Svet w-'C" turns white, and blows fuses...

[edge59]

Hi folks, I'm new hear but have built an amp that all voltages seem correct but the new set of 6L6's, blows the fuse 5 amps, after running it at meadium volumes, one of the 6L6's turns white, looses vacuum? I have the bias set at 29-30-ma. each, with a 1 ohm resistor from the cathode to ground. The plates are 465-477 VDC. 3.3k grid leaks seem ok, as the screen grids (500 ohm). Is there something I might have missed? Or is Svetlana winged "C"'s production control going bad? Any thoughts would be greatly appreciated!
thanks
Edge

[Matt T.]

That's my understanding...it lost vacuum. I've only had one tube do it though...and it was only a preamp tube...and I never plugged it back in.

[Enzo]

Let me get it straight, your amp fired up and a tube failed.

Once they fail they don't heal.

Or did a string of tube after tube fail in this amp?

One bad tube means nothing. Svet makes good tubes as far as my experience, but NO tube maker has zero failure rates.

Get rid of the white tube. Try a different set of tubes, even old one if you know they work.

[Mars Amp Repair]

White tubes

Right,
Unless the side of the tube is melted & sucked in (I haven't seen that since the old days of horizontal output tubes in TV's) , chances are the tube took a physical shot that broke the vaccum...this usally occurs near the base or at the nipple inside the Key in the center of the pins. Usually happens if that key is broken out from being wiggled too far side to side when being removed. (or water/beer dripped in the top w/hot at a gig )

Probably just a fluke that the tube lost its vacuum. Replace & move on.

Just as a sidebar since I mentioned it ;
Interestingly enough the reason the Horiz out tubes did melt & suck in the side is that they are essentially beam power tubes that are driven with a 15khz signal which acts as the 'bias' for the tube just like our power tubes & their applied negative bias. When the horizontal drive disappears, the tube looses its bias & conducts like mad, It gets so hot it actually melts the glass & the vacuum sucks it in until the glass is too thin & air rushes in.

Has it been that long ....glen