Pictorial of Biasing a Marshall Jubilee.

[J Moose]

Re: Pictorial of Biasing a Marshall Jubilee.

Gnarly.

How do you set the volt meter up? Auto-ranging? DC current?

And where can I find one of those slick little bias tools? Is that what it's called? search Fleabay for 'bias tool'?

Thanks!

[skyydogg01]

Re: Pictorial of Biasing a Marshall Jubilee.

very informative, brother.

yes. this must be in the vault.

[StrangeSound]

Re: Pictorial of Biasing a Marshall Jubilee.

That's so vault worthy..

[JammerMatt]

Re: Pictorial of Biasing a Marshall Jubilee.

You wanna know why I put a towel on the cab? A few years ago, I was biasing a head like this on an angle. ....Tube amps throw out some serious amperage!

That story is METAL! :

[Gearjoneser]

Re: Pictorial of Biasing a Marshall Jubilee.

Those vertical trimpots always make me a bit nervous when I'm working on a Marshall. Kinda nerve-racking working on an amp like that when its on .

You wanna know why I put a towel on the cab? A few years ago, I was biasing a head like this on an angle. Stupidly, I used a metal handled screwdriver, and accidentally touched something, while my other hand was holding the chassis up. I got a zillion amps running through me, and was stuck to the chassis for a few seconds. Naturally, I let go of the chassis, and it slammed down on the cab, and I fell backwards!

I was afraid the tubes might have been damaged, but everything was fine. However, there was a few marks on the top of the cab, where the head slammed down on it. It scared the crap outta me, and coulda killed me.
I threw that metal handled screwdriver in the trash, and went and bought a set of plastic handled ones at the 99 cent store.

So, let this be a lesson to you guys wanting to bias your own amps. You've got to be VERY careful with a live amp, and poking a screwdriver in there.
You could literally die if you make a stupid mistake. Tube amps throw out some serious amperage!

[krankguitarist]

Re: Pictorial of Biasing a Marshall Jubilee.

Those vertical trimpots always make me a bit nervous when I'm working on a Marshall. Kinda nerve-racking working on an amp like that when its on .

[Luke Duke]

Re: Pictorial of Biasing a Marshall Jubilee.

Nice job Joe! So slightly OT I was wondering if you change out your cabs on your Jubilee. you know slant for 3 months, straight for 3 months. Something like that.

Luke

[B2D]

Re: Pictorial of Biasing a Marshall Jubilee.

Vault. Now.

[Gearjoneser]

Tonight, after playing my Jubilee, which has had the same JJ E-34L's and Tungsol pre's for a couple years, I decided to do a tuneup on it. I have a bias tool I bought off Ebay for around $15, my volt/amp/ohm meter, and some contact cleaner. After pulling the chassis out, I sprayed a little bit of contact cleaner in the pots and pushed a cable in and out of each jack, so they're good as new. Next, I put the bias tool in between a power tube and the amp, turned on my meter, and noticed that the bias was on the cold side after a couple years of using it lightly.....it read 29mA. EL-34's and E-34L's usually sound best between 35 - 42mA.

My method is to have the amp chassis plugged up with the cab and guitar, and hit some chords, while adjusting the bias pot. I hold the chassis up on an angle with my meter laying on a transformer. I'm EXTREMELY careful to not electrocute myself by using a plastic handled screwdriver, and I protect the cab by laying a towel on it. I always bias by ear, trying to get the amp to sound as punchy and strong as possible, but make sure the numbers fall between 35-42mA. After a half hour of making adjustments and hitting chords on the guitar, I decided that it sounded best at approximately 40mA.
These aren't new tubes, so it was just an interim bias/checkup. It sounded best between 38 and 40, so I went a little hot, and loved the sound at 40mA.
I checked it on both channels and both tubes, just to make sure it sounded healthy as ever. Here's some pics!

Set to DC Amps

It reads a little low at 28-29mA. 6L6's are usually 32mA, but EL-34's should be around 38mA

I turn the chassis up on it's side, while turned on, plugged into a cab, with guitar plugged into the front.


Find the bias pot, but be careful not to touch ANYTHING. Use a plastic handled screwdriver. I hit the guitar to make it Kerrrang!...then tweak the pot so the amp is sounding bold as can be. Stay in range though.





Ah, the British tone just kills. 97 Matchless Chieftain, and 87 2550 Jubilee!!!