NOS tubes

[Obsessive Compulsive]

I'm not affiliated, but here is a relatively quick explanation. Don't skip ahead. Cathode biased is the common, "change tubes without having to bias."

http://carlscustomamps.com/cathode-vs-fixed-bias

Thanks for the link. I think mine is a cathode biasing, just a simple switch selecting 6L6 or EL34.

[jeremy]

i dont think its the hp printer model where the printers are cheap the they kill you on ink, i think its more they want their amps to be consistent so by setting the amp a certain way and selling tubes in a certain range that suit those settings, they are controlling the consistency of the sound their amps produce. mesa tubes arent crazy expensive compared to other rated and matched tubes.

most big amps, 50w and up, are fixed biased. some smaller amps are cathode biased but not as many as ya think

[dave74]

I really like the mesa system. It allows you to mix and match different models of the same tube type within the same amp, for example in my 90's 2 channel rectifier I've settled on a pair of 440 in the inner slots and 420 for the outers. For my tremoverb I've combined 430 and 420.

[Ewizard]

The term fixed bias is misleading. Its description is meant to describe how the bias voltage is applied to the grids of the tubes. It is fixed because the bias voltage supply does not scale, move/shift, or change based on the current flow through the tube. The grid's bias is set for the ideal idle bias operation. As the tubes work harder and harder this bias voltage actually shifts more positive ( simply as a result of power-supply sag, not intentionally ) causing the tubes to bias hotter and hotter the harder they are run. When you play the amp at full volume, the voltage sag in the power supply causes the bias voltage to shift up ( less negative ) in relation to ground, making the tube bias hotter. This is why 60% - 70% of dissipation is the ideal range for fixed bias amps; any hotter and the tube could bias above 100% when run hard. Under nominal or median ranges of operation, this bias shift is minimal or not worthy of concern.

Cathode biased is not necessarily able to have " any " tube placed in it and function properly. Cathode-biased amps work by raising the power tube's cathodes above ground potential by a small amount. The power tubes grid is also referenced to ground by a slightly larger resistor. This is converse to what you think that means. Because the cathode is now held above ground potential and the grid is also held above ground potential, a point of equilibrium is met with regard to current flow through the cathode to the plate. The Plates conduction is controlled by the grid which as far as the plate is concerned is negative in relation to ground from the cathode. The cathode current flow is controlled by the cathode resistor and is what sets is ground potential difference between the cathode and the grid. The grid's relation to ground is fixed, so it does not change its relation to the plate. The cathode resistor is what sets the relative difference. A larger cathode resistor reduces the difference in ground potential making the cathode seem closer to the same ground potential as the grid relative to the plate, shutting the tube off MORE ( the grid becomes more negative in relation to the cathode to ground ). With a smaller cathode resistor, the cathode becomes closer to ground potential making the difference in ground potential to the grid larger, shutting the tube off LESS ( the grid becomes less negative in relation to the cathode to ground ). The cathode bias resistor sets the difference between that particular tube and the grid to ground. Not every tube is the same and it is very possible to have one tube set to be biased perfectly, but another set can be too hot, or potentially too cold.

Cathode-biased amps are generally set to have the bias at between 85% - 95% of dissipation at idle ( the amp is simply on with no signal ). The closer you bias a set of tubes to 100%, the higher the likelihood that the next set you install will bias above 100%. Conversely, the colder you bias, the higher the likelihood that the next set can bias too cold. The range of bias is more forgiving in cathode-biased amps because the way they work, they self-regulate quite well. Most techs will shoot for 90% - 95% of dissipation as the ideal range, although some will shoot for 100% if the client desires. As a cathode biased amp is run harder ( full volume ), the voltage sag in the power supply will actually cause the bias to shift colder ( more negative, shifting dissipation from perhaps 90% down to 70% or lower ). Cathode biased amps bode well with being run at full volume because the louder they are, the more conservatively the bias will shift. This is why you can bias as high as 95% or more of dissipation, as you operate the amp, it will bias colder. Biasing a cathode biased amp too cold will actually cause it to bias MUCH too cold at higher volume settings because the bias will shift downward.

The bias of a tube is a parameter set on a per tube basis. No two sets of tubes are identical, but can be very close, or close enough. ANY time you change the tubes, you should double check the bias of them regardless of fixed, or cathode bias operation. Cathode-biased tubes amps have a larger range of tolerance to any given set of tubes, but can still be biased too cold, or too hot for any given set. Fixed bias is less forgiving, and requires a little more precision in the setting of bias to ensure proper operation at higher volume settings. Each topology has a pro and a con over the other. I really like cathode-biased amps and think they offer something that meets the demands of most musicians, unbeknownst to them. Fixed bias designs are obviously much more suited to other musical styles and musician preferences, also for reasons not so obvious to them. There is no better topology, only one that is more suited to the musician and their musical needs. My desire lands more on lower wattage, cathode-biased designs.

I think the reason you don't see many, if any, modern cathode biased amp designs above 50 watts, is that the wattage at that point, no longer allows the things that make cathode biased amps what they are become easily apparent. At about 50 watts, they just turn voltage into heat more than anything. I.E. what people love about cathode-biased amp designs is lost at around 50 watts of power. Fixed bias is not absent from any issues, but it does make more power and is considerably more able to control bias shift, making an amp much more stable at any given volume setting.

I'm officially drunk now and will have to come back to this later, but this should hit many people's wonders.

[beaubrummels]

I'm officially drunk now and will have to come back to this later, but this should hit many people's wonders.

Possibly the most informative drunk post this forum has ever seen.

(Though it still doesn't get at confirming what Mesa is doing. ;-) )

[dave74]

Mesa's color system does allow you to closely replicate whatever bias-point you prefer within their range (determined by the fixed-bias resistor system as Ewizard detailed).

The range of allowed draw from the tubes is essentially subdivided into 6 more precise ratings. Red and then yellow are the coldest (yes it is counter-intuitive),,,,,,,and white and grey are the warmest, or maybe slightly less chilly we should say. Blue and green are in the middle of their tolerance range.

I don't know the numbers, but from what I remember the el34 setting runs quite a bit warmer than the 6L6. They suggest in some of the manuals to use tube-rectification if running el34 so it doesn't run so hot. Perhaps this recommendation was just to cover their a** on the warranty.


edit: I should add that my series2 single rectifier has no tube rectifier and runs the str-447 (eh el34) just fine.

[NegativeEase]

The term fixed bias is misleading. Its description is meant to describe how the bias voltage is applied to the grids of the tubes. It is fixed because the bias voltage supply does not scale, move/shift, or change based on the current flow through the tube. The grid's bias is set for the ideal idle bias operation. As the tubes work harder and harder this bias voltage actually shifts more positive ( simply as a result of power-supply sag, not intentionally ) causing the tubes to bias hotter and hotter the harder they are run. When you play the amp at full volume, the voltage sag in the power supply causes the bias voltage to shift up ( less negative ) in relation to ground, making the tube bias hotter. This is why 60% - 70% of dissipation is the ideal range for fixed bias amps; any hotter and the tube could bias above 100% when run hard. Under nominal or median ranges of operation, this bias shift is minimal or not worthy of concern.

Cathode biased is not necessarily able to have " any " tube placed in it and function properly. Cathode-biased amps work by raising the power tube's cathodes above ground potential by a small amount. The power tubes grid is also referenced to ground by a slightly larger resistor. This is converse to what you think that means. Because the cathode is now held above ground potential and the grid is also held above ground potential, a point of equilibrium is met with regard to current flow through the cathode to the plate. The Plates conduction is controlled by the grid which as far as the plate is concerned is negative in relation to ground from the cathode. The cathode current flow is controlled by the cathode resistor and is what sets is ground potential difference between the cathode and the grid. The grid's relation to ground is fixed, so it does not change its relation to the plate. The cathode resistor is what sets the relative difference. A larger cathode resistor reduces the difference in ground potential making the cathode seem closer to the same ground potential as the grid relative to the plate, shutting the tube off MORE ( the grid becomes more negative in relation to the cathode to ground ). With a smaller cathode resistor, the cathode becomes closer to ground potential making the difference in ground potential to the grid larger, shutting the tube off LESS ( the grid becomes less negative in relation to the cathode to ground ). The cathode bias resistor sets the difference between that particular tube and the grid to ground. Not every tube is the same and it is very possible to have one tube set to be biased perfectly, but another set can be too hot, or potentially too cold.

Cathode-biased amps are generally set to have the bias at between 85% - 95% of dissipation at idle ( the amp is simply on with no signal ). The closer you bias a set of tubes to 100%, the higher the likelihood that the next set you install will bias above 100%. Conversely, the colder you bias, the higher the likelihood that the next set can bias too cold. The range of bias is more forgiving in cathode-biased amps because the way they work, they self-regulate quite well. Most techs will shoot for 90% - 95% of dissipation as the ideal range, although some will shoot for 100% if the client desires. As a cathode biased amp is run harder ( full volume ), the voltage sag in the power supply will actually cause the bias to shift colder ( more negative, shifting dissipation from perhaps 90% down to 70% or lower ). Cathode biased amps bode well with being run at full volume because the louder they are, the more conservatively the bias will shift. This is why you can bias as high as 95% or more of dissipation, as you operate the amp, it will bias colder. Biasing a cathode biased amp too cold will actually cause it to bias MUCH too cold at higher volume settings because the bias will shift downward.

The bias of a tube is a parameter set on a per tube basis. No two sets of tubes are identical, but can be very close, or close enough. ANY time you change the tubes, you should double check the bias of them regardless of fixed, or cathode bias operation. Cathode-biased tubes amps have a larger range of tolerance to any given set of tubes, but can still be biased too cold, or too hot for any given set. Fixed bias is less forgiving, and requires a little more precision in the setting of bias to ensure proper operation at higher volume settings. Each topology has a pro and a con over the other. I really like cathode-biased amps and think they offer something that meets the demands of most musicians, unbeknownst to them. Fixed bias designs are obviously much more suited to other musical styles and musician preferences, also for reasons not so obvious to them. There is no better topology, only one that is more suited to the musician and their musical needs. My desire lands more on lower wattage, cathode-biased designs.

I think the reason you don't see many, if any, modern cathode biased amp designs above 50 watts, is that the wattage at that point, no longer allows the things that make cathode biased amps what they are become easily apparent. At about 50 watts, they just turn voltage into heat more than anything. I.E. what people love about cathode-biased amp designs is lost at around 50 watts of power. Fixed bias is not absent from any issues, but it does make more power and is considerably more able to control bias shift, making an amp much more stable at any given volume setting.

I'm officially drunk now and will have to come back to this later, but this should hit many people's wonders.

You're out of control Ewizard, clearly you cant balance your love for alcohol and amplifiers