12AT7 Qs

[glassman]

Re: 12AT7 Qs

and a question for Glassman - is there an "ideal" idling current for a given tube, and if so, how it relates to biasing an amp for that tube, and for all the tubes in the power section. Thanks!

The short answer in no. The output bias variables include:

(1) The power supply capability and design
(2) The plate voltage
(3) The reactance of the output circuit
(4) The tube used
(5) The players style

Ranked in the approximate order of importance/impact.

All have to be considered when setting the output bias and the difference in setpoints can be very wide. Notice that the item I rank as #1 doesn't factor in to any bias formulas...primarily because many modern amplifiers have power supplies that exceed the requirements of the amp while some of the truely great vintage amps (Plexis, tweeds, etc) have "right" sized power supplies. Setting up an amp with a "right" sized power supply will yeild a "magical setting" somewhere in there while setting up an oversupplied amp is usually an exercize in comprimise with only a marginal benefit.

[PVFan]

Re: 12AT7 Qs

The short answer in no. The output bias variables include:

(1) The power supply capability and design
(2) The plate voltage
(3) The reactance of the output circuit
(4) The tube used
(5) The players style

Ranked in the approximate order of importance/impact.

All have to be considered when setting the output bias and the difference in setpoints can be very wide. Notice that the item I rank as #1 doesn't factor in to any bias formulas...primarily because many modern amplifiers have power supplies that exceed the requirements of the amp while some of the truely great vintage amps (Plexis, tweeds, etc) have "right" sized power supplies. Setting up an amp with a "right" sized power supply will yeild a "magical setting" somewhere in there while setting up an oversupplied amp is usually an exercize in comprimise with only a marginal benefit.

okay. take the player out of the equation.

what do most techs *essentially* mean when they say "bias your amp for the tubes in them"

and how is that important for the sound, a) when playing, b) when not playing

thanks

[glassman]

Re: 12AT7 Qs

okay. take the player out of the equation.

what do most techs *essentially* mean when they say "bias your amp for the tubes in them"

and how is that important for the sound, a) when playing, b) when not playing

thanks

Geez...that's a tough question! I personally can't see myself ever taking the player out of the question.

When I set up an amp for a music store that will be going up for sale I tend to set the amp up neutrally for that amp, not the type of tube...that's about as close as I come to setting an amp up without a specific player in mind. For instance, if I have a Fender Twin and a Peavey 5150, each to be set up neutrally, the Twin will be set up with a neutral to warm bias setting while the 5150 would be on the colder side of neutral. The Twin will need to sound warm with a rounded bass character while the 5150 will need to have a stiffer bass character.

If I consider player style, I may set the Twin a little cooler for a country player or a jazz player to make it more articulate. I may also set the 5150 a little warmer for bigger bass but I will need to use the owner's cab arrangement with the amp so that I can determine (1) what volume they play at and (2) if thier cab combination can handle the increased bass without "playing the cab".

[PVFan]

Re: 12AT7 Qs

thanks!