EF86 TUBE GUITAR AMPLIFIER – DESIGN CONSIDERATIONS
I just completed the several builds of this high quality guitar amplifier circuit and I love it. I’m now trying to push this amplifier further to create the most beautiful sound quality possible and I’m probably focusing on the last 5% that’s still available for change through further amp design considerations and parts quality increase.
When this next build is completed, I’ll know which approach sounds best and whether the additional cost is warranted. This amplifier is already getting expensive from a parts cost standpoint (now around $1,500) and increasing the cost further (another $500 or so) can only be justified if the end result is worth it.
I’m using all of the techniques I’ve learned from the audiophile component design and hands on building experience that I’ve done plus I’m reading as much as I can about what other high end amp builders are doing with their guitar amplifiers.
My very first observation about guitar amps (the way they’re made currently) is that they make absolutely no sense. Heat is the biggest enemy of a tube amp and guitar amps are built to maximize detrimental heat destruction of the components used to create the circuits. This makes absolutely no sense to me.
Also, vibrations generated by speakers when an amplifier is installed inside the speaker cabinet makes no sense and is destructive for the amplifier. What a stupid mess in my opinion and yet, builder keep building guitar amps and speaker cabinets in this way. Guitar guys have wanted to carry everything in one box and the builders have designed a way to do that (amps inside the speaker cabinet with tubes pointed down and components above the amp in tight enclosures that have inadequate ventilation) and it leads to all kinds of long-term failure problems.
The first thing I needed to do was to go back to an amplifier design approach that makes sense and remove all of the heat related and vibration related problems. That was the easy part of this new design since it didn’t require doing anything unusual other than separating the amplifier from the speaker cabinet and building the amplifier as I would an audiophile amp, with tubes pointing up, components underneath in a really solid chassis, and no outer cabinet to hold and contain heat. That’s how I build audio focused archetypal amplifiers and this similar approach allows me to continue forward using what works best based upon my many years of experience and observation.
Tubes should be pointed up and not be placed upside down where they can fall out of their tube sockets (or need base scratching tube retainers to stay in place) and where the heat from the tubes rises up to basically “fry” the amplifier resistors, capacitors, and other components. This guitar combo approach basically shortens the life of expensive NOS tubes and the circuit components themselves. In my opinion, don’t do that.
The chassis for this amplifier should also be robust (to create something that can last a long time, look good, and add vibrational control) and not be a thin and malleable “like a beer can” (i.e. typical Hammond chassis) enclosure. It should be made out of aluminum and not steel. It should look good and if you can drive a truck over it without severe damage, so much the better. There is no need to spend money on bling since just using cnc’d 1/8” aluminum in the correct way will provide the longevity, beauty, and robustness needed. Also good industrial design goes a long way and easily replaces expensive bling with an approach that works really well and costs less.
Really great sounding NOS tubes are expensive. Most guitar amps use matched pairs or quads of power tubes. The really great NOS power tubes are difficult to find and afford let alone finding them so they match each other in pairs or quads. Are NOS tubes worth using? Yes, some are. Are new production tubes really lesser than NOS ones? Yes, most of the time but not always. To determine which to use, however, is an expensive proposition and requires considerable buying and selling to figure out what works best. I’ve already done that for you and am using a NOS/new production combination of tubes in this amplifier that removes your need to tube roll and spend unnecessary money (unless you really want to of course).
Yes, a really loud amp is great for playing on stage in a large room or auditorium. But in reality, for the 10,000 practice hours it takes to play at a professional level, how many of those hours are played through a practice amp in a small room at home? Probably most all of them with a few hundred hours played (if the player is talented and lucky) in front of an inspiring and critical audience.
My approach is to use one superb power tube, one NOS extremely incredible rectifier tube, and one NOS EF86 tube for the preamp section and another NOS 12AX7 tube to make up the gain lost by including a full tone control section (bass, midrange and treble). That’s it. I don’t believe in using more tubes since any addition only degrades the sound quality and complicates the circuit build beyond what is really needed. A superb clean amplifier tone can be used alone or with pedals in front of it to create a myriad of tones.
A really great distorted amp should remain a distorted amp since its clean tone will be a compromise compared to owning two amplifiers instead of just one. For me, I only want the clean tones with pedals to modify that tone. For some of you, owning more than one amp makes sense. Two is all that you would need and more than that allows you to explore different equipment if that is your goal. There really are no rules.
The cost of buying and selling equipment is what restricts us more than anything. So my approach is to get one part of this equation right and to provide the absolute best emotionally attractive clean tone type of guitar amplifier possible. That’s what this guitar tube amplifier represents and is.
A NOS Mullard GZ34 will cost upward of $200 and will last a very long time (maybe the life of the player) so this is a great investment. Don’t compromise here.
I’ve had the pleasure of using most all really old and expensive NOS power tubes to determine which one provides the most linear sound quality as well as the greatest emotional enjoyment when played using this amplifier.
After considerable testing (i’ve even used the $400+ Genelec Gold Lion KT88 during my R&D) I found that a new production Psvane EL34-PH was the best sounding power tube of the whole bunch. This made me very happy since this tube is easy to source and I no longer needed to accumulate a stash of expensive and rare NOS power tubes with the fear of not being able to purchase one later on when I need to replace it let alone afford to do so.
This amplifier is designed around a NOS EF86 nine pin input tube. This tube provides the ultimate clean tone that is possible particularly when run at lower than normal voltages. The EF86 guitar amps that are out there typically don’t use this tube as the first input stage tube. Usually it’s a 12AX7. However, the EF86 tube I use is run at less than typical voltages and has a beautiful sound quality that can’t be created when using the 12AX7. It is the ultimate clean tube but when used typically at higher voltages, it is microphonic and this leads to sound problems.
The EF86 has a mu factor of 300 and a 12AX7 has a mu factor of 100. A whole lot more gain can be achieved with the EF86 and that’s what its typically used to do. I run the EF86 around 200 VDC and this substantial reduction in gain allows me to obtain a sweet beautiful and linear sound quality that I can’t get with a normal 12AX7 or an EF86 run toward the higher 300 volt range that it was designed to use. The EF86 tube is one of the most microphonic tubes you could use until you lower the operating voltage and then use only well tested low microphonic EF86 tubes.
Fortunately, one of the best ways to reduce or eliminate the microphonic issue is to operate the tube at around 200 volts rather than closer to 300. Also mounting the tube socket on a vibration type of custom platform that is separated by the top chassis plate by vibration isolation connectors (much like a trampoline) and then placing a Herbies guitar tube damper around the outside of the EF86 pretty much eliminates any microphonic problem. That is, if you use a brand of NOS EF86 that already has been tested and has low microphonic problems.