Strange Bias Setting

[FourT6and2]

So, one of my fav. guitarists biases his Super Lead to 25ma. Assuming a plate voltage of about 480-500, that's only 48% plate dissipation, as apposed to usual 65-70% at around 35ma or so. That's pretty damn cold, isn't it? What does this accomplish in terms of tone and tube life? Or do you think it's 25ma per tube, not per pair?

 

[HeimBrent]

Biasing it cold will increase tubelife and decrease the dynamics of the amp, but what goes for the rest of your post... I have no idea.

 

[duesentrieb]

His plate voltage could be much higher . . .

 

[jet66]

I've got a couple amps that actually sound better with a slightly colder bias. I don't know if it is by design (specs at where you read them makes them read low when double-checked with a bias probe) or what, but they do sound better closer to 50% (and slightly less) than at 60-70%. Biasing 'in the middle,' they almost sound like they are biased really hot.

 

[Vrad]

What Olaf said....

I know on my Super Lead, the plate voltage is somewhere around 510 volts. It sounds better biased cold as opposed to hot. Hotter bias seems to sound messy.

 

[Bogged]

I have my Bogner Alchemist 112 biased at 50%, any hotter and I'll have heat issues. Matter of fact it sounds more open than at 60% where it sounded husky but sweet and get hot. I'm using SED winged Cs.

 

[FourT6and2]

Yeah, I guess the plate voltage in his amp could be much higher, over 500V. So, that would put it up between 50-60% dissipation.

 

[duesentrieb]

the more plate volts, the less diss. I use. Over 480V I usually go down to 55% or so.

 

[jasonP]

Yeah, I guess the plate voltage in his amp could be much higher, over 500V. So, that would put it up between 50-60% dissipation.

What are you running for plate voltage?

 

[Vrad]

the more plate volts, the less diss. I use. Over 480V I usually go down to 55% or so.

My PV has been holding steady at 498 for the last couple hours... Just biased it to about 55% or so. I like a colder bias I think. Sounds clearer.

 

[FourT6and2]

Yeah, I guess the plate voltage in his amp could be much higher, over 500V. So, that would put it up between 50-60% dissipation.

What are you running for plate voltage?

Without tubes, I'm running 500V. With power tubes and biased to 65-70% (about 35-36ma) I'm at 480V. I might drop it down to 28-31ma or so.

 

[lester]

I have Phillips 7581A tubes in my Bruno amp and Tony biased it at 36ma where it should be around 50ma. He siliconed it in that position, so it all comes down to taste as long as it's safe. And it sounds amazing. Amp whiz Bruce Zinky says(regarding his Mofo). "Best done by ear. Set the voltage at pin 5 of the power tubes to about -50 volts or so. Now, set the amp to a low volume clean tone. Play a clean chord, let it sustain, and adjust the bias voltage numerically higher than -51 Volts. The trick is to get the voltage as negative as possible before the sustain goes away and the sound suffers. Raising the voltage above -50 (numerically lower) will increase the power amp gain, but it will lower the maximum power and reduce tube life. Getting the voltage as negative as possible without reducing sustain will give you maximum power, headroom and dynamic response. Of course, if you never use the amp anywhere near full volume , you might like the numerically low "Hot" biasing sound at the volumes you play."

 

[FourT6and2]

I have Phillips 7581A tubes in my Bruno amp and Tony biased it at 36ma where it should be around 50ma. He siliconed it in that position, so it all comes down to taste as long as it's safe. And it sounds amazing. Amp whiz Bruce Zinky says(regarding his Mofo). "Best done by ear. Set the voltage at pin 5 of the power tubes to about -50 volts or so. Now, set the amp to a low volume clean tone. Play a clean chord, let it sustain, and adjust the bias voltage numerically higher than -51 Volts. The trick is to get the voltage as negative as possible before the sustain goes away and the sound suffers. Raising the voltage above -50 (numerically lower) will increase the power amp gain, but it will lower the maximum power and reduce tube life. Getting the voltage as negative as possible without reducing sustain will give you maximum power, headroom and dynamic response. Of course, if you never use the amp anywhere near full volume , you might like the numerically low "Hot" biasing sound at the volumes you play."

Why would someone use silicon to lock an amp's bias in place? A new set of tubes will never bias up to the exact same setting as your previous set. All that does is make things worse... What if you put in a new set of tubes that draw more or less current? Then you're fucked.

The other stuff makes sense, though. About biasing an amp cold. Though, I've never played through the amp while, at the same time, attempting to bias it. I don't have enough hands... And I don't like the idea of 500V pulsing through me.

 

[lester]

Bruno's a trip. It's kind of like a suggestion I guess, but it's silicon, just take it off if you don't like it. Have you checked out those Phillips 7581's? At that bias setting, I think it'll last for a bit.