Pictorial of Biasing a Marshall Jubilee.

[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!!!

 

[B2D]

Re: Pictorial of Biasing a Marshall Jubilee.

Vault. Now.

 

[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

 

[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 .

 

[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? :27: 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! :yell: :fingersx:

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. :fingersx: :laugh2:

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!

 

[JammerMatt]

Re: Pictorial of Biasing a Marshall Jubilee.

You wanna know why I put a towel on the cab? :27: 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! :firedevil:

 

[StrangeSound]

Re: Pictorial of Biasing a Marshall Jubilee.

That's so vault worthy..:bowdown:

 

[skyydogg01]

Re: Pictorial of Biasing a Marshall Jubilee.

very informative, brother.

yes. this must be in the vault.

 

[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!

 

[bbking1021]

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!

Look on ebay for the bias tools. You can look under "tube bias" "amp bias". They usually can go for $15-50 depending on features.

+1 for vault

 

[dgk2448]

Re: Pictorial of Biasing a Marshall Jubilee.

i want one of those SOOOO bad. awesome thread GJ.

 

[joelap]

Re: Pictorial of Biasing a Marshall Jubilee.

nice thread, vault-worthy.

 

[Gearjoneser]

Re: Pictorial of Biasing a Marshall Jubilee.

I came home tonight, and turned the amp on. I biased it last night around 11, so I couldn't turn it up too loud. Now that I've gotten to hear it at a decent volume level, I can tell that it sounds thicker and richer, and the gain has tightened up. I figure 40mA isn't too hard on E-34L's, which are 10% higher output than regular EL-34's, which I probably would have left at 38mA.

Bias tools come in the mA or voltage versions. I like the mA version better, so if you shop for one, buy the mA tester. I think I paid about $15 for it.
I think my volt/ohm/amp meter was around $50, but I've had that since I was 18, going to electronic tech school. Every guitarist with tube amps should have those two things. There's no reason to pay $40 for a bias, when a person with basic electronics skills can handle that task.

I can't say it enough though....BE EXTREMELY CAREFUL when poking around inside a live tube amp. Those amperages are no joke...they're very lethal.
The most important thing is to have plastic handled tools, and some people even wear gardening gloves to insulate themselves from being grounded accidentally.

 

[Christobevii3]

Re: Pictorial of Biasing a Marshall Jubilee.

What do you do if an amp doesn't have a bias pot?

 

[Farkus]

Re: Pictorial of Biasing a Marshall Jubilee.

Bob over at Eurotubes is making bias probes for around $25.00, which is cool if you already have a meter. If you don't he is selling cheap multimeters for $10.00 as well. Pretty sweet deal as you know the stuff will work when it gets to you (and if it doesn't he'll make it right). I'm not an employee or anything, but I think it's cool that he's offering a (relatively) low cost solution for the average player.

Nice post Gearjoneser.

 

[Gearjoneser]

Re: Pictorial of Biasing a Marshall Jubilee.

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

Set it to DC Amps. The Auto Range means that the meter will change the metric range automatically. On cheaper meters, if the correct range isn't selected and the amperage exceeds that range, the fuse will blow.

 

[BigAlTheBird]

Re: Pictorial of Biasing a Marshall Jubilee.

I'd be too scared of ****ing up and dying, but that is a great post Joe! Definitely vault-worthy.

 

[J Moose]

Re: Pictorial of Biasing a Marshall Jubilee.

What do you do if an amp doesn't have a bias pot?

Nuttin'.


Really...if the amp doesn't have a bias pot then it's a 'fixed' or otherwise self-adjusting bias. At that point, changing a tube is no different then changing a lightbulb.



Joe ~ What kinda volume were you running that thing at while biasing? I always thought tech's did that with no signal running through, or at least having the amp on standby...but that wouldn't apply full current to the plates would it?

 

[Gearjoneser]

Re: Pictorial of Biasing a Marshall Jubilee.

Joe ~ What kinda volume were you running that thing at while biasing? I always thought tech's did that with no signal running through, or at least having the amp on standby...but that wouldn't apply full current to the plates would it?

The amp has to be running to get a reading on the amp meter, but the volume level doesn't seem to change it very much. Ideally, it's nice to have it running at stage volume, when biasing by ear, so you can hear the tightness of the low end thump, which is what I aim for. It's hard to have the guitar strapped on while doing this, so I usually just lean it right next to where I'm working, and pluck the A/D or D/G, and listen while I'm tweaking the pot.

Here's one thing to know. You never want to turn the bias pot drastically, because it can destabilize the circuit, and you'll get erradic readings on the meter. If the numbers start jumping around, you're better off turning the amp off for 15 minutes and letting the tubes cool, then starting again later.
Another technique is setting it at 35, play and listen, 36, play and listen, 37, play and listen etc. The main point of biasing is to get the amp sounding as healthy as possible, and sometimes it's hard to tell when you're just fiddling with the bias pot and whacking the strings on the guitar.

The most important thing, besides NOT electrocuting yourself, is NOT overheating the tubes. If you ever go past the desired numbers, you could fry your tubes. You'll know you're doing the wrong thing if the plates start turning orange. This is why it's not a good idea to do this without a meter on it. At least you know if you're in the ballpark then.

 

[Lespaul1981]

Re: Pictorial of Biasing a Marshall Jubilee.

great thread!