How does pulling tubes affect bias?

[astrozombie]

My amp has 4 tubes. it runs at 40w.

i can pull two and run it at roughly 20w.

but how does this change the amps bias? do I need to bias it with 2 tubes?

 

[crusty philtrum]

Re: How does pulling tubes affect bias?

Cathode bias ?

 

[Funkfingers]

Re: How does pulling tubes affect bias?

Glenn. Are you discussing a Class A VOX AC30 or a Class A/B Marshall?

 

[astrozombie]

Re: How does pulling tubes affect bias?

my marshall dsl401.

sorry for the delay.

this is not a "self biasing" amp even though it uses el84s.

thank you

 

[Chickenwings]

Re: How does pulling tubes affect bias?

it will also affect the impedance. I think it doubles it...but you might want an amp guru to chime in on that one.

 

[astrozombie]

Re: How does pulling tubes affect bias?

it will also affect the impedance. I think it doubles it...but you might want an amp guru to chime in on that one.

I was told to run the 16ohm speaker at 8ohm for this operation

 

[Lake Placid Blues]

Re: How does pulling tubes affect bias?

It may affect the plate voltage which will cause a slight change to the bias.

 

[dystrust]

Re: How does pulling tubes affect bias?

It may affect the plate voltage which will cause a slight change to the bias.

LPB is right that it may cause a change in plate voltage and in turn a small bias drift, but I doubt it'd be noticeable.

I was told to run the 16ohm speaker at 8ohm for this operation

You're also correct about needing to change the output impedance setting.

 

[Chickenwings]

Re: How does pulling tubes affect bias?

I dunno man. If you want to reduce your volume you are safer to let the amp do what it is designed to do, but run it through a less efficient speaker.
What speaker are you running right now?

 

[SlyFoxx]

Re: How does pulling tubes affect bias?

40 watts to 20 is a 3dB reduction. A tiny difference ...not worth the effort and since you are asking these questions outside your area of expertise as well. Leave your amp be.

 

[astrozombie]

Re: How does pulling tubes affect bias?

It's about headroom and being able to run the power stage harder. Not volume.

Did I mention volume? ;(

 

[6p5p]

Re: How does pulling tubes affect bias?

It's about headroom and being able to run the power stage harder. Not volume.

Did I mention volume? ;(

And what about an attenuator between the amp and the cab?
EDIT: ok, forget it, I see it is a combo

 

[Kamanda~SD]

Re: How does pulling tubes affect bias?

And what about an attenuator between the amp and the cab?
EDIT: ok, forget it, I see it is a combo

You can use an attenuator with a 401.

 

[astrozombie]

Re: How does pulling tubes affect bias?

You can use an attenuator with a 401.

between the spearer out and the speaker right?

 

[Kamanda~SD]

Re: How does pulling tubes affect bias?

Correct