Tech Tips

Always use the standby switch on your amp in order to lengthen the life of the tubes. Hybrids are guitar amps that use a tube in the front end of the pre-amp. This is used to give the overdrive channel a smooth compressed sound. The rest of the amp is transistorised or solid state. If you fit output tubes yourself get the bias done by your Service Technician for best sound and tube performance. Purchase only the best tubes so you can get the best sound quality and overall reliability.

Pre-Amp Tubes

The same tube type can have two different part numbers - one for the USA and another for the UK. The following types of tubes can be found in the pre-amp section of most tube amps :

  • USA Tube Part No. EEC81 - Uk Tube Part No. 12AT7
  • USA Tube Part No. EEC82 - Uk Tube Part No. 12AU7
  • USA Tube Part No. ECC83 - Uk Tube Part No. 12AX7
  • USA Tube Part No. 7025   - Uk Tube Part No.  12AX7

Make sure you purchase reputable brand tubes that have been tested and checked for stability, microphonics, distortion tests, hum and so on. If you buy a set in a box, the tubes are usually a matched set but the bias still needs be checked and set by your guitar amp technician. A good indication of your output tubes needing replacement is when the low notes are sounding muddy and losing definition. Another indication is when the amp is not sounding as loud as it used to, or the amp is distorting un-naturally. Keep in mind that other factors can also contribute to low power output.

  • Keep spare tubes with you because tubes can fail at any time.
  • Keep spare fuses for all of your mains powered equipment.
  • Use the same fuse rating when replacing blown fuses.
  • Replace SLO BLO fuses with SLO BLO types - these fuses have the letter T before the fuse rating eg. T630mA.
  • Replace FAST BLO fuses with FAST BLO types - these fuses have the letter F before the fuse rating eg. F630mA.
  • Use proper speaker cables/leads to connect your amp to the speaker box - Do not use guitar leads.

Common Faulty Tube Symptoms

  • There's no glow inside the glass when the amp is turned on.
  • Dark spots appearing on the plate structure of the tubes (valves).
  • Black burn marks on the glass.
  • Low output - not as loud as it used to be.
  • Crackle or noise coming from the speakers.
  • High pitch squeal - caused by microphonic tube (valve).
  • Noise and crackle when faulty tube is tapped and the amp is turned up high.

Please observe standard safety measures and have a healthy respect for the high voltages. WARNING - NEVER work on any mains powered Guitar Amplifiers or Electrical Equipment unless you know exactly what you are doing! Even with the plug out of the socket, many appliances are capable of holding and producing LETHAL charges!


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