The Deuce

The Ultimate Guitar Amp?

All Tube, True Stereo, Single Cabinet Guitar Amp with Very Well Optimized Adjustable Distortion Generation for the Bluesman, and five tone controls per channel for optimal tone with any guitar. 35 watts rms per channel.

by Robert B. Richards, AE

copyright (c) 2008, Robert B. Richards, All rights reserved internationally.

(It's the one on the left)

Intro :

For well over a decade there have been many sound effect devices on the market that synthesize a great sounding stereo output. This was true when I bought my Boss SE-70 digital effects processor back in about 1996. Most guitarists I've known (me too) can't be bothered with dragging multiple amps to a gig, so we stay "traditional" - monophonic. Since the guitar itself is very probably monophonic, a stereo amp will still put the direct "core" sound out in mono, but then the digital reverb, delay taps, chorusser, phlanger, and etc. will radiate an ambience that is easier to hear and connect with, easier to be carried away by.

When it comes to the question of what sound effects an amp should have, it's arguable that the days of built in tremolo and reverb are a bit obsolete. Every guitarist I know of has accumulated many footpedals and/or bigger effects processors. They all have their favorites, and probably don't need or want the main amp cabinet to have somebody else's idea of what looks good on paper or in an advertisment. I'm talking about reverbs, chorusers, flangers, delay effects, hybrid effects, etc. So I decided that this amp should do a more limited part of the total process very well (tone, distortion and be stereo), and leave the rest up to the external footpedals or digital processors. There's a lot to be said for keeping things simple. This amp has a stereo preamp with five tone controls per channel (four passive Baxandahl (James) and a 6 position switchable high pass shelf), an input gain, a master volume, a stereo/mono switch, a pilot light and an On/Off switch. No other clutter on the front panel. The poweramp is 70watt stereo (35w/ch.), and there's two 10 inch vintage reissue Jensen speaker drivers for the best of bluesy compression, cone breakup and bubbly fat distortions. I tried to make an amp that Eric Clapton would be thrilled with.

This is the only all tube, true stereo guitar amp, with very good tone controls and of truly useful power levels, that I know of. And it's all in a vintage tweed style cabinet that is about the size of a Fender Twinamp or Twin Reverb.

More Features:

Although it goes quite loud clean, it's also very optimized for bringing the very best distortion sounds out of tubes. Harmonic distortion spectrum shape and intermodulation-modulation is optimized throughout, and choice of speaker driver was very specific for several reasons.

This is a no feedback design in both the preamp and poweramp sections, so well voiced distortion is there, but none of the drawbacks of feedback are there (slewing related distortions, loss of second harmonic distortion product, harmonic distortion products that extend substantially out from the fundamental, phase margin instability, recirculating I.M. generation, etc.).

The very slightly outward angled speaker panels create a better sense of stereo spread when you're back 10 feet or more, without the cabinet getting bulky and awkward to carry.

External speakers can be added or used instead if desired.

A second set of preamp outputs can be footswitched on and off. This can be nice when you want to switch on a P.A. system connection or additional guitar amps at each end of the stage for soloing.

 

Cabinet Construction:

 

Shell is solid pine, all bracing is oak.

Speaker panels are half inch plywood, to minimize weight and have good strength and not be at all warped. Any warp at all is no good..

The front is 3/4 inch plywood. My own old world design, as far as I know. Low-key vintage tweed look. Not horribly distracting, just hopefully artistically inspiring.

Out where the audience is, there will be noticeably more stereo separation due to the slightly angled driver panels.

Tweeding the front.

 

The Stereo Poweramp:

I tried to keep it simple for the sake of reliability. Since there's no negative feedback, I needed fewer stages.

I added a separate power supply for bias on the E34Ls. Otherwise I'd have to derive the approximately 20 volts bias (subtract) from the B+, which would reduce the final power out substantially. You want the amp circuit to see zero ohms from DC to infra-red, when it looks at the power supply, so it's frequency response at the speaker is somewhat consistant (this would substantially affect phase margin if it were a feedback circuit). I don't recommend chokes. They were useful back in the days when the biggest cap you could get was 25-50 uF. Now two 500uF J.J. brand (or equiv.) electrolytics with about 20 ohms between them and a 0.1 polypropylene across them makes more sense to me (guess I need to update this schemo a bit). If you use chokes and caps, you are creating a 2 pole filter. (If you don't know what you are doing with multi-pole filters, you could be creating a resonant circuit. Something that rings significantly every time a rectifier diode switches on or off - which they do 120 times a second.)

 

 

It's definately worth the time to lay everything out and see what fits where.

 

There's really only barely enough room in most places, to fit what you probably want to have.

 

I used all white dry-transfer lettering for the nomenclature. Fabricating, painting and lettering the chassis is a lot of work. I always put two coats of satin clear over the lettering.

 

Oh shit, now I gotta wire the dam thing...

 

I couldn't find a chassis that was a reasonable size and shape, so I bolted two 8 X 12 X 2 inch Hammond chassis together. I cut two big holes on the mating surfaces with a saber saw, so power and gnd wires could go where they need to. Three large bolts hold them together on the common surface, and the power transformer stradles the two chassis, thereby adding strength. Perfecto.

What isn't obvious is the 25 black wires returning to the ground star center lugstrip. It's near the lower-middle area. You want to have a separate ground wire returning to the star center for each section of circuitry, and have the star center generally near the main 500uF filter cap(s), not in the direct charging current path for the big filter caps (take a tap from there to what you will use as the actual star center "signal ground" bolt). Avoid having the star center ground very near where the third prong of the AC plug (the earth ground pin) ties to the chassis. This made a BIG difference in hum level.

The orange things are bias testpoints. There's a ground testpoint too, but you can't see it in the photo. Each tube has it's own bias pot right next to it. If you disconnect the speaker cables while the amp is in use, the built in load resistors automatically switch in, to protect the circuit from potential self destruction. I decided to add a Stereo/Mono switch in the lower right corner, for more flexibility. I could have both poweramp channels in this cabinet do one preamp output and a second amp on the other side of the stage doing the other preamp output (the other amp being optionally foot switchable). That's one of several ways the system as a whole could be configured.

Waiting for the preamp chassis to be completed and installed.

 

The Stereo Preamp Circuit :

This is where a big part of the magic happens. I wanted to have the 6 position highpass switch ahead of the 12AX7 distortion stage so I could vary the charactor or frequency balance of the distortion, and then have the regular tone controls after the distorter in case I want to roll off the higher order distortion products, create more of a reverse fletcher munson curve or just deal with whatever peculiarities a given instrument might have. The EF86 turned in a whopping high gain of 237, I double checked it. I chose to go big on the cathode bypass cap so sub-harmonics and I.M. difference products could effectively modulate the distortion generation in the next stage. Part of the distortion "casserole". The 1 meg resistor feeding the Input Gain pot is so the range of the pot is how I like it. The control will be at about its mid position when the distortion is just getting thick with a normal Stratocaster guitar. It also reduces I.M. distortion in the front end (EF86)by lightening the load on it. Some I.M. can be good, too much is not good. I want a good clean sound option too.

The transition into distortion is pretty gradual. I like the way it sounds. The passive tone sections have about a 12 dB loss per. The 12AX7 gain/buffer distortion generating stage has a measured gain of 71 or 37dB. The final output buffer is a self biased follower, which will have less distortion than the direct coupled type in the 12AX7 stage. I used 6SN7 octal tubes for the tone buffer and output buffer because anyone knows that octals are very sexy. 6SN7's are very linear so don't mess things up. They have been worshipped by certain hard core audiophiles for the pleasantness of their sound. They also provide a lower and more stable output impedance, which is definitely an issue when driving long cables. I wanted this to be able to drive 20 foot cables going to external guitar amps put else where in the jam room, when the footswitch enables that function (good for lead solo). 12AU7's might have been fine here. I later decided to put 50X attenuators on the foot switchable outputs (not shown here), since the additional guitar amps have way too much gain to handle that preamp output level (10K in series and a 200ohm to gnd.). Slightly less attenuation would have been better.

Although as shown, the circuit sounds pretty darn good, there were some final tweaks done to the distortion section (the 12AX7) that changed the nature of the distortion sound a little bit. The actual values and topology are very similar to the 1959 Fender Bassman or 1957 Fender Twinamp.

The footswitch connector could be wired for a double footswitch, if a second function was desired. The jack is stereo and the extra connection is wired as a redundant ground connection here. I may eventually modify this so the second footswitch bumps up the pre-distortion gain by maybe 3-5 dB (could make that adjustable). I'm using a Marshall double footswitch that I bought years ago for another project that got dismantled.

I've actually changed the B+ filter cap arrangement since I drew this. Now I have two 100uF caps in place of the one 470uF cap, with about a 200 ohm 5W resistor between them. It works much better (much less ripple) than the single bigger cap. It's hard to find exactly the transformer you want. Custom's are too spendy for me. The shortcoming is the current rating of the 5 volt winding, and height of the B+ windings. When you convert the 5V winding to DC, you approximately double the current draw because of the loading of the large filter cap, so the winding could only handle the filiments of the pentodes and 12AX7's. The 6SN7's have AC on their filiments. The hum is quite low for a guitar amp. How I grounded the star center of the circuit to the chassis (in each chassis), and where I tied the green wire (earth ground) to the chassis made a huge difference in hum level. That's something that doesn't show on a schematic. You need a Ouija Board to get that right.

Oh shit... now I've gotta wire the dam thing...

 

Parts on the left, tools on the right. I had to remove all the pots and rotary switch from the one channel in order to wire the first channel. It's pretty tight in there (4 X 3 X 17 inch Hammond chassis).

You want to fully test the first channel before you wire the second channel. Changing parts later in such a tight space will be major surgury - not fun.

Below is part density before adding the 2nd channel tone circuit components and wiring.

I guess I like challenges.

 

You want to actually put it in the cabinet and use it for at least a week, do pink noise cal'd mic tests and etc., before wiring that 2nd channel.

Below it's all done - both channels.

Deep inside the forest is a secret garden... or so they say.

Too late for changes now.

Oops no, I changed my mind, the Hi-treble control had too little effect in the range that the speakers reproduce.

Later: Oops no. Now it's too sqeaky sounding. I went too far. I'll change it again.

Modelling the circuits with a Duncan program or equivelent is a great idea, but there's no substitute for real-world listening.

Never did find a secret garden.

I almost forgot to ground my pentode shock-plate. When it wasn't grounded, touching it would cause a small but noticable increase in hum. The shock plate, made of heavy 1/8 inch brass on very fancy soft rubber grommets, keeps the amp from getting microphonic feedback when the gain is set to maximum (it actually works too). It was pointed out to me that you don't want to use tube shield covers on the somewhat microphonic front end pentodes because the spring inside them will resonate when stimulated physically by the speakers. The EF806S (J.J.'s version of the EF86) which I'm using, already has an internal shield. I think the shield compatible sockets are nice though since they shield the pin area of the tube better.

Above, you can barely see the footswitch jack between the filter cap and the fuse. The IEC AC connector on the short cord is for the poweramp. That way you can have the ON/OFF switch on the preamp turn on and off both the preamp and the poweramp.

Above you can see the relay board for the footswitch mounted right on the footswitch jack. The orange thing is the relay. The big filter cap inside the chassis is for the 6.3 volt DC for the front end tube filiments and pilot light.

 

Preamp Waveforms and discussion:

Below is the EF86 being slightly overdriven. Very smooth asymetric distortion .

This particular distortion is not likely to happen much without an external gain pedal, which might be a nice addition later. The Input Gain pot being right after this stage makes that possible. Because of the way the screen grid of the pentode is wired, there is some modulation of the attack and decay, which I believe is an important part of the sound. Tone burst tests will show this. I haven't gotten around to taking pictures of this yet.

After the EF86 pentode, hi-pass shelf switch and input Gain control, the signal goes to the 12AX7 distortion generating stage.

Below is the 12AX7 "Full-Wave Saturator" circuit distortion waveforms:

To my knowledge, the 12AX7 gain stage direct coupled to a follower buffer was first used in the 2nd or 3rd Fender Twinamp (the '57 e-type, for example, which appears to be Eric Clapton's favorite these days), and in most of the early Bassman circuits (the '59 for ex., which was essentially copied in the early Marshall JTM 45). Certain other Fenders as well, but not all. Twin Reverbs and most Bandmasters didn't have this. Showman's either. It's arguable that any "blues" amp should have this, regardless of what else it has. It distorts gradually and sounds good (not recommended for Hi-Fi). I call it a "full wave saturator" because both half cycles get clipped by a tube saturating, even though the positive half cycle saturation happens indirectly.

Left photo is 12AX7 gain stage plate just short of clipping, right is same just in clipping.

 

Below left is the follower output when it just starts to have the "flat spot" on the negative half cycle. Middle is with a little more input drive. Right is same with even more drive.

The follower starts distorting noticeably before the gain stage would actually clip. The positive half cycle gets lopped off when the direct coupled follower starts drawing grid current, and the negative half cycle gets the flat spot when driving a load that is even slightly capacitive (30Kohm in series and .01uF to ground), such as a tone control circuit.

 

Pink Noise Tests:

The test setup:

Not an ideal setup at all, but maybe revealing enough if you know how to interpret the results.

So you get the preamp to look good on the bench, waveforms and such, but what does this "e-phase" version sound like in use, and how does it measure acoustically with pink noise and a calibrated mic? Again, now is the time to do any tweaking of frequency response, tone circuit, 12AX7 distortion mechanism, and whatever else, before finishing wiring that 2nd channel.

Amp about a foot out from wall in my living room, mic at about 5 ft. Not exactly "Hi-Fi". Tone controls not centered.

I usually have my amp about a foot out from a wall, since my place is small and it's in the way anywhere else. To see how many of the frequency response anomolies are primarily a function of the acoustics created by the wall, I then pulled the amp out and rotated it so there's no wall near its rear or front, and mic'd it far and close. I was hearing the possibly resonant peak around 1kHZ, and it was bugging me when I played either my metal string acoustic (Taylor) or my Strat. I wanted to measure just how much of it was my room and how much was the amp. The wavelength of about 1kHZ is about 1 foot, so maybe putting some acoustic absorbtive material inside the cabinet would take the hurt out of that some.

Amp away from walls and mic 5 ft. out. Tone not at mid.

 

Same as above but close mic'd. Tone not at mid.

Then I played with the tone controls and close mic'd that.

Tone controls all centered, shelf off, close mic'd, minimal walls.

This is not a perfect test setup (Behringer Ultra-Curve Pro RTA and Pink noise with Behringer cal'd mic on stand), but close enough if you know how to interpret the results in my opinion and pretty important (much better than nothing).

All tone controls at minimum position.

 

All tone controls at max position.

The speaker drivers I used are the 40 watt version of the Jensen vintage reissue P10R, called the P10Q. I wanted to have the sound be as close to the P10R as I could, since I loved how they sounded in another amp I built. The Jensen 100watt Neodymium driver didn't sound nearly as good (very poor high frequency extension - very "honky" sounding). I tried those, and then sent them back. I really like the sound of the P10Q. It pretty much has the same high end extension of the P10R, with a more practical power rating.

 

After Acoustic Padding is added:

I've glued some 1/4 inch thick wool felt to the inside of the cabinet wherever there was a place to put it, in the top area and sides. I wanted to be relatively thorough so I might learn something from it. After playing for a whilewith all tone controls midranged, I turned up the lower midrange tone control to where I know it to be mostly flat with the calibrated mic and pink noise (3 o'clock). I played it like that for a while as well. Playing my strat through it with the insulation, it sounds very good to my ear. Although the difference is small, and generally doesn't show up on the very improperly done calibrated mic tests, I think it helped some but not a big change. Here's some pics:

 

The added insulation (the amp is upside down). The poweramp covers pretty much the whole bottom, so no insulation there.

 

Some days it seems like my whole world is sideways.

 

With

Close mic'd, tone controls all centered, cabinet away from wall with insulation (above). The 170HZ peak is probably about the room (due to the wavelength (about 7ft.) and absence in other graphs), rather than the amp.

 

Without

Same thing but no insulation. I'd only trust this graph comparison in the mid frequencies due to variations in the angle of the amp in the room and exact location of the mic relative to the speaker. From say 300HZ - 4kHZ maybe. Not a big difference that shows with this test method, but I think I hear a significant difference when playing through it. I'm pretty sure I like it better with the insulation. It may have lowered the Q on whatever resonances there are in the cabinet, that might have been active around 400HZ - 7kHZ. I mention those frequencies because they are the ones who's wavelengths fits inside the cabinet dimensions in various ways, and can therefore resonate.

 

As above but amp near wall; about a foot out, mic at about 5 ft. Carpeted floor.

I would think it could still benifit from an attenuation of the frequencies around 500 - 1.2kHZ, but the test with no wall and mic at 5 ft. suggests that the amp is not that bad and it's the nearby wall that is actually pumping up the 500 - 1.2kHZ frequency region. Looking at the wavelength of those frequencies (6 inches to 2 feet), I think it's believable. Those frequencies can be effectively attenuated somewhat by turning up the lower and upper midrange tone controls a bit.

Tone controls set by ear to sound better with strat (close mic'd). This graph suggests that the low-Q slight notch should be around 1.3kHZ.

I'm not sure what caused the notch at about 2kHZ, but since that's above the highest note on the guitar (somewhere around 800HZ), I'm not real worried about it. I think that section of frequency, 1kHZ - 5kHZ, has a lot to do with the overall sound of the amp, since those are the frequencies that the ear is most sensitive to. The 2kHZ notch probably has more to do with the fact that two physically displaced speakers are feeding one mic.

I tried to have a Deco look.

Poweramp in the bottom, preamp in the top. This amp, with 35 watts per channel, goes very dam loud. The distortion circuit also sounds very good at very low levels (when the poweramp and speakers aren't at all overdriven).

A musician wants an amp that is intuitive, easy to adjust, and doesn't distract him or her away from the creative part of their brain. Achieving that on a stereo amp is perhaps even more challenging.

You've got Input Gain, a high-pass Shelf 6-position rotary switch, James style tone; Lo, Mid-Lo, Mid-Hi, and Hi variable slope, then a Master volume and a Stereo/Mono switch. Just what is needed and no more. The signal flows from left to right through the knobs. As intuitive as it gets.

 

I feel pretty good about what I ended up with. The distortion is bubbly and thick but not whimpy, not at all fingernails on the blackboard like many amps I've heard (possibly all transistor amps and some tube amps). It has lots of "bluesy charactor". It makes you feel good. It's probably the best sounding amp I've ever played through. It also sounds exceptional when adjusted for no distortion and used with either an electric guitar or an acoustic guitar (perhaps especially with my nylon string acoustic guitar). Every guitar has it's own frequency response "signiture". They vary a lot. You want to have the tone control flexibility to bring out the best in any instrument, whatever that is.

Warning to amp builders:

It's been horribly labor intensive to build (all point to point wiring), and parts alone were over $1500. .Also, despite a strong effort to keep the weight down, it ended up being 68 lbs (a '57 Fender Twinamp e-type reissue is spec'd at 52lbs). I guess that's why nobody makes all in one stereo tube amps of this power rating. Be warned if you're a builder. If I were to go into production with this, I'd definately use circuit boards and switching power supplies (to reduce the weight by almost 10 lbs. and make assembly much more cost effective). I had decided to just go buy a Fender Blues Deville for around $700, until I went and played through one. It was very bright and just not pleasant to listen to. I felt that I had no choice but to research, design and build an amp of my own. My amp sounds WAY better to my ear.

 

This amp doesn't do everything, but what it does do, it does pretty dam well.