Aum Acoustics

EF86 Guitar Amplifier

NOS Tesla/Telefunken EF86, NOS Philips Miniwatt 12AX7, Psvane EL34PH, NOS Mullard GZ34


This is a new guitar amplifier that I build and sell. It’s designed to provide a really beautiful “clean” tone for a hollow body arch top guitar or a semi-hollow guitar. It’s voiced to create an excellent jazz tone and will not include a master/volume knob to send the amp into distortion. Headroom is excellent and this is a 10 watt amplifier. It’s plenty loud for home or studio practice or playing.

 It is also an excellent platform for attaching guitar pedals, and can use a distortion pedal to obtain excellent tonal shifts if you desire but you won’t have a send/return loop to use. However, it’s not really designed for distortion. Instead, its one of the finest clean guitar amps you’ll ever encounter and I can guarantee you’ll love the tone quality and the feel of this amp if you’re looking for the best clean tones possible. It’s extremely dynamic and responsive. I’m using a Polytone pedal to fine tune the tone even more than the included tone stack and its absolutely beautiful.

I’ve removed all of the tone sucking features that plague most guitar amps. That was one of my primary design goals and I’ve accomplished it. I’m also an extremely critical audiophile. I listen to everything and work on gaining tone through almost an infinite number of changes that I go through. Everything needs to be properly balanced including the components that I use. Since this is an amp focused on creating superb clean tones, I’m using more high grade resistors and capacitors than you’ll normally end up finding in a guitar amp. A combination of carbon film, metal film, and wire wound resistors (much larger in value and quality that you’d typically find in a guitar amp) plus the right capacitors is really a hands on challenge that I’ve already gone through by building my many audiophile amps. Instead of removing distortion I now can focus on getting the most beautiful tone without the compression and artifacts you’ll find with most guitar amps. 3 dimensional sound stage is exccelent as well as width and depth even though this is a mono signal and not stereo. The beauty of using tubes as well as a NOS Mullard GZ34 rectifier really come through.

I’ve even built a version of this guitar amp using the new really expensive V-Cap guitar tone caps. Yes, those caps are nice but the oil in paper sound quality that results just has too much overhang. I don’t like it and took them out and replaced them with my favorites, the SoZo Blue caps. That was a several hundred dollar mistake but is one example of what I go through when trying to optimize any of my OEM projects. I spend the money myself and if it doesn’t work, I eat the cost. That’s what you now have as a proven platform for creating a really beautiful tone.

Another R&D attempt was a redesign of the chassis to accomodate a laying in style of power transformer rather than the standup transformer you see in the photo above. I used an even more expensive transformer set made by a more expensive manufacturer. The output transformer is larger and after careful layout changes to my cad/cam design files, I got everything to work well. In the end, the improvement in sound quality was minuscule and I’m now going back to using the Edcor transformer set that you see in the unit in the photo above. I actually like the sound quality a tiny bit better.

I did end up replacing the Hammond choke that I started out with with a slightly higher Henry value choke (5H instead of 4H) and had that choke custom built by a different manufacturer. It does sound better. So that’s a keeper and again, another R&D loss item.

That’s not a way to run a business (incurring R&D exploration costs without an effort to recoup them) but when I build anything, making money isn’t my goal. Achieving the absolute best result is and I’m lucky if I break even. Usually I don’t and instead end up with a loss for each component that I custom build. This is just one example of how this happens with just one component. No, I don’t ask you to participate in this loss but rather and I begin my marketing of components worth sharing by selling a few units at my actual parts cost (spreadsheet sent to you upon request) without markups for my time, my R&D expenses, or any kind of marketing that I might do. If there is an interest in building more than these initial units, then I’ll mark up the product to capture some effort for my time. Even at that level, I don’t recoup any of my losses or R&D cost. So consequently, my OEM production is a done from a passionate interest in what I’m doing and only shared with others if it works. If it doesn’t, it becomes one of my own explorations that I learn from and I take full responsibility for the loss.

There isn’t reverb or a send/return effects loop or a master/gain pair of pots. The tone control circuit includes bass, mid and treble and really is a necessity and I do include it because in my opinion, its necessary to sculpt the best tone. Normally, adding a tone circuit reduces volume and to prevent that from happening I’ve added a 12AX7 tube as the 2nd tube stage.

I’m using a 12AX7 tube and a cathode follower circuit to replace the gain lost by inclusion of the bass/mid/treble tone controls. I’ve owned several amps with the ability to switch out the tone circuit and found that by removing the tone circuit (using an on/off switch on the front panel) an amp sounds its best without the tone controls and hence my aversion to adding them. My Carr Rambler didn’t let me to do that but it was way too loud for home use (even though I really like the sound quality) and hence one of the key reasons I got rid of it. I really need tone controls to “sculpt” the ideal tone for my guitars and the 12AX7 makeup gain that’s added back is a perfect compliment and excellent solution. Of course, the particular 12AX7 tube that I use now has a direct bearing on the resulting sound quality and I’m now using the excellent Phillips Miniwatt 12AX7 as my recommended tube for the 2nd stage. Its all a delicate balancing act when designing an amp to sound the way you envision and the combination of tubes I’m using has gone through an extensive “tube rolling” process before I did the final tuning for this amp and came up with the best tube combination.

The 15W single ended 8 ohm output transformer that I use (and of course the really large power transformer) is ideal for this circuit and it allows the use of many large output tubes. However, after much listening, tube rolling, and playing with component values, the new production Psvane EL34PH ended up being my favorite and is the power tube I then used to “fine tune” this circuit and final component values. The slight sag that the NOS Mullard GZ34 provides along with the more romantic sound quality of this tube combine to provide a beautiful clean tone with some nice tonal modification. Not much but just enough to smooth and allow the clean tone to bloom really well. No, this isn’t a Fender Deluxe sound quality where distortion breakup happens early. Headroom and volume are excellent in this amp and the tone stays clean during most of its spectrum. This is a beautiful jazz piano type of sound quality (Steinway grand and not a Bechstein that is more bell like and crystal clear) that you might expect to get using a world class jazz style grand piano.

The EF86 is the first stage input tube (in my opinion) does a better job of providing a beautiful clean tone than when using a typical 12AX7 first stage tube. However, it is potentially very microphonic and to prevent potential problems, I use a much lower operating voltage on that tube than is typical. That really helps and then I also suspend the socket on a separate tube socket plate that is mounted underneath the chassis and isolated from the top plate by special vibration isolation supports that I order from the UK. Then I add a Herbies guitar vibration damping ring to the EF86 and only use NOS EF86 tubes that test and sound good and don’t have the typical microphonic issues. The end result is worth it! I also do this for the 12AX7 tube minus the Herbie’s vibration damping ring.

Yes, this is a lot of work but endowing this first stage with the EF86 tube is what makes this amplifier really special. The few commercial alternatives that you’ll find that use a EF86 tube typically place that tube in the 2nd tube stage with the 12AX7 as the first stage input tube. Here, I’m using the EF86 in the first stage and I absolutely love it. I also highly recommend using the NOS Tesla EF86 for this 1st stage preamp tube since if graded well, it doesn’t have the microphonic problems other NOS brands do.

Even in the 2nd stage, the 12AX7 that is used for the tone controls still must be really good. It effects the tonality of this amp and the nicest sound quality I’ve found is when using the NOS Phillips Miniwatt 12AX7. Other new production 12AX7 tubes don’t sound good at all (very flat, uninteresting, and too scratchy and your pick clicks really are emphasized) and really detract from the potential sound quality this amp is able to generate. The Miniwatt tube has some “sweetness” added to the treble frequencies that are excellent without losing detail or dynamics. If you’re looking for an alternative, the NOS Mullard 12AX7 is a possibility as well as a NOS RCA clear top 12AX7. I wouldn’t recommend less than what these 3 beautiful NOS tubes offer. The sweet sound these 12AX7 tubes create in the 2nd stage provided the “icing” on the cake so to speak and enhances the other tubes I’ve chosen. Do not compromise with this tube selection since it works really well. I’ve spent a lot of time and money to end up with this superb combination.

 I’ve used my Sadowsky SS-15 to fine tune this amp and it now sounds beautiful. I’ve also had the opportunity to utilize many of the large NOS power tubes to find out which one works the best in this amp. I even had the rare Genelec KT88 in the amp. I liked the NOS Mullard EL34 even better than all of the 6550 and the KT88s I used. However, the absolute best sounding tube was a new production Psvane EL34PH. The treble is less “in your face” and yet very detailed and beautiful. I can understand how some of these rare NOS tubes would work well for audiophile components but not for guitar. In the end, I found this Psvane EL34PH to be the best for the power output tube.  I’ve used this particular tube to “fine tune” the components I use in this amp and I highly suggest that you too purchase this tube. I’m not including it with this purchase but it is readily available online so sourcing it is not a problem.

One of the keys to getting the best tone was to run the EL34PH just below 400 VDC. Too much lower and it was too loose and didn’t sound good. Much higher and it sounded too “hi fi” and again, not good. The real sonic beauty emerged when this tube ended up running at around 400VDC on the plate and that’s what I’m now using. If I set this voltage lower, it ends up with my G string not full or fat enough. At the right biasing every string and every fret now sounds good. The last measurement I took found this tube running at 400.6 VDC (minus the reduction for the cathode resistor) and that’s absolutely perfect to obtain the best sound quality and is the reason I highly recommend that you use this exact same power tube for this amp. I’ve worked very hard at arriving at this combination and have really fine tuned this amp. Use what I’ve used to get the best tone. Once you have that, you can explore to your heart’s content and most important, have fun with this process. You never know what you’re going to come up with with other tubes. This amplifier is truly a musical instrument and one that provides a superb tonal quality when playing an electric guitar.

 The EL34PH that I use provides a really nice sound quality yet isn’t too thick or muddy as is typical of this tube in many amps. The KT88 provides the best overall clean tone I’ve come across so far but its too “glass like” and “pingey” and not a really good solution for a guitar amp. In my opinion, don’t use it. The operating voltage that I use is too high for a NOS 6V6 and I really like the EL34 run at the specs that I use much better. I’d have to change the cathode resistor value if using a 6V6 and then use a new production 6V6 that can take the higher voltages. This amp isn’t designed for a good NOS 6v6 that is typically run at 250 volts so don’t use one. The voltage is too high and the cathode resistor value is too low.

I love the EL34PH sound quality much better and I find it perfect for a guitar amplifier! Plate voltage is around 400 VDC in this circuit and this EL34PH has a maximum voltage rating of 800VDC. So this is an ideal tube and the plate voltage, current, etc I’ve ended up using allows it to sound its best yet not be overly “pushed”. I’ve also settled on a power tube cathode resistor value that allows for the perfect “tweaking” for this tube. If you have a hollow body or semi-hollow guitar and are looking for the sweetest most beautiful “clean” sound quality, this amp will provide that for you.

I have used many NOS power tube types to come up with the best tube. Another really good tone comes from the Electro Harmonix 6V6 new production tube. This tube is able to use higher voltages well. I wouldn’t put an expensive NOS 6V6 tube in this circuit since the operating voltage is higher than ideal for these older tubes. They like to be operated around 250V and 350V is too high in my opinion. Even though it works, they would shorten the life of these rare tubes. The new EH 6V6, however, does well and is another good possibility if you want to use it. It’s sound quality is somewhat similar to my favorite, the EL34PH made by Pavane and because of that similarity and the even better bass I get from the EL34PH, I like the EL34PH better. A change in the cathode resistor value should be made if you really want to use the EH6v6 and this would affect the large number of other power tubes (EL34, KT66, KT88 etc) that this amp will easily accept.

Yes, a good Winged C KT88 works well in this amp but sounds way too glassy!

This amps sound quality is much better than a solid state amp even though I appreciate the sound quality provided by my Quilter ToneBlock 202 and use it for a reality check. The 2nd order harmonics from the tubes in my amp creates a beautiful 3 dimensionality and wider sound stage that isn’t there with the Quilter or other solid state amps. Without tubes, things are a bit mental sounding and as a result don’t really have the soul of a tube. When I play the Quilter, I initially really like it for the first few minutes. Every fret is even and very exacting. However, after only a few minutes of playing, I don’t feel the emotional attachment to my sound quality that I get when using this EF86 tube amp and I get really bored. I then have a need to change amps and head for the tube amp. That’s where I gain the greatest enjoyment – playing the EF86 tube amp and hearing that beautiful sound quality is what inspires me to play! This is the perfect practice and studio guitar amp to log on tons of hours as you head toward proficient playing.

A solid state amp is good and technically exacting but it always ends up with a boring and flat sound quality and this results in a non-emotional attachment to my playing. Both amp designs are good solutions but when properly using tubes as I have within a superb circuit design, much more enjoyment is possible. It’s really good to own both styles of amps so you can experience this yourself. Once you realize that what I’m sharing is true, you’ll end up appreciating both amps but find that they indeed are different and each has its own purpose – one to hone your skills (ruthlessly if you can take it) and the other to increase your musical enjoyment while you’re playing and improving your touch sensitivity and your enjoyment of the tone you’re creating. I know which one I prefer. Do you?

I run the voltages in this amp at a lower level than other amps (for both the power and preamp tubes) to keep the signal as clean as is possible. This creates the least microphonic problems and keeps all tubes within their most linear range. The results is a beautiful tasteful bloom (with some slight sag due to the GZ34 rectifier tube) that provides a superb clean tone. The voltage and current specs are not too much or too little (even 5 VDC makes a difference) and the result provides the best tone. This definitely is a clean amp design but as a result, it also works well as an excellent pedal platform even though it doesn’t have an effects loop. I considered installing an effects loop but the loss of tone quality I’d receive wasn’t worth the effort. This is THE CLEAN AMP to use and is better than a circuit you’ll find in an amp that does both clean and distortion well. If you really want good distortion, you can use various pedals to accomplish that but in all honesty, a different amp design would be the ultimate solution but again very limiting since you’ll end up with a one trick pony with distortion being the goal and the clean tone a compromise. Own both types of amps and you have the best solution you can obtain.

I’ll post voltage levels at all test points when I have time so you’ll be able to see exactly where I’ve set the limits for this amp.This approach provides a really beautiful clean tonal quality (full of really good tone). The EL34PH power tube I’ve chosen sounds excellent and is easy to obtain so you don’t need to worry about finding progressively harder to source expensive matched tubes like you find in many other amps. Here you can use the best without “breaking the bank”. Also, using the EF86 as the front end first stage input tube really makes a big difference. Run at a voltage that is lower than normal and use the vibration techniques that I have and you’ll end up eliminating microphonic problems that come with this tube type.

I’m using all really high quality parts that are extremely unique to include in a guitar amp so this is not a cheap build placed in a “tin can” type of chassis with a heat imposing enclosure. This is an amp designed and built correctly considering all of the possible design goals. My parts cost is around $1,600. Adding an amount for my time and R&D costs and you have a fairly expensive amp. However, this is a musical instrument just like your guitar and speaker are. Don’t compromise if you’re already using a really expensive guitar! Your entire system must be well balanced and your guitar amplifier and speaker cabinet and speaker chosen are all just as important as is your guitar. If you’ve paid thousands of dollars for a really superb guitar, why would you now compromise  your guitar amplifier and speaker?

All tubes are upright (as they should be) and the chassis is not enclosed to allow for really excellent heat dissipation. I just can’t understand why someone would hang a guitar amp chassis upside down (with tubes pointing down) and have all of the heat rising upward while frying and baking the components above it. I also don’t understand why an amp would be placed in an enclosure that can’t ventilate well like most heads or combo amps. HEAT IS YOUR #1 ENEMY with all tube equipment.

Tube amps have generate plenty of heat and really takes a toll on expensive amps and tubes. Really excellent ventilation is your savior and is very important to include. Creating a superb cnc’d 1/8″ thick aluminum chassis that looks good and allows for the best ventilation and component layout possible makes the most sense. It also yields an excellent design approach that allows me to make the chassis bigger than normal guitar amps so components can be spread out properly (away from each other) and heat as well as electrical interference (both external and internal) can be minimized.

Why vastly limit the life of your components and basically “fry” their insides. Good NOS tubes are rare and expensive and care should be used to optimize their life span. These are issues that have always bothered me and why I’ve built audio components for listening (audiophiles) instead of guitar amps (for music creators). However, if you are in alignment with my concerns, I’m finally providing you with a proper way to use a well designed guitar amp. My approach is very different than most guitar amp builders and I’m sure you can relate to it. I use only 1 superb power tube and also use a 100db sensitive guitar speaker to obtain beautiful 8-9 watts of clean power. This is plenty loud for most any use (practice or recording) in a small to medium sized room. Increasing the efficiency of your speaker by 3db is equivalent to doubling the power of your amplifier so this alone adds a considerable amount of volume back to your amp. Using a high efficiency speaker is the best way to utilize a lower powered tube amp for guitar playing. A speaker rated at 100db is perfect. 3db less and you half your guitar amp volume. The Celestion Creamback 75 that I use is my favorite 100db speaker and the one I use. Consider that speaker if you’re also considering this amp.

This EF86 amplifier marks the beginning of my work with guitar amps. It removes me from building and selling  just really high-end audiophile oriented equipment and I’m really enjoying it. I’m also going back to my roots in music (as a creator and not just a listener) and am now representing recording studio gear and will be representing and selling just a few really stellar component solutions – ones that I use myself and really really like.

I’m now more interested in helping others create beautiful sounding music than I am in helping them to just listen to music passively. Too many audiophiles don’t know what really good recorded music sounds like and they tilt their audio systems to an unrealistic sound quality they desire. That really bugs me since the actual recorded tracks were never meant to sound like that. Going back to my professional piano playing days and to when I built several recording studios is what I’m now doing and is the reason for changing to building guitar amps and selling recording gear.

You will notice that I use a really high quality custom 1/8″ aluminum cnc’d chassis (in all of my prior audiophile amps and again for this guitar amplifier) and that it is extremely solid and robust. Not many guitar amps are built this well. I also have laid out the interior of this amplifier so it can be re-capped when it gets to maybe 15 years of age and endow several generations of players.

 However, I don’t go overboard on embellishing the chassis to waste money. The chassis is created this way to provide proper design, longevity, and excellent vibration control. In my opinion (and through much hands on experience) the circuit design utilized is the most important element to include first. Then parts selection becomes important – but only if the circuit design is way above average. Bling isn’t necessary beyond making something ergonomic to use and really beautiful to look at. I’m really good at creating my designs so they look beautiful even though I could make more money if I was interested in adding more “bling” and marketing my creations through typical sales channels.

I’d rather build for longevity than waste money needlessly on exotic and unnecessary chassis design even though I do love beautiful design. You can see that in my work as I try to make anything that I create beautiful.

My own built components already have enough money in just their parts cost to justify their existence and higher price tag. Yet, they are a bargain compared to alternatives. Adding labor and other required costs (to stay in business) makes the resulting price level really high and its already high enough. So I don’t use a dealer/distributor network that triples the cost and I price what I make just to get by. So anything that you purchase from me will provide a superb value or I won’t offer it. It’s as simple as that. The Pro Audio gear that I also represent has such low margins that I really don’t make a lot of money selling it. I sell it because I believe in using it.

Let me know if you have any questions. This guitar amplifier will be available by the end of 2020 and be a custom build (available by preorder) anytime thereafter. I will make only one of these amps available for sale at parts cost without any markups for my time or profit. The construction time frame on all custom order amps will be around 10 weeks and will require full payment for parts cost up front. The remaining amount will be due prior to shipping and will be for my time involved plus the actual shipping cost.

The chassis cnc metal and anodizing work and transformer construction takes the longest time with a 6-8 week approximate delivery time (from them to me) as a minimum. While waiting, however, I do build out the amp as far as I can (as you see in the photos shown below) so the final build doesn’t take as long and I can test and ship the amp in a reasonable time period thereafter.

I’ll update this page when this amp is ready for new orders

I Built A 2nd Top Plate To Fit Larger Rectifier & Power Tubes Base Diameters
I’ve Replaced The 2 eight pin tube hanging plates you see here with a new chassis design (photo at the top).

The Ideal Speaker & Cabinet For This Amp?

Here’s a photo of the new speaker cabinet that was created just for this amplifier:

The Ideal Speaker Being Mounted In This Cabinet?
The Very Musical 12″ Celestion Creamback 75, 8ohm


Repair Information

If you have any problems connecting this guitar amplifier to your current system, please let me know. If you perform any repairs yourself, you void any kind of warranty or repair policy that I might have. Also, if you purchase a component from me at parts cost without any markup for profit or my time, you also are on your own since there is no margin for my spending time or investing additional parts cost to repair something.

However, I will be glad to help you on the phone regarding possible problems that you might have with any of your audio components and I’ll be glad to share the circuit design layout, schematic, and parts list for this piece of equipment – always! This will allow you to easily repair (or get repaired locally) something that might have been “fried” from improper connections or other issues. This component is built to last a very long time and was one of my goals for its design. It is archival in build quality and should provide many dedcades of pleasurable use. All parts used are of the highest possible quality.

Ground loops are the typical issues that someone will have when connecting components that are designed “outside of the normal electrical specifications”. Some manufacturers do this to get their equipment to sound good. However, then you are pretty much captive to using their components. In my opinion, that’s not a good way to create a superb audio system. Make sure all of your components are built according to normal standards and work well with all other equipment (if of course, those components are designed properly).

I will be glad to discuss equipment compatibility with you prior to your purchasing anything that I build or sell