Is there a "Biasing 101" or "Biasing for Dummies" reference?

[skhan007]

My task is simple: Swap out the stock EL34's for a pair of Winged C EL34's in my 1987x reissue Marshall plexi head.

I have a working knowledge of amps, having built my 18 watt from a kit.

My understanding is that I need to get a bias tool/bias meter (recommendations would be greatly appreciated), determine my idle plate voltage, apply the bias formula (25 Watts X 0.7/ voltage = milliamps I should adjust to) from http://metroamp.com/wiki/index.php/Bias_Formula

Am I missing anything here? I know where the bias trim pot is on my 1987x (it's labeled as such). Thanks in advance!!

[louderock]

Just finished biasing my Marshall with new power tubes as I type this. Here's what I use:

http://www.amp-head.com/

Works great.

[drgonzoguitar]

Here are a few good resources:

http://www.duncanamps.com/technical/lvbias.html

http://www.aikenamps.com/Biasing.html

[rockon1]

My task is simple: Swap out the stock EL34's for a pair of Winged C EL34's in my 1987x reissue Marshall plexi head.

I have a working knowledge of amps, having built my 18 watt from a kit.

My understanding is that I need to get a bias tool/bias meter (recommendations would be greatly appreciated), determine my idle plate voltage, apply the bias formula (25 Watts X 0.7/ voltage = milliamps I should adjust to) from http://metroamp.com/wiki/index.php/Bias_Formula

Am I missing anything here? I know where the bias trim pot is on my 1987x (it's labeled as such). Thanks in advance!!

Thats only if you want 70% dissapation of coarse. Its not any "golden rule" as lower might be better sounding and easier on the tubes. Heres a link to Eurotubes site . He actually has some informative,hands on video's to watch. Bob

http://www.eurotubes.com/euro-video.htm


.......

[Old Tele man]

...70% is an "upper" limit not a "set here" value.

...anything between 50% and 70% is OK, just adjust between those levels to suit your sonic preference.

[GenoBluzGtr]

...70% is an "upper" limit not a "set here" value.

...anything between 50% and 70% is OK, just adjust between those levels to suit your sonic preference.

Ditto... that range is a good one, but varies due to taste and which amp. I have found that in my Deluxe Reverb, closer to 50-55% seems to be best. In my VibroVerb (BF) 70% is the one, and for my Tweed Bassman, right on 65% dissipation from the 5881s is where it sounds best.

[skhan007]

Thats only if you want 70% dissapation of coarse. Its not any "golden rule" as lower might be better sounding and easier on the tubes. Heres a link to Eurotubes site . He actually has some informative,hands on video's to watch. Bob

http://www.eurotubes.com/euro-video.htm


.......

...70% is an "upper" limit not a "set here" value.

...anything between 50% and 70% is OK, just adjust between those levels to suit your sonic preference.

Ditto... that range is a good one, but varies due to taste and which amp. I have found that in my Deluxe Reverb, closer to 50-55% seems to be best. In my VibroVerb (BF) 70% is the one, and for my Tweed Bassman, right on 65% dissipation from the 5881s is where it sounds best.

Thanks guys. Those Eurotube videos were great!

Here's a quick question: When choosing a biasing tool, do I want to get one that will show me both plate voltage as well as idle current? I found a used Alessandro bias meter, but it will only tell me the idle current in mA.

I think I need to know my plate voltage to accurately bias...or can I simply solve for the unknown variable (plate voltage) by assuming 50-70% dissipation x 25 watts/ unknown = mA reading? (Realizing per the above statement from Geno allows for tailoring the setting to sonic preference).

Do I want a meter that will tell me more than mA (like amp-head, weber, etc.)? Or do I go old school and use a multimeter at pin 3 (which scares me)?

[rockon1]

Thats up to you. Sure would be nice to have a "compu-bias" or other bias tool that measures plate voltage as well as current draw but they are a heck of a lot more expensive than a 25$ bias probe.

As for calculating the bias you need to know the plate voltage to "accurately" set it. That said an approximation will get you close based on known PV's the the same amp. A difference of 20-30 volts doing a calculatiuon will only be a few ma's difference.

For instance
25watt/ 425pv x605= 35ma
25watt/ 450pv x60%= 33ma
25watt/ 470pv x60%= 32ma

Bob