# new Bassman LTD bias question..



## GTmaker (Apr 24, 2006)

Attention all you amp bias masters....

I just got a new bassman LTD and just want to check the bias to make sure everything is as it should be.

I have a new bassman LTD and I have a volt/ohm meter...now what?

I've never done this before so I do need specific instructions. As I am just checking to see if everything is OK, I would prefer the simple check as opposed to the complicated one if such a choice exists.

thanks in advance.


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## Xanadu (Feb 3, 2006)

eww..a bassman


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## GTmaker (Apr 24, 2006)

Xanadu said:


> eww..a bassman


is it the amp or my question that seems to have you upset?


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## Xanadu (Feb 3, 2006)

GTmaker said:


> is it the amp or my question that seems to have you upset?


oh it's the amp, I've played a couple and I don't like them as much as my yorkie. Plus I have two friends that play bassman, and they're always in the shop for something or other. (they being the amps, not my friends)


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## Robert Bogdan (May 25, 2006)

The 4x10 Tweed Bassman is the best all around amp ever made IM not so HO.


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## Wild Bill (May 3, 2006)

*Don't run with scissors!*



GTmaker said:


> Attention all you amp bias masters....
> 
> I just got a new bassman LTD and just want to check the bias to make sure everything is as it should be.
> 
> ...


The short answer is...there's no simple test!

What's more, the voltages around where you have to test can hurt like blazes and in extreme cases kill!

This is the reason why many American websites will not tell newbies how to do this. Their's is a country where you can sue anybody for anything, after all. McDonald's hot coffee, anyone?

There are a number of good techie sites, like 

http://www.aikenamps.com

or R G Keen's excellent site at geofex that describe biasing. What you need to know is that most amps do NOT provide a simple and safe test point where you just put your meter on two points and crank an adjustment to a specific number!

If your amp doesn't have test points installed you can put some in. Or you can measure the resistance of the output transformer windings with no power on, then measure the voltage drop across each winding with the amp powered up, using alligator-style clip leads so you don't have your fingers near the clip points, where the voltage will likely be between 400-500 volts or more. Ohm's Law will then let you calculate the idle current flowing in the windings.

What is the right idle current? Different for every amp, including two amps of the same model. You have to refer to the tube data sheets for the plate dissipation rating, divide it by the plate voltage (that's where even two amps of the same model can vary a bit) and then take 60%-70% of that answer as the preferred and safe maximum.

Even if you have test points they don't always directly read the idle current. One common way is to break the cathode connections on the output tubes and install a 1 ohm 1 watt resistor. If you read 35 mv across a 1 ohm resistor Ohm's Law says that represents .035 amps of cathode current, or 35 ma. This method also includes a couple of ma.'s of screen current but for practical purposes you can ignore that in a guitar amp. It will put you only slightly off, on the safer side for the tube. Close enough for rock and roll! 

If you know about all this stuff it's no big deal. If you've never poked inside a tube amp before then I've probably lost you so far. It's like doing your own car tuneup, except that you can't see any moving parts like fan blades that can hurt you. All you have to do is just TOUCH the wrong one!

This is why most non-techies just let someone experienced bias their amp. If you want to get experienced you might want to check out techie sites like Ampage and AX84 that are great for helping out newbies. Building amps is a fun hobby!

But if you're looking to know how to just do a quick bias adjustment, as I said it's just not that simple. 

Even if you measure things without shocking yourself you best be absolutely sure of your math when calculating the idle current for the bias adjustment setting. I've seen tubes where the bias pot was cranked SO wrong the tubes melted inside!

Not trying to scare you, just trying to give you the straight dope. If yours was just a casual question I'd say let some tech do it. $40 or so won't kill you but high voltage will. And a burnt pair of 6L6's or EL34's costs about the same. 

But if you want to become a techie - welcome, brother! Let's build an amp and blow one of those lameass dual rectifiers off the stage!


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## GTmaker (Apr 24, 2006)

looks like theres a bias test point labled on the board of the bassman LTD.
when I get a chance, I'll test to see if I get a reading anywhere between 65 and 70 mV. If not in that range, I'll turn the little knobby of the Bias Adjust screw and make it so.
thanks.


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