Pictorial of Biasing a Marshall Jubilee.

[Simon_F]

Re: Pictorial of Biasing a Marshall Jubilee.

Vaulted

Thanks Joe!

[Gearjoneser]

Re: Pictorial of Biasing a Marshall Jubilee.

Why do you prefer the mA version? The mV ones are the ones with a 1-Ohm shunt resistor installed, right? I'm just wondering why one would be better than the other.

I like the mA version because I like to know how hard the tube is getting hit with current. The Voltage Bias Tool is just fine, but by knowing the amperage (mA), I can tell if those plates are going to heat up too much or not.

Before I put the chassis back in the cabinet, I like to use it for the evening and observe the glow of the tubes and listen carefully to the tone. Once I know it's sounding better than ever and the plates aren't turning orange at all, then I put the amp back together, and it's good to go for a year or two.

This Jubilee is not my main amp. I just play it regularly at home and don't put too many hours on it, so those JJ E-34L's should last 5 years.....maybe more.

[bbking1021]

Re: Pictorial of Biasing a Marshall Jubilee.

What are these units called? I'd like to check out if there are any on ebay.de

I don't have to buy a multimeter and a bias probe then!

Look under "tube bias" or "amp bias" and you should find them. BUT you still need a multimeter for it to plug into.

[StrangeSound]

Re: Pictorial of Biasing a Marshall Jubilee.

This is definitely headed to the vault, once we've finished it.

Joe...

Why do you prefer the mA version? The mV ones are the ones with a 1-Ohm shunt resistor installed, right? I'm just wondering why one would be better than the other.

What are these units called? I'd like to check out if there are any on ebay.de

I don't have to buy a multimeter and a bias probe then!

[Simon_F]

Re: Pictorial of Biasing a Marshall Jubilee.

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.

This is definitely headed to the vault, once we've finished it.

Joe...

Why do you prefer the mA version? The mV ones are the ones with a 1-Ohm shunt resistor installed, right? I'm just wondering why one would be better than the other.

[Imp]

Re: Pictorial of Biasing a Marshall Jubilee.

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.

know what it should be for EL84s dude? extra bonus points, thanks for the cool photos

tom

[StrangeSound]

Re: Pictorial of Biasing a Marshall Jubilee.

This thread must be banned...in the vault

[bbking1021]

Re: Pictorial of Biasing a Marshall Jubilee.

bump for vault

[casblah]

Re: Pictorial of Biasing a Marshall Jubilee.

As always, great info GJ!

[Vasshu the humanoid typhoon]

Re: Pictorial of Biasing a Marshall Jubilee.

Cool post Joe
And one very nice amp I might add!
Hehe have been zapped a few times as well....me not like high voltage very much...lol

[StrangeSound]

Re: Pictorial of Biasing a Marshall Jubilee.

I stopped biasing my amps after being shocked once.... ONCE

My left arm felt like it was zapped by lighting and was a dull numb appurtance to my body for about one hour.


HURTS!

You did touch something on the amp..or did you use a non-plastic screwdriver?

[Stratman]

Re: Pictorial of Biasing a Marshall Jubilee.

I stopped biasing my amps after being shocked once.... ONCE

My left arm felt like it was zapped by lighting and was a dull numb appurtance to my body for about one hour.


HURTS!

[skyydogg01]

Re: Pictorial of Biasing a Marshall Jubilee.

As far as I know, the Blues Jr. (which should be there soon) is fixed bias. Don't hold me to that, but I believe it is.

But this is still all greek to me, and all but one of my amps have been tubers.

yeah, it's fixed bias. Just more info that i'd like to know if I decide to do the "variable bias" mod.

[Sludgenutz]

Re: Pictorial of Biasing a Marshall Jubilee.

Vault Worthy! Great info!

I would like to add a measure of safety by suggesting fiberglass screwdrivers be used in "live" chassis. A metal tool shaft can fry an amp or cause injury as was already noted!

"Fiberglass screwdrivers" are the safest way to turn the trim pots. They are really inexpensive too! Many use the smallest amount of metal in the just the tip.

http://www.tecratools.com/pages/datacom/alignment.html

[BigAlTheBird]

Re: Pictorial of Biasing a Marshall Jubilee.

shameless bump.

this thing NEEDS to be in the vault.

someone like me, who is just getting into tube amps, this thread is very valuable.

Thanks Jonesy

As far as I know, the Blues Jr. (which should be there soon) is fixed bias. Don't hold me to that, but I believe it is.

But this is still all greek to me, and all but one of my amps have been tubers.