Embedded Files
The BML project is about creating a light-weight 5F6a chassis. The idea is to leverage all the wonderful cabs and speakers out there for the 5F6a chassis, but use a light-weight chassis instead. A normal tweed Bassman gets almost half its 40+ LB weight from the chassis. The Princeton AOT project is a model for how to get high performance from a light-weight chassis. I'm pursuing a similar approach for the 5F6a. The project is evolving as I write. This page will be something of a log tracing the evolution of the ideas and designs. Check back frequently and feel free to email me with suggestions or comments.
The BML project is about creating a light-weight 5F6a chassis. The idea is to leverage all the wonderful cabs and speakers out there for the 5F6a chassis, but use a light-weight chassis instead. A normal tweed Bassman gets almost half its 40+ LB weight from the chassis. The Princeton AOT project is a model for how to get high performance from a light-weight chassis. I'm pursuing a similar approach for the 5F6a. The project is evolving as I write. This page will be something of a log tracing the evolution of the ideas and designs. Check back frequently and feel free to email me with suggestions or comments.
NEW: BML4 (6/2020)
NEW: BML4 (6/2020)
Initial Design: 9/3/16
Initial Design: 9/3/16
The primary goal is a drop-in chassis replacement for the tweed Bassman. Of course there are many (semi) custom combos and heads built around the same chassis, so the goal applies to those as well. Keep the main features of the Bassman, but add extensions and options as possible without compromising performance and cost.
The primary goal is a drop-in chassis replacement for the tweed Bassman. Of course there are many (semi) custom combos and heads built around the same chassis, so the goal applies to those as well. Keep the main features of the Bassman, but add extensions and options as possible without compromising performance and cost.
The build should be easy to replicate so I'm going for a printed circuit board. There is no better way to ensure consistent performance and ease of construction. The challenge is to also make it reliable and easy to service/tweak. The latter points are often overlooked in mass production to keep costs down, but I think it's doable at minimal cost.
The build should be easy to replicate so I'm going for a printed circuit board. There is no better way to ensure consistent performance and ease of construction. The challenge is to also make it reliable and easy to service/tweak. The latter points are often overlooked in mass production to keep costs down, but I think it's doable at minimal cost.
The Bassman puts out 40-50 watts RMS into 2 ohms. This amp should do the same for 4 or 8 ohms using a new AOT output stage and higher current power supply. An 8 ohm load will produce slightly less power, but a higher voltage power supply can compensate for that. The preamp will have a standard 3x 12ax7 Bassman circuit at it's core with embellishments to deal with known issues and commonly desired features. This design targets a full feature and capability amp. Several aspects of the design are new and not available in any commercial amps.
The Bassman puts out 40-50 watts RMS into 2 ohms. This amp should do the same for 4 or 8 ohms using a new AOT output stage and higher current power supply. An 8 ohm load will produce slightly less power, but a higher voltage power supply can compensate for that. The preamp will have a standard 3x 12ax7 Bassman circuit at it's core with embellishments to deal with known issues and commonly desired features. This design targets a full feature and capability amp. Several aspects of the design are new and not available in any commercial amps.
- A wider range of tone stack control is achieved with user fine-tuning through internal pots and switches. Basically, the preamp is a tweaker's delight. The front panel Treble-Mid-Bass controls are a standard Bassman tone stack, but internally, a second tone stack (switched in/out) will modify the overall EQ. This allows tailoring the amp for specific guitar and speakers/cabinet variations. The focus is better control of low-bass frequencies (200-400 Hz) that often cause boominess and muddiness, midrange frequencies that can add nasal qualities (700-1k Hz), and overall EQ balance. High frequency fret-noise and harshness (ice-pick) is controlled by careful HF roll-off - adjusted by component additions/omissions.
- A wider range of tone stack control is achieved with user fine-tuning through internal pots and switches. Basically, the preamp is a tweaker's delight. The front panel Treble-Mid-Bass controls are a standard Bassman tone stack, but internally, a second tone stack (switched in/out) will modify the overall EQ. This allows tailoring the amp for specific guitar and speakers/cabinet variations. The focus is better control of low-bass frequencies (200-400 Hz) that often cause boominess and muddiness, midrange frequencies that can add nasal qualities (700-1k Hz), and overall EQ balance. High frequency fret-noise and harshness (ice-pick) is controlled by careful HF roll-off - adjusted by component additions/omissions.
- A distortion stage (as in the Princeton AOT) can be switched in. It's designed for a slightly aggressive tone and highly responsive to the guitar volume control. Internal controls for distortion quality and gain are being considered.
- A distortion stage (as in the Princeton AOT) can be switched in. It's designed for a slightly aggressive tone and highly responsive to the guitar volume control. Internal controls for distortion quality and gain are being considered.
- A pre-phase-inverter master volume allows the preamp gain and distortion to be controlled independent of room volume.
- A pre-phase-inverter master volume allows the preamp gain and distortion to be controlled independent of room volume.
- A high quality spring reverb is driven and mixed in just prior to the phase inverter. A variety of spring pans can be used (8-10 ohm). See the Spring Reverb page or the Amp-Aid ReverbFX3 for details of a similar design.
- A high quality spring reverb is driven and mixed in just prior to the phase inverter. A variety of spring pans can be used (8-10 ohm). See the Spring Reverb page or the Amp-Aid ReverbFX3 for details of a similar design.
- A high quality bias tremolo is added to the 12ax7 phase inverter (similar to the Amp-Aid BMTrem2). Concentric knobs on the front panel control speed/depth using the space of only one control.
- A high quality bias tremolo is added to the 12ax7 phase inverter (similar to the Amp-Aid BMTrem2). Concentric knobs on the front panel control speed/depth using the space of only one control.
- The preamp power supply is unusual for a guitar amp. It's semi-regulated and employs a capacitor multiplier (look it up!) for low noise and ripple. Front panel switches select variable sag threshold and depth for controlling the dynamics of signal compression, voltage shifts, and distortion
- The preamp power supply is unusual for a guitar amp. It's semi-regulated and employs a capacitor multiplier (look it up!) for low noise and ripple. Front panel switches select variable sag threshold and depth for controlling the dynamics of signal compression, voltage shifts, and distortion
At this point, two PC boards and an AOT power supply are the major chassis layout elements. The preamp board is taking shape as a 12"x 3" board. The power supply and power amp board is estimated at around 5"x3". The AOT power supply is a 150w module measuring about 8"x2.5"x1.5". All of these fit comfortably in the 5F6a chassis, but they can also fit other chassis formats.
At this point, two PC boards and an AOT power supply are the major chassis layout elements. The preamp board is taking shape as a 12"x 3" board. The power supply and power amp board is estimated at around 5"x3". The AOT power supply is a 150w module measuring about 8"x2.5"x1.5". All of these fit comfortably in the 5F6a chassis, but they can also fit other chassis formats.
9/21/16
9/21/16
The circuits and chassis layouts are taking shape. Here is a chassis layout - it's about 95% done. It's still missing the preamp circuit board mounting holes. The preamp board is still in layout so the hole pattern isn't defined yet.
The circuits and chassis layouts are taking shape. Here is a chassis layout - it's about 95% done. It's still missing the preamp circuit board mounting holes. The preamp board is still in layout so the hole pattern isn't defined yet.
The four orange rectangles show placement of the 2 PC boards, the 24v PS, and the filament transformer within the chassis. Only the holes need to be drilled in the chassis. This will be a drop-in replacement for a 5F6A chassis. Chassis cost estimates are pending. Of course other chassis configurations are possible.
The four orange rectangles show placement of the 2 PC boards, the 24v PS, and the filament transformer within the chassis. Only the holes need to be drilled in the chassis. This will be a drop-in replacement for a 5F6A chassis. Chassis cost estimates are pending. Of course other chassis configurations are possible.
The power supply and power amp board is ready for prototype build. Parts are on order. I'll send the PCB order once I get the parts and verify my layout sizes agree with the actual parts.
The power supply and power amp board is ready for prototype build. Parts are on order. I'll send the PCB order once I get the parts and verify my layout sizes agree with the actual parts.
Here is the PCB silkscreen. The board is 5.4"x3" and it has the B+ power supply and the 50w AOT output stage. With the output transistors it has a footprint of 6"x3" and fits in the left side of the chassis. Estimated board parts cost is ~$50.
Here is the PCB silkscreen. The board is 5.4"x3" and it has the B+ power supply and the 50w AOT output stage. With the output transistors it has a footprint of 6"x3" and fits in the left side of the chassis. Estimated board parts cost is ~$50.
The circuit is below. Note that all the filaments and AC wiring are point-to-point on the chassis. The PL connections are molex wire harness plugs that connect to the board J headers. The 24v PS is an LED Power Supply 151.2W 24V 6.3A at a cost of $36.
The circuit is below. Note that all the filaments and AC wiring are point-to-point on the chassis. The PL connections are molex wire harness plugs that connect to the board J headers. The 24v PS is an LED Power Supply 151.2W 24V 6.3A at a cost of $36.
9/28/16
9/28/16
The preamp is taking shape. The PCB is still being checked and refined but the a complete draft layout is shown below.
The preamp is taking shape. The PCB is still being checked and refined but the a complete draft layout is shown below.
Controls are at the top. Pots are soldered directly to the board, switches are connected by short wires to the SW pads. The bottom has Molex connectors for wire harness to tube sockets (V1-V6), back panel connections, and the power supply/power amp board.
Controls are at the top. Pots are soldered directly to the board, switches are connected by short wires to the SW pads. The bottom has Molex connectors for wire harness to tube sockets (V1-V6), back panel connections, and the power supply/power amp board.
The left side of the board has tremolo, reverb, and other circuits outside of the main signal path. These are densely packed.
The left side of the board has tremolo, reverb, and other circuits outside of the main signal path. These are densely packed.
The main signal path circuits are from the right to the center areas of the board and these are more loosely arranged to minimize cross-talk and optimize trace lengths. The current draft of the preamp circuit is shown below.
The main signal path circuits are from the right to the center areas of the board and these are more loosely arranged to minimize cross-talk and optimize trace lengths. The current draft of the preamp circuit is shown below.
This should give a sense of the whole design. Clearly there is more than a simple Bassman here - although if you look carefully, you'll see the Bassman in there, and the amp can operate in "Bassman mode". There are lots of other options and features too. More about all of this once I get prototypes completed and tweaked. I'm planning to order 2 board sets and stuff one set for debugging and tweaking.
This should give a sense of the whole design. Clearly there is more than a simple Bassman here - although if you look carefully, you'll see the Bassman in there, and the amp can operate in "Bassman mode". There are lots of other options and features too. More about all of this once I get prototypes completed and tweaked. I'm planning to order 2 board sets and stuff one set for debugging and tweaking.
10/9/16
10/9/16
Power supply and Power Amp board is tested and working as hoped. Minor part value changes and minor adjustments were made to the PCB for part spacing and hole sizes. V1 boards are fine (as shown below) and v2 boards should be perfect.
Power supply and Power Amp board is tested and working as hoped. Minor part value changes and minor adjustments were made to the PCB for part spacing and hole sizes. V1 boards are fine (as shown below) and v2 boards should be perfect.
The power supply puts out up to 480v, and is now tweaked for ~400vdc to run the preamp tubes. Very clean, it measures only ~20mv ripple. The power amp has only one pair of output caps to configure it for 8 and 4 ohm speakers. If 2 ohm loads are driven, the additional pair of caps can be added to improve bass response (if needed). Bias is set for 33ma per amp - just enough to warm the output transistors.
The power supply puts out up to 480v, and is now tweaked for ~400vdc to run the preamp tubes. Very clean, it measures only ~20mv ripple. The power amp has only one pair of output caps to configure it for 8 and 4 ohm speakers. If 2 ohm loads are driven, the additional pair of caps can be added to improve bass response (if needed). Bias is set for 33ma per amp - just enough to warm the output transistors.
10/17/16
10/17/16
After some "real-life" interruptions and diversions, I'm back on this. I've made some final changes to the chassis and sent it out for fab. I'm having two chassis made.
After some "real-life" interruptions and diversions, I'm back on this. I've made some final changes to the chassis and sent it out for fab. I'm having two chassis made.
Here is the design drawing and the artwork for a brushed-brass face plate. Fab is being done by Red River Precision Manufacturing, Inc. in Round Rock, Texas. (www.redriverprec.com). They have been very helpful and I'm looking forward to getting the proto pieces.
Here is the design drawing and the artwork for a brushed-brass face plate. Fab is being done by Red River Precision Manufacturing, Inc. in Round Rock, Texas. (www.redriverprec.com). They have been very helpful and I'm looking forward to getting the proto pieces.
Here is a PC layout of the preamp board (component side) that shows the ground plane I added to help keep most all of the signal path quiet - up to the phase-inverter. I've been rather obsessed with hum reduction and I hope I've thought of all the ways possible to keep things quiet. We'll see when the proto board gets here. I'll order two boards once I check it, and check it again, and again, ...
Here is a PC layout of the preamp board (component side) that shows the ground plane I added to help keep most all of the signal path quiet - up to the phase-inverter. I've been rather obsessed with hum reduction and I hope I've thought of all the ways possible to keep things quiet. We'll see when the proto board gets here. I'll order two boards once I check it, and check it again, and again, ...
The preamp circuit itself has also been tweaked - but I'll wait until the proto is up and running before I post another schematic version.
The preamp circuit itself has also been tweaked - but I'll wait until the proto is up and running before I post another schematic version.
It's also time to focus on the cab - this is going to be a "short" bassman cab. I'm pretty sure I can use a standard narrow-panel tweed bassman width, depth, and chassis cutout. The only difference will be the height. I'm trying to cut the height by 5-6" which will make the cab size closer to a Deluxe Reverb. That should cut some weight and make the amp easier to carry and transport.
It's also time to focus on the cab - this is going to be a "short" bassman cab. I'm pretty sure I can use a standard narrow-panel tweed bassman width, depth, and chassis cutout. The only difference will be the height. I'm trying to cut the height by 5-6" which will make the cab size closer to a Deluxe Reverb. That should cut some weight and make the amp easier to carry and transport.
I have plans to make two baffles for the final cab - one for a 1x12 and another for a 12"+10" speaker arrangement. More about that once it takes shape.
I have plans to make two baffles for the final cab - one for a 1x12 and another for a 12"+10" speaker arrangement. More about that once it takes shape.
11/5/16
11/5/16
Preamp PCB has arrived and is now stuffed with all its parts (see pic below). No problems were noted in the stuffing. The jumpers are intentional - they are shielded signal connections. I've seen too many PCB's with long traces that pickup hum/noise. A careful layout minimizes lengthy traces, but there are cases where signals need to travel some distance, and for those cases a shielded jumper makes sense to me. It's just another component to stuff since the board has pads and holes for signal and shield on both ends.
Preamp PCB has arrived and is now stuffed with all its parts (see pic below). No problems were noted in the stuffing. The jumpers are intentional - they are shielded signal connections. I've seen too many PCB's with long traces that pickup hum/noise. A careful layout minimizes lengthy traces, but there are cases where signals need to travel some distance, and for those cases a shielded jumper makes sense to me. It's just another component to stuff since the board has pads and holes for signal and shield on both ends.
The chassis is needed before I can power up and test the preamp. The chassis is in fab and delivery is expected in about 10 days. I also designed and ordered a cab from Kendrick Amps. They're making a custom "short" Bassman cab with two baffles - 1x12 and 12+10. Now it gets real as all the planning, design, and waiting shifts to assembly, test, and tweaking with actual hardware. This is gonna be fun.
The chassis is needed before I can power up and test the preamp. The chassis is in fab and delivery is expected in about 10 days. I also designed and ordered a cab from Kendrick Amps. They're making a custom "short" Bassman cab with two baffles - 1x12 and 12+10. Now it gets real as all the planning, design, and waiting shifts to assembly, test, and tweaking with actual hardware. This is gonna be fun.
12/3/16
12/3/16
Chassis has arrived and it looks to fit all the components. Again, there were some tweaks during fab so the final version will be posted once everything is up and running. The workmanship is very good. Red River did a great job. The image below shows the first stage of assembly with the filament transformer and the 24v power supply in place.
Chassis has arrived and it looks to fit all the components. Again, there were some tweaks during fab so the final version will be posted once everything is up and running. The workmanship is very good. Red River did a great job. The image below shows the first stage of assembly with the filament transformer and the 24v power supply in place.
The next image shows the Power Amp/Power Supply board in place with partial AC wiring. I previously tested and tweaked the board for DC voltage and power amp operation, so I'm pretty sure I don't have to remove it. Although all harnesses have Molex connectors, it would take some time to remove the board and I'm hoping I don't have to.
The next image shows the Power Amp/Power Supply board in place with partial AC wiring. I previously tested and tweaked the board for DC voltage and power amp operation, so I'm pretty sure I don't have to remove it. Although all harnesses have Molex connectors, it would take some time to remove the board and I'm hoping I don't have to.
The view below shows the 24v feed and the speaker connections on the power amp board, close to the output transistors. The design gets the 24v onto the board, through the transistors, and off the board to the speakers as quickly as possible (meaning with short traces on the board). This minimizes stray pickup and unwanted voltage (power) drops.
The view below shows the 24v feed and the speaker connections on the power amp board, close to the output transistors. The design gets the 24v onto the board, through the transistors, and off the board to the speakers as quickly as possible (meaning with short traces on the board). This minimizes stray pickup and unwanted voltage (power) drops.
The image below shows the wire harness to the two speaker jacks (red/black wires). The reverb pan and pedal jacks are in place but not wired yet. The three tube sockets are wired for filaments. The left tube is the Phase Inverter with AC filament power.
The image below shows the wire harness to the two speaker jacks (red/black wires). The reverb pan and pedal jacks are in place but not wired yet. The three tube sockets are wired for filaments. The left tube is the Phase Inverter with AC filament power.
A bridge rectifier and two filter caps produce DC filament power for the two preamp tubes on the right. The filament reference is the 24v supply for a bit of elevation. The use of DC filament power on the right side of the chassis removes a major source of stray hum pickup in the high-impedance preamp circuits and wiring near the right two tubes.
A bridge rectifier and two filter caps produce DC filament power for the two preamp tubes on the right. The filament reference is the 24v supply for a bit of elevation. The use of DC filament power on the right side of the chassis removes a major source of stray hum pickup in the high-impedance preamp circuits and wiring near the right two tubes.
12/16/16
12/16/16
It's alive! I completed the build and it survived two days of practice studio sessions without a hitch. It has a killer clean tone - think super reverb - and it gets dirty with overdrive - but in a smooth creamy way (like a Bassman!). I have not fully explored all the tone variations yet, but its pretty pleasing so far -- and it far exceeds the range of a normal tweed Bassman.
It's alive! I completed the build and it survived two days of practice studio sessions without a hitch. It has a killer clean tone - think super reverb - and it gets dirty with overdrive - but in a smooth creamy way (like a Bassman!). I have not fully explored all the tone variations yet, but its pretty pleasing so far -- and it far exceeds the range of a normal tweed Bassman.
The final build pics are below. I'll call this build version 1.1 and I'll update all the documents and drawings below to reflect this version, and I'll add some circuit descriptions and explanations. There are a few changes from the original design (version 1) posted above, but the v1 PC boards work great with only 2 trace cuts and jumpers -- so my spare boards are highly usable.
The final build pics are below. I'll call this build version 1.1 and I'll update all the documents and drawings below to reflect this version, and I'll add some circuit descriptions and explanations. There are a few changes from the original design (version 1) posted above, but the v1 PC boards work great with only 2 trace cuts and jumpers -- so my spare boards are highly usable.
The first image is the gut-shot. It shows the completed cab with 12" Gold (8 ohm) and 10" Weber Neo (16 ohm). Each speaker has a separate phone plug so it's easy to run the amp with one or both speakers connected. The max output power is inversely proportional to speaker impedance.
The first image is the gut-shot. It shows the completed cab with 12" Gold (8 ohm) and 10" Weber Neo (16 ohm). Each speaker has a separate phone plug so it's easy to run the amp with one or both speakers connected. The max output power is inversely proportional to speaker impedance.
It produces clean sin wave rms power of 60w @ 3.2 ohm / 50w @ 4 ohm / 34w @ 6 ohm / 25w @ 8 ohm / 12w @ 16 ohm. My current speaker configuration offers 6, 8, and 16 ohm loads. It's super loud with a 6 ohm load (both speakers driven). Driving the 16 ohm Weber alone is a nice option since it limits the power to 12w so the amp can be cranked into PI overdrive and compression in a studio without causing pain.
It produces clean sin wave rms power of 60w @ 3.2 ohm / 50w @ 4 ohm / 34w @ 6 ohm / 25w @ 8 ohm / 12w @ 16 ohm. My current speaker configuration offers 6, 8, and 16 ohm loads. It's super loud with a 6 ohm load (both speakers driven). Driving the 16 ohm Weber alone is a nice option since it limits the power to 12w so the amp can be cranked into PI overdrive and compression in a studio without causing pain.
The reverb tank is a short 3-spring tank I've grown to like.
The reverb tank is a short 3-spring tank I've grown to like.
The chassis layout worked out as I had hoped. The ten Molex connectors facilitate simple removal/install of the preamp board. In a production setting, those connections could be hard-wired to save a few $. Either way, the board swings out while fully connected, which simplifies probing or tweaking or changing components.
The chassis layout worked out as I had hoped. The ten Molex connectors facilitate simple removal/install of the preamp board. In a production setting, those connections could be hard-wired to save a few $. Either way, the board swings out while fully connected, which simplifies probing or tweaking or changing components.
Although the BML circuits are far more complex than the tweed Bassman, access and manual wiring are actually simpler since the PCBs make almost all of the connections. The power amp is super clear and completely quiet. I'm really happy with it. I installed the PAPS board early in the assembly process and I have not had to touch it since.
Although the BML circuits are far more complex than the tweed Bassman, access and manual wiring are actually simpler since the PCBs make almost all of the connections. The power amp is super clear and completely quiet. I'm really happy with it. I installed the PAPS board early in the assembly process and I have not had to touch it since.
The power transistors run cool to the touch - the aluminum chassis is an effective heat sink. The only component that seems to run hot is T1, the small power transformer. It's running within it's specified power limit, so I assume it's designed to operate that way. (I've since called the manufacturer to confirm that the heating is "normal" and well within the max component tolerance.) The filament transformer, T2 (on the speaker side of the chassis), runs cool to the touch.
The power transistors run cool to the touch - the aluminum chassis is an effective heat sink. The only component that seems to run hot is T1, the small power transformer. It's running within it's specified power limit, so I assume it's designed to operate that way. (I've since called the manufacturer to confirm that the heating is "normal" and well within the max component tolerance.) The filament transformer, T2 (on the speaker side of the chassis), runs cool to the touch.
The complete chassis wired with boards and tubes weighs under 6 Lbs. Ignoring the cabinet itself, the component with the biggest weight is the 12" Gold speaker (9.3 Lbs). The complete combo, as shown, weighs in at around 34 Lbs.
The complete chassis wired with boards and tubes weighs under 6 Lbs. Ignoring the cabinet itself, the component with the biggest weight is the 12" Gold speaker (9.3 Lbs). The complete combo, as shown, weighs in at around 34 Lbs.
The cab is essentially a "short" tweed Bassman cab. It was made by Kendrick (Kendrick Amps) and they did a great job. They kept the specs so it could be ordered again with either a 1x12 or a 1x12 + 1x10 baffle. I ordered both baffles to facilitate future experiments and needs.
The cab is essentially a "short" tweed Bassman cab. It was made by Kendrick (Kendrick Amps) and they did a great job. They kept the specs so it could be ordered again with either a 1x12 or a 1x12 + 1x10 baffle. I ordered both baffles to facilitate future experiments and needs.
The view below shows the control panel layout. There is no label plate yet - I'm still working on the artwork and wording for switch functions. Some of these switches control circuits and functions not found in other amps, so coming up with names and labels is not that easy.
The view below shows the control panel layout. There is no label plate yet - I'm still working on the artwork and wording for switch functions. Some of these switches control circuits and functions not found in other amps, so coming up with names and labels is not that easy.
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Update 5/5/19: v1.3 circuits and descriptions
Update 5/5/19: v1.3 circuits and descriptions
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PAPS v1.3 - the chassis wiring and power amp and power supply board (PAPS) schematic is below. The main differences from the initial design and drawing are in the chassis wiring. The PAPS PCB itself is unchanged - it turned out perfect. The chassis wiring deals with mains, switches, and heaters. Everything up to PL1 and PL2 is hand wired. The PL1 and PL2 harness connector provide power to the PAPS PCB.
PAPS v1.3 - the chassis wiring and power amp and power supply board (PAPS) schematic is below. The main differences from the initial design and drawing are in the chassis wiring. The PAPS PCB itself is unchanged - it turned out perfect. The chassis wiring deals with mains, switches, and heaters. Everything up to PL1 and PL2 is hand wired. The PL1 and PL2 harness connector provide power to the PAPS PCB.
The main change for v1.3 is the heater supply. The 6.3V transformer has been removed and the heaters are now connected to the 24V power supply. V1 and V2 are simply in series. V3 and the 6.3V pilot light are connected to a current limited source (Q19). The current limit ensures that the V3 heater warms up without burning out the pilot light.
The main change for v1.3 is the heater supply. The 6.3V transformer has been removed and the heaters are now connected to the 24V power supply. V1 and V2 are simply in series. V3 and the 6.3V pilot light are connected to a current limited source (Q19). The current limit ensures that the V3 heater warms up without burning out the pilot light.
The earth (chassis) ground from PL1 has also been removed since it created a ground loop. Chassis grounds are now only made through the 3 preamp PCB mounting screws with star washers that connect to the PCB ground plane.
The earth (chassis) ground from PL1 has also been removed since it created a ground loop. Chassis grounds are now only made through the 3 preamp PCB mounting screws with star washers that connect to the PCB ground plane.
PRE v1.3 - the preamp board circuits are shown on the two sheets below. Version 1.3 is constructed on the v1 PRE PCB, so it requires a few trace cuts and jumpers. The schematic makes note of the changes and I'll describe some of them below.
PRE v1.3 - the preamp board circuits are shown on the two sheets below. Version 1.3 is constructed on the v1 PRE PCB, so it requires a few trace cuts and jumpers. The schematic makes note of the changes and I'll describe some of them below.
A number of components are not stuffed - they are marked by *. Most of the unstuffed * items are caps that can be used to roll-off high frequencies and/or noise. I see no need for adding them at this point. The high freq's are clean and pleasant. The highs add significant "touch" sensitivity and cut through a mix beautifully.
A number of components are not stuffed - they are marked by *. Most of the unstuffed * items are caps that can be used to roll-off high frequencies and/or noise. I see no need for adding them at this point. The high freq's are clean and pleasant. The highs add significant "touch" sensitivity and cut through a mix beautifully.
V1 is a simple gain stage with a slightly lower than usual bias. Q1 and V2 form a LIO overdrive circuit that can be switched in/out with SW1. The amp can go from super clean to very nasty with that switch setting. SW2 provides bit of EQ adjustment prior to the DCCF stage. The 3 options are: LF cut, or HF boost, or straight through.
V1 is a simple gain stage with a slightly lower than usual bias. Q1 and V2 form a LIO overdrive circuit that can be switched in/out with SW1. The amp can go from super clean to very nasty with that switch setting. SW2 provides bit of EQ adjustment prior to the DCCF stage. The 3 options are: LF cut, or HF boost, or straight through.
V3 and V4 form a direct-coupled cathode follower (DCCF) stage. A similar stage gives the '59 Tweed Bassman it's classic warm tone and it does the same here. P1 is the gain or drive control that determines how hard this stage is driven into overdrive/distortion.
V3 and V4 form a direct-coupled cathode follower (DCCF) stage. A similar stage gives the '59 Tweed Bassman it's classic warm tone and it does the same here. P1 is the gain or drive control that determines how hard this stage is driven into overdrive/distortion.
The DCCF stage drives a tone shaping EQ circuit that favors middle and high frequencies to give the amp a brighter tone than the original Bassman. P8 is an internal adjustment pot to fine tune the high end response to taste. Too much high end can be piercing and emphasize fret/string noise. This setting also helps to adjust the amp response for variations in speakers.
The DCCF stage drives a tone shaping EQ circuit that favors middle and high frequencies to give the amp a brighter tone than the original Bassman. P8 is an internal adjustment pot to fine tune the high end response to taste. Too much high end can be piercing and emphasize fret/string noise. This setting also helps to adjust the amp response for variations in speakers.
Q2 buffers the EQ stage output and drives a 3-knob tone stack. SW4 controls the mid-scoop frequency, which varies from about 250Hz - 500Hz. There are lots of EQ options here, especially in combination with the SW2 settings.
Q2 buffers the EQ stage output and drives a 3-knob tone stack. SW4 controls the mid-scoop frequency, which varies from about 250Hz - 500Hz. There are lots of EQ options here, especially in combination with the SW2 settings.
Q3-5 provide gain and reverb send/recovery and mixing. P5 is a master volume control that controls the signal level fed into the phase inverter.
Q3-5 provide gain and reverb send/recovery and mixing. P5 is a master volume control that controls the signal level fed into the phase inverter.
Q21 is a current source for V5 and V6 configured as a phase inverter (PI). The current source is modulated by the tremolo oscillator (Q15 and Q16). SW5 attenuates the PI output (like a post-PI master volume) so that the PI can contribute to the amp tone.
Q21 is a current source for V5 and V6 configured as a phase inverter (PI). The current source is modulated by the tremolo oscillator (Q15 and Q16). SW5 attenuates the PI output (like a post-PI master volume) so that the PI can contribute to the amp tone.
Chassis - The chassis design is shown in this xps file and obtained from Red River Precision Manufacturing, Inc. in Round Rock, Texas. (www.redriverprec.com). They have NC files for this chassis now and they can reproduce them on demand.
Chassis - The chassis design is shown in this xps file and obtained from Red River Precision Manufacturing, Inc. in Round Rock, Texas. (www.redriverprec.com). They have NC files for this chassis now and they can reproduce them on demand.
Cab - As mentioned above, the cab can be ordered from Kendrick (Kendrick Amps) with either a 1x12 or a 1x12 + 1x10 baffle.
Cab - As mentioned above, the cab can be ordered from Kendrick (Kendrick Amps) with either a 1x12 or a 1x12 + 1x10 baffle.
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