Frying 100 hom resistors

[rfgordon]

When an otherwise healthy amp suddenly starts cooking the 100 ohm resistors in the heater artificial CT when high volts are flipped on--is the culprit usually a bad tube?

[VacuumVoodoo]

EL34/6L6 family? Could be arcing or a short betwen pin 1 & 2 on one of power tube sockets or internally in the tube.

[rfgordon]

The amp runs two 12AX7s and one KT88 in cathode bias.

[rfgordon]

Svoloch! So I put in new 100 Rs and took the KT88 out. Fired up without incident. Popped in a good 6L6 and it's fine. Ding dang durn it! Brand new KT88 dead and gone. javascript:emoticon(':evil:')
Evil or Very Mad

[CaseyJones]

Some o' those KT88s are junk from the get-go. I'll take my chances with well cooked vintage tubes before I mess with the new stuff... especially KT66s, 77s, 88s, stuff like that with hungry filaments.

[rfgordon]

So let me ask y'all this--when a tube shorts plate to filament like this, should it show any visible signs of electrical mayhem? Mine looks fine--but looks can be deceiving. I'm gonna send it back to CEDist where I got it.

The thing is, that when I've been really cranking the amp and hit a power chord, the volume would drop momentarily. It did this just before toasting the resistors, too. Perhaps that was a sign that an early death was in the cards for this tube? Cathode bias amps don't sag like fixed bias amps do, so I figure the fade out was from some other cause.

[CaseyJones]

So let me ask y'all this--when a tube shorts plate to filament like this, should it show any visible signs of electrical mayhem? Mine looks fine--but looks can be deceiving.

Sure it will show visible signs of electrical mayhem... on a tube tester. The cheapest crappiest emissions tester will light up a short circuit light if the filament is shorted to anything. That's why any place that sold tubes "back in the day" (like Rat Shack or Lafayette) had a console type tube tester right there. It was there as a courtesy, customers could come in and test their suspect tubes then they could test the new tubes to make sure they were o.k. before leaving the store. Of course every so often you run into a situation where the new tubes tests exactly like the "suspect" tube which just adds to the confusion.

It's gettin' down to the point where every TV-7 or 539C on Earth has been dragged out, calibrated and pressed back into service. Any tube vendor worth a damn screens every tube before shipping it, that's just good business. If you screen and QC yer merch you're reasonably sure that you'll catch the DOAs before they cause problems, if not then there are plenty of vendors who do screen their tubes. Oh, I forgot... the guys who sell better quality stuff charge a bit more and of course we all want to save a buck. Some days I feel like I'm pissin' into the wind, I'm tryin' to build really outstanding amps while some of my supplier's quality slips ever lower.

I'm gonna send it back to CEDist where I got it.

And that's that.

I keep hopin' that the quality of new tubes catches up to NOS tubes but it makes no sense that it would. The market is ever smaller, the materials are increasingly expensive and depending on where yer factory is located business is becoming increasingly difficult. Europe has RoHS, we have OSHA and the EPA. Western Electric still turns out good stuff here but it sure ain't cheap!

My solution is to seek out less desirable NOS, for instance 807s are so common you can use 'em to decorate yer Christmas tree. For SE... why not?! It's a cool lineup, with a 6SL7 on the front end you'd have the big bottle thing goin' on. I'd even use an 80 rectifier, why not keep yer technology pre-historic?!

[rfgordon]

I put my ohm meter across the pins of the KT88, and guess what I found? From plate to one filament is 1k5 ohms, and from plate to the other end of the filament is 2k ohms. Definitely a bad bottle.