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-   -   300B failed again (http://www.diyaudio.com/forums/tubes-valves/206335-300b-failed-again.html)

the_manta 9th February 2012 11:04 AM

300B failed again
 
Hi folks,
The next 300B died in my dad's amplifier yesterday. This time a EH with ceramic base. Not even a year old.
Well it did fail only 50%. Look:

http://img6.imagebanana.com/img/jy20...phundred_b.jpg

Interesting fact is that the sound isn't that much worse as one would think. Wouldn't have noticed it right away. At higher levels it starts to distort pretty fast.

Another strange thing is, that there is no heater-cathode short. I connected it to my fluke short tester and shook the hell out of the tube - nothing. Also the springs on the top are all tightend up. The filament wire cannot be broken. I really don't know where the open connection is.

Is this a known problem in 300Bs ? Anyone experienced the same fault ?

Even the chinese 300B (very early production) lasted longer but ended with a bang (heater cathode failure)

And what bothers me most: You can get a new production EL84 for under 10$. An EL84 is tiny and needs 3 exactly wound grids. A 300B only needs one big grid and can be produced much more easy but costs a lot more.

What's the cheapest 300B out there ?

Regards, Simon

FullRangeMan 9th February 2012 11:19 AM

Putz! Really bad news, I sorry read it. The cheapest 300B I now is the NGG300B (guiguang), at 96USD/pair in HongKong:
Guiguang tube

If I understant right these your tubes are the SOVTEK 300BEH??
Where do you bought them??
Thanks

Doz 9th February 2012 11:27 AM

300B is directly heated... how did the heater (technically a filament here) fail with regards to the cathode?

tubelab.com 9th February 2012 12:01 PM

Quote:

Is this a known problem in 300Bs ? Anyone experienced the same fault ?
How many do you want. I have about 5, ALL SOVTEK!!!!!!! The early ones failed within a few months of the 90 day warantee expiring. Sovtek refused to help even suggesting that I BUY the new EH 300B since it had a much better filament.....but there was nothing wrong with the Sovtek filament.

I used only cheap 300B's, which have gotten expensive so I don't build 300B amps at all any more. I had only one Shuguang fail in this manner and the vendor replaced it. The problem seems to be where the lead wires are crimped over the filament. Maybe the coating wasn't cleaned off completely or the filament wire was contaminated. The Shuguang was intermittent. The second half of the filament would occasionally light up once the tube got hot.

the_manta 9th February 2012 03:28 PM

Quote:

Originally Posted by FullRangeMan (Post 2898739)
Putz! Really bad news, I sorry read it. The cheapest 300B I now is the NGG300B (guiguang), at 96USD/pair in HongKong:
Guiguang tube

If I understant right these your tubes are the SOVTEK 300BEH??
Where do you bought them??
Thanks

Thanks for the link ! I hope these will last longer.

The failed tube is an Electro Harmonix (EH) 300B. The standard one, not the gold-grid or whatever one. I made a photo. It's one of the left pair. (The right ones with the carbon plates are some early production chinese.)
I looked up some pictures of the Sovtek 300B and the internals look exactly the same. Well, no suprise - they both came from the Reflector/New-Sensor plant in Saratov I guess.

Quote:

Originally Posted by Doz (Post 2898743)
300B is directly heated... how did the heater (technically a filament here) fail with regards to the cathode?

Sorry, Doz, I meant heater-grid short.

@Tubelab:
5 tubes....What a waste of money. It's a shame.
Thanks for the useful information!

picowallspeaker 9th February 2012 03:41 PM

Hi ,
I was reading just today that best positioning of these tubes is horizontal ,not vertical as usual ; this because the heater won't be facilited to 'relax' from heat ,and proning to do an arc...and arching !
:o

kevinkr 9th February 2012 04:34 PM

Quote:

Originally Posted by picowallspeaker (Post 2899055)
Hi ,
I was reading just today that best positioning of these tubes is horizontal ,not vertical as usual ; this because the heater won't be facilited to 'relax' from heat ,and proning to do an arc...and arching !
:o

No, this isn't really true, the best orientation is definitely vertical, however it can be operated horizontally if specific positioning guidelines are met. Have a look at the WE300B specification and recommendation here: http://www.westernelectric.com/products/300b/300B.pdf and here: Western Electric, the filament must be oriented such that plane of the filament is vertical, that way when the filament sags it will not immediately end up contacting the grid wires.

My experience with various brands of 300B over the years is that all except the old JJ type are unreliable. (Excepting NOS or slightly used vintage WE300B which were rugged and reliable, but rather more expensive) I can't vouch for the current production but I have quartet of JJ 300B that have lasted over 11yrs and have something like 5000hrs on them total and they still work and sound fine.

The Guigang seems to be well regarded, but I have no experience with this tube at all.

I built small quantities of SE and PP amps commercially with 300Bs until about 2001 when I went out of business. (And lost about a dozen 300B over a couple of years, excluding out of warranty customer failures.)

tubelab.com 9th February 2012 05:11 PM

Quote:

5 tubes....What a waste of money. It's a shame. Thanks for the useful information!
Granted it was about 10 years ago, but it still seems to be a problem. That experience angered me to the point of refusing to buy another New Sensor tube for almost 5 years. I was repairing and building guitar amps at the time and their KT88's were good, the Chinese were still making vacuum firecrackers and the Svetlana's were more expensive. I later bought 5 used Sovtek 300B's at a hamfest for $100. They were all different sizes (over an inch difference in height) but they worked. 4 of them went into my 300Beast amp and they still work today.

About 5 years ago there was some reliability issues with JJ and Shuguang 5AR4's. Ned at Triode electronics sold me some Sovtek 5AR4's that he guranteed that I wouldn't blow up. 3 of them went into my 845SE where 2 of them form a voltage doubler to get 1100 volts from a 480 volt 1 AMP transformer. They are still alive!

JimW 9th February 2012 06:29 PM

hasn't been my experience!
 
I have two mono blocks, and two stereo amps using 300B (that's 6 EH 300bs total), one of the stereo amps sees daily service, and I can honestly say I have not had an electrical failure. Four of these tubes have been in use since 2006. I have had two broken (kids involved) tubes, one EH and one JJ. I do occasionally rotate JJ's in and out of service, as they do sound a bit better, but the EH have been satisfactory for the most part (they are more microphonic than the JJ). Now I do run the tubes conservatively, perhaps that makes a difference, the hottest of the amps runs them at 28 watts, the monoblocks at 25 watts, and the remaining amp runs them at 20 watts, and I also have made sure the filament voltage doesn't exceed 5.1-2 volts. I do have a stereo 2a3 amp that has had the same tubes in for 8-9 years (EH), but this amp doesn't see a lot of action these days although I used it in my office for 9-10 hours a day for about 5 years. I do make it a point to periodically reseat the tubes, as I have had sockets lose contact, which can be a big deal if it happens to be the grid pin that makes poor or no contact.

Bouquet 9th February 2012 06:41 PM

Had three Sovtek 300B's with exactly the same failure , al at the same side of the filament too.
I reckon Sovtek has a real manufactoring problem the should have solved by now.
The at least could offer you a replacement for a decent price.


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