Bias vs non bias

[andrew]

What are the pros and cons of having a power section using a bias control and those that don't (Rocket)? Also how do Cut differ from Presence and is it pointless to use both of them on one amp? Thanks!

[Bob-I]

What are the pros and cons of having a power section using a bias control and those that don't (Rocket)?

An amp with a bias control is called "Fixed bias". I know that sounds wrong but it means that the bias is "fixed" by setting the adjustment and it doesn't change during normal amp operation. Amps without a bias adjustment are typically "cathode" or "self" biased. This means that the bias is set by a resistor on the cathode of the tube and can change depending on how hard you drive the amp.

Typically fixed bias amps have more punch and are more dynamic where cathode biased amps have more "sag" and compression when pushed hard.

Also how do Cut differ from Presence and is it pointless to use both of them on one amp? Thanks!

I'm not sure what you mean by "cut". Presence allows more highs to be amplified by the power section.

[mlp-mx6]

Cut controls REDUCE highs by cross-canceling them from the phase inverter outputs (power tube grid inputs). Look at a schematic for a Vox AC-30 and you'll see the CUT control wired as a series cap and variable resistor (pot) across the two sides of PI output.

Presence controls work by reducing the negative feedback above a certain frequency. The freqs below the cutoff are still attenuated by the negative feedback, but the higher ones are not.

[andrew]

Thanks!!!

[David Root]

Fixed bias also comes in a "variable fixed bias" form. All this is is a fixed bias circuit with a pot partially replacing a fixed resistance.

The best arrangement for flexibility and adjustment throughout the power tubes' life is a separate bias pot for each power tube, with a pair of one ohm resistors in the power tubes cathodes to ground, and three meter tip jacks so you can measure the actual cathode currents individually (by measuring DC mV as a proxy thru the one ohm resistors).
Sounds complex but not really. It's in the TUT books.

[andrew]

Thanks for the help! The variable fixed bias appears to be what is on the AX84 20 watt SE power amp: off of pin 8 on a 6v6: 1ohm/1w into a 1k/2w pot into a 100ohm/5watt to ground. My new build has an AX84 20wattPP with a cathode bias( the cathode bias is not in the AX84 design) Do you think the same values in the SE would apply to each 6v6 tube in a PP? Thanks for all your time.Maybe one day I can offer some answers.