EVH LBX III Bias drift

[slimchicken5150@]

:feedback: I've got an EVH LBX III Stealth. Here is my issue. Whether the amp is on 1/4 or full power the clean green channel bias is steady. It doesn't ever change type of steady :32:. Then, when I flip to the red channel, my bias makes it's way normally to what it's set at (usually I set it at 30mV) So, up to 30mV it hits:32:However it only stays at this voltage for about 30-45 seconds then it starts creeping up, 32,36,37,40,42mV etc. It will do this if the amp has just warmed up or if it's been on for hours. It came factory with 2, el84 Groove Tubes and 4 unmarked 12ax7's. Fender does this sometimes I found out. I swapped the el84's with JJ's. Same damn issue k:. It still creeps up. Is this normal? Please advise . :bowdown:

 

[JMP/HBE]

You won't fix it here.
Take it to an amp tech.

 

[Rex_Rocker]

:feedback: I've got an EVH LBX III Stealth. Here is my issue. Whether the amp is on 1/4 or full power the clean green channel bias is steady. It doesn't ever change type of steady :32:. Then, when I flip to the red channel, my bias makes it's way normally to what it's set at (usually I set it at 30mV) So, up to 30mV it hits:32:However it only stays at this voltage for about 30-45 seconds then it starts creeping up, 32,36,37,40,42mV etc. It will do this if the amp has just warmed up or if it's been on for hours. It came factory with 2, el84 Groove Tubes and 4 unmarked 12ax7's. Fender does this sometimes I found out. I swapped the el84's with JJ's. Same damn issue k:. It still creeps up. Is this normal? Please advise . :bowdown:

I didn't notice the bias drifting in either my LBX or LBX II, but I did set the bias using the red channel (or blue channel in the II).

The stock GT EL84's and unbranded 12AX7's are actually JJ's, LOL. Compare them side by side, and you'll see.

 

[Mincer]

Welcome to the forum!

Is this causing the amp to not work properly?

 

[beaubrummels]

Welcome to the forum!

Is this causing the amp to not work properly?

It would change the sound for sure, and be audible while playing. Bias cold sounds darker, bias shifting hot would sound great, bright and tight, but it would also burn up tubes quick if it gets too high. I've never had an EVH LBX so I can't comment further, though.

 

[Mincer]

What, in general, causes bias to change or increase over a short period of time like that? I know there are people here who know more about this stuff than I, so I enjoy learning.

 

[beaubrummels]

My guess would be the bias resistor is getting hot and not maintaining it's value. Perhaps need a higher temp spec bias resistor? Or wire it away from whatever the heat source is? But note, that is just a guess. It could be something else I'm not aware of - I don't repair amps for a living and am not familiar with the EVH LBX circuit. Maybe try putting a fan on it and see if it slows the drift?

 

[Rex_Rocker]

It would change the sound for sure, and be audible while playing. Bias cold sounds darker, bias shifting hot would sound great, bright and tight, but it would also burn up tubes quick if it gets too high. I've never had an EVH LBX so I can't comment further, though.

WIth the LBX, I found the other way to be case. I never adjusted the bias past a certain point or below a certain point, but I noticed the hotter I got, the more lower-middy it got (and slightly louder). The colder, it got crisper and slightly harsh.

I thought that was the norm? I know people bias mod their Peavey 5150/6505 to get them to sound fatter and less fizzy because they're non-adjustable fixed bias from the factory, and they come biased super cold. Same with Rectos and people running 6L6's on the EL34 setting to get them to bias hotter.

 

[jeremy]

ive always heard cold bias as cleaner/drier/tighter

 

[Rex_Rocker]

ive always heard cold bias as cleaner/drier/tighter

I guess "cleaner" if you get past crossover distortion too. Because too cold, and the poweramp sounds bad too.

 

[JMP/HBE]

What, in general, causes bias to change or increase over a short period of time like that? I know there are people here who know more about this stuff than I, so I enjoy learning.

Malfunction, bad SGR or bias resistor. Could be anything.
Its like trying to cure cancer over the internet.
If he really wants it fixed take it to someone qualified.
Just the fact he's here means he's cutting corners or doesn't have the money.
Not a fan of the mexican VH amps at least that where they used to be made.
Now china, indo, VN ?
Unfortunately some have no access to an amp Doctor.

 

[Rex_Rocker]

Not a fan of the mexican VH amps at least that where they used to be made.
Now china, indo, VN ?
Unfortunately some have no access to an amp Doctor.

They're still Mexican.

I didn't have any problems with mine. Decently made and killer-sounding. Good transformers as well. Heyboer and Schumacher.

Certainly no Soldanos, but better-made than the Peaveys, for sure.

 

[JMP/HBE]

They're still Mexican.

I didn't have any problems with mine. Decently made and killer-sounding. Good transformers as well. Heyboer and Schumacher.

Certainly no Soldanos, but better-made than the Peaveys, for sure.

Those are excellent Transformers. I had no idea.
To be fair i hear of issues with 20+ yr old PV 5150's but not really so much with the newer EVH stuff.
But seriously get that amp in the shop pronto if you want it fixed.

 

[beaubrummels]

WIth the LBX, I found the other way to be case. I never adjusted the bias past a certain point or below a certain point, but I noticed the hotter I got, the more lower-middy it got (and slightly louder). The colder, it got crisper and slightly harsh.

I thought that was the norm? I know people bias mod their Peavey 5150/6505 to get them to sound fatter and less fizzy because they're non-adjustable fixed bias from the factory, and they come biased super cold. Same with Rectos and people running 6L6's on the EL34 setting to get them to bias hotter.

Yeah, I'm largely going by my experience with my JCM900 SL-X. It came biased hot from the factory and sounded freaking amazing. But it burned up 2 power tubes within a couple months. When I got those replaced and had it rebiased, the guy (a licensed Marshall repairman, not any hack) said he set it to exactly the factory spec, which was colder, and it sounded darker and much less amazing. It sounded pretty good, it sounded like a Marshall, but it was a darkly filtered Marshall, like listening to a great amp through an open window outside the house, rather than right in the room making your bell bottoms flap.

 

[JMP/HBE]

Most tube amps sound spectacular just before the tubes blow.
EVH in the day had boxes of power tubes he would just swap in & out going for whatever sounded best.
Once i plugged my DSL into the wrong speaker out.
Amp sounded like never before, glorious in every way them my nose picked up that roasting electrical smell.
I looked inside the head & 2 of 4 EL-34's were glowing red.
Fortunately i had other tubes and the amp wasn't hurt but it got my attention.