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The EF86 is a high gain pentode designed to amplify low-level audio signals. It is typically used as a preamp in audio equipment and instrument amplifiers. Because of its design, the EF86 is much more sensitive than other common preamp tubes like the 12AX7, and achieves more gain.
With all these good things going on, why don’t more designs feature this tube?
The reason is simple.
A complex mechanical structure, combined with a high gain design, make the EF86 very susceptible to vibration and the resulting microphonics.
A triode has three active elements while a pentode has five. Most people know that a 12AX7 is a dual triode, that is, two preamp tubes in one small bottle. The EF86 is a single, very large preamp tube by comparison. The EF86 also has an internal shield structure that is physically tied to pins 2 and 7. There is a lot of precision hardware that is carefully aligned to prevent physical damage or electrical failure. Older tubes from Telefunken and Amperex used to flash metal directly onto the base of the tube between pins 2 and 7. I can only assume this was to reduce the number of physical leads inside the bottle. The JJ EF806-S is a very good replica of a Telefunken and uses this method today.
Here is a quote from an old Amperex data sheet for the EF86:
“The tube can be used without special precaution against microphonic effects, in amplifiers in which the input voltage is greater than 10 millivolts, when the tube is mounted in the vicinity of a 5 watt loudspeaker with an efficiency of 5%” The VOX AC-4 is the only guitar amp close to these specs.
This tube was designed for radios, tape recorders and home audio. Anything that has low volume or the ability to isolate the tube from vibration. The EF86 made its way into guitar amplifiers because it had a lot of gain and a good track record in quality audio equipment. VOX, with the early AC30 is perhaps the best example. You would have to talk with the original designers, but an EF86 can easily drive a signal through a simple tone control and high-cut filter into the output section. If you take the tremolo and vibrato circuits out of a vintage VOX using an EF86, that is exactly what you will find.
Modern amp builders generally do not bother with this tube because it is so hard to find one that will work in a tube combo amplifier. There are a few like Dr. Z, Matchless, Bad Cat and VOX that still use this tube, and that may be because those amps often have their roots in classic British amps and EL84 output tubes. Some things are so good; you are willing to put up with their shortcomings.
Hope is not lost, because there are things you can do to make this tube work. There are no substitutes. If your equipment uses the EF86, you have to like it enough to deal with microphonics or get something else. Vintage Mullard design notes suggested using a tube socket that is shock mounted. You will see this in old Gibson and Fender guitar amps that used the 6SJ7 pentode, but the market could use a good modern alternative. One way to do it is to place small silicone O-rings between the tube socket and chassis. This helps isolate the socket from vibration while maintaining good electrical connections. Tube shields do not offer any reduction in vibration and may make the problem worse. If your equipment has sockets mounted to a printed circuit board, your only choice is to dampen the tube.
If you cannot isolate the tube socket then your only alternative is to add some treatment to the tube that will dampen the vibration of the bottle. There are a number of ways to do this and the cost varies from pennies to many dollars. There are a number of products on the market designed for this purpose such as Duende Criatura tube rings. Others like Kryna, also add passive heat radiation while dampening vibration. If you are on a budget then you can buy silicone O-rings that you stretch over the bottle or you can take a lead from Peavey and put a piece of heat-shrink wrap over the end of the tube.
Thetubestore.com sells a variety of EF86 tubes that will work in your audio equipment. It is important to choose the one that will be best suited for your gear. If your equipment uses tube sockets the have an extended base to accommodate a tube shield, you should not buy the Electro-Harmonix or Tung-Sol brands. The reason is that they are produced with a bottle that is right at the maximum design width for a 9-pin miniature tube. If they run on the wide side and the socket is off spec or has any dents, the tube will be difficult to seat in the socket and microphonics may be exaggerated. The JJ EF806-S, NOS from RFT, Tungsram and Tesla should be your group of choice. The JJ and NOS tubes are all around 20 mm while the EH and Tung Sols’ run around 22-23 mm.
The JJ, Tung Sol and Electro-Harmonics are designed with a plate assembly that is very similar looking to vintage Telefunken production. The plate is about 13 mm tall, circular, made of solid metal with a single round hole on one side and mica spacers on the top and bottom. The RFT and Tesla tubes have taller plate structures and mica spacers that come through the wall of the plate. This gives them a little more gain and sometimes a little more noise.
The Winged C EF86 has a taller plate with mesh shield that is similar to what you find in vintage Amperex or Philips tubes. If you want an affordable alternative to Philips or Amperex, the Japanese Matsushita/GE EF86/6267 is the real deal with that delicate mesh shield. They were built on the same equipment as the Philips. If subjected to vibration, I find versions with a mesh shield can be more prone to microphonics.
Here is a quick comparison / review chart of EF86 tubes thetubestore.com currently offers:
Tung Sol Gold*
A Bit Darker
A Bit Darker
A Bit Brighter
*Do not use with a socket that has an integrated tube shield element, ie. Fender style.
You can see that some careful consideration will improve your chances of making the right choice for your equipment. I have been using the EF86 in my amps for 20 years and just love the rich, detailed sound they produce, and the negligible amount of hiss or hum. You can see my recent DIY amp project as an example.
They are not at their best in combo guitar amps but if you follow my recommendations, it will help. If you use a head only guitar amp, place the head on something other than the speaker cab, or put a piece of high-density foam between the head and cab. Audiophiles have been using these tricks forever.
The EF86 is not for everyone, but if you need it, a thoughtful selection will help you get the best tube for your needs.