How to tell if EL34 tube is dead or dysfunctional?

[ogerestein]

Hello all, first time poster here. Need some expert advice!

I'd like to know if there is any way to determine if an EL34 tube is either dead, or not functioning properly, using basic tools and without using a tube tester (which I don't have). Would just sticking it into my amp (Jolida JD202) and giving it a go risk damaging the amp?

Physically, there is no damage that I can see at all, and I did a short-circuit test using a multimeter and everything seems fine.

Some background: I bought four SED Winged C tubes, slightly used. After about one minute of powering up, one started glowing very brightly, so I turned the amp off immediately. Turns out the Jolida manual had a mistake in it and I was setting maximum plate voltage to start biasing, instead of minimum, for two out of the four tubes. Anyways, I just put my old tubes back in and boxed the SEDs. I labelled the misfit tube, but, being a genius, used a post-it on the box which fell off. So, now I have no idea which of the four is potentially a bad tube. Again, there are no physical differences I can see, and all tested fine for shorts.

Any help or advice would be greatly appreciated! Thanks in advance.
Oliver G.

[Willabe]

Some background: I bought four SED Winged C tubes, slightly used. After about one minute of powering up, one started glowing very brightly, so I turned the amp off immediately. Turns out the Jolida manual had a mistake in it and I was setting maximum plate voltage to start biasing, instead of minimum, for two out of the four tubes.

I'd bet it's fine.

With the EL34's removed, 1st set the bias on an amp that takes EL34's for maximum -bias or largest, say -63dcv, what ever it has, then shut off amp. Then put in the tubes and turn on amp watching for sparks/flashes inside the tubes, with your hand ready to shut off the amp. Now slowly bring up the -bias to where you want it, still watching the EL34's and ready to shut down the amp. If all seems good plug in and play for a while and keep an eye on the tubes, be ready to shut down the amp.

BUT if you need to shut it down REMEMBER you have a guitar in your hands, DONT touch the strings with 1 hand and reach for the open chassis with the other!!!!!!!!!!!!!

Might be best to put the chassis back in it's cab before you test it with a guitar.

HBP had a tube red plate on him that actually burned a hole in it's plate before he shut it down. The tube still works.     


                                   Brad     

[HotBluePlates]

HBP had a tube red plate on him that actually burned a hole in it's plate before he shut it down. The tube still works.     

Well, I actually bought it in a lot of tube on ebay. So the hole in the plate was already there before I got it.

But you're right... tube still works and sounds pretty nice, too!

[Willabe]

But you're right... tube still works and sounds pretty nice, too!

 

There ya go Oliver G.



              Brad     

[HotBluePlates]

I'd bet it's fine.

I agree. Follow the procedure Willabe outlined, and just try the tube in your amp.

After about one minute of powering up, one started glowing very brightly, so I turned the amp off immediately.

It sounds like you are describing some red-plating (gray plate starts glowing cherry-red, usually in one or 2 areas of the plate).

If you read around, you'll find folks saying, "If a tube red-plates, it is bad and/or can never be trusted again." Like many things tube-related, there is a basis for the statement (which is usually not provided) but then the notion is taken out of context and exaggerated somewhat.

The problem with overheating tube elements is that the materials may emit gas ions which were bound up in the material (outgassing). These ions could contaminate the control grid in a manner that would tend to counteract the bias voltage, making the tube behave as if it had less bias (more resulting plate current). If that situation is carried to an extreme, the additional current could run into the control grid, further reduce apparent bias, heat the grid, cause secondary emission from the grid, which furthers plate current. And on, and on, into a runaway situation.

So that's the possible problem, described in old tube books as a "gassy tube". But your tube also has a getter flash which is there to combine with and trap such gas molecules, and prevent the gassy tube condition. And a tube's vacuum is never absolute to start with...

So in the absence of other info and/or experience, you get the statement, "If a tube ever red-plates, it is forever suspect." It's an overstatement in my opinion, and you'll throw away perfectly good tubes. It's a safe (and profitable) recommendation if you're a tube vendor.

It might sound silly, but your amp is the best tube tester. Ensure the fuse is the correct, rated value. Watch the tube at power-on and be prepared to power off if you see signs of trouble. But I'd guess you have an 85-90% chance the tube will operate perfectly fine.

If you're really concerned about the risk of gassy tubes, cathode-bias is inherently self-limiting and will either prevent a failure or minimize the scope of any resulting damage. There probably exist some exotic hi-fi amps which use fixed bias (for greater output power), but use some form of servo circuit to lock tube current to a safe value (which would then also be safe in the face of a gassy tube).

I can think of cases where I had trouble with amps where there was an actual failure of some component in the fixed bias circuit, but I can't recall any situation where I had trouble or damage caused by a gassy tube. And that's over an almost 25-year span of tinkering amps.

[mresistor]

Is it under warranty?

[2deaf]

I have had several tubes go into runaway mode usually after an extensive period of abuse.  Every time the grid was shorting the bias.  Sometimes a bad tube will runaway as soon as you give it plate voltage and sometimes it will slowly increase current.  A couple of them had to have a large signal to set them off.  One returned to normal with no signal but there was no stopping the other one once it got to a certain point.  I had a 6550 that did the fast runaway with no signal then returned to normal and repeated the pattern with about a three second period.  At least one tube continued to short the bias with the power off and then slowly released it.  Once cool enough to touch, everything was normal when tested with a DMM. 

I use the good old light bulb limiter when I suspect a bad tube because I got tired of replacing cathode fuses.   

[ogerestein]

Thanks all for your replies. Very interesting and informative. I think I have enough confidence now to just try it on my amp, since I don't have easy access to a tester. Will report later with results.

Thanks again...!

[eleventeen]

Very often, a tester will not give a good indication of tube being much good as a final power amp tube in a guitar amp.


Oh, it will indicate whether gassy or not or short or not. But most tube testers supply no more than 300 volts to the plate of a TUT tube under test. Thus it will not indicate for a short that *only * occurs after warmup where and something inside the tube bends (with heat) and shorts something else. Rare but not unheard of.

[ogerestein]

Well, I've tried all 4 SED Winged C tubes in my amp, and here are the results:

- Tubes 1 and 2, no problem, was able to bias them normally to 40 mV approx.
- Tubes 3 and 4 however, do not respond to any biasing. I get a zero reading (well, 0.9 mV actually) that doesn't budge, even when I've turned the adjustment screw almost all the way to the end. Visually there is nothing wrong with them, the heating element fires up and everything looks normal when turned on.

I put my old Tung-Sol tubes back in and was able to bias all 4 of them normally, eliminating the amp as the problem.

So, I guess I'm looking at two dead tubes? 

[PRR]

> "If a tube red-plates, it is bad and/or can never be trusted again."

The factory severely red-plates them in final clean-up and activation. Hangs a big RF coil around the tube and nukes it, later runs substantial plate current until red-hot.

So I guess all tubes are bad?

> I get a zero reading (well, 0.9 mV actually)

How does it sound? Soft? Loud?

> I did a short-circuit test using a multimeter

Just FYI: I had an output pair which had melted the PT. Ohm-meter said everything was un-shorted. About that time I scored a basic stupid tube-checker, which heats the heater and neon-lamps for leakage. (It also tied all the stray bits to 150V and displayed a current, which would sort utterly-dead duds, but isn't a trustworthy good/weak indicator, and can harm tubes.) When warmed, there was big glow for G2. My assumption is that a G2 strand got loose and touched the cathode, but only when hot. So meter is good first-check but not conclusive.

[mresistor]

Quote PRR (It also tied all the stray bits to 150V and displayed a current, which would sort utterly-dead duds, but isn't a trustworthy good/weak indicator, and can harm tubes.)

PRR  what was this tube checker?

[HotBluePlates]

PRR  what was this tube checker?

He just described any "Emission Type" tube tester. There's probably hundreds of them. They're the ones that aren't a bunch of money on ebay.

[Ritchie200]

When I took Electronics II in High School the instructor had us repairing TV's, tube amps, and tube radios.  Why? Because that's what he did for extra money in his spare time.  Or I should say, that's what WE did for his extra money...  He was very knowledgable and had built all his own ham radio gear and all sorts of other cool things, most from scratch from his own designs.  When we ran into some bad tubes, he would sometimes throw them into the tester and red plate them.  He would always say, "You gotta burn the crap off them and sometimes they will come back.  Kind of like blowing the carbon out of your engine getting on the highway!"  He was right, sometimes they did come back.  Sometimes we were replacing the fuse in the the tester....  On a new tube, I agree with the consensus here that it should be fine.


Of course we spent the rest of the time charging capacitors from the breadboard kits and tossing them to the newbie Electronics I kids.  Or reversing the electrolytics when they went to the bathroom and waiting for the inevitable pop and the subsequent pale faces and possible bowel movement of the victim.  A well earned right of passage.  Safety glasses?!?!  What are those?  These days that would get you expelled, thrown in jail for some sort of hate crime, and sued by the precious snowflake's parents.....


Jim

[HotBluePlates]

...  When we ran into some bad tubes, he would sometimes throw them into the tester and red plate them.  He would always say, "You gotta burn the crap off them and sometimes they will come back.  Kind of like blowing the carbon out of your engine getting on the highway!"  ...

Are you sure he didn't just run the heaters at higher-than-rated voltage? It will make them glow brighter...

Some Hickok tube testers have a Life Test. You test the tube at normal heater voltage, then while the tube is still registering a reading on the tester, you flip the Life Test switch. A strong tube is one where the test result either doesn't drop, or drops very little.

The Life Test switches the tube's heater to a lower voltage. This also causes the heater to draw less current, and lower voltage times less current equals less heater power and less heating of the cathode. A strong cathode coating will still emit enough electrons under these conditions to give a good test reading.

As tubes age, their cathodes emit less (assuming they don't die of something else first). Sometimes you can rejuvenate an old cathode by "super-heating" it by running on higher-than-rated heater voltage for some time. All tubes will show higher emission when the cathode temperature is raised in this way. Some, which may not be completely on their way out, will still show improved emission when the heater voltage (and resulting heater heat) is returned to normal.

Old TV tube rejuvenators worked on a similar principle. Carry it too far and the heater will burn open. No big loss, cause the tube was needing replacement anyway... Truly dead, dead tubes show minimal improvement on higher heater voltage, and no improvement when returned to normal heater voltage.

[Ritchie200]

HBP, you do realize that was like a million years ago!!!  Just ask my kids!   And my memory....    If I am not mistaken it was the plates because he had a long conversation about the rejuvenators with an assistant teacher that was there for a semester.  He also made the statement that some transmitter tubes run like that all the time so it's safe to burn them off.  He was laughing a fiendish laugh at the time...  Heck I don't know....  Now you got me second guessing!


I do vividly remember catching my first charged cap tho!!!


Jim

[shooter]

one handed or two?