Resources > The Secret Life Of Valves

Top Tips For Valves

  • Power valves which have been used in Marshall amplifiers (JTM45 & 50/100 watt models) from the beginning are KT66, KT77, KT88, 6550,6L6, 5881 & EL34.
  • Pre-amp valves that have been used in Marshall amplifiers from the beginning are ECC81, ECC82 & ECC83.
  • The JTM45 models (2245 & 1962) are fitted with a GZ34 rectifier valve.
  • In the older small valve amplifiers we produced a 20-watt fitted with an EL84 valve and a 10-watt fitted with an ECL86 valve.
  • EL84 valves are fitted in the new power amplifier EL84 20/20.
  • When replacing power valves, they must be fitted with the same type that have been removed. You can not mix different types of power valve e.g. EL34 & 5881 valves, in the same amplifier.
  • When replacing power valves you can not mix different manufacturers valves e.g. Svetlana & Sovtek.
  • When replacing power valves with different manufacturers valves you must replace all power valves with the same make and the amplifier must be then re-biased to suit the new valves.
  • Always unplug your amplifier from the mains before replacing any valves (pre-amp or power valves).
  • When replacing the valves, leave mains and standby switches in the on position for 30 minutes with the mains lead unplugged before replacing any valves.
  • Never run a 100-watt amplifier with two of the power valves removed. This can cause damage to both output and mains transformers.
  • Where possible when replacing power valves we would strongly suggest that all of the valves in the power stage are replaced at the same time.
  • It is not necessary to bias pre-amp valves.
  • Never run a valve amplifier without a load such as a speaker.
  • Always match the speaker impedance to the amplifier impedance to prolong the life of the power valves.
  • The average life of a power valve if used three to four times a week should be approximately 18 months to 2 years.
  • The average life of a pre-amp valve if used three to four times a week should be approximately 3 to 4 years.
  • If a power valve has a slight bluey purple glow it does not mean that the valve is necessarily faulty.
  • Always replace fuses with the correct type and value to prolong the life of the amplifier.
  • When replacing valves, ensure that the locating pin on the valve lines up with the slot on the valve base.
  • If when power valves are removed there is a burn mark on the valve base, the valve base must be replaced and the speaker lead and speaker cabinet being used must be serviced. This is a sign that the amplifier has been run without a load.
  • The small red glow at the top and bottom of the valve is not an indication that the valve is working - this is merely the heater.
  • The old amplifiers can have new power valves fitted (5881 or EL34) with some circuit adjustment and re-biasing.
  • The valve line-up in a 100 watt amplifier is shown in pairs - 2 on the left (1 & 2) and 2 on the right (3 & 4).
  • When replacing power valves in matched pairs, replace (1 & 4) and (2 & 3).
  • If a power valve in a 100W amplifier starts to glow bright red this usually indicates that the other power valve in the matched pair is faulty (1 2) and (3 4).