EL84M?

[JustMike]

I have an AC30CC2x with a blown hT fuse and 2 white cloudy EH el84's, one if which has a burnt 470 ohm grid resistor. In looking at a schematic, the grid resistors are shown as 100 ohms.
Preliminary check shows iron to be good. I think a tube went bad and took out the resistor.  I replaced the 470 grid resistor and the ht fuse. With the 84's pulled, plugged it in and turned it on (in standby). B+ voltage at the standby switch is nominal (cant remember exact value, but its 300+ish).
 So I believe a new set of tubes is all it needs now...hopefully. I've heard that the m's are more robust than the el84's but they are cleaner. Any AC30 folks have opinions on these tube in this amp? The owner is a touring musician and plays cleaner 60's style music.

[sluckey]

Any receiving tube that is white/cloudy has lost it's vacuum and allowed air into the tube. It's bad. Throw it away. Most likely cause is mechanical stress, some obvious, some not so obvious.

[JustMike]

 Of course. The tubes are gone and are going to be replaced. The question now is which 84 variant will be the best as a replacement. I'm new to AC30's and it looks like they run the tubes pretty hot, in fact too hot for some!

[acheld]

I recently bought a pair of Soviet EL84Ms, and there is no discernible visual or audible difference to me --- but I guess the reliability factor will be played out over the next couple of years. 

My personal experience with EL84 tubes over the past 6 years has not been great.  I play through a variety of Fender BJs, as well as a Hoffman BJ clone (love that amp), and the EL84s last about 18 months.  (This behavior seems much different than the 6V6 tubes in my DR, which seem to last forever.)

LOL,  after I saw how that often the tubes failed, I modded each amp to a cooler bias, and that has helped . . . but not so much.

I've used all the major brands, haven't seen much difference in tone or reliability.

But, I'm willing to take advice.   Is there a currently manufactured EL84 that seems better to you folks? 

[tubeswell]

EL84s wear out pretty fast compared to other output tube types of similar operation conditions. It’s the miniature envelope and all that heat. The Russian mil-spec ‘EL84M’ can take higher (400ish) plate voltage, but still squeezes into a miniature envelope, so runs fairly hot 🥵.

[pdf64]

Can’t compare those fixed bias amps to cathode biased types.
Fixed bias can easily spiral off into self destruction if the valves get too hot.
Whereas cathode bias tends to self regulate.

[JustMike]

AFAIK, the AC30's are cathode biased. If it's the heat and not the plate voltage that is the killer, then perhaps the "M" isn't worth the extra $$$?

[sluckey]

It's the current that kills the tubes. That amp (as well as most AC30s) use a 50Ω cathode resistor and that biases the EL84 insanely hot. Sounds good for a while though.

[Bieworm]

Fwiw... I have a vibro king with the EL84 reverb driver. It eats standard EL84 tubes for breakfast, but since it sports an EL84 M the thing didn't break down ever since. It's like 3 years ago.. so I am convinced the M is a tougher tube.

[tubeswell]

I am convinced the M is a tougher tube.

Agree the M is a tougher tube - tends to function more reliably at higher plate volts.


But the heat ultimately kills EL84s because its packed into a miniature envelope.


And like as already been said numerous times, pumping the current through, all other things being equal, will make them wear out faster (because it makes more heat)

[Bieworm]

I am convinced the M is a tougher tube.

Agree the M is a tougher tube - tends to function more reliably at higher plate volts.


But the heat ultimately kills EL84s because its packed into a miniature envelope.


And like as already been said numerous times, pumping the current through, all other things being equal, will make them wear out faster (because it makes more heat)

Good thing they sound so incredible when overdriven.  Otherwise they wouldn't be around anymore...