About pulling power tubes to lower power...

[korovamilkdud]

Does this work in class A operation, or is this modification exclusive to class AB amps?

Does this require re-biasing?

I do know this doubles the amp's impedence.

 

[aleclee]

Re: About pulling power tubes to lower power...

Operating class doesn't matter but cathode biased amps (e.g., Vox) need to be modded to safely pull power tubes.

 

[callous_frigid_chill]

Re: About pulling power tubes to lower power...

True class A amps only have one power tube, no?

 

[aleclee]

Re: About pulling power tubes to lower power...

True class A amps only have one power tube, no?

Though all single-ended amps are true Class A, there are amps such as the THD Bivalve that have >1 power tube and operate in Class A.

 

[korovamilkdud]

Re: About pulling power tubes to lower power...

Operating class doesn't matter but cathode biased amps (e.g., Vox) need to be modded to safely pull power tubes.

Errr...I don't have a clue how you would be able to tell...

 

[aleclee]

Re: About pulling power tubes to lower power...

Errr...I don't have a clue how you would be able to tell...

Most amps that are advertised as Class A are cathode biased . . . and do not actually run in Class A.

 

[drew_half_empty]

Re: About pulling power tubes to lower power...

whats the difference between class A & class B anyways?

 

[Quencho092]

Re: About pulling power tubes to lower power...

it's A and AB push pull.

I think it has something to do with the way that the tubes 'share the load' by pushing and pulling, as a result also having an idle current.

True class A doesnt 'push or pull', and it doesn't idle, it's always running steady. (correct me if i'm wrong this is all from foggy website reading memory...)

 

[Lake Placid Blues]

Re: About pulling power tubes to lower power...

In Class A the powerrrr tube(s) amplify the both sides of the signal, both the crest and the trough of the wave. In Class B one tube (or pair) amplifies the crest, or top half of the wave, and the other tube (or pair) amplifies the bottom half or trough of the wave. This is much more efficient from a power production perspective, but sounds more harsh, with a distortion occurring at the "cross over point" between the top and bottom 1/2 of the wave. In Class A/B each tube (or pair) amplifies a small portion of the other half of the wave. This creates a smoother more class A type tone. On a fixed bias amp, the hotter the bias is set the more each tube shares a portion of the other 1/2 of the signal wave. The colder the bias is set the less so, or more pure class B. IIRC, Class A/B I is less Class A, and more Class B, than Class A/BII when properly biased. Cathode Bias usually allows closer to class A operation by a Class A/B amp than fixed bias. Most Fender and all Marshall tube models are Class A/BII