idea to try doing this with the line adjust control as that is a global adjustment and the voltage selector only changes the filament voltage.

As an aside on tube life, the Dragon Lady worked in a place in Long Beach that was built in the middle fifties. It had a Rauland PA amp on a shelf in the office that was installed when they built the place, and aside from the odd power outage had been in continuous operation since that time. That was in 1993.

[SatelliteAmps]

Originally Posted by strat a various

Notice my corrected edited post. I just miss the good old days.

I miss them too. There is a local guy here in San Diego that has two of those old testers. They are working perfectly, and it is hilarious to see how tubes test on them.