Pictorial of Biasing a Marshall Jubilee.

[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!

[Fresh_Start]

Re: Pictorial of Biasing a Marshall Jubilee.

Hoffman Amps sells the parts and has instructions for making your own bias current checker.

Here's an interesting and off-the-beaten-path explanation of biasing an amp from the Tone Lizard. Don't have an oscilloscope yet though...

Chip

[SonofaMitch]

Re: Pictorial of Biasing a Marshall Jubilee.

+1 to all, Very Informative, Deffinatly Vault Worthy

~Mitch~

[skyydogg01]

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

[brit84]

Re: Pictorial of Biasing a Marshall Jubilee.

do you just use a single bias probe on one tube or do you test on both? Im new to this but am keeping up so I can do it myself.

Thanks

[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.

[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

[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.

[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!

[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?

[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