New Power Tube Flashed

[boyon00]

I installed 4 new SED Winged C power tubes in my Dynaco ST 70 amp this morning. I was going to wait for the tubes to warm up to bias them when one of the tubes lit up very brightly and caused a great hum in the left channel. It was only in there for less than 5 minutes. I immediately turned off the amp. When things cooled down I put the old power tubes back in. Thankfully the amp sounds fine. Now I was wondering this has to be a bad tube,right? I bought a quad of matched tubes and it is possible it is defective.
Thanks
Brian

[Old1625]

It is probably a bad tube if the old ones still work reasonably well in the application.

Another test of the new tubes, rearranging them so that the tube that acted up is in a different position, and verifying that that particular tube is going to go into runaway no matter where it is placed, will prove its defect.

If another of the new set of tubes acts up in the same socket as before then you will have probable cause to believe that there is an issue with the amp--leaky blocking capacitor, control grid resistor that has gone up in ohmic value, etc...--which could be effectively masked by slightly different characteristics of the old set of tubes.

Bear in mind with the ST-70 that if you have an old original driver PC board that has gotten dark with heat and age that anything is possible; IME those old boards get leaky between foil tracks as they get dark. Flexure of the board may produce noise in many cases, or there may be subtle rumbling background noise with such leakage, but not necessarily.

[similost]

Another thought, the older tubes the bias may have been set higher, and then when you put in the new ones, the bias was too high for them. I had put in a new set of tubes, and instantly had to turn the bias down quite a bit. They started glowing way too bright... when I swap back to my older tubes, I have to turn the bias back up... you may have been over driving them..

[boyon00]

Another thought, the older tubes the bias may have been set higher, and then when you put in the new ones, the bias was too high for them. I had put in a new set of tubes, and instantly had to turn the bias down quite a bit. They started glowing way too bright... when I swap back to my older tubes, I have to turn the bias back up... you may have been over driving them..

Actually the bias for the older tubes was down to 1.42 v. when I checked it after, which I corrected to 1.56.

[boyon00]

I forgot to add something. Does this mean that tube is pooched? I would be a little reluctant to try another new tube in that socket as I would be afraid of damaging something.

[fsjonsey]

Occasionally, brand new tubes will fail prematurely. I'd have the suspect tubes tested on a mutual conductance tester.

[boyon00]

Occasionally, brand new tubes will fail prematurely. I'd have the suspect tubes tested on a mutual conductance tester.

I just checked it on my tube taster and it failed the s/c test...

[FalconEddy]

I just checked it on my tube taster and it failed the s/c test...

There ya go.

. . Falcon

[Old1625]

Not much more to say....

Just make sure you have an eagle-eye on the tube in that socket when you plug in and fire up the next set..... :thmbsp:

[boyon00]

Not much more to say....

Just make sure you have an eagle-eye on the tube in that socket when you plug in and fire up the next set..... :thmbsp:

Duly noted... Now I hope my tube supplier will at least replace the defective one with a tube of the same value...