EXPIRED - FOR SALE or TRADE: 6267/EF86 / 6DA6/EF89 Tubes
Item #649328519Info: 6267/EF86 / 6DA6/EF89 Tubes
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Jul 24, 17 9:28am (PST)
Edited: Jan 19, 18 11:28pm
North York,Toronto, ON
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Vintage 1960's 6267/EF86 + 6DA6/EF89 Tubes @ $50.00 / pair
Shipping included to anywhere in Canada
Cash GTA Toronto pick-up is always welcome!
I will pack most securely for safe arrival.
Cash/EMT/Canada Post money order grams preferred
Jan 19, 18 11:28pm
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.
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