I have a 5E3 kit that is cathode biased. I played through it and AB'd it with Fender's Eric Clapton Vibro-lux which is the 5E3 but the power section is grid biased. I couldn't really hear a difference which surprised me. I began to think maybe the type of biasing an amp uses isn't that important to the sound. ...
TLDR: Fixed-bias on a Class A amp will sound basically the same as Cathode-bias on a Class A amp, especially if the power supply and other stuff is the same. But trying to Cathode-bias an amp that otherwise was optimized to run as a Class AB amp making
large output power will result in obvious sag/squish/sonic-difference. And often amps that allow use of both bias methods aren't trying to intentionally hobble one bias setting for a sonic-effect.
Bias Method
alone doesn't dictate sound, though cathode bias cannot be used far into Class AB.
Class A idling at 100% plate dissipation means the tube swings from "Idle Current" to "About Double Idle Current" and down to "Almost Zero Current."
- Full Power Current from the power supply is nearly the same as Zero (or Idle) Power Current drawn from the power supply. So power supplies are lightly-built.
- Find the "Average Current" over the signal cycle, and it is roughly the same as the Idle/Zero-Power Current. So bias
volts across the cathode resistor stays essentially constant.
- Tube Rectifiers can be used with impunity in Class A amps because the more-constant current-draw means less voltage-change across the rectifier from idle to full-power.
- Any sense of "sag" mostly comes from pushing beyond max clean output power:
- The 5E3 strays a little beyond Class A into Class AB, but only at max power and/or when distorting.
- If one side tries to pull much more than 2x idle current, this creates a bigger bias voltage-drop across the cathode resistor, which reduces plate current.
- High current-draw drains the charge from the filter caps, maybe faster than they can be recharged, especially when filter caps are small-ish (like 16µF).
Class B amps
theoretically idle at zero plate current, and swing to very high peak plate current. Class AB amps are "something between Class A and Class B" so higher power Class AB amps pull a peak plate current very much higher than 2x idle current.
- Because the peak plate current might jump up to 3-4 times idle current in a Class AB amp, cathode bias is infeasible because the bias voltage developed would counteract the plate current. So fixed bias (higher parts count, more expensive, less safety) only really gets used on higher-power Class AB amps.
- Big swings in current-draw from idle to full-power means the bigger Class AB amps will tend to solid-state rectifiers to limit power supply voltage sag.
- Big swings in current-draw from idle to full-power means the bigger Class AB amps will generally get stiffer power supplies with more and larger filter caps to deliver those big current peaks. Which also means less voltage-sag (except for the earlier 1960s-style amps that might still have smaller filter caps, even with Class AB).
I know we "all" ignore bias
voltage from grid-to-cathode these days, but we shouldn't.
- The early 1950s
5B6 Bassman biased its barely-into-Class-AB 5881s with -27v grid-to-cathode.
- The mid 1960s
AB763 Super Reverb biased its 6L6GCs with -52v grid-to-cathode.
- If you tried to develop a large bias voltage of 52v for the Super Reverb with a cathode resistor to tame its idle current, it would squish and compress like hell when driven hard enough to shut one side off (after which the average plate current rises above total idle current), and the cathode resistor began developing a large counteracting-voltage that would constrain plate current.
Some amps offer both bias options but try to keep "all else the same," and if all else really is the same then there's no real sonic difference. Sonic differences arise when "all else is
not the same."
Topic: Grid Bias Versus Cathode Question (Read 1633 times)