Can 6AK6 tubes take 250 volts?

[PRNDL]

I now have two bad 6AK6 tubes and recall reading somewhere that they can't take high B+ voltages even though the spec sheet says max plate voltage is 300 volts and screen 250 volts.

My design fairly simple: 12AX7 into LTP into a pair of 6AK6's. B+ is 289, 250 and 240 volts.

The main problem is the amp crackles when playing guitar. It doesn't crackle at idle, although there is a bit of hum, but not a lot. The crackling gets louder when the volume is turned up.

At first, they were wired in triode, but I changed that to pentode after reading that it shortens tube life considerably.

The amp originally had 6AQ5's and I switched to 6AK6's to drop the output power in half.

I have several questions:

Are available 6AK6's able to handle the high B+?
Also, I'm wondering if the issue might be putting too much voltage into the 6AK6 grids.

Is it worthwhile to get GE JAN military grade 6AK6's
http://www.partsexpress.com/pe/showd...number=072-606

Or, will the ones from tubesandmore.com be fine (no manufacturer specified)?

The ones I have are Motorola.

Thanks!!!

PS
Here's a link to the amp photos
http://www.naturdoctor.com/Chapters/Amps/DynoTweed.html

PPS
I've tried a lot of things without success, which leads me to believe it is an output tube problem.

Perhaps 6BA6 is a better choice, since it has 330 max screen and grid, and is fairly low watts (3.4 each).

[jonnyeye]

I seem to recall that Adam Alpern's site has an amp design that used 6AK6's, and he mentioned something about frying them when exceeding a B+ of 180V. I'm curious where you're operating these tubes - could you tell us what your load and what voltage are where? I ask because these tubes have a max. screen dissipation of only 0.75W, so if you're plunking 250V onto the screens they're dissipating about 0.65W at idle - on the peaks they'll exceed their max dissipation, and fail. If that's what you're doing, I'd recommend lowering your screen voltage to 180V or less (hopefully less). I say 180V because that's where the screen voltage is on all the screen curves I can find, so you can do a loadline and see if there are other problems.

[PRNDL]

Yes, I was referring to Adam's comments.

It's cathode biased with 280 ohms and 47µF.
The anode is connected to the output transformer.

If I recall correctly, there was only about 2 volts on the cathode, which meant about 3.5 mA per tube. 3.5m x 280 = .98 watts

You might be correct about the screen voltage.
I've attached a schematic.

Would this be a good solution?
Replace the second 20µ cap with two 10µ caps and a resistive divider (180/250 x 40 = 7K/10K) along with increasing the 280 cathode resistor to 1K or so.

Also, would the same problem occur if I swapped in a different tube - 6AN5's have a max plate and screen voltage of 300. With them, I'd use the 1K cathode resistor.