Idiots guide to biasing an amp

[fenderiarhs]

I have a Marshall JCM2000 TSL601 combo and i am going to change the power amp tubes and i would like to bias it myself......I live in an island so it is quite difficult to carry the amp and travel 300km to the marshall service.....The Marshall tsl has on the back pannel three pins and a trim pot from which it is supposed to be biased.Here is how it looks from the rear pannel of the amp:

Considering i know how to use a multimeter could someone give me an idiots guide on how should i proceed to bias the amp????

(Here's what i have found on the marshall forum regarding biasing butit is not very clear for my modest knowledge of english...... http://marshallampforum.com/forum/viewtopic.php?t=17 )

 

[MDS]

Re: Idiots guide to biasing an amp

That description looks pretty clear...what parts don't make sense? It looks like your amp only has one pot, so I'd imagine you just connect negative to the center pin, and positive to the pin closest to the pot. Turn the pot until the multimeter says 80mV and you're done...

Let me just add that as a recent owner of a JCM2000, this bias setup rules...beats the crap out of what I have to do to bias my peavey (I converted it to adjustable bias)....

 

[Gearjoneser]

Re: Idiots guide to biasing an amp

I think there's two bias pots on the 100W amps and one bias pot on 50W......I'm just talking about the modern Marshalls with the test points on the back panel.

I think the reason there's a Positive/Negative/Postive is so you can test both tubes, or both duo's of tubes if it's got 4 power tubes. (inner 2 and outter 2).

I usually go by the numbers that are suggested ratings, but I also turn the amp up and play it while turning the pot in tiny increments. I set it so the amp sounds as healthy as possible, while still being around those numbers.

Things to remember when biasing....ALWAYS use a miniscrewdriver with a plastic handle. If the numbers start jumping all over the place and not settling, turn the amp off for half an hour, and resume later. Sometimes when making small changes on the bias pot you destabilize the circuit, making it hard to get a stable reading.

 

[MDS]

Re: Idiots guide to biasing an amp

I've got two pots on my DSL-50, go figure!

 

[Erlend_G]

Re: Idiots guide to biasing an amp

Just smash the amp with an hammer until it stops making funny noises and the snow on the screen dissapears.

Hey, it worked on my 1980' Grundig TV.

 

[Gearjoneser]

Re: Idiots guide to biasing an amp

I've got two pots on my DSL-50, go figure!

Yeah, I thought there was two pots on all the DSL's, so I was surprised to see only one on that TSL. I think there's two so you can balance them perfectly.

In any case, the only things you don't want to do are bias it too cold or so hot that the plates start turning red. I like to bias amps in a dimly lit room, so I can see exactly what the tubes look like as I'm making adjustments.

 

[Erlend_G]

Re: Idiots guide to biasing an amp

"Eh.... a bee's arse?"

 

[MDS]

Re: Idiots guide to biasing an amp

Yeah, I thought there was two pots on all the DSL's, so I was surprised to see only one on that TSL. I think there's two so you can balance them perfectly.

In any case, the only things you don't want to do are bias it too cold or so hot that the plates start turning red. I like to bias amps in a dimly lit room, so I can see exactly what the tubes look like as I'm making adjustments.

What color do you shoot for? I've messed a lot with biasing on my peavey after I modded it for the adjustable bias, and I got it where I liked how it sounds, but still don't totally know how far is too much...I've wanted to mod my Mesa for adjustable bias for years, but I've never gotten around to it...my Mesa is so nice that I don't want to futz with it too much too, ya know? Kudos to Marshall for the easy adjustment!

Mike

 

[Gearjoneser]

Re: Idiots guide to biasing an amp

What color do you shoot for?

The main thing I'm looking at is the gray plates inside the tube. If I turn the pot and see the gray turning orange AT ALL......I back the pot down and shut the amp off immediately, so I don't ruin the tubes. I like dim light so I can see the gray plates clearly. The normal look of the tubes should just be an even orange glow amongst all of them. If they're biased cold, the amp is grainy sounding and weak. If biased too hot, they glow too bright, the amp sounds pushed/distorted, the gray plates are turning red, and the surrounding area is getting overly hot.

Not surprising, but most amps bias somewhere in the middle of the pot's travel, so it's not rocket science. It's just getting the amp to sound nice and healthy without being weak or pushed.

 

[Erlend_G]

Re: Idiots guide to biasing an amp

I prefer my tubes to glow a brigth blue\green.

 

[MDS]

Re: Idiots guide to biasing an amp

I rebiased my DSL last night. It was like 41/43, so it wasn't too far out of spec, but I've got it at 45/45 now. I'm not sure how much of a difference in tone it made, but what a cool biasing system!

Mike