ModD
This change addresses two issues - gain and overdrive control, and preventing tube failures. These are independent mods that can be added to any Univalve.
This change addresses two issues - gain and overdrive control, and preventing tube failures. These are independent mods that can be added to any Univalve.
I recently had a conversation with Jerome F. who brought up the idea of adding a master volume control. I couldn't find any postings or comments from anyone that had added one to the Univalve, but I spent some time thinking about what would be gained by adding a MV and how it could be done. I posted about the idea on the Univalve forum, but got no responses.
I recently had a conversation with Jerome F. who brought up the idea of adding a master volume control. I couldn't find any postings or comments from anyone that had added one to the Univalve, but I spent some time thinking about what would be gained by adding a MV and how it could be done. I posted about the idea on the Univalve forum, but got no responses.
It seems few Uni/Bivalve owners are obsessing over adding a MV control to their amps. :-) Perhaps this saga will change a few minds. I have to give credit to Jerome for his "out of the box" thinking. An MV control for the Uni never occurred to me until he mentioned it.
It seems few Uni/Bivalve owners are obsessing over adding a MV control to their amps. :-) Perhaps this saga will change a few minds. I have to give credit to Jerome for his "out of the box" thinking. An MV control for the Uni never occurred to me until he mentioned it.
In doing Mod C, I went beyond tweaking around the edges of the amp - it turned the Uni into a different animal. This MV mod also has relatively major impact in that it alters the amp's external appearance and provides new control over the signal levels in the amp stages. The MV lets you produce new tone qualities the original Uni can not produce. This mod exposes the "amp-within" that's in every Uni that you can finally hear and enjoy. The ModD schematic is an extension of ModC. It includes all of ModC, which you can read about on the ModC page, and it includes the two new extensions described below. You can download it here or at the bottom of this page.
In doing Mod C, I went beyond tweaking around the edges of the amp - it turned the Uni into a different animal. This MV mod also has relatively major impact in that it alters the amp's external appearance and provides new control over the signal levels in the amp stages. The MV lets you produce new tone qualities the original Uni can not produce. This mod exposes the "amp-within" that's in every Uni that you can finally hear and enjoy. The ModD schematic is an extension of ModC. It includes all of ModC, which you can read about on the ModC page, and it includes the two new extensions described below. You can download it here or at the bottom of this page.
Below the mod descriptions you will find a series of waveform pictures that illustrate the signal passage through the stages of the Univalve. The images illustrate the impact of controlling signal levels and how these levels affect the distortions produced at different points in the amp.
Below the mod descriptions you will find a series of waveform pictures that illustrate the signal passage through the stages of the Univalve. The images illustrate the impact of controlling signal levels and how these levels affect the distortions produced at different points in the amp.
Master Volume
Master Volume
The stock Uni has a single gain control between V1a and V1b. V1a is before the volume control and is unlikely to distort significantly, with a guitar as input. (A high-gain pedal or preamp into the Rock input can cause overdrive of V1a.) Using the Rock input (or external preamp/pedal into Roll), you can get enough gain out of V1 to push V2a into distortion.
The stock Uni has a single gain control between V1a and V1b. V1a is before the volume control and is unlikely to distort significantly, with a guitar as input. (A high-gain pedal or preamp into the Rock input can cause overdrive of V1a.) Using the Rock input (or external preamp/pedal into Roll), you can get enough gain out of V1 to push V2a into distortion.
The DC-coupled cathode follower stage (V1b-V2a) has a unique overdrive quality described in many books and sites. The squishing of half the waveform is perceived as warm or rich, and this stage produces lots of this tonal quality as you drive it hard. A hard-driven V2a produces lots of harmonics for the tone stack to control, and it drives the V2b driver stage into distortion as well. Once V2a and V2b are distorting, it's likely that V3, the power tube, is also driven to distortion. So, getting overdrive distortion from a stock Uni drives all three stages (V2a, V2b, and V3) to distortion at around the same input level or volume setting.
The DC-coupled cathode follower stage (V1b-V2a) has a unique overdrive quality described in many books and sites. The squishing of half the waveform is perceived as warm or rich, and this stage produces lots of this tonal quality as you drive it hard. A hard-driven V2a produces lots of harmonics for the tone stack to control, and it drives the V2b driver stage into distortion as well. Once V2a and V2b are distorting, it's likely that V3, the power tube, is also driven to distortion. So, getting overdrive distortion from a stock Uni drives all three stages (V2a, V2b, and V3) to distortion at around the same input level or volume setting.
The speaker signal is a mix of all these distortions, regardless of the hot plate setting. The hot plate only controls the speaker volume, not the signal or distortion levels in the amp stages. Swapping tubes will affect the balance of stage drives and their contribution of distortion to the speaker signal. This is one of the appealing features of the amp, the ability to tweak the tone and distortion mix by changing tubes.
The speaker signal is a mix of all these distortions, regardless of the hot plate setting. The hot plate only controls the speaker volume, not the signal or distortion levels in the amp stages. Swapping tubes will affect the balance of stage drives and their contribution of distortion to the speaker signal. This is one of the appealing features of the amp, the ability to tweak the tone and distortion mix by changing tubes.
Swapping tubes is great, but adding a MV control between V2b and V3 provides much more control over the relative V2 and V3 levels of distortion, for any mix of tubes. A MV control allows the V2 stages to distort without necessarily driving V3 hard. This is the hidden Uni sound - the amp within - a hard-driven preamp with a variable clean-to-dirty power stage. The MV mod provides a whole new tonal landscape that lies between the clean and hard-drive tones of the standard Uni.
Swapping tubes is great, but adding a MV control between V2b and V3 provides much more control over the relative V2 and V3 levels of distortion, for any mix of tubes. A MV control allows the V2 stages to distort without necessarily driving V3 hard. This is the hidden Uni sound - the amp within - a hard-driven preamp with a variable clean-to-dirty power stage. The MV mod provides a whole new tonal landscape that lies between the clean and hard-drive tones of the standard Uni.
The preamp distortion from V2 has a "light" or smooth quality to it, while the power tube typically adds a much rougher grainy distortion quality. With a MV control, you can adjust the power tube contribution to the tone. A side effect is that clean power tube settings also reduce the speaker volume without use of the hot plate. This gives you preamp drive and distortion at bedroom levels, without the tone-killing effects of a cranked-down hot plate.
The preamp distortion from V2 has a "light" or smooth quality to it, while the power tube typically adds a much rougher grainy distortion quality. With a MV control, you can adjust the power tube contribution to the tone. A side effect is that clean power tube settings also reduce the speaker volume without use of the hot plate. This gives you preamp drive and distortion at bedroom levels, without the tone-killing effects of a cranked-down hot plate.
For example, with power tubes that distort at relatively low levels (JJ KT77 and EH EL34), I can drive the preamp hard, cut back the MV to get just a touch of V3 distortion, and set the hot plate at ~50%, to get room-friendly volume levels with a full tone and tons of gain, squeal, and crunch. The MV just provides more control, and many amps have them for that reason (the Flexi-50 has one). The Uni becomes a better amp (IMO) with a MV control because it can produce a much greater range of tone and play "feel".
For example, with power tubes that distort at relatively low levels (JJ KT77 and EH EL34), I can drive the preamp hard, cut back the MV to get just a touch of V3 distortion, and set the hot plate at ~50%, to get room-friendly volume levels with a full tone and tons of gain, squeal, and crunch. The MV just provides more control, and many amps have them for that reason (the Flexi-50 has one). The Uni becomes a better amp (IMO) with a MV control because it can produce a much greater range of tone and play "feel".
Implementing a MV control is inexpensive and reasonably easy. The Uni circuit is setup for a MV control replacing R18 (220K) as the grid load resistor for the V3 stage. Pull R18 off the PCB and wire the board connections to a 220K pot (log or audio taper). The pot wiper is connected to R25 and the V3 grid (see images below).
Implementing a MV control is inexpensive and reasonably easy. The Uni circuit is setup for a MV control replacing R18 (220K) as the grid load resistor for the V3 stage. Pull R18 off the PCB and wire the board connections to a 220K pot (log or audio taper). The pot wiper is connected to R25 and the V3 grid (see images below).
It's a simple electrical modification, but the MV control needs a front panel location. The Uni has a full front panel so there's no obvious place to mount a new pot. I considered removing the light and mounting the MV in that hole. Another possibility is trading the hot plate control for the MV control. I decided against both of these options since the light is part of the THD "look" and I still find the hot plate very useful, even with a MV control.
It's a simple electrical modification, but the MV control needs a front panel location. The Uni has a full front panel so there's no obvious place to mount a new pot. I considered removing the light and mounting the MV in that hole. Another possibility is trading the hot plate control for the MV control. I decided against both of these options since the light is part of the THD "look" and I still find the hot plate very useful, even with a MV control.
I settled on fitting a small 220K pot between the Attitude control and the Noise Reduction switch. Drilling the panel is the hard part. If you're not mechanically equipped, I suggest getting a pro to do it. It is possible to destroy the THD steel face plate in the drilling process. I drilled the face plate and the chassis together, but it may be safer to remove all the front panel controls and knobs and pull off the steel face plate and drill the chassis and face plate separately.
I settled on fitting a small 220K pot between the Attitude control and the Noise Reduction switch. Drilling the panel is the hard part. If you're not mechanically equipped, I suggest getting a pro to do it. It is possible to destroy the THD steel face plate in the drilling process. I drilled the face plate and the chassis together, but it may be safer to remove all the front panel controls and knobs and pull off the steel face plate and drill the chassis and face plate separately.
Another option is to find a micropot with a 1/4" inch bushing. That size hole is much simpler (safer) to drill than the standard 3/8" pot bushing hole. If I were to do it again, I'd go that route. The chassis inside is tight too. I had to rotate the Attitude pot a bit to make room for the MV control.
Another option is to find a micropot with a 1/4" inch bushing. That size hole is much simpler (safer) to drill than the standard 3/8" pot bushing hole. If I were to do it again, I'd go that route. The chassis inside is tight too. I had to rotate the Attitude pot a bit to make room for the MV control.
Wires to/from the pot are shielded. Note that one end of R18 is ground anyway so it makes the shield connection simple. I suggest connecting the pot wiper wire to the unused pin in the center of the V2 socket (see picture below). That's a convenient tie point for connecting to the (blue) wire running to R25 mounted on the V3 socket. (Note that the blue V3 wire has to be removed from its PCB pad near the R18 pads.)
Wires to/from the pot are shielded. Note that one end of R18 is ground anyway so it makes the shield connection simple. I suggest connecting the pot wiper wire to the unused pin in the center of the V2 socket (see picture below). That's a convenient tie point for connecting to the (blue) wire running to R25 mounted on the V3 socket. (Note that the blue V3 wire has to be removed from its PCB pad near the R18 pads.)
The final challenge is finding a small version of the THD knobs for the MV pot. Fortunately these are a common knob style and smaller sizes are available.
The final challenge is finding a small version of the THD knobs for the MV pot. Fortunately these are a common knob style and smaller sizes are available.
Below is a picture of the Master Volume control added to front panel (yes, that is a KT88 standing tall)
Below is a picture of the Master Volume control added to front panel (yes, that is a KT88 standing tall)
Here is a picture of the chassis inside showing the MV pot location.
Here is a picture of the chassis inside showing the MV pot location.
Note that the chassis shield plate is bent slightly (it is soft metal) to provide some extra space for the pot.
Note that the chassis shield plate is bent slightly (it is soft metal) to provide some extra space for the pot.
Below is a picture of the V2 socket. Note the shielded cable connections to the R18 pads, and the blue wire removed from the PCB V3 pad and connected to the V2 center pin tie point. If you're up for it, replace the blue wire running to R25 on V3 with a shielded conductor. Grid wires should get special treatment. If you're wondering about those other parts tied to the V2 socket pins, they are the subject of the next section.
Below is a picture of the V2 socket. Note the shielded cable connections to the R18 pads, and the blue wire removed from the PCB V3 pad and connected to the V2 center pin tie point. If you're up for it, replace the blue wire running to R25 on V3 with a shielded conductor. Grid wires should get special treatment. If you're wondering about those other parts tied to the V2 socket pins, they are the subject of the next section.
V2 Protection
V2 Protection
V2a is a direct-coupled common cathode stage. As described in Merlin Blencowe's books, this circuit has the potential for damaging the tube if the B+ is turned on before sufficient cathode warm up. Accidents do happen. I've have at least two 12ax7s fail in the Uni, that I can remember.
V2a is a direct-coupled common cathode stage. As described in Merlin Blencowe's books, this circuit has the potential for damaging the tube if the B+ is turned on before sufficient cathode warm up. Accidents do happen. I've have at least two 12ax7s fail in the Uni, that I can remember.
At the time I just replaced the bad tube and thought nothing more of it... but it happened again as I was testing the MV mod and this time I remembered the failure discussion at the valve wizard site. The failure mode may vary, but in this case the preamp became very noisy - like someone clearing their throat. I tested the two preamp tubes and sure enough, V2a was the problem and I recalled hitting the B+ switch (Play) out of sequence just before the failure... aargh.
At the time I just replaced the bad tube and thought nothing more of it... but it happened again as I was testing the MV mod and this time I remembered the failure discussion at the valve wizard site. The failure mode may vary, but in this case the preamp became very noisy - like someone clearing their throat. I tested the two preamp tubes and sure enough, V2a was the problem and I recalled hitting the B+ switch (Play) out of sequence just before the failure... aargh.
I implemented the fix described on the valvewizard site in hopes of never having this problem again. It's cheap and easy and involves only two resistors and a diode. The parts are mounted as shown in the picture above. A 100 ohm resistor connects V2 pin 2 to the PCB pad for pin 2. (Note the original wire connection is removed.) Insulate exposed lead segments as needed. A 10K resistor and diode are soldered together and attached to V2 pins 2 and 3. Some heat-shrink tubing (yellow) around the pair serves as insulation. See the ModD schematic link below for the circuit details.
I implemented the fix described on the valvewizard site in hopes of never having this problem again. It's cheap and easy and involves only two resistors and a diode. The parts are mounted as shown in the picture above. A 100 ohm resistor connects V2 pin 2 to the PCB pad for pin 2. (Note the original wire connection is removed.) Insulate exposed lead segments as needed. A 10K resistor and diode are soldered together and attached to V2 pins 2 and 3. Some heat-shrink tubing (yellow) around the pair serves as insulation. See the ModD schematic link below for the circuit details.
The ModD schematic includes a few other changes from ModC. The resistor in series with the V2b grid is upped to 10K (shown with the red shrink tubing in the image above). That resistor was added as part of ModC, but with a MV the V2 circuits are likely to be driven harder and a higher value grid resistor will be more effective at limiting grid currents. In addition, the calculations of the low frequency break-points for the cathode bypass networks for the V1 stage have changed. Thanks to Pete and Steve (music-electronics-forum) for catching an error in my calculations for the ModC schematic notes. The error has no impact on the amp, but you will notice that the frequencies differ on the ModC and ModD schematics, although the component values are the same. Lastly, the ModD schematic includes some notes for the DC voltages at key circuit points for HiV/LoV switch settings.
The ModD schematic includes a few other changes from ModC. The resistor in series with the V2b grid is upped to 10K (shown with the red shrink tubing in the image above). That resistor was added as part of ModC, but with a MV the V2 circuits are likely to be driven harder and a higher value grid resistor will be more effective at limiting grid currents. In addition, the calculations of the low frequency break-points for the cathode bypass networks for the V1 stage have changed. Thanks to Pete and Steve (music-electronics-forum) for catching an error in my calculations for the ModC schematic notes. The error has no impact on the amp, but you will notice that the frequencies differ on the ModC and ModD schematics, although the component values are the same. Lastly, the ModD schematic includes some notes for the DC voltages at key circuit points for HiV/LoV switch settings.
Below is a picture of the tube areas of the chassis in its current state with ModC and ModD implemented.
Below is a picture of the tube areas of the chassis in its current state with ModC and ModD implemented.
Waveform Images
Waveform Images
Below are some o'scope shots of waveforms at different points in the amp under hi-drive and normal conditions.
Below are some o'scope shots of waveforms at different points in the amp under hi-drive and normal conditions.
You can see what you hear. All the images are taken with a 440Hz sine wave (A above middle C) coming into the Roll input via the TLA/GSP system I currently use for recording and playing. Tone controls on the TLA/GSP are flat, but the sine wave signal is actually getting the nominal bit of tremolo and reverb that my guitar usually gets in this equipment chain. The "normal" and "full" drive levels are created by setting the GSP output control to mid (noon) and max, respectively. The normal drive level going into the Uni is about what you'd get from a humbucker running straight into the amp. The full drive level is about 4-5x the norm level, so it's like a pedal adding that much gain or using the Rock input, which adds more than 10x. Note that at either level, the sine wave doesn't get visibly distorted, so the input to the Uni is visually "clean". The Uni is set to LoV for all shots.
You can see what you hear. All the images are taken with a 440Hz sine wave (A above middle C) coming into the Roll input via the TLA/GSP system I currently use for recording and playing. Tone controls on the TLA/GSP are flat, but the sine wave signal is actually getting the nominal bit of tremolo and reverb that my guitar usually gets in this equipment chain. The "normal" and "full" drive levels are created by setting the GSP output control to mid (noon) and max, respectively. The normal drive level going into the Uni is about what you'd get from a humbucker running straight into the amp. The full drive level is about 4-5x the norm level, so it's like a pedal adding that much gain or using the Rock input, which adds more than 10x. Note that at either level, the sine wave doesn't get visibly distorted, so the input to the Uni is visually "clean". The Uni is set to LoV for all shots.
Image 1 (below) is the signal on R11, the output of V2a, the cathode follower feeding the tone stack. The vertical scale is 50v/div. The waveform is riding on ~150vdc level. The waveform is visibly clean.
Image 1 (below) is the signal on R11, the output of V2a, the cathode follower feeding the tone stack. The vertical scale is 50v/div. The waveform is riding on ~150vdc level. The waveform is visibly clean.
Image 2 (below) is again taken at R11, but this time the input signal is at full drive, and you can see the squishing of the waveform on the negative portion where the voltage reaches about 100v. The squish occurs because V1b is biased hot and therefore grid current flows for the positive excursions of large input signals. This causes the soft clipping on the negative side of the V1b plate signal which is also seen at the V2a cathode. Further increase in the input level just creates more squishing. The "soft clip" is what we hear as "warm" or "rich in harmonics". It's a key element of the THD amp sound (and most amps for that matter). (See the section on tube-amp sound for related discussion.) The vertical scale is 50v/div. Note the signal excursion is over 100v peak to peak at this point. There is also a soft clip occurring on the positive swing due to classic DC-coupled cathode follower behavior, although this is less visible on the scope.
Image 2 (below) is again taken at R11, but this time the input signal is at full drive, and you can see the squishing of the waveform on the negative portion where the voltage reaches about 100v. The squish occurs because V1b is biased hot and therefore grid current flows for the positive excursions of large input signals. This causes the soft clipping on the negative side of the V1b plate signal which is also seen at the V2a cathode. Further increase in the input level just creates more squishing. The "soft clip" is what we hear as "warm" or "rich in harmonics". It's a key element of the THD amp sound (and most amps for that matter). (See the section on tube-amp sound for related discussion.) The vertical scale is 50v/div. Note the signal excursion is over 100v peak to peak at this point. There is also a soft clip occurring on the positive swing due to classic DC-coupled cathode follower behavior, although this is less visible on the scope.
Image 3 (below) shows the normal drive signal result at R17 (V2b grid side), at the output of the tone stack. The tone settings are mid (noon) and the vertical scale is 10v/div. The screen center line is 0v. You can see a dc bias level of ~10v, which is applied to the grid of V2b through R17. Note that the tone stack attenuates the signal of Image 1 by over 5x, but it still looks visually clean.
Image 3 (below) shows the normal drive signal result at R17 (V2b grid side), at the output of the tone stack. The tone settings are mid (noon) and the vertical scale is 10v/div. The screen center line is 0v. You can see a dc bias level of ~10v, which is applied to the grid of V2b through R17. Note that the tone stack attenuates the signal of Image 1 by over 5x, but it still looks visually clean.
Image 4 (below) shows the full drive result at R17. The tone settings are mid and the vertical scale is 10v/div. Attitude is set to mid. The squished waveform of Image 2 is altered by the tone stack. It's apparent that the tone stack impact the shape of the signal, even in a neutral tone setting. Also note that the waveform does not show symmetric excursion - it maintains its positive peak at about 10v, the bias voltage set by R16. Here again some "soft" clipping occurs in the V2b stage, this time squishing positive signal excursions.
Image 4 (below) shows the full drive result at R17. The tone settings are mid and the vertical scale is 10v/div. Attitude is set to mid. The squished waveform of Image 2 is altered by the tone stack. It's apparent that the tone stack impact the shape of the signal, even in a neutral tone setting. Also note that the waveform does not show symmetric excursion - it maintains its positive peak at about 10v, the bias voltage set by R16. Here again some "soft" clipping occurs in the V2b stage, this time squishing positive signal excursions.
Image 5 (below) again shows the full drive signal at R17. This time the tone settings are max treble and mid bass, and the vertical scale remains 10v/div. The negative excursion is spiky while the positive excursion is still "soft" clipped. Recall, the input to the stack (Image 2) had a squashed negative half. The harmonic content of that input waveform is altered by the stack to emphasize the high freq components in that squashed signal, producing the spiky result shown.
Image 5 (below) again shows the full drive signal at R17. This time the tone settings are max treble and mid bass, and the vertical scale remains 10v/div. The negative excursion is spiky while the positive excursion is still "soft" clipped. Recall, the input to the stack (Image 2) had a squashed negative half. The harmonic content of that input waveform is altered by the stack to emphasize the high freq components in that squashed signal, producing the spiky result shown.
Image 6 (below) again shows the full drive signal at R17. This time with both tone controls at max. The vertical scale is still 10v/div. Both excursions are "soft" clipped now. Recall, the R11 output that feeds the stack (Image 2) had a squashed negative half and the grid current at V2b squashes the positive excursion. This is the input to the driver stage V2b, not it's output, but this is what happens in the preamp stages when you "dime" everything... and it can sound great.
Image 6 (below) again shows the full drive signal at R17. This time with both tone controls at max. The vertical scale is still 10v/div. Both excursions are "soft" clipped now. Recall, the R11 output that feeds the stack (Image 2) had a squashed negative half and the grid current at V2b squashes the positive excursion. This is the input to the driver stage V2b, not it's output, but this is what happens in the preamp stages when you "dime" everything... and it can sound great.
Image 7 (below) shows the signal at the Master Volume input (R18) for the normal input level and mid tone settings. The signal is centered around ground and the scope is set for 50v/div. The increase in signal level comes from the driver stage (V2b) gain. Recall the normal level input to V2b is shown in Image 3. It's clear that the preamp has passed a normal level signal all the way to the power stage with little visual change. Looking clean doesn't mean "hi-fi" clean, but anyone playing an amp that passes signals that look this clean will consider the amp tone "clean". Changing the tone settings of an amp operating in a clean mode will mostly impact the relative levels of different frequency sine waves, not their shapes.
Image 7 (below) shows the signal at the Master Volume input (R18) for the normal input level and mid tone settings. The signal is centered around ground and the scope is set for 50v/div. The increase in signal level comes from the driver stage (V2b) gain. Recall the normal level input to V2b is shown in Image 3. It's clear that the preamp has passed a normal level signal all the way to the power stage with little visual change. Looking clean doesn't mean "hi-fi" clean, but anyone playing an amp that passes signals that look this clean will consider the amp tone "clean". Changing the tone settings of an amp operating in a clean mode will mostly impact the relative levels of different frequency sine waves, not their shapes.
Image 8 (below) shows the V2b output signal feeding the Master Volume input (R18) for the full drive input level and mid tone settings. The screen center line is ground and the scope is set for 50v/div. The V2b driver stage output clips on the positive excursions due to large negative excursion grid signals driving the tube to cut off.
Image 8 (below) shows the V2b output signal feeding the Master Volume input (R18) for the full drive input level and mid tone settings. The screen center line is ground and the scope is set for 50v/div. The V2b driver stage output clips on the positive excursions due to large negative excursion grid signals driving the tube to cut off.
The negative squashing observed is caused by grid current effects on positive grid signal excursions. Recall the input to this stage is shown in Image 6. With full drive on the amp input, the latter amp stages go into hard-drive mode where highly-asymmetric waveforms are produced (non-linear distortions). This continues through the output stage producing characteristic Single-Ended (Class A) tone.
The negative squashing observed is caused by grid current effects on positive grid signal excursions. Recall the input to this stage is shown in Image 6. With full drive on the amp input, the latter amp stages go into hard-drive mode where highly-asymmetric waveforms are produced (non-linear distortions). This continues through the output stage producing characteristic Single-Ended (Class A) tone.
This is what you pay the big bucks for! *Ummmm.... highly-asymmetric output waveforms* The MV let's you produce this driver output, without forcing V3, the power stage to clip. This driver clipped signal is a cleaner-sounding clip than V3 produces. Of course, you can still crank the MV and get V3 to clip/play along.
This is what you pay the big bucks for! *Ummmm.... highly-asymmetric output waveforms* The MV let's you produce this driver output, without forcing V3, the power stage to clip. This driver clipped signal is a cleaner-sounding clip than V3 produces. Of course, you can still crank the MV and get V3 to clip/play along.
Image 9 (below) shows the signal at the Master Volume input (R18) for the full drive input level and max tone settings. The screen center line is ground and the scope is set for 50v/div. This just shows that the impact of the tone controls at this point are to visually sharpen the transitions (steepness and corners) of the curve and to flatten the slow changing sections (low freq components).
Image 9 (below) shows the signal at the Master Volume input (R18) for the full drive input level and max tone settings. The screen center line is ground and the scope is set for 50v/div. This just shows that the impact of the tone controls at this point are to visually sharpen the transitions (steepness and corners) of the curve and to flatten the slow changing sections (low freq components).
Note the asymmetric quality of the waveform and that both waveform excursions show obvious squashing (clipping) although the positive and negative distortions are different. The top half is a "hard clip" due to V2b cut off, while the bottom half is a "soft clip" due to V2b grid current effects. Of course earlier stages also contributed distortion and harmonics, so the tone of the signal at this point reflects all those contributions.
Note the asymmetric quality of the waveform and that both waveform excursions show obvious squashing (clipping) although the positive and negative distortions are different. The top half is a "hard clip" due to V2b cut off, while the bottom half is a "soft clip" due to V2b grid current effects. Of course earlier stages also contributed distortion and harmonics, so the tone of the signal at this point reflects all those contributions.
Image 10 (below) shows the signal at the grid of V3 input (R18) for the full drive input level and max tone settings. The screen center line is ground and the scope is set for 50v/div. The MV is set to max and we can see that the V3 grid signal is driven by over 100v pp.
Image 10 (below) shows the signal at the grid of V3 input (R18) for the full drive input level and max tone settings. The screen center line is ground and the scope is set for 50v/div. The MV is set to max and we can see that the V3 grid signal is driven by over 100v pp.
Additional clipping on the positive signal excursions is due to V3 grid current and this causes additional power-stage distortions that are added to the signal. Reducing the MV level will present a variable size version of driver output (Image 9) as a grid signal to the V3 stage, and this will be amplified by V3 while adding a controlled amount of clipping and distortion.
Additional clipping on the positive signal excursions is due to V3 grid current and this causes additional power-stage distortions that are added to the signal. Reducing the MV level will present a variable size version of driver output (Image 9) as a grid signal to the V3 stage, and this will be amplified by V3 while adding a controlled amount of clipping and distortion.
That's it... I hope you found this interesting and at least amusing...
That's it... I hope you found this interesting and at least amusing...
You now should have a picture of what it means to "dime" your amp and what those glowing glass bulbs are doing for you... enjoy.
You now should have a picture of what it means to "dime" your amp and what those glowing glass bulbs are doing for you... enjoy.
A final note about tube choices... while many tube combos sound good, my current favorite is two Electro-Harmonix 12AX7EH's and a JJ KT-66.
A final note about tube choices... while many tube combos sound good, my current favorite is two Electro-Harmonix 12AX7EH's and a JJ KT-66.