NEW AMP DAY, PART 2! PROBLEM SOLVED!!!! :) last page.

  • Thread starter Thread starter guitarslingerans
  • Start date Start date
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Oblivion DC":2bdef said:
What you do is fire up your multimeter and click over to DC volts.
Turn all the knobs on the amp to 0 and unplug the guitar cord from the amp input.
Put it on Standby and let it warm up for a few minutes.
Unscrew the rear grill and look for the bias adjustment pot on the left side of the amp (left when you're standing behind it).
You'll see a black socket and two red sockets in the back of the amp - those are the test points you use to check the bias.
Take it off of Standby.
Plug the black probe into the black socket and the red probe into one of the red sockets.
You should be getting a reading of 72-74 mA.
That is the combined reading for two of the tubes on one side.
This means that I set each tube to idle at 36-37 mA.
If it's not where I left it, try adjusting the bias knob so that the reading on the Multimeter changes.
Pull the red probe out and put it in to the other red socket.
You should see that the numbers are already either the same or at most one or two numbers off.

That's all there is to it.
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It needs to be plugged into a speaker cabinet while you're doing this.
 
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Marshall Freak":7c2f8 said:
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It needs to be plugged into a speaker cabinet while you're doing this.
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Fuck me - yeah that's absotively true!
Do not fire up the amp without an cab hooked up to it.
Thanks Marshall Freak.
:thumbsup:
 
Alrighty. Well I guess I will give it a try tomorrow after work, I'll go snatch a multimeter at radio shack on lunch break. I have a set of GT 6L6s in my other 5150, if I took those out to test the II out, would I set the bias in the same same range as you had it, or no?

-A-
 
Yeah man.
Setting it at 72-74 per side should be fine. Give the tubes that are in there a look before you change them out. I'd be interested in seeing what they were reading. Also, after you look at the original set, turn the bias down til it's running cold. Shut the amp down and wait for the tubes to cool off before you swith them out. Power the amp back up and wait for it to warm up again, then use the multimeter to set the bias.
This way if the bias pot did drift, you won't blow your new set of tubes while you're waiting for them to warm up.
Good luck man.
 
ans you should get a bias rite just to have, its easy to use and every amp whore should have one.
 
Alright, so I'm gonna go at it today, and I think I'm gonna give Jerry a call sometime today and talk to him and make sure I'm gonna be doing it right, but here are a couple of assumtions Ihave, and if you can confirm them, or tell me they are wrong, that would be cool.

Since there are 2 red bias points, I assume each one is the bias measure for a pair of tubes, correct?

Since each red point is for a pair, I assume one is for the outside, and one for the inside pair, correct?

A blown tube would make the bias read at half of what it should be reading, since each bias point is measuring the combined mAs of each pair of tubes, correct?

Any and all help is appreciated, I've got a show Weds, next week, and I wanna try to get this figured out before then and fixed so I can use it. Thanks again,

-Ans-
 
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guitarslingerans":9bec5 said:
Since there are 2 red bias points, I assume each one is the bias measure for a pair of tubes, correct?

Since each red point is for a pair, I assume one is for the outside, and one for the inside pair, correct?

A blown tube would make the bias read at half of what it should be reading, since each bias point is measuring the combined mAs of each pair of tubes, correct?
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Correct, the bias points measure for a pair of tubes.

I don't know which testing point is for which pair, but I think you're correct in that one is for the outside and one is for the inside pair.

Also not sure about how the blown tube would affect the reading as I've never had that happen to me before.

Definitely give Jerry a call. Shoot him an email to let him know you're gonna be calling, and leave a message if he's not there when you call. He's really good about call backs.

Hope that helps.
 
Give me a shout if you need any help with it. It's pretty easy.
Jerry
 
Cool beans, thanks again Jerry, I will let you know how everything works out.



-Ans-
 
Crisis averted!

Bad preamp tube. Phase Inverter, bad JJ tube was causing the problems. But damn dude, the bias moved! I checked it and it was at almost 79mAs!

Anyways, I cooled the bias down, replaced the phase inverter, and everything is dandy right now.

-Ans-
 
Sweet!!
I'm glad things are working out and it was as simple as a bad PI tube.
:D

Had me worried there for a sec!!
 
It is still weird how the bias went from where you set it at around 72-74mAs all the way to 79 though, isn't it? Or can that happen pretty easily?

-Ans-
 
That's normal, that's not really a big jump. It takes a small movement of the pot to change a pretty good amount. You're good to go.
Jerry
 
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JerryP":76c72 said:
:thumbsup: You can't kill a fuckin 5150!
Jerry
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Some can :gethim: :no: :aww:

BTW, Jerry, I've been meaning to email ya back but I've had a lot of stuff going on so nothing is really concrete just yet.
 
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4nkam":f90e5 said:
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Some can :gethim: :no: :aww:

BTW, Jerry, I've been meaning to email ya back but I've had a lot of stuff going on so nothing is really concrete just yet.
Click to expand...

No sweat, give me a shout when ever you're ready.
Jerry
 
IT SURE SUCKS WHEN YOU GET A NEW AMP AND UPS MESSES IT UP and then you start to get that sick feelin. but its all good when its easy to fix.
 
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