There are two types of tubes used in an ED amp - 12AX7 (ECC83) in positions V1 and V2 and 6BM8 (ECL82) in positions V3, V4 and V5. 12AX7's are easy to find and can be purchased at most music stores. The 6BM8 tube will most likely not be found at a music store as it is a more unique tube.

The 6BM8 (ECL82) is a pentode/triode combination that looks like an EL84. The 7 Watt pentode section is actually a tetrode like a 6V6 and exhibits plate characteristics that are more similar to a 6V6 than an EL84. The triode section has gain characteristics somewhere between the 12AT7 and 12AX7 and very good drive capability.

There are several different makes of 6BM8/ECL82 tubes available from various distributors and sellers you will find online as well as many available from online auction sellers. The Russian made version is desgnated as 6F3P. All these different makes have varying levels of quality so I will attempt to clarify the most important features you need to know in order to have your ED amp working at its best.

When considering 6BM8 tubes the three most important parameters for the ED amp are; cathode current, heater (filament) to cathode insulation and microphonics. Each parameter will be explained in more detail.

Cathode current is the amount of current the cathode is emitting while the amplifier is operating. As a very general rule, most Russian made 6BM8 tubes will have lower cathode current than most any NOS tube by any other make. Your ED amp was designed with the Russian made, Svetlana 6BM8 tube. Higher cathode current will provide slightly higher output and more distortion however, these tubes will operate at a higher temperature and the tubes will wear out faster. You just can't get something for nothing.

Heater insulation is not something mentioned often but is an important consideration for the ED amp. Most guitar amplifiers, such as the ED, operate tube heaters with an AC voltage. Tubes with poor heater to cathode insulation will actually have some of the AC current on the heater impressed on the cathode which will show up in the audio signal. You may have noticed that after replacing a tube in your amp there is more humming noise from the amp. Some amplifiers come equipped with a "Hum Adjust" to cancel out any heater-cathode imbalance. The ED amp was not designed with a Hum Adjust feature. The tube in position V4 of the ED amp is the most sensitive to poor heater-cathode insulation because this tube provides the gain required for output stage. Careful selection of the V4 tube is required. The V3 and V5 position tubes are not nearly as critical.

One of the design characteristics of the 6BM8 tube is the structure of the triode section. The triode section has a long plate structure and makes it much more susceptible to picking up surrounding vibrations. As the tube ages and parts begin to loosen up, you may even hear a "jingling" sound while you play at low levels. All 6BM8 tubes will wear down over time and produce varying amounts of microphonic related noise. This is also known to occur in other types of tubes that have been extensively used/worn.

For your ED amp, Svetlana tubes are perhaps the best all around tube because they have a good life span, good heater-cathode insulation and low microphonics. However, not all Svetlana tubes are created equal - look for Svetlana tubes made in Russia or the USSR (CCCP) labelled in Russian as 6Ф3П with the winged "C" logo, made after 1972. NOS Svetlana tubes made prior to 1972 will have a very high heater noise making them in-appropriate to use in the ED amp. New manufacture Electro-Harmonix 6BM8 tubes are somewhat comparable to Svetlana made tubes but, as we have discovered, have a very inconsistent heater-cathode insulation which makes it difficult to find a useful tube. The Electro-Harmonix 6BM8 tends to become noisy as well, therefore, Electro-Harmonix 6BM8 tubes are not recommended at this time. Nearly all NOS 6BM8 or ECL82 tubes from other manufacturers will work very well with low heater noise and low microphonics but with a higher cathode current that will wear out the tubes faster. NOS tubes are great for recording. Yugoslavian made ECL82's will run hotter and are more prone to glowing red on the plates and slightly higher microphonics. Japanese made 6BM8 tubes are very good and have very low microphonics. The very best tube we have tried in the ED amp is the very rare, Hungarian made, 6BM8/ECL82 by National Electronics. If you have this tube, use it only when absolutely necessary!