The E34L has a slightly higher grid voltage rating (-13.5 to-16.5 vs -10 to -13.5 volts) than the traditional EL34 vacuum tube. The blue glass is purely for aesthetics. For the same idle plate current value, the E34L is typically biased with a more negative grid voltage than the EL34. This allows the E34L to offer more headroom (later breakup) than the traditional EL34. Available in Singles, Matched Pairs or Matched Quads - please select from list.