Showing posts with label guitar. Show all posts
Showing posts with label guitar. Show all posts
Saturday, October 18, 2014
Simple 100W Guitar Pre Amplifier Circuit Diagram
This is a simple Simple 100W Guitar Pre-Amplifier Circuit Diagram. Guitar amplifiers are always an fascinating challenge. The tone controls, gain & overload characteristics are individual, & the ideal combination varies from guitarist to the next, & from guitar to the next. There is no amp that satisfies everyones requirements, & this offering is not expected to be an exception. The preamp is now at Revision-A, & although the whole schematic of the new version is not shown below, the essential characteristics are not changed - it still has the same tone control "stack" & other controls, but now has a second op amp to reduce output impedance & improve gain characteristics.
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If the bright switch is bright ( much treble), increase the 1k resistor (R5) to tame it down again. Reduce the worth to get more bite. The tone control arrangement shown will give zero output if all controls are set to maximum - this is unlikely to be a common requirement in use, but be aware of it when testing.
The input, effects & output connections are shown in Figure 1B.
The connections shown are similar (ok, virtually identical :-) to those used in my prototype. Noise is low, & probably might have been lower if I had made the amp a tiny bigger. All connectors must be fully insulated types, so there is no connection to chassis. This is important ! You will notice from the above diagram that I didnt include the "loop breaker" circuit shown in the power supply diagram. For my needs, it is not necessary, for your needs, I shall let you pick. In case you select to make use of it, then the earth (chassis) connection marked * (next to the input connectors) must be left off. A few important points The main 0 volt point is the connection between the filter caps.
This is the reference for all zero volt returns, including the 0.1 ohm speaker feedback resistor. Dont connect the feedback resistor directly to the amps GND point, or you will generate distortion & feasible instability. The supply for the amp & preamp must be taken directly from the filter caps - the diagram above is literal - that means that you follow the path of the wiring as shown. Although mentioned above, you might well ask why the pots dont mount directly to the PCB to save wiring. Simple . Had I done it that way, you would require to make use of the same type pots as I designed for, & the panel layout would must be the same , with the exact same spacings. I figured that this would be limiting, so wiring it is. The wiring actually doesnt take long & is simple to do, so is not an issue. I didnt include the "Bright" switch in Figure 1B for clarity. I expect that it will cause few issues.
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One major difference from any "store bought" amplifier is that in case you build it yourself, you can alter things to fit your own needs. The ability to experiment is the key to this circuit, which is although introduced in complete form, there is every expectation that builders will make modifications to suit themselves.
The amp is rated at 100W in to a four Ohms load, as this is typical of a "combo" type amp with 8 Ohm speakers in parallel. Alternatively, you can run the amp in to a "quad" box (four x 8 Ohm speakers in series parallel - see Figure five in Project 27b, the original editorial) and will get about 60 Watts. For the adventurous, two quad boxes and the amp head will provide 100W, but will be much louder than the twin. This is a common combination for guitarists, but it does make it hard for the sound man to bring everything else up to the same level.
The Pre-Amplifier
A picture of the Revision-A preamp is shown below. Youll see that theres dual op amps, but the schematic only shows. This is the main part of the Rev-A update - the output section now has gain (which is basically selected), and a better buffered low output impedance. The remainder of the circuit is unchanged.
Guitar Pre-Amplifier Board
The preamp circuit is shown in Figure one, and has a few fascinating characteristics that separate it from the "normal" - assuming that there is such a thing. This is simple but elegant design, that provides excellent tonal range. The gain structure is designed to provide a immense amount of gain, which is ideal for those guitarists who like to get that fully distorted "fat" sound.
However, with a couple of simple changes, the preamp can be tamed to suit any style of playing. Likewise, the tone controls as shown have sufficient range to cover very anything from an electrified violin to a bass guitar - The response can be limited in the event you wish (by experimenting with the tone control capacitor values), but I recommend that you try it "as is" before making any changes.
Figure 1 - Guitar Pre-Amplifier
From Figure one, you can see that the preamp makes use of a dual op amp as its only amplification. The lone transistor is an emitter follower, & maintains a low output impedance after the master volume control. As shown, with a typical guitar input, it is feasible to receive a fat overdrive sound by winding up the volume, & then setting the master for an appropriate level. The general frequency response is deliberately limited to prevent extreme low-end waffle, & to cut the extreme highs to help reduce noise & to limit the response to the normal requirements for guitar. In case you use the TL072 op amp as shown, you may find that noise is an issue - at high gain with lots of treble boost. I strongly recommend that you use an OPA2134 - a premium audio op amp from Los angels Instruments (Burr-Brown division), you will then find this possibly the quietest guitar amp you have ever heard (or not heard :-). At any gain setting, there is more pickup noise from my guitar than circuit noise - & for the prototype one used carbon resistors!
Notes:
one - IC pin outs are industry standard for dual op amps - pin four is -ve supply, and pin 8 is +ve supply.
two - Op amp supply pins must be bypassed to earth with 100nF caps (preferably ceramic) as close as feasible to the op amp itself.
three - Diodes are 1N4148, 1N914 or similar.
four - Pots ought to be linear for tone controls, & log for volume and master.
one - IC pin outs are industry standard for dual op amps - pin four is -ve supply, and pin 8 is +ve supply.
two - Op amp supply pins must be bypassed to earth with 100nF caps (preferably ceramic) as close as feasible to the op amp itself.
three - Diodes are 1N4148, 1N914 or similar.
four - Pots ought to be linear for tone controls, & log for volume and master.
The power supply section (bottom left corner) connects directly to the main +/-35V power amp supply. Use one Watt zen-er diodes (D5 and D6), and make positive that the zen-er supply resistors (R18 and R19, 680 ohm one Watt) are kept away from other parts, as they will get warm in operation. Again, the preamp PCB accommodates the supply on the board.
The pin connections shown (either huge dots or "port" symbols) are the pins from the PCB. Normally, all pots would be PCB types, and mounted directly to the board. For a do-it-yourself project, that would limit the layout to that imposed by the board, so all connections use wiring. It may look a bit hard, but is simple and looks fine when the unit is done. Cable ties keep the wiring tidy, and only a single connection to the GND point ought to be used(several are provided, so select that suits your layout. VCC is +35V from the main supply, and VEE is the -35V supply.
In the event you dont require all the gain that is available, basically increase the worth of R6 (the first 4k7 resistor) - for even less noise and gain, increase R11 (the second 4k7) as well. For more gain, decrease R11 - I recommend a maximum of 2k2 here.
If the bright switch is bright ( much treble), increase the 1k resistor (R5) to tame it down again. Reduce the worth to get more bite. The tone control arrangement shown will give zero output if all controls are set to maximum - this is unlikely to be a common requirement in use, but be aware of it when testing.
The diode network at the output is designed to permit the preamp to generate a "soft" clipping characteristic when the volume is turned up. Because of the diode clipping, the power amp needs to have an input sensitivity of about 750mV for full output, otherwise it wont be feasible to get full power even with the Master gain control at the maximum setting.
Make positive that the input connectors are isolated from the chassis. The earth isolation parts in the power supply help to prevent hum ( when the amp is connected to other mains powered equipment).
If issues are encountered with this circuit, then you have made a wiring mistake .. period. A golden rule here is to check the wiring, then keep on checking it until you find the error, since I can assure you that if it does not work properly there is at least mistake, & probably more.
The input, effects & output connections are shown in Figure 1B.
Figure 1B - Internal Wiring
This is the reference for all zero volt returns, including the 0.1 ohm speaker feedback resistor. Dont connect the feedback resistor directly to the amps GND point, or you will generate distortion & feasible instability. The supply for the amp & preamp must be taken directly from the filter caps - the diagram above is literal - that means that you follow the path of the wiring as shown. Although mentioned above, you might well ask why the pots dont mount directly to the PCB to save wiring. Simple . Had I done it that way, you would require to make use of the same type pots as I designed for, & the panel layout would must be the same , with the exact same spacings. I figured that this would be limiting, so wiring it is. The wiring actually doesnt take long & is simple to do, so is not an issue. I didnt include the "Bright" switch in Figure 1B for clarity. I expect that it will cause few issues.
Friday, September 19, 2014
Hammonator Organ to Guitar Amp Conversion Wiring diagram Schematic
In the world of electronics, vacuum tubes are almost obsolete. Nearly the last holdout, the cathode ray tube (CRT), is rapidly being replaced by the LCD and other new technologies. Despite this trend, the vacuum tube has seen a big revival in the field of guitar amplifiers, and to a lesser extent, hi-fi amplifiers. Vacuum tubes and related parts have become more readily available in recent years as numerous companies have tapped into this market.
The reason for the popularity of tubes in guitar amps involves the nice tones that are produced when tubes are driven to the point of distortion. For some background on this, follow some of the links on The Strat Monger. There are numerous solid state "modeling amps" that try to simulate vacuum tube amps with digital signal processing (DSP) techniques, but in the end, that method is never more than a simulation. It just aint the same as the real thing.
One can spend a large amount of money and time building a tube amp from scratch. Hammond organ ampifiers chassis are available on the surplus market for a reasonable price, they make a good starting point for a guitar amp. The difficult job of cutting chassis holes for the tubes and transformers is already done, one just needs to drill a few holes for the potentiometers and connectors. This project started with the amplifier from a Hammond M2 organ, chassis model AO14-1B.
Hammonator Organ to Guitar Amp Conversion Circuit Diagram
The output stage of this amplifier resembles a fusion between a Fender Princeton Reverb, Fender Vibroverb and ham radio transmitter. With 6V6 output tubes running at a 420V plate voltage, it puts out approximately 18 watts of audio power. The 17" reverb tank provides a deep echoey sound. The "simpler is better" philosophy was used in the design, multiple inputs with their own preamp stages were intentionally avoided to reduce hiss. The amp is plenty loud, and the sound quality is excellent. The Hammonator amp has worked well driving both 12" and 15" guitar speakers.
The Hammonator Model 1 amp is a simplified version of the Hammonator 2RVT schema. Builders can start with the Model 1 schema and easily add the Model 2RVT Vibrato/Tremolo schemary at a later date.There are a few unique features in this amp, and some slight deviations from the aforementioned simplicity goal. An optional fluorescent EM87/6HU6 "magic eye" tube (EM87 in action) is used for an output level meter, it is fun to stare at while playing. The EM87 uses a peak reading schema that was inspired by this design then modified somewhat. There is a reverb send control (Dwell) that can be used to expand the variety of reverb sounds. Most Fender amps send only a full-strength signal to the reverb spring. By turning the reverb send signal down a bit, a less "clangy" and more "spacey" reverb sound results.
The Hammonator also features a negative feedback control. With the feedback control turned all the way to the left (max negative feedback), the amp compresses the signal and the waveform peaks are reduced. With the feedback control turned all the way to the right, the sound is louder and less compressed and approaches that of the popular Fender Tweed Deluxe (5E3) amps. The feedback control could also be called "Clarity", "Gain" or "Presence".
This amp uses four octal base 6SN7 dual triode tubes for most of the low level signal amplification instead of the more common 12AX7 or 12AU7 tubes. This was done because the chassis was already set up for the octal sockets. Boutique amp enthusiasts will probably like this feature since the 6SN7 tubes are older and may have more of a vintage amp sound. Fortunately, the 6SN7 is still easy to acquire. This amp has been "tuned" for good sound, the bias settings of all of the tube stages were tweaked while a guitar was plugged in. This process was used to optimize the musical qualities of the amp. Not all vintage 6SN7 tubes are the same, quieter Sylvania tubes were used for VT1 and VT3 to reduce the hiss, nosier RCA and GE tubes were used elsewhere. You can test for noisy 6SN7 tubes by putting them in the VT1 socket, listening to the hiss level and tapping on the tube to listen for microphonics.
It is possible to change VT1, the first preamplifier and tone recovery tube, from a lower gain 6SN7 dual triode to a higher gain 6SL7 dual triode without any wiring changes. This allows the amplifier to work better with low output guitar pickups. This trick is often done with other amps by swapping 12AU7, 12AT7, 12AY7 and 12AX7 tubes, they all share the same pinout but have different gains.
The newer and more common AO-29 (M3 organ) chassis would also make a good chassis for a guitar amp conversion. The three 9 pin tube sockets could be used for 12AX7 or 12AU7 dual triode tubes and the five 7 pin tube sockets could be filled with common 6AV6 tubes (similar to a single 12AX7 triode) or 6C4 tubes (similar to a single 12AU7 triode). A similar schema layout could be used on the AO-29 chassis but the cathode bias resistor values on the 7 and 9 pin preamp triodes would need to be changed from the values used on the 6SN7 tubes. The AO-29 power transformer and output transformer are very similar to those used in the AO-14.
Connections:
Power Input - grounded 120VAC
Guitar Input - High Impedance
Reverb Send
Reverb Return
Speaker Output - 8 ohms
Controls:
On/Off (on the back)
Input Volume
Bass
Treble
Reverb Send (Dwell)
Reverb Return
Feedback (Gain)
Theory:
The AC power input schemary was modified from the original Hammond schema. The power transformer is old enough that it was designed to run on 110V-115V mains instead of the 120V mains found today. Running the stock amp on 120V produces higher filament and B+ voltages, the higher filament voltages can shorten the life of the tubes. This problem can be easily fixed by putting the 5V rectifier filament winding in series with the AC primary winding. The 5V phasing must be correct, the easy way to test this is to try both orientations and monitor the 6.3V filament winding, use the lower wiring that produces the lower voltage. When the tubes are plugged, the filament voltage should be very close to 6.3V.
A grounded plug was used, this is critical for safety. A 2 amp fuse and switch are used to provide a standard fused disconnect. The varistor on the transformer primary protects against line voltage transients, those can get multiplied on the high voltage output winding and cause damage.
The transformer high voltage winding is sent to a center tapped full wave rectifier consisting of two 1N4007 diodes. The high voltage DC is dropped through a typical chain of resistors and capacitors to produce the voltages used in the amp. The first resistor (150 ohms/2 Watt) is used to set the initial B1+ voltage that drives the power output tubes.
There is a lot of misinformation on the net about tube rectifiers vs solid state rectifiers and the effect on amp sound. This probably derives from the more efficient nature of solid state diodes and the resulting higher voltage when a direct substitution is done. Putting a resistor after the diodes drops the B+ voltage to a level that is closer to that achieved with a 5U4 rectifier. The diodes have the advantage of better efficiency due to the lack of a high power filament, the power transformer will also run cooler using diodes. The 1nF/1KV capacitors across the diodes protect against high voltage transients and eliminate RF rectification issues.
The 5H inductor choke is used to reduce hum in the preamp stages, the value is not especially critical. The 220nF capacitors in the power supply are fairly unique to this design, they improve the high frequency response of the amp. This is a trick that was borrowed from solid state schemary. If you dont have any 220nF caps, 100nF caps should do the job.
The Vbias- negative voltage is derived from a half wave rectifier and a resistive ladder. The 25K bias control can be adjusted to set the idle bias level on the power tubes. Bias levels for both 6V6 and 6L6 tubes can be generated.
The guitar input stage (VT1b) is a standard class A triode amplifier. The 1K cathode resistor was chosen to bias this most important amplifier stage into the "sweet spot". The tone controls use the Baxandall tone stack configuration. This schema has a much more distinct boost and cut operation when compared to many of the traditional Fender diagram. A guitar player friend had the amusing suggestion that the "Bass" and "Treble" labels should be changed to "Balls" and "Grit". The post-tone amplifier stage VT1a is another class A triode amplifier. Again, the 1K bias resistor was chosen for the best sound.
The reverb send amp VT2 gets its input from the tone control recovery amplifier VT1a. The 500K linear pot is used to adjust the reverb send level from half way to full. An audo taper pot was tried here, the linear pot had a better response. Both halves of VT2 are run in parallel, the 560 ohm bias resistor was chosen for the best tube drive level. VT2 runs slightly warm, with a bit of blue glow showing. A standard Fender "Twin Reverb" reverb transformer can be used to drive the reverb, I used a slightly heavier Buddy MC500 transformer.
The reverb return signal goes to two class A triode stages formed by VT3b and VT3a. The reverb return level is set with the 100K audio pot and mixed into the phase splitter stage (VT4) through a 5nF capacitor. The clean (non-reverb) signal is amplified by VT7, a 6AV6 triode wired as a floating cathode-biased stage. The 6AV6 isolates the reverb send and receive signals to prevent feedback, it also forms the heart of the vibrato/tremolo schema in the Hammonator model 2RVT design.
The balanced phase splitter schema is formed by VT4a and VT4b. This stage combined with the power tube stage is fairly close to the Fender Vibroverb schema. The two opposite-phase drive signals are sent to the control grids of the 6V6 power output tubes. An RF power amp trick is used here to reduce potential radio frequency oscillation issues, 10nF capacitors bypass the Lcd grids to ground. These caps should not be confused with the unpopular tone-deadening control grid caps that were added to Post-CBS Fender Twin Reverb amps.
A triple feedback loop is used between the output transformer and the input of the phase splitter. The low and high cut loops reduce the sub-sonic and ultra-sonic gain, eliminating any tendencies to oscillate and generate radio frequencies. While experimenting with the schema, some nearly dead power tubes were used, the tubes tended to oscillate when biased to a useful setting. These additions reduced that problem and improved the sound, RF superimposed on audio does not sound good.
A fairly heavy modem isolation transformer from a 300 baud vintage of modem was wired in series to make the low-cut inductor. When the amp is driving a speaker, there can be large resonances in the low bass part of the spectrum. A 12" speaker in an open-backed cabinet had a natural resonance around 70 Hz. Audio at the speaker resonance frequency is amplified to about twice the level as other frequencies, resulting in an exaggerated bass response and distortion. The low-cut feedback schema offsets this resonance effect.
An earlier version (obsolete) of this amp used a different anti-resonance feedback (ARF) loop that consisted of a 300 ohm resistor, a series-wired modem transformer and a 1.32uF stack of capacitors that was tuned to cancel the speaker resonance. When feeding a purely resistive load, the amplifier has a fairly flat frequency response. The low-cut/high-cut feedback loop eliminates the need to tune the amp for individual speakers.
The 6HU6 eye tube schema gets its control signal from the output transformer. The signal is rectified, low-passed and sent to the tubes control grid. The 10M bias resistor opens the tubes display farther during quiet operation. The 5K trimmer should be adjusted so that the eye tube display closes completely when the amp is played to maximum power.
Biasing the Power Tubes
If you want more than 18 Watts of power, it is possible to replace the 6V6 tubes with 6L6 tubes, simply re-adjust the bias control. The bias is set by putting a DC volt meter between the Imon1 terminal and ground. The Imon2 terminal can be checked to see if the power tubes are well matched. Both Imon1 and Imon2 should have similar voltages. The 6V6 tubes work well with a bias of around 0.17V (17 mA) and 6L6 tubes work well at around 0.35V (35mA). Tube bias setting is a trade-off between loudness and tube life. Generally, the bias should be set so that the tubes dont become too warm when there is no signal going through them.
Construction:
Here is a photo of the wiring side of the Hammonator 2RVT amp, it is essentially the Hammonator 1 schema with a few additions.The stock Hammond amp chassis that this project was built on was dirty, rusty and filled with mostly useless parts. A wire brush was used to scrape off the rust and dirt. Leave the original filament wiring from the power transformer to the 6V6 tubes intact. You will need to move one of the filament wires on some of the 6SN7 tube sockets (formerly other tube types). The power transformers high voltage leads can be left connected to the 5U4 socket, the 1N4007 rectifier diodes can be wired to the pins of the 5U4 socket. The output transformers primary wiring should be left as-is.
The ground wires that connect all of the tube sockets should be left intact. Just about everything else can be clipped off, leave all of the transformer wires as long as possible. There were two plug-boards in the center of the amp. All of the wires between the plug-boards and the tube sockets were clipped at the tube sockets and the boards were removed. The wires to the screw terminals were also clipped off. Some of the plug-board capacitors were scavanged for use elsewhere.
A new 3-wire power cord power switch were installed in the small metal wiring box that is located behind the power transformer. Two of the downward-facing holes in the wiring box were expanded to fit the power cords strain relief and the switch. A plastic "pigtail" type of fuse holder was also installed in the box. The power cables green ground wire was connected to the chassis with a solder lug.
The two tall electrolytic capacitors were removed from the chassis. The silver capacitors hole was filed out and drilled to fit the 6HU6 eye tube socket. A sheet metal filler was installed in the black capacitors hole (the photo above was taken before this was done). The volume pedal tower was disassembled and the empty space was used as a "doghouse" for most of the electrolytic capacitors. The caps were secured to the towers bakelite spacers with panduit ties. The tower allows the amp to sit upside down without resting on the tubes, this is very useful when working on the amp.
The Hammonator Model 1 amp is a simplified version of the Hammonator 2RVT schema. Builders can start with the Model 1 schema and easily add the Model 2RVT Vibrato/Tremolo schemary at a later date.There are a few unique features in this amp, and some slight deviations from the aforementioned simplicity goal. An optional fluorescent EM87/6HU6 "magic eye" tube (EM87 in action) is used for an output level meter, it is fun to stare at while playing. The EM87 uses a peak reading schema that was inspired by this design then modified somewhat. There is a reverb send control (Dwell) that can be used to expand the variety of reverb sounds. Most Fender amps send only a full-strength signal to the reverb spring. By turning the reverb send signal down a bit, a less "clangy" and more "spacey" reverb sound results.
The Hammonator also features a negative feedback control. With the feedback control turned all the way to the left (max negative feedback), the amp compresses the signal and the waveform peaks are reduced. With the feedback control turned all the way to the right, the sound is louder and less compressed and approaches that of the popular Fender Tweed Deluxe (5E3) amps. The feedback control could also be called "Clarity", "Gain" or "Presence".
This amp uses four octal base 6SN7 dual triode tubes for most of the low level signal amplification instead of the more common 12AX7 or 12AU7 tubes. This was done because the chassis was already set up for the octal sockets. Boutique amp enthusiasts will probably like this feature since the 6SN7 tubes are older and may have more of a vintage amp sound. Fortunately, the 6SN7 is still easy to acquire. This amp has been "tuned" for good sound, the bias settings of all of the tube stages were tweaked while a guitar was plugged in. This process was used to optimize the musical qualities of the amp. Not all vintage 6SN7 tubes are the same, quieter Sylvania tubes were used for VT1 and VT3 to reduce the hiss, nosier RCA and GE tubes were used elsewhere. You can test for noisy 6SN7 tubes by putting them in the VT1 socket, listening to the hiss level and tapping on the tube to listen for microphonics.
It is possible to change VT1, the first preamplifier and tone recovery tube, from a lower gain 6SN7 dual triode to a higher gain 6SL7 dual triode without any wiring changes. This allows the amplifier to work better with low output guitar pickups. This trick is often done with other amps by swapping 12AU7, 12AT7, 12AY7 and 12AX7 tubes, they all share the same pinout but have different gains.
The newer and more common AO-29 (M3 organ) chassis would also make a good chassis for a guitar amp conversion. The three 9 pin tube sockets could be used for 12AX7 or 12AU7 dual triode tubes and the five 7 pin tube sockets could be filled with common 6AV6 tubes (similar to a single 12AX7 triode) or 6C4 tubes (similar to a single 12AU7 triode). A similar schema layout could be used on the AO-29 chassis but the cathode bias resistor values on the 7 and 9 pin preamp triodes would need to be changed from the values used on the 6SN7 tubes. The AO-29 power transformer and output transformer are very similar to those used in the AO-14.
Connections:
Power Input - grounded 120VAC
Guitar Input - High Impedance
Reverb Send
Reverb Return
Speaker Output - 8 ohms
Controls:
On/Off (on the back)
Input Volume
Bass
Treble
Reverb Send (Dwell)
Reverb Return
Feedback (Gain)
Theory:
The AC power input schemary was modified from the original Hammond schema. The power transformer is old enough that it was designed to run on 110V-115V mains instead of the 120V mains found today. Running the stock amp on 120V produces higher filament and B+ voltages, the higher filament voltages can shorten the life of the tubes. This problem can be easily fixed by putting the 5V rectifier filament winding in series with the AC primary winding. The 5V phasing must be correct, the easy way to test this is to try both orientations and monitor the 6.3V filament winding, use the lower wiring that produces the lower voltage. When the tubes are plugged, the filament voltage should be very close to 6.3V.
A grounded plug was used, this is critical for safety. A 2 amp fuse and switch are used to provide a standard fused disconnect. The varistor on the transformer primary protects against line voltage transients, those can get multiplied on the high voltage output winding and cause damage.
The transformer high voltage winding is sent to a center tapped full wave rectifier consisting of two 1N4007 diodes. The high voltage DC is dropped through a typical chain of resistors and capacitors to produce the voltages used in the amp. The first resistor (150 ohms/2 Watt) is used to set the initial B1+ voltage that drives the power output tubes.
There is a lot of misinformation on the net about tube rectifiers vs solid state rectifiers and the effect on amp sound. This probably derives from the more efficient nature of solid state diodes and the resulting higher voltage when a direct substitution is done. Putting a resistor after the diodes drops the B+ voltage to a level that is closer to that achieved with a 5U4 rectifier. The diodes have the advantage of better efficiency due to the lack of a high power filament, the power transformer will also run cooler using diodes. The 1nF/1KV capacitors across the diodes protect against high voltage transients and eliminate RF rectification issues.
The 5H inductor choke is used to reduce hum in the preamp stages, the value is not especially critical. The 220nF capacitors in the power supply are fairly unique to this design, they improve the high frequency response of the amp. This is a trick that was borrowed from solid state schemary. If you dont have any 220nF caps, 100nF caps should do the job.
The Vbias- negative voltage is derived from a half wave rectifier and a resistive ladder. The 25K bias control can be adjusted to set the idle bias level on the power tubes. Bias levels for both 6V6 and 6L6 tubes can be generated.
The guitar input stage (VT1b) is a standard class A triode amplifier. The 1K cathode resistor was chosen to bias this most important amplifier stage into the "sweet spot". The tone controls use the Baxandall tone stack configuration. This schema has a much more distinct boost and cut operation when compared to many of the traditional Fender diagram. A guitar player friend had the amusing suggestion that the "Bass" and "Treble" labels should be changed to "Balls" and "Grit". The post-tone amplifier stage VT1a is another class A triode amplifier. Again, the 1K bias resistor was chosen for the best sound.
The reverb send amp VT2 gets its input from the tone control recovery amplifier VT1a. The 500K linear pot is used to adjust the reverb send level from half way to full. An audo taper pot was tried here, the linear pot had a better response. Both halves of VT2 are run in parallel, the 560 ohm bias resistor was chosen for the best tube drive level. VT2 runs slightly warm, with a bit of blue glow showing. A standard Fender "Twin Reverb" reverb transformer can be used to drive the reverb, I used a slightly heavier Buddy MC500 transformer.
The reverb return signal goes to two class A triode stages formed by VT3b and VT3a. The reverb return level is set with the 100K audio pot and mixed into the phase splitter stage (VT4) through a 5nF capacitor. The clean (non-reverb) signal is amplified by VT7, a 6AV6 triode wired as a floating cathode-biased stage. The 6AV6 isolates the reverb send and receive signals to prevent feedback, it also forms the heart of the vibrato/tremolo schema in the Hammonator model 2RVT design.
The balanced phase splitter schema is formed by VT4a and VT4b. This stage combined with the power tube stage is fairly close to the Fender Vibroverb schema. The two opposite-phase drive signals are sent to the control grids of the 6V6 power output tubes. An RF power amp trick is used here to reduce potential radio frequency oscillation issues, 10nF capacitors bypass the Lcd grids to ground. These caps should not be confused with the unpopular tone-deadening control grid caps that were added to Post-CBS Fender Twin Reverb amps.
A triple feedback loop is used between the output transformer and the input of the phase splitter. The low and high cut loops reduce the sub-sonic and ultra-sonic gain, eliminating any tendencies to oscillate and generate radio frequencies. While experimenting with the schema, some nearly dead power tubes were used, the tubes tended to oscillate when biased to a useful setting. These additions reduced that problem and improved the sound, RF superimposed on audio does not sound good.
A fairly heavy modem isolation transformer from a 300 baud vintage of modem was wired in series to make the low-cut inductor. When the amp is driving a speaker, there can be large resonances in the low bass part of the spectrum. A 12" speaker in an open-backed cabinet had a natural resonance around 70 Hz. Audio at the speaker resonance frequency is amplified to about twice the level as other frequencies, resulting in an exaggerated bass response and distortion. The low-cut feedback schema offsets this resonance effect.
An earlier version (obsolete) of this amp used a different anti-resonance feedback (ARF) loop that consisted of a 300 ohm resistor, a series-wired modem transformer and a 1.32uF stack of capacitors that was tuned to cancel the speaker resonance. When feeding a purely resistive load, the amplifier has a fairly flat frequency response. The low-cut/high-cut feedback loop eliminates the need to tune the amp for individual speakers.
The 6HU6 eye tube schema gets its control signal from the output transformer. The signal is rectified, low-passed and sent to the tubes control grid. The 10M bias resistor opens the tubes display farther during quiet operation. The 5K trimmer should be adjusted so that the eye tube display closes completely when the amp is played to maximum power.
Biasing the Power Tubes
If you want more than 18 Watts of power, it is possible to replace the 6V6 tubes with 6L6 tubes, simply re-adjust the bias control. The bias is set by putting a DC volt meter between the Imon1 terminal and ground. The Imon2 terminal can be checked to see if the power tubes are well matched. Both Imon1 and Imon2 should have similar voltages. The 6V6 tubes work well with a bias of around 0.17V (17 mA) and 6L6 tubes work well at around 0.35V (35mA). Tube bias setting is a trade-off between loudness and tube life. Generally, the bias should be set so that the tubes dont become too warm when there is no signal going through them.
Construction:
Here is a photo of the wiring side of the Hammonator 2RVT amp, it is essentially the Hammonator 1 schema with a few additions.The stock Hammond amp chassis that this project was built on was dirty, rusty and filled with mostly useless parts. A wire brush was used to scrape off the rust and dirt. Leave the original filament wiring from the power transformer to the 6V6 tubes intact. You will need to move one of the filament wires on some of the 6SN7 tube sockets (formerly other tube types). The power transformers high voltage leads can be left connected to the 5U4 socket, the 1N4007 rectifier diodes can be wired to the pins of the 5U4 socket. The output transformers primary wiring should be left as-is.
The ground wires that connect all of the tube sockets should be left intact. Just about everything else can be clipped off, leave all of the transformer wires as long as possible. There were two plug-boards in the center of the amp. All of the wires between the plug-boards and the tube sockets were clipped at the tube sockets and the boards were removed. The wires to the screw terminals were also clipped off. Some of the plug-board capacitors were scavanged for use elsewhere.
A new 3-wire power cord power switch were installed in the small metal wiring box that is located behind the power transformer. Two of the downward-facing holes in the wiring box were expanded to fit the power cords strain relief and the switch. A plastic "pigtail" type of fuse holder was also installed in the box. The power cables green ground wire was connected to the chassis with a solder lug.
The two tall electrolytic capacitors were removed from the chassis. The silver capacitors hole was filed out and drilled to fit the 6HU6 eye tube socket. A sheet metal filler was installed in the black capacitors hole (the photo above was taken before this was done). The volume pedal tower was disassembled and the empty space was used as a "doghouse" for most of the electrolytic capacitors. The caps were secured to the towers bakelite spacers with panduit ties. The tower allows the amp to sit upside down without resting on the tubes, this is very useful when working on the amp.
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