Louder than a single 6V6 tube yet not so loud that it doesn’t work well in a studio setting or home practice room. Really great musicians spend at least 10,000 hours to build their chops and most of that time is spent with a practice amplifier and not on stage. This amp must really sing and allow for tone shaping (by changing tube types and the use of a full tone circuit) to keep a player long-term interested and inventive.
Why The EF86 Preamp Tube & Hence The Name EF86 Amp?
The EF86 is a superb preamp tube. It also is notoriously problematic when it comes to mechanical noise. It’s a pentode not a dual triode and the large plate that it has is what causes the noise issues. However, the tone is amazing and it has great headroom with a ton of gain.
A normal 12AX7 preamp tube has a mu (amplification factor) of 100, while the EF86 has a mu of 300. This gain factor allows you to use the pre-amp section of an amp to drive a simple tone control, and on to the output tube without any additional make-up gain stage.
However, in this amp, the EF86 is being operated at reduced voltages to eliminate microphonic problems and a 12AX7 based second tube stage is then being used to increase the gain lost from adding the tone control circuit. The EF86 as the first stage preamp tube will provide incredible detail from your playing as well as a very responsive and touch sensitive front end.
Most amplifiers that use the EF86 use it in the 2nd tube stage rather than the first like done here. This is what makes this a very unique sounding and incredible playing guitar amplifier.
It’s hard to find an amp like this that has 8 to 10 watts of very efficient power and uses one EF86 preamp tube, one 12AX7 (or variant) tone control tube, one GZ34 rectifier tube, and one (not two) all large power tubes (EL34, 6550, KT88, etc) with automatic self biasing.
If you keep the Plate voltage under 400 volts DC, you can run almost any popular output tube. Having the ability to operate different output tubes was a key design goal. Octal based output tubes provide the greatest variety. With the voltage below 400 VDC you can go from an EL34 to a KT88. In order to change tubes on the fly a cathode biased design was used.
The common tube that’s not included for use in this amp is the 6V6. That tube has a much lower volume output and requires a much higher cathode resistor. I could include a switch to increase the cathode resistor value but plate voltage is too high to use NOS 6V6 tubes without vastly reducing their tube life. This can be very expensive when using NOS 6V6 tubes and a real shame to use up limited supply. quality tubes. To gain a more romantic sound quality, use the EL34 tube instead of the 6V6. The EL34 in this circuit provides a louder presentation and a beautiful romantic thick sound, yet still is “clean” due to the circuit design and the lower EF86 operating plate voltage.
This amplifier is built to sound its absolute best and is a superb “clean” amp (with varying degrees of character and tone depending on tubes chosen). It also provides an excellent pedal platform solution. It is a world class practice amp when considering tone and complex sound quality.
Above Average Build Requirements
This amplifier is hand wired using the best components. It is built for longevity and is definitely worth passing on to other players particularly those who you care about most. The chassis is cnc’d out of 1/8″ thick aluminum and can be driven over by a truck without collapsing. Yet, there isn’t a waste of money spent. Everything that’s included is there for a specific purpose and it meets those goals really well and most than most other guitar amps.
A Simplified Circuit Is The Best!
All extraneous features that typically reduce volume or sound quality have been removed. There is no send/return effects loop, no reverb, and most important, no loss of preamp loudness since there is a 12AX7 tube dedicated to making up the loss of gain when adding the tone controls. That might initially sound like a negative description but its not. All of those other circuits, reduce the beauty of an amplifiers sound quality. Tone controls are, however, a necessity and must be included. Adding the 12AX7 tube circuit replaces gain that is lost from adding a full tone control complement and is the only meaningful add-on that must be included with this amp. Everything else must go.
Heat Reduction Is A Primary Concern
The build up of heat is the biggest enemy of any hot running amplifier. Using expensive NOS tubes becomes an expensive proposition with most guitar amplifier designs since two or more matched power tubes are required and many of the tubes are either hanging up side down (while getting tube holder scratches on their bases and heating up all of the components hanging above them) or enclosed inside tight poorly ventilated cabinets that also contain vibrating speaker membranes.
This circuit design separates the speaker from the amplifier (head and separate speaker cabinet concept) and is spread out with ergonomically and optimally placed components. Tubes are pointing up with plenty of space around them to dissipate heat. This amplifier is also designed to function without a cabinet and that also substantially reduces heat problems.
Big Fat Open Sound – No One Trick Pony
Low power miracle amps sound too small and thin. They might produce a distorted sound but not a big fat open sound and most have a tone control that is too simple and doesn’t adjust the full frequency spectrum. So you usually end up with a one-trick pony and that’s not what we’re dealing with here. This amp can be used for really good tone shaping and includes controls for treble, mid, and bass.
The class A design produces a very rich tone with nice harmonic content. It’s very simple and can be wired up without an elaborate circuit board. This simplified circuit reduces the number of components involved and this in turn increases the fidelity. The more stuff you cram into an amp, the more you will have to deal with noise and colouration of the signal.
- Class A single ended with an output of about 10 watts – beautiful tone & respectable loudness making it an incredible practice amp as well as a great amplifier for small venues as well as in the studio.
- Hand wired with turret boards for ease of maintenance long-term (includes both schematic & layout for reference). Extra circuit boards for future re-capping and repair use are available at additional cost only with the purchase of each amp. Longevity is encouraged in this way since this amp will last for many decades if parts are available. Purchasing these boards now makes sure the parts are in hand for future use with no need to worry about availability. ONLY A NEW AMP PURCHASER WILL BE ALLOWED TO PURCHASE AN ADDITIONAL SET OF BOARDS (but must be purchased at the same time the amp is purchased).
- Plate voltage is maintained just below 400 volts DC to be able to operate numerous power tube types
- A variety of power tubes (EL34, EL37, 6550, KT88, etc) can be used without any adjustment (self biasing). With only 1 power tube needed for this amp, tube cost is substantially reduced plus there is no need to find a pair or quad of matched tubes. Even though its expensive, you can purchase a single rare NOS power tube if you desire. If you can’t afford this luxury or chose not to, a good new production tube can be used instead.
- The pre-amp tube of choice is the EF86. This is a unique tube that isn’t found too often in guitar amplifiers due to its delicate microphonic issues. To reduce microphonics and to be able to use this superb preamp tube, its operated at lower voltages than typically spec’d and is vibration isolated from the chassis by using a separate custom tube socket hanging plate with sorbothane washers and a Herbies guitar tube damper. Then the only microphonic reduction requirement is to use a good EF86 tube.
- A 12AX7 is used to replace the gain lost through the inclusion of the full tone control circuit (treble, mid and bass).
- A tube rectifier is used to provide both slow startup and a superb tone. A NOS Mullard GZ34 is the ultimate tube for this amp while a GZ32 is a possible alternative (providing a bit more sag).
- Above average chassis design and circuit layout provides a unique and long lasting archival amplifier
- Optimum heat reduction concepts are used throughout the design and layout
- This amp is also very quiet for close up practice or studio use – no hum, no hiss, no mechanical noise. This would turn out to be the single biggest consideration and challenge for this circuit design. Single ended amps can be noisy by design due to a number of factors and this particular design eliminates those potential problems.
- It provides a great pedal platform to add back in any kind of preferred sound quality that isn’t included in the original design. You can’t add back beautiful clean tone if it isn’t included in the amps inherent design. It is with this amp. However, great distortion (that isn’t part of this amps design) can always be added back in through the inclusion of external pedals rather than trying to make one amp provide both types of tone. Why compromise?
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