Guitar companies are making models for the masses while the instruments artists play are typically modified to get special tones in the instrument rather from amps or software (see the 'Schematics' page). All companies have custom shop brands or options where you can get this done for $5 - $10 thousand +. Since 2008 a lot of info has been available on the internet revealing schematics and modifications to get the artist tones. This opens the window to have special instrument tones in your guitar or have a mod guitar that has the features of a $$ custom shop model. This blog will pick a few projects to get classic tones and increased flexibility on board. Not for everyone but it is a matter of style, understanding what goes into a custom shop $10 thousand guitar and why great players use a hand full of onboard modifications to trademark their sound.

Second Hand -"It's probably a well-known story . . . I went into a shop in Nashville called Sho Bud which was owned by Buddy Emmons – the famous pedal steel player – and they had things like Rickenbackers in the front of the shop going for quite high prices. In the back they had this second-hand department, and there was a row of Stratocasters, and I bought them all. Blackie was made out of three of these guitars – the body of one, the neck of another and the pickups of another." Eric Clapton



Blues Junior Amplifier Modification




Intro and Background- Why are tubes still around and what is in the box


"Once upon a time, vacuum tubes were used all over the place. They glowed their little hearts out in our television sets, car and home radios, hi-fi systems, and guitar amplifiers, and were crucial components in myriad military applications, from radar technology to missile guidance systems and more. Bit by bit they have been replaced in all of these functions by other forms of more compact and more stable technology… except in guitar amps, where they maintain their preeminence over all kinds of far more advanced electronics. Is this just nostalgia, or mere perversity on the part of guitarists? Not in the least: when used to amplify electric guitars, tubes still simply sound better than anything else out there. Sure, there are some respectable sounding solid-state amps, and digital modeling amps have also made inroads into the market, but ninety-nine out of a hundred serious pros (if not more) continue to use tube amps for both recording and touring, and these little glowing bottles still define the cornerstone tones of rock, blues, and country guitar." David Hunter article Premiere Guitar  
Historically tube amps were put together using a relatively small selection of tubes taken from military and radio applications. Advances in the technology after the war enabled "hifi" in the fidelity power range. Widespread low cost use during the 60s was the hayday then eventual decline as transistors and digital circuits took over.   The tubes amplify the input signal and do this with a 'voice' rather than crystal clean amplification (voice=some signal distortion resulting in nice round waves instead of square waves). Variations in capacitors and condensers in their circuits give them different types of tones which have become the characteristic sounds of Fender or Marshall or other signature amps. 

 The origin of the tube amp business for guitars started with Fender amps during the 40s as the electric guitar business developed. Later Jim Marshall modified a Fender Bassman circuit changing the preamp tubes and started Marshal Amps (British company 1962). Randal Smith had a small California shop modifying Fender amps and did a few changes to the Fender Princeton that caught on launching Mesa Boogie (California 1969 termed 1st boutique amp company now mass production).  These 3 amp companies are the real legacy kings and if you look up any famous guitarist they have spent time on stage with one of these amps at one time in their careers if not throughout their careers.

Whats inside the box- the typical circuit is:

 Signal from mic or instrument  >  Preamp Stage  >  Amplifier Stage  >  Speaker
Typically before the signal hits the preamp you can adjust the gain (power to the preamp tubes), treble, midrange, bass to change “shape” of the sound. The preamp then sends the modified signal to the power amp, which amplifies it in preparation to drive the speaker. Using the master volume control, you adjust the amount of power that is sent to the amplifier stage tubes which increases or decreases the speaker volume.

 The preamp is needed since the instrument signal is to low to efficiently be the input to the amplifier stage to give you nice high volume.  Preamp tubes  are the smaller tubes in the amp and most are 12AX7, 12AY7 or 5751s which basically operate at different gain levels.  They tend to have limited impact on voice quality but many musicians - including Stevie Ray Vaughn- and boutique amp companies play with different tubes in this stage to drive more distortion before the signal hits the output stage.  We will skip the preamp for now and look at other changes that make a bigger difference. 

 Where it gets a little weird is there are hundreds of amps out there and they all use a relatively small number of amplifier stage tube types.  There are just not that many types available in the power ranges needed for a sound amplifier.  Almost all amps use these main power tubes: 
  • 6V6 & 6L6: Fender is known for using 6V6's in their smaller amps and 6L6's in their larger amps. They have a lot of clean headroom so you get more volume from the amp before they start to distort. 
  • EL34 & EL84: Marshall is known for using EL34's and EL84's in their heads. These have less clean headroom, so they'll distort at lower volumes, but the distortion is usually warm and smooth.
  • The EL-84 power tube is little brother to the EL-34, the tube found in Marshall and Orange heads. The lower-powered EL-84 is often characterized as having a little bit of the EL-34′s trademark “bite” and mid-range push, but with a little less low bottom and a slightly more supple sound. Fender also uses the EL84 with circuits that optimize a cleaner Fender legacy sound.
The standard stock amps made by the big companies today are the common sound for mass production but often there are issues with the models like some cheap parts, poor bass response or limited high end. This usually results in a player having someone do amp modifications to give the amp a better sound or response profile. This is not about the effects you put on the sound it is about opening up the dynamic range of the amp.  A classic example is an amp that is always played with the bass, mid or treble wide open. It would be much better if you got this sound with the controls set half way.  The other common example is an amp where the bass sounds the same at any setting above 2 or 3. 

 The flabby sound quality has resulted in an industry of amp techs that modify these amps to meet a players needs.  It also results in high end custom/boutique amps to get better sound from the same tubes.  The reality of custom/boutique amps is they are expensive and you are really paying for a few high quality resistors, capacitors, potentiometers and circuit wiring changes.  Granted the boutique amp companies will stress the importance of woods used in the cabinets, US assembly and handmade point-to-point wiring but this is really minor when compared to quality circuits. 

 If you are buying a tube amp first decide which tubes you prefer by listening to amps or artists that favor certain amps and you can look up those amp models and tubes.  Here is a neat video of Billy Corgan who has used many different amps through the years and for touring he had the tube amps custom modded into different programmable heads- like a very high end custom Line 6 amp.  It shows how he used all these different tube amps for signature tones in his music and not many pedal effects. 

 Billy Corgan rig rundown

Project- Improving a Small Combo Tube Amp -  Selecting the Amp


 Everyone should have a small combo tube amp. Something that would be good for home practice, studio or a small venue gig without dragging around big suff and speaker cabinets.  Ideally the combo amp can also act as a amp head so you can plug into other cabinets or slave to other amps if needed. These models are small grab-and-go amps that give all the volume you need before you add effects.  The volume and sound from a tube amp based on the watt rating is much louder than a solid state amp so 12 to 25 Watts is plenty for a small venue.  
  • Fender Champ- 1948 through 82 and vintage model since 2006. 5 watts and single 6V6 tube $400 to $1000 used.
  • Fender Deluxe- Stevie Ray and Neil Young favorite- 22 Watts, 1X12 speaker 6V6 tubes and 2 channel $969
  • Fender Princeton – 12 Watts, 1X12 speaker 6V6 tubes and 2 channel $999
  • Fender Blues Junior ‘II’ 2005 to 2010- 15Watts, 1X12 speaker EL84 tube $400 before the ‘III’ came out.
  • Fender Blues Junior III-15Watts, 1X12 speaker EL84 tubes $529
  • VOX AC 15C1- 15Watts, 1X12 speaker EL84 tubes $600 to $900
  • Marshall DSL 15C - 15Watts, 1X12 speaker 6V6 tubes $500
  • Mesa Boogie Transatlantic- 15Watts, 1X12, 2 channel EL84 tubes $1700
  • Orange Tiny Terror combo- 15Watts, 1X12, 2 channel EL84 tubes $900
  • Matchless Lighting 112 – 15 Watts one channel 1X12 EL84 tubes $2500
  • Top Hat Royal 20- 20 Watts 1X12 EL84 tubes $2000
  • 3rd Power Dream Weaver- 12” speaker 18 & 38 Watts using EL34s at 60 pounds and $3300
If you want to spend more time with writeups and videos of small EL84 tube amps this has 9 amps in one article  Premier Guitar Champions of Chime EL84 Amps

 So these are all single speaker 1X12 amps up to 25 Watts with prices that range from $500 to $3000 using just a few tube combinations.  Some have a lot of flexibility with 2 channels and impedance switching 4/8/16 ohms so you can run off different speaker cabinets.  Some are very limited with only a volume and tone knob (Fender Champ and some of the boutique amps). The high priced amps also use point-to-point wiring where they are using soldered wire circuits instead of circuit boards to make connections.  The low cost amps suffer from cheap components that limit sound quality. 

 Warning- You can kill yourself modifying an amplifier since the circuit stores high voltage.  Nothing here is an instruction on how to stick your hands inside any electrical equipment and make changes since it can make you fly 10 feet across the room or burn down your house.

 The project is to take a low cost tube amp that comes standard with lousy parts and do some modifications opening up and improving the sound to see if it can approach some of the sound qualities of the crazy high priced amps.  The first step is to pick an amp to work with and we will use the Fender Blues Junior. Below in the Background section is the Fender specs and manuals are available on the Fender site. 
Blues Junior Combo Tube Amp


Control Panel- on the Blues Junior the 'volume' adjusts the preamp power so it is 
the same as the gain control on most other amps and the 'master' adjusts the power to the main power tubes so it is the same as the volume control on most other amps.  
Dont ask me why they did this.
























We are using the Blues Junior because it is cheap and has a good foundation to work with. There are a lot of them available as used on the internet so you can pick one up for a good price.  There are ‘collector’ models with tweed or different cases but they all have the same circuit and tubes. I was able to purchase a 2010 for $300 in excellent condition after looking for 2 weeks. 

 There is a lot of info on the internet with modifications for amps but the info on the Blues Junior is very detailed with lots of options. Here are a few good sources:
Here are some videos of Blues Junior amps with mods

 Amp Upgrades and Modifications

 After a lot of web research here is what will be added/changed in the amp:
  1. Output transformer- TO22 from Dave Allen switchable for 8/4 ohms http://www.allenamps.com/parts.php
  2. Presence control- mod based on BillM info
  3. Standby switch- mod based on internet info
  4. Reverb tank replacement - MOD 8EB2C1B
  5. Auxiliary out jack- for connection to external cabinets or slaving the amp and will automatically turn the internal speaker off
  6. Line out jack - for connecting to mixer, computer or other device that needs a low signal
  7. Tone stack upgrade - Fromel
  8. Bias modification for running the tubes cooler - Fromel
  9. Electrolytic Capacitor upgrade to higher quality power caps and increasing the capacitance of 1st stage filter - Fromel
  10. Wire Dressing to reduce hum – BillM board pics

Cost for Parts- $170 total 

  • The parts in the Fromel supreme kit is $25 and the actual kit with instructions from Fromel (recommended) is $70
  • Standby switch and presence control parts $10
  • Line out and auxiliary cabinet jacks with resistors $8
  • New reverb tank $17
  • T026 4 ohm/8ohm output transformer $65 from David Allen
2nd Warning- People have died sticking their hands in amp circuits that store high voltage.  Nothing here is an instruction on how to modify any electrical equipment.

 Reverb Tank
The stock reverb tank is a thin 2 spring tank that has a history of failure at the RCA connectors.  I went with a MOD 8EB2C1B and here is a description of the tank:
“Replacement Reverb Tank For 8EB2C1B, Black Finish Voiced To Sound Like the Original Tanks From the 60's Solid, Sturdy Construction Short (9 1/4") 3 Spring Unit, Medium Decay Input Impedance 800 Ohms, Output Impedance 2,575 Ohms.

Testing the amp with the original tank, 
MOD 8EB2C1B and an old larger 
70s amp  reverb tank.  The MOD tank
was a big improvement and sounded best
The Transducers are wired directly to their respective RCA jacks as opposed to current production tanks where Transducers are connected by a detachable plug to their respective RCA jacks. This makes the tanks less receptive to any outside interference. The original Hammond Accutronics tanks from the 1960s were also wired directly to their respective RCA jacks. In addition very close attention has been paid to the spacing and size of the lamination of the Transducers resulting in a more vintage like tone.”
Standby Switch

 It is not required to have a standby switch but it does help to heat the tubes and also use it as a mute switch.  

This is the wiring for the existing switch which is a 2 position 4 lug on/off switch and this can be changed with a on/standby/off switch using a Carling 3 position 4 lug switch.  








I got the wiring from looking at internet pictures of modified amps and seeing where they made the changes.  




Output transformer- TO22 from Dave Allen switchable for 8/4 ohms

This swap achieves 2 things. The amp is noticeably louder and gives the option for a 4 or 8 ohm output so you can hook up to external speaker cabinets.  There is plenty of power here and with the new transformer playing at home is usually on volume and master settings of 2 or 3.



Stock output transformer - left
Allen 4ohm/8ohm -  right

Power spectrum T022 from BillM
as Bill notes low distortion in the
bass and improved tone throughout
Power spectrum stock from BillM





Adding the auxiliary cabinet and line out jacks

 You can damage a tube amp by turning it on without a speaker plugged in so the safe configuration is to keep the internal speaker in the circuit and automatically turn it off when you plug into a new cabinet. Most amps use this and that's how this circuit is set up.

 The wiring here puts a switch in the circuit to select 4 or 8 ohm output so it will normally be set at 8 ohm for the internal speaker.  The signal then goes to a switchable jack as the external auxiliary for the extension cabinet that when in use will cut the signal to the internal cabinet.  This allows the internal speaker to stay plugged in and off while the external speaker is used. The line out allows you to plug into a mixer or other low signal device knocking the signal down by a ratio of the resistors.  The output is knocked down by about a factor of 4.5. 


















If you want to use the internal speaker while using external speakers you need to wire the external jack in series or parallel then do the calculation and set the impedance switch to the proper impedance before you plug in.   Most amps just turn off the internal speaker so you only have to set the impedance switch to what ever lines up for the external cabinet- less to think about. 

 Another approach is needed if you want to use the internal speaker but balance the volume of the internal speaker independently of the extension cabinet. In this case you need to install an L-pad between the output transformer and the internal speaker which will allow control of the power (volume) of the speaker while looking like 8 ohms to the transformer. 

 Parts Express Speaker L-Pad Attenuator 50W Mono 1" Shaft 8 Ohm Part # 260-255 





Presence control


 This is a BillM mod and you should buy it from him.  His site is temporarily suspending orders due to his health so checking the internet you can get the details of the change but basically R25 is replaced with a 10K pot.  

 This achieves the following from BillM " The presence control affected overtone frequencies in the human voice range, making the voice more “present,” hence the name.  But the presence control is not part of the tone stack. Instead, it controls the amount of negative feedback in the output stage. Negative feedback controls the accuracy of the output stage’s reaction to the signal sent to it by the prior stages in the amplifier. With no negative feedback, the output stage tends to overshoot and exaggerate the high midrange and sharp, percussive sounds. With too much negative feedback, the amp sounds slow and mellow; pick attack is muted.

 The presence control lets you adjust presence from a brighter, louder tone that puts an aggressive edge on your pick attack to a more laid-back sound that’s ideal for jazz or a warm, mellow tone that swallows pick attack and lets the note bloom out. The stock value for presence is at about the 1 o’clock position on the control"





Capacitor and Resistor Changes - http://www.fromelelectronics.com/



 Tone Stack - Replace the awful Bass and Mid caps with high quality WIMA caps. The new caps have corrected values to open up the tone and get rid of that boxy sound. Note other high quality caps rather than WIMAs can be used but the values are important. 

 The Treble cap is replaced with a high quality Silver Mica cap. The adjusted value shifts the mids up very slightly, the result is an amp that sounds huge and warm, not boxy and thin.

 Twin Mod - The mids on the amp have a flabby bottom. This mod enables you to take all the mids out of the tone allowing more flexibility and versatility. This is accomplished with just a short jumper wire on pins 2 & 3 of the treble control but the result is far better control of your tone.
From BillM shows where to short
the treble pot on the back of the board
Power Stiffening - Doubles the capacitance in the first filter stage, this reduces hum and increases reserve power for solid bass and strong picking, 

 Cooler Bias - . This mod lets the amp run cooler without sacrificing tone by changing R52 to 27K ohms. The new bias resistor sets the on the tubes to about 8.5w should let them run cooler like most Fender amps and add to tube life.

Electrolytic Cap Upgrade - Replaces all of the power filter and cathode bypass caps with high quality F&T and Nichicon caps. The current Blues Juniors made in Mexico are known to have leaky filter caps.  The Fromel kit rectifies these issues.


























Proper Lead Dress - 


 Take a look inside a boutique tube amp and you will see all the wires carefully twisted and placed for greatest reduction in oscillation and noise. The Blues Jr has all the internal wires zip tied together in parallel so the dressing needs to be improved. 

 I checked online photos including some of Bill Machrones work to see what the lead dressing looked like in addition silicone was added at the base of the ribbon connectors and the flabby connections on the wire to the reverb tank.

Stock wiring with a few nylon ties removed

Amp after all mods and lead dressing

Sound Test 

 I had a stock amp around and was able to compare it to the fully modded amp. The changes in the sound of the amp are dramatic- my wife even thinks so and she rarely comments on any equipment stuff.   Here are a few videos testing the amp compared to a stock Blues Junior III and comparing it to the Fender settings on the Egnator Tweaker 15 amp which uses 6V6 tubes common to smaller old Fender amps.

 Blues Junior Mod Compared to Stock


Blues Junior Amp Specs and details from the Fender website:

ELECTRONICS
VOLTAGE: 120V
WATTAGE: 15 Watts
CONTROLS: Reverb, Master, Middle, Bass, Treble, "Fat" Switch, Volume
CHANNELS: One
INPUTS: One - 1/4"
SPEAKER JACK: One 1/4" Mono
FOOTSWITCH: 1-Button Footswitch for “FAT” boost

 HARDWARE
CABINET MATERIAL: 7-Ply 3/4" Birch/Maple Plywood
AMPLIFIER COVERING: Black Textured Vinyl
GRILLE CLOTH: Silver
AMPLIFIER JEWEL: Red Jewel
FRONT PANEL: Black
HANDLE: Molded Rubber "Dog Bone" Style
SPEAKERS: One - 12" 
TOTAL IMPEDENCE: 8 ohms

 TUBES
PREAMP TUBES: 3 x 12AX7
POWER TUBES: 2 x EL84
RECTIFIER: Solid State
REVERB DRIVE: Solid State

 MEASUREMENTS
LENGTH (IN INCHES): 9.81
HEIGHT (IN INCHES): 16
WIDTH (IN INCHES): 18
WEIGHT (IN POUNDS): 31
LIST PRICE: $529






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2 comments:

  1. AnonymousDecember 5, 2019 at 6:59 PM

    The Sound Test video link doesn't work.

    ReplyDelete
    Replies
    1. john3hr33December 8, 2019 at 12:39 PM

      Sorry I pulled it and other videos off youtube after too many troll comments.

      Delete

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