300B tube not working after only 30 hours

[docbob52]

My BeePre sounded and tested wonderful on resistance and voltage checks.  I put 30 hours on it and then  one channel died.  After swapping tubes around, I discovered the dead channel followed one of the 300B tubes which is proof positive to me that the problem is the 300B tube.  The bad tube glows and tests ok with a DVM resistance checks, but I don't have a tube tester here that I trust test it and I did do the thump test. 

Before I plug this new 95$ 300B tube in what would you suggest that I test in the circuit?  I checked out the stacked wire resistors and they were spot on. 

All advise appreciated.

 

[Paul Birkeland]

Sometimes it's just luck (or lack there of)  of the draw.

You'd want to measure the "pos out" on the filament regulator board on that side with a working tube, then the voltage across the paralleled 4 Ohm resistors.  They should be within 10% of the suggested manual values.

The last Eh I ran into that did this needed some additional solder flowed into a loose in in the base, and it has been running strongly since (roughly one year).

 

[RPMac]

I like to check the DC voltage across the .62 resister on the filament board. Divide that voltage by .62 to get the amperage on the filaments.

 

[docbob52]

Thanks for the help.  By the way RP, what should the amperage to the filaments be?

 

[Paul Joppa]

BeePre is designed around the WE specification for the 300B, which is 1.2 amps current. (Note that the 0.62 ohm resistor is a 5% part, so the calculated current  will be +/-5% from the actual current.)

 

[docbob52]

Paul, thanks so much.

 

[2wo]

The last Eh I ran into that did this needed some additional solder flowed into a loose in in the base, and it has been running strongly since (roughly one year).
[/quote]

To amplify a bit, heat the 2 smaller pins of the tube itself and add a little solder to the bottom of the pin...John   

 

[docbob52]

When you say "the bottom of the pin"  do you mean the very tip of it or at the other end where the pin enters the base of the tube?

 

[Doc B]

The tip of the pin actually has a hole that is filled with solder. If you reheat it you can add a little more solder through the hole.

 

[docbob52]

Thank you all.  I tried your re-soldering trick and the 300B is restored like new with both channels working great.  How did yall  learn about this trick?  I have never heard of it before.  I am curious: why would a new tube work at first but develop a bad solder connection later? 

You saved me 95$ for this new tube that I will send back for a refund and use the money to put on my next order-- an Eros phono preamp.

Again, thank you so very much for all your great help.  This forum rules for great advise and ideas and unlike many others, in a very timely manner.

Doc you have built one very great company and provided me and so many others  great fun, knowledge and wonderful state of the art components over the past 17 years.

 

[Paul Birkeland]

Thank you all.  I tried your re-soldering trick and the 300B is restored like new with both channels working great.  How did yall  learn about this trick?  I have never heard of it before.  I am curious: why would a new tube work at first but develop a bad solder connection later? 

I was helping Clark Blumenstein with his BeePre, and he had a 300B that he was thwacking periodically when it would drop out periodically.  I was able to measure the voltages when the tube dropped out, and noticed that the filament was peachy, but the plate voltage was really high (so no plate current).

I took my best 100 Watt soldering iron and cooked the hell out of the socket pin, adding some solder through the little hole in the base pin, and the tube has been reliable since. 

 

[Grainger49]

Woo!  Hoo!  This is good information for all of us. 

Thanks Dan and Bob!

 

[Doc B]

Man, I can't remember the first time of many I have used that technique. That goes back to my antique radio restoration days, maybe 20 years ago. You got used to resoldering pins and regluing bases back then, as there weren't so many new production tubes.

My favorite fix, though, was one I saw at a swap meet. My bud Crazy Eric bought a globe style 50 in which the entire structure was leaning over at an angle. It was pretty clear that the filament would short against the grid. Consequently he got it for a few bucks. He walks over to me and says something like "nothing to lose" and smacks the tube hard into the palm of his hand. Holds it up and the structure is perfectly vertical. The guy probably made the tube worth about 20X as much just by smacking it.

While a 300B is a serious investment, after doing this for almost 25 years it kinda cracks me up to see how much angst some customers go through when the $5 tube we put in a Crack kit makes a little noise or isn't perfectly balanced. One of the facts about the vacuum tube era that has been contorted over the years is why tubes plug into sockets. The current lore seems to be that it's because that way you can roll them. That's not why, it's because you typically replaced them fairly often. That of course was one of the big marketing ploys that SS amp manufacturers have played ever since they first appeared - "those tube amps are tricky, you have to replace tubes all the time and they are really difficult to bias." Thus causing the tube guys to respond - "the worst tube amp is better than the best SS amp".