A friend of mine, Scott, had an old Squire practice bass amp that was no longer working and he did not feel like fixing it. In light of this he decided to give me the remnants of the amp so I could do my bidding on it. Living in campus dorms or small apartments is not conducive to volume of the VHT 50W Head or the 100W Custom Head or even to the ~8W SE AX84 Head. All of these amps are so overpowered for bedroom applications and can overheat the 25W 1x12 cabinet I haul around. The broken practice amp I acquired from Scott was a perfect candidate for what is affectionately called a Flea Amp, or a guitar amp with less than 5w usually with a triode output section using a tube in the 12aXX family (12ax7, 12au7, 12at7....)
A little bit of research through a couple of amp forums suggested that an ECC99 tube is an upgrade over a 12aXX type tube in the output stage. The ECC99 tube has a higher Gm, lower Ra, and much higher voltage handling capabilities. Subjective sound testing also suggested that it has a better bass response. The picture below shows the shell of the amp, gutted with the chassis and back removed.
I wanted this little amp to be simply constructed but with some versatility. I opted for using terminal strips instead of eyelets or turrets so it was a true point to point build ala Matchless. The cabinet which housed the old bass amp was extremely tiny so I had to work with an 11x8x2 inch chassis. Hammond makes a good line of aluminum chassis in this size range which are easy to tap and drill. I decided to use a Baxandall tone stack to minimize the number of knobs. In a Baxandall stack, there is only a Bass and Treble control unlike a FMV type tone stack commonly found in Fender, Marshalls and Voxs all which have 3 controls, Bass, Mids, and Treble. The neat thing about Baxandall stacks is that the Bass and Treble controls have a much larger range of adjustment than FMV stacks and are almost completely independent of each other. The only drawback of a Baxandall is the addition of a few more components and a lack of a mid range control.
The output section of the Flea Amp is a self split push pull Class A stage. The amp does not require a phase inverter circuit in order to function. The plates of the ECC99 are wired to the primaries of the output transformer and the cathodes are tied together and wired to a bias pot. One of the grids is attached to the output of the last gain stage (master volume in this case) and the other grid is grounded. In this configuration there is not much efficiency but that is the goal of this project. In this manner the tube is in Class A but is push pull.
I developed a drill layout for the chassis in Adobe Illustrator which allowed for a 1:1 ratio drawing with precise spacing and sizes. The gist of the layout was done on the Adobe but I did not plan every little wire and detail. I did most of the wire layout once the amp was coming together but the locations of the transformers, pots and switches were all determined before hand and incorporated into the drill layout. The picture below shows the front drill layout.
One design decision which should be changed in subsequent builds based on this amp is the power supply capacitors. For this amp I bought TVA Sprague electrolytic caps which are one of the classic standards for vintage and modern amps. However, they are also monstrous and over sized and the values I choose, (100uF and 47uF) are overkill for the filtering required for such a low current amplifier. Due to this, I had to mount all of the power supply capacitors on the outside of the chassis instead of on the inside. This created a more difficult ground scheme since a well designed amp will have filter stage grounds near their subsequent stages rather than separate and far away from the individual stages. However since the amp is not overly gainy I did not pay much price in terms of ground noise. The picture below shows a preliminary power supply layout before the amp was drilled out.
A flea amp was pretty straightfoward to build despite very compact chassis. The price of all the components was under 300$ so this was quite a budget build. The power transformer and output transformers were made by Hammond, all JJ tubes, TVA Sprague filter caps, Orange Drop 716 series coupling caps, and silver mica tone caps. All resistors were carbon film 1W minimum but the power supply resistors were 3W metal film. All the pots were 1M with push pull switches from Apex Jr. All the wire used was Teflon coated silver plated solid and stranded core wire 18 gauge. I used the 1M push pull pots to add even more flexibility into the amp. Each pot can be pulled activating a switch which can do several different things. The gain knob pulled out gives a touch more gain, the master pulled out increases the bass into the output tube, the bass and treble controls pulled out change the frequency content of the tone stack. There are 2 mini switches which also affect the tone stack. The first switch can lift the tone stack ground, ground it or ground it through the first stage cathode. The second switch can bump the mids up or down. Another addition to the amp is the on-off-on standby switch which has two different modes. In the Green Light Mode, the B+ voltage is ~330 and in the Red Light Mod the B+ is ~270V. Changing the B+ voltage can change the dynamics and feel of the amp. In Green Light Mode there is more headroom and dynamics while in the Red Light Mode the amp sounds warmer and a little more browned out. Below is a picture of the finish amp which was painted with a mix of silver and black spray paint to match the silver/black grill cloth.
Since the 8" speaker built into the cabinet could hamper the tone of the amp I installed a switch so that an external cabinet could be used and the internal speaker could be muted when an external speaker is selected. Sound tests later confirmed that the internal speaker is no where near as good sounding as any 12" or 10" speaker and through a 2x12 or 4x12 the amp really shines. However, the 8" speaker is solid enough for practice were ultimate tone is not the goal. The picture below shows the amp upside down in order to see the power supply and top layout.
The reader may wonder if a 1 or 2W amp is even loud enough for practice. Much to nearly most peoples surprise the amp is quite loud even though the 8" internal speaker. The amp is too loud to play over 1/2 volume if people are home. Through a 4x12 the amp would be easily usable for studio and maybe, just maybe a small club show with a quiet band. A lot of this has to do with the sensitivity rating of the speakers as a very sensitive speaker >93 db 1W/1m than the amp will become perceptively louder. Another thing to consider is that perceived loudness is not linear but logarithmic. 1W is ~20db less power than 100W (To compute this use your calculator and insert 10*log(100/1) and make sure its log base 10). Similarly a 50W amp is only ~3db less power than a 100W amp (10*log(100/50) = ~3 db) The major difference as the wattage goes down is clean headroom and frequency response. The Flea Amp cant have a huge signal slammed into the grids without getting some uglier clipping the ECC99. Putting a distortion pedal or a delay pedal in front of the amp sounds great and combining the pedals and the amp distortion can push the amp into metal territory.
Below is a video I made where you can hear a little bit of the amp. It is mostly clean and when I have the video camera up and running I will make a full demo.
A little bit of research through a couple of amp forums suggested that an ECC99 tube is an upgrade over a 12aXX type tube in the output stage. The ECC99 tube has a higher Gm, lower Ra, and much higher voltage handling capabilities. Subjective sound testing also suggested that it has a better bass response. The picture below shows the shell of the amp, gutted with the chassis and back removed.
I wanted this little amp to be simply constructed but with some versatility. I opted for using terminal strips instead of eyelets or turrets so it was a true point to point build ala Matchless. The cabinet which housed the old bass amp was extremely tiny so I had to work with an 11x8x2 inch chassis. Hammond makes a good line of aluminum chassis in this size range which are easy to tap and drill. I decided to use a Baxandall tone stack to minimize the number of knobs. In a Baxandall stack, there is only a Bass and Treble control unlike a FMV type tone stack commonly found in Fender, Marshalls and Voxs all which have 3 controls, Bass, Mids, and Treble. The neat thing about Baxandall stacks is that the Bass and Treble controls have a much larger range of adjustment than FMV stacks and are almost completely independent of each other. The only drawback of a Baxandall is the addition of a few more components and a lack of a mid range control.
The output section of the Flea Amp is a self split push pull Class A stage. The amp does not require a phase inverter circuit in order to function. The plates of the ECC99 are wired to the primaries of the output transformer and the cathodes are tied together and wired to a bias pot. One of the grids is attached to the output of the last gain stage (master volume in this case) and the other grid is grounded. In this configuration there is not much efficiency but that is the goal of this project. In this manner the tube is in Class A but is push pull.
I developed a drill layout for the chassis in Adobe Illustrator which allowed for a 1:1 ratio drawing with precise spacing and sizes. The gist of the layout was done on the Adobe but I did not plan every little wire and detail. I did most of the wire layout once the amp was coming together but the locations of the transformers, pots and switches were all determined before hand and incorporated into the drill layout. The picture below shows the front drill layout.
One design decision which should be changed in subsequent builds based on this amp is the power supply capacitors. For this amp I bought TVA Sprague electrolytic caps which are one of the classic standards for vintage and modern amps. However, they are also monstrous and over sized and the values I choose, (100uF and 47uF) are overkill for the filtering required for such a low current amplifier. Due to this, I had to mount all of the power supply capacitors on the outside of the chassis instead of on the inside. This created a more difficult ground scheme since a well designed amp will have filter stage grounds near their subsequent stages rather than separate and far away from the individual stages. However since the amp is not overly gainy I did not pay much price in terms of ground noise. The picture below shows a preliminary power supply layout before the amp was drilled out.
A flea amp was pretty straightfoward to build despite very compact chassis. The price of all the components was under 300$ so this was quite a budget build. The power transformer and output transformers were made by Hammond, all JJ tubes, TVA Sprague filter caps, Orange Drop 716 series coupling caps, and silver mica tone caps. All resistors were carbon film 1W minimum but the power supply resistors were 3W metal film. All the pots were 1M with push pull switches from Apex Jr. All the wire used was Teflon coated silver plated solid and stranded core wire 18 gauge. I used the 1M push pull pots to add even more flexibility into the amp. Each pot can be pulled activating a switch which can do several different things. The gain knob pulled out gives a touch more gain, the master pulled out increases the bass into the output tube, the bass and treble controls pulled out change the frequency content of the tone stack. There are 2 mini switches which also affect the tone stack. The first switch can lift the tone stack ground, ground it or ground it through the first stage cathode. The second switch can bump the mids up or down. Another addition to the amp is the on-off-on standby switch which has two different modes. In the Green Light Mode, the B+ voltage is ~330 and in the Red Light Mod the B+ is ~270V. Changing the B+ voltage can change the dynamics and feel of the amp. In Green Light Mode there is more headroom and dynamics while in the Red Light Mode the amp sounds warmer and a little more browned out. Below is a picture of the finish amp which was painted with a mix of silver and black spray paint to match the silver/black grill cloth.
Since the 8" speaker built into the cabinet could hamper the tone of the amp I installed a switch so that an external cabinet could be used and the internal speaker could be muted when an external speaker is selected. Sound tests later confirmed that the internal speaker is no where near as good sounding as any 12" or 10" speaker and through a 2x12 or 4x12 the amp really shines. However, the 8" speaker is solid enough for practice were ultimate tone is not the goal. The picture below shows the amp upside down in order to see the power supply and top layout.
The reader may wonder if a 1 or 2W amp is even loud enough for practice. Much to nearly most peoples surprise the amp is quite loud even though the 8" internal speaker. The amp is too loud to play over 1/2 volume if people are home. Through a 4x12 the amp would be easily usable for studio and maybe, just maybe a small club show with a quiet band. A lot of this has to do with the sensitivity rating of the speakers as a very sensitive speaker >93 db 1W/1m than the amp will become perceptively louder. Another thing to consider is that perceived loudness is not linear but logarithmic. 1W is ~20db less power than 100W (To compute this use your calculator and insert 10*log(100/1) and make sure its log base 10). Similarly a 50W amp is only ~3db less power than a 100W amp (10*log(100/50) = ~3 db) The major difference as the wattage goes down is clean headroom and frequency response. The Flea Amp cant have a huge signal slammed into the grids without getting some uglier clipping the ECC99. Putting a distortion pedal or a delay pedal in front of the amp sounds great and combining the pedals and the amp distortion can push the amp into metal territory.
Below is a video I made where you can hear a little bit of the amp. It is mostly clean and when I have the video camera up and running I will make a full demo.
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