Category Archives: Technical Things

EF86 Vox AC15

I’ve recently become interested in guitar amps that use a pentode input stage. Compared to a triode, such as a 12AX7, a pentode usually will have much more gain but the drawback is they can be noisy.
What I’v noticed is the pentodes seem to lend themselves to playing full guitar chords. Where an overdriven 12AX7 seem to work well with 5ths, a pentode handles more complex chords better to my ear. I believe it has to do with the even order harmonics that pentodes produce so well.

Here is a demo of an AC15 that has a nice comparison of the EF86 pentode channel and the top boost channel that uses more traditional 12AX7s.

Guide to Single Knob Tone Controls

This is a listing of some one knob tone controls found in tube guitar amps.
Most are designed to control the high end, meaning they either add high end or roll off high end. Toward the bottom of the list there are also two tone controls that control the bass and two that control the mids.

NOTE: The example schematics I show might have different component values than the original from the schematic.

I’ve added a note indicating if the control is complicated to implement and how “lossy” it is. More “loss” means more signal is lost and the volume is reduced.

1) Tweed
The tweed tone control is found in many amps, most notably early Fenders like the 5E3 and the Princeton 5F2A. Turning the tone control up causes it to act like a bright cap on the volume knob increasing the highs. Turning it the other way rolls off the highs.
Complexity: Simple
Loss: Very Low Loss




2) Gibson GA-30
The GA-30‘s treble control uses a small cathode bypass cap on one end of the pot to provide a treble boost. The closer the cathode bypass cap gets to ground the more high end boost there is. The other side of the pot connects a cap to ground so as you turn that way it rolls off treble to ground.
Complexity: Simple
Loss: Low Loss




3) Bitmo / Fender 5F2
The Bitmo, which takes it name from a kit for the Valve Junior, is very similar to the Tweed except instead of being built on a volume pot, it is built on a resistor. The size of the resistor can make a difference in how the effective the tone control is. I like using a 470k resistor because at it’s highest setting it simulates a traditional Marshall treble peaking circuit. The Fender 5F2 is very close to this design also but the treble cap connects at the other end of the coupling cap.
Complexity: Simple
Loss: Moderate





4) Specialist
I found this tone control in the Marshall 2046 Specialist. It’s a stand alone tone control and works reasonably well but it’s not my favourite.
Complexity: Simple
Loss: Low Loss




5) Vox – Cut
The Vox cut control is a pot and capacitor put between the two outputs of a phase inverter. The two outputs cancel each other out as the pot is turned. Without the capacitor it’s a master volume but with the capacitor its a tone
control because the capacitor limits the frequencies being cancels to the highs. Obviously this only works in a push/pull amp.
Complexity: Simple
Loss: Low Loss




6) Treble Cut
This is probably the most basic tone control and found in many old, low end amps. It’s just a pot and capacitor like the Vox Cut but it bleeds treble to ground rather than the inverting side of the phase inverter.
Complexity: Simple
Loss: Low Loss




7) 18 Watt – Normal Channel
This is the tone control from the original 18 watt and used on the normal channel. I’ve never tried this one.
Complexity: Simple
Loss: Low Loss




8) 18 Watt – Tremolo Channel
This is the more popular of the 18 Watt tone controls and it’s used the the 18 Watt Lite. It’s very similar to the Tweed tone control but it wired slightly differently and uses different value pots.
Complexity: Simple
Loss: Low Loss




9) Big Muff/Supro
In the Big Muff tone control the signal goes into both a high pass filter and a low pass filter. The pot acts a bit like a balance control between the two filters. The Supro Thunderbolt uses this tone control. Since the Supro predates the Big Muff maybe this should be called the Thunderbolt tone control?
Complexity: Moderate
Loss: Moderate




10) TMB with fixed bass and middle resistors
The classic Fender (and later copied by everyone) TMB tone stack, used in hundreds of amps, can easily be used as a single knob tone control. By replacing the bass and middle pots with resistors you are left with just the treble control. When replacing a 250k bass pot an 82k resistor works pretty well simulating a pot in the center position. 120k works pretty well for replacing a 1M bass pot in a Marshall tone stack to simulate that pot at noon. When choosing the mid resistor, usually half of the pot value is a good starting point. Make it bigger for more mids and smaller for less.
Complexity: High
Loss: High Loss




Bass Controls

11) Gibson Bass Control
Most tone controls on this list deal with the high end. This one deals with the lows. Also found in the Gibson GA30, this is a stand alone bass controls that uses capacitors of different sizes to control the bass. As the knob is turned more signal is sent through the smaller capacitor thereby reducing the low end. The Matchless Chieftain uses this bass control.
Complexity: Moderate
Loss: Moderate to Low




12) Coupling Capacitor Selector
This is also more of a bass control. Instead of a pot it uses a multi-position switch to select different size coupling capacitors. The smaller the capacitor the less low end with pass to the rest of the circuit. It’s recommended to use
high value resistors (10M) between the capacitors to eliminate “pop” noises when using the switch. The Matchless DC30 uses a switch like this.
Complexity: High (depends on how many capacitors are used)
Loss: Low Loss





Middle Controls

13) Gibson Mid Control
Another handy circuit from the Gibson GA-30. This is a stand alone mid control.




14) Framus Mid Control
This one is found in the Framus Cobra and I greatly prefer it over the Gibson mid control. It’s possible to get very nice Fender scooped tones with this one.


How Tube Amps Work

I came across a pretty well done article about how tube amps work. If you are curious about what is going on under those knbs and tubes head over to Premiere Guitar and there is a 5 page article that covers the basics. They even discuss a tremolo circuit. The article focuses on a Vox AC4 as an example but the information can be applied to just about any tube amp.

Bud Boxes

Recently I came upon an Amazon gift card for $25 bucks. Being an amp head I immediately decided to get something towards amp building. I need a chassis and after a little research found I could get a Bud Box from amazon and basically get free shipping too. They have a bunch of sizes but I just got this little one. It’s about the size of a Valve Junior chassis.

Free shipping over $25 bucks.


Reamping for Guitar Tone Goodness

There is a nice article at Sonic Scoop about reamping techniques. This is sending your recorded guitar signal back through an amp again to fix/alter the tone.

He uses an example where the guitar tones were not tweaked to the fullest potential due to studio budget restraints so they used reamping later to be able to get the tones they wanted.

It’s good info so check it out.

Electro-Harmonix 6V6 EH

I am continuing to tweak my little single-ended build and I’m thinking power tubes now. Currently there is an old Admiral tube in there that I got on Ebay a while ago. I don’t have any complaints with it but at some point I’ll need to do some experimenting.

One 6V6 that I’ve had terrific luck in the past with is the Electro-Harmonix 6V6 EH. I’ve used them in single ended amps and push-pull. A a really nice thing about them is that they can be run at all sorts of voltages and still sound great. I’ve run them at 425vdc and at 335vdc and it always delivers. They have a nice crunch to them, almost like an EL84.

At some point I know I’ll try one in my current project. It’s tough to beat a great old tube but I’ll give the EH a chance for sure.

How a Vacuum Tube Works

I’m about to get a little technical about tubes here but bear with me.

The vacuum tube is, without question, an ancient electrical component. RCA sold the first tubes commercially back in 1920, almost 100 years ago! That being said, the vacuum tube is still the standard when it comes to building guitar amplifiers. As DR. Z said in the video in the previous post, tubes have a warmth that solid state can’t match.


So they go in guitar amps and make them sound good but what do they do actually do? Surfing around Youtube I found this video that tries to explain how a vacuum tube works. It begins with an explanation of how a diode works and then moves on to explain the triode. Triodes are very common in guitar amps, mostly in the preamp but also used in the output section of some amps such as the Vox Lil’ Night Train.

What makes this video stand out to me is that he uses a water analogy. He compares the function of a vacuum tube with the function of a hydraulic system and I think it really helps to explain how a tube works and what the different components of a tube do.

I don’t think you need a technical background to understand what he is explaining. I think he gives just the right amount of information without it being overwhelming or not explaining something clearly.