"Bad" NEW TUBES

[DRBILL]

I have been working fitfully on a pair of MARK III's for over a year. Most of the time was spent waiting on out-of-stock parts. Also, a lot of time was spent returning defective or mistaken orders. Unfortunately, this litany included some of the oldest and most trusted names in the HAM and audio trade.

My most vexing incident happened during the last three weeks. I ordered four KT88's for which I paid a premium in order to get the "prefered" imported brand. All four tested perfect on my B&K-700. No shorts or grid emmission and with expected meter readings. I popped them in the MARK III's, set the bias and checked the under-chassis voltages, and one tube in each amplifier exhibited a runaway rosy plate. The red plate followed them to other sockets. They were bad tubes.

No questions asked, the vendor swapped them for two more. Both passed the tube tester, but one was also a runaway in the amplifier. I have ordered ONE KT-88 from another vendor with the hope that the tubes in his bin are from a better lot.

I do a fair amount of business with a guy that specializes in NOS tubes. I have never had to send anything back. The reason that I don't do all of my tube orders from him is "$$$$!" Perhaps I am being penny-wise and pound-foolish. I am about to the point that the higher price might offset the frustration. Some words have formed in my mouth, recently, that surprised me!

Are any of you having similar experiences?

DR BILL (aka DR GLOW in the local band business!)

[JJkizak]

Are you using the correct meter? are you measuring according to the book?

If the tube checker checks them fine maybe the problem isn't the tubes and you toasted some cathode or plate resistors. Anything burned? A marginal type Radio Shack multimeter I would not use. The new good multimeters will not load down the circuit. And if they were glowing red you might have to replace some parts anyway. Resistors don't like to be juiced with extra current.

JJK

[DRBILL]

Dear JJ,

I have a fairly decent bench of test equipment, all in proper calibration. I always have a schematic in front of me on a clipboard in case my mind wanders.

You must understand that when the good tubes were in place, everything worked according to specs. By substituting a bad tube for a good tube, the plate glowed in the bad tube. You can do that in any socket position. I presently have three tubes in two amplifiers that work perfectly. I'm hoping to get a fourth good tube to complete the set. And for the record, I don't own any "Radio Store" test equipment!

Thanks for your interest.

DR BILL

[oscarsear]

Well Dr. Bill, I cannot assist in any way other than to share my tube failure tales. I don't build or bench test squat and necessarily rely upon my suppliers integrity. In 6 mos I've had a KT-88, KT-90 and a rectifier tube fail, most upon initial outings. I wondered whether it might not be part-in-parcel with vintage product. Got everything running and alls well...for now.

[DRBILL]

Yo, Oscar,

Try this theory. What if every time a vendor put returned tubes back in the bin. The time would come when one out of four picks would be a dud. Eventually, every pick would be a dud! As you say, we have to depend on the integrity of the people we deal with.

How do you like the KT90's? I've heard mixed reports. They have a reputation for being very accepting of high plate and screen-grid punishment. I thought one "authority" was reviewing wine instead of vaccum tubes. Something like "a warm and smooth tube that quickly cloyed the ear with its monotonous sameness!" Me? I never met a tube I didn't like!

DR BILL

[sunnysal]

it is not usual that new tubes can be bad, it is an ufortunate fact of life.

it is also true that VERY few tube testers put a significant load on a tube while testing so they can test good and fail under load in the amp.

part of the "fun" of owngin tube electronics I guess.

i blew up my beloved JFL amp once when a perfect-testing 2A3 went south...grrr...

regards,

tony

[DRBILL]

Tony,

I really wanted to hear that you got your amp back up and running! Yes?

DR BILL

[D-MAN]

Are they Chinese or Eastern Block? Name some names, dude!

I have had reasonable luck with duet Russian glass (6550). Never went for the NOS due to cost.

DM

[Coytee]

New to tubes, so I have a question.

When tubes go bad, do they run the gamut of dying? What I mean is, do they just burnout like a lightbulb might, do they (internally) pop like a lightbulb might?

Might they literally "blow up", as in, pop inside and that action actually cause them to shatter in some way or another?

What's the most catastrophic way, a tube can fail on it's own?

[Marvel]

Dr. Bill,

Why don't you contact Manley Labs and see what they would charge for the KT-90. Might be a little higher, but I'll bet they will be good. One of their monoblock amps uses 10 KT-90s, for 275 watts in triode mode!

=================

FAQ #16p. Can I call in my spare tubes order?

Sure. Give a call to the factory (909) 627-4256 and ask for Manny in the parts department if you already know what you need.

FAQ #17. Do you accept credit cards?

Yes we do: Visa, Mastercard, and American Express. But don't be emailing those numbers. Call or FAX them in.

==================

They might not sell you tubes since you don't have a Manley amp.

Or try Kevin at Upscale Audio, as they test their tubes, and have KT-90s and KT-88s:

http://www.upscaleaudio.com/view_category.asp?cat=39

Cheers,

Marvel

[D-MAN]

The WORST thing that they do is short the plate to the grid. Then everything, including the speakers, start to smoke. Rather an unpleasant event better left unexplored.

I've heard that Chinese tubes tend to do this more than some others. However, I have not seen it, but I don't use Chinese tubes, either.

DM

[Coytee]

hmm...smoking speakers... doesn't sound terribly fun to me. As I reflect on what you say, I presume mechanically, what's happening there is somehow the tube and the "grid" (the grid perhaps being the transformers??) short out and by doing so, direct current flows through the system trying to disguise itself as perhaps Bee Gees music? When said disguised music gets to speaders, they wake up, realize it really DOES sound like the Bee Gees so the whole system tries to commit suicide rather than play the sound??

It's only a theory I'm working on, but do I have it?

[Coytee]

speaders = speakers

[JJkizak]

How do you get DC (Plate short) through an output transformer to smoke the speakers?

JJK

[DRBILL]

Yes, sometimes they just fail because the filiment burns out, like in an incandescent lamp. Actually, this is fairly rare. Internal shorts and "gas" are the most frequent failures. The cathode coating can flake off like dandruf and short things. You know the mirror coating on the inside of a tube? The "getters" are supposed to suck up the last molecules of gas in the tube, and they also are supposed to direct the stuff that boils off the incandescent filiment to a safe place away from the elements in a tube. Thus, the silvering on the top and sometimes the side of the tube. However, after many hours of service, some of this stuff begins to plate out down where the wires come through the glass envelope. Shorts. There is no such thing as a perfect vacuum, so stuff boils off all of the tube elements and sometimes gets so rich that it ionizes. That's a gassy tube.

The most catastrophic failure is a "runaway tube". This is probably caused by gas. Current flows unimpeded to the plate which turns bright red-orange. This can become so hot that the glass envelope shatters. It can take out an output transformer. This is not likely to happen if you have protected your device with a proper fuse.

I can think of no tube failure that would deliver DC to your speakers.

I haven't thought about this in perhaps fifty years. I may have left something out. in any case, I hope this helps.

DR BILL

[dodger]

IMHO, "BAD" new tubes or "BAD" NOS tubes receive more publicity now that the number of tubes, whether New or NOS, has diminished.

A number expect and talk a lot about Chinese tubes either going bad quickly or bad to begin with. The same for the Russian tubes.

Also there are quite a number of Counterfeit NOS tubes.

What may have been a bad tube installed by a TV Repair Tech during a home visit, is now a consumer problem. For many there is no recourse.

I have the original Genalex Gold Lion KT-88s from my McIntosh MC 60s. On a Sencore "Mighty Mite" tester, they test fully brand new - Power, grid, gas, lifespan left.

There are some that replace tubes too often. Then there are those in one blues Group where the lead guitarist has the tubes changed prior to every show. NOS Genalex is the usual choice. The old tubes - all of one night's use - are tossed into the trash.

And we have the last dishonest tube sales person that will sell a used NOS tube at auction by listing NOS?

Legally gets him off the hook. No Ethics.

Luckily, I have not encountered, heard of, or witnessed anyone of the Forum Members doing any of these practices. There is integrity and knowledge that even a new tube may fail.

But tube sellers do have an obligation to stand behind their product for a reasonable length of time. I don't mean 1 year guarantee, but reasonable.

Something an Episcopalian can live with, Dr. Bill. LOL

dodger

[sunnysal]

dr. bill,

yes the amp was restored to full function by jeff himself, the failure took out the power trannie and a cap or two...grrr...

well, I am enjoying the amp immensely now and have to accept that tube amps can and will fail more frequently than SS gear, it's the price we pay for the tube sound. one can be lucky or not, the tubes can last virtually forever or not...

I use the tube tester when I buy a new quad of 2A3 or 6SN7 input tubes but I still know that I cannot by sure until they are in the amp under load...fun, fun, fun.

warm regards, tony

[D-MAN]

We had all tubes when I was in the Navy (late 70's).

Never had a tube actually melt down, as they were all mil-spec stuff...

But I did read where Chinese power output tubes were failing in a catastrophic manner, shorting and taking out everything else down the food chain. This could be a heater-based short or whatever, I believe that the article said plate to grid.

So I am not completely making this up, although I cannot cite the actual source, so I might as well be making it up, I guess.

DM

[DRBILL]

Yo, Tony,

"well, I am enjoying the amp immensely now and have to accept that tube amps can and will fail more frequently than SS gear, it's the price we pay for the tube sound. one can be lucky or not, the tubes can last virtually forever or not..."

PWK was antsy about the introduction of SS amplifiers. He told me that tube amps seldom caused any trouble with his speakers except for "clipping". Low power amplifiers took out more Klipsch tweeters than the brute tube gear. What he dreaded was the failure of high powered SS gear with no output transformers. These were deadly to woofers. When they shorted, the voice coil could actually catch fire! Hundreds of watts of DC rushing into an eight ohm load. Not pretty.

"On the whole, I'd rather be in Philadelphia" as said WCF.

DR BILL

[lynnm]

FWIW

I have been a bottlehead for about 50 years ( to one degree or another ). In the begnning my interest was in radio,later TV,and more recently Audio. In all that time I can recall only one really catastrophic tube failure.

One day I turned on my ASL AQ1003DT amp and headed for the kitchen to get a coffee. BLAM!! One of my Sovtek EL34s developed what the techs diagnosed as a plate to cathode short. I was running KLF30s at the time and can honestly say that the event sounded as if someone had slammed a door as hard as humanly possible!

That said tubes rarely die in such a catastrophic manner. http://www.Divertech.com (the distributor fixed the amp and provided a new quad of EL34s) and my dealer provided a loaner amp while mine was in for repair.