Rocket with verb eating tubes...

[eddie25]

Having trouble with a Rocket with verb type amp (based on the normster layout). Three EL84s in a day, two redplated and one was blue fireworks. All in different spots and all after the amp has been played for a while pretty loud. Everything tests and sounds normal right until it happens and voltages are normal. The tubes are at almost at exactly 12W from my calculations. The fishy thing is that it happened after I changed the 100 ohm 5 watt screen resistors to 470 ohm 3 watts (ironically, to make the tubes last longer). I've since changed back and nothing yet for a few hours, but I can see my screens glowing and pulsing to the signal on two of the tubes still.

Could the 3 watt screen resistors possibly be the culprit? Or does an amp that runs this hot just eat cheap tubes and I got unlucky? I'm gonna try some 1K 5 watt screen resistors soon and maybe some sovtek el84M's, but any insight might be good. Are your guys' tubes lasting? Ultimately I'd like to be able to use any el84's and have them last. Thanks.

[M Fowler]

Eddie there must be a wiring error to be redplating like that. That amp is cathode bias and should not give any problems with redplating. Can you post a gut shot?

Your running the 50R/25W resistor with cap?

[dartanion]

The Rocket has quite low B+ to start with, around 330V, and the screens should be just a smidge lower, so the 100 ohm screen resistors should be fine. I have built EL84 amps where the plates are around 360V and screens around 340V and haven't had problems eating tubes. I think you may have gotten some bad tubes. Using 470 ohm 3W would lower the screen voltage, and would probably get really stinkin' hot. If you burnt some of those resistors, that could cause problems, but not like what you describe.

[eddie25]

The Rocket has quite low B+ to start with, around 330V, and the screens should be just a smidge lower, so the 100 ohm screen resistors should be fine. I have built EL84 amps where the plates are around 360V and screens around 340V and haven't had problems eating tubes. I think you may have gotten some bad tubes. Using 470 ohm 3W would lower the screen voltage, and would probably get really stinkin' hot. If you burnt some of those resistors, that could cause problems, but not like what you describe.

Yeah I keep coming back to the 470r 3w resistors because that's the only thing I've really narrowed it down to so far. But I feel like 3 watts should still be ok, and they measure fine and show no sign if burning. My plates are about 294 or so... reading some tube info it seems like max is listed between 250 and 300, so that's why I'm sorta wondering if its just the tubes getting overworked in general...

The one tube that I got to watch go, in the dark, started with what seems like the screen glowing (looking through the little holes in the plate) and a blue pulsing in the tube according to the signal, which eventually led to an orange glow on the plates, very slowly. In the first instance it followed the tube when I re-arranged them, but then later happened to another tube in another spot.

The cathode resistor is a 50ohm 25w with a 220uf bypass. I suspected the wiring at first, but all my voltages are spot on and it happens only after a while which has me thinking something else.

One more thing to consider is that the two redplating tubes were from the same matched quad of sovtek el84's. The one with the blue fireworks was a JJ, a pair I had to use for troubleshooting (which worked in other amps). Two 'Orange' tubes have been in there for troubleshooting as well and seem to do fine in any spot, but one brethren of the JJ and one brethren of the sovtek are the ones I can see what I think may be the screens pulsing blue to the signal again. This makes a case for bad tubes, but are others running cheap tubes at 100% ok?

I'll get it with some more time, but I appreciate the replies. Thanks!

[dartanion]

Get yourself a matched set of Saratov Russian Military EL84s and be done with it. They are not super expensive either.

[eddie25]

I hear ya. But my stubborness makes me want this thing to work with current production tubes. I'm gonna try the 1K screen resistors with some new sovteks and if they last for a while I'll switch to some quality tubes. I'm wanting this thing to go to a gigging pro who's not a gearhead.

[mclmk8d]

Get yourself a matched set of Saratov Russian Military EL84s and be done with it. They are not super expensive either.

+1 on the Saratovs

[M Fowler]

I seriously doubt it is current production tubes that are at fault. I have never had any of the current production tubes fail on me. You need to poke around some more and make sure everything is correct.

I agree the Saratov would be favored over the rest of the tube line up though.

[mlp-mx6]

I would double-check your cathode resistor on the power tubes. Make sure the connection is solid and that the resistor is measuring as you would expect. That resistor is the source of your "bias" and is key to limiting plate/screen current.

[eddie25]

So the 3 watt screen resistors wouldn't cause redplating if they got too hot? If they lost resistance wouldn't that cause a higher plate current?

Since I've changed back to the 100 ohm 5 watts everything has seemed normal. Although reading R. Aikens EL84 comparison he makes it seem to me like glowing screens and 100 ohm screen resistors are unsafe, and I still feel like it's something screen related.

I'll post some pics soon though.

[jjman]

I wouldn't expect el84 screens resistors to dissipate even 1 watt, even when cranked. And even shorted screen resistors should not trigger red plating at idle, right? Many amps don't have screen resistors.

I suspect a gremlin.

[eddie25]

Yeah, it's driving me a little nuts since that's what I thought, but now that I'm back to normal 100r 5w, it's been running all day, pretty loud, and everything sounds great. I've checked every voltage and it all seems normal. I guess time will tell.

It wasn't necessarily red-plate at idle, since it happened when I was playing every time, always more than halfway up.

Note the mirror image layout from normal wreck style amps.