ModA
Begin by looking at the stock amp schematic that shows the first two gain stages.
Begin by looking at the stock amp schematic that shows the first two gain stages.
The amp has two inputs, one “Rock” input for high-gain overdrive, and the more modest gain “Roll” input. Since I often use an external preamp, the Roll input is perfect, but I also use the Rock input without the preamp. The Rock input stage is modified to increase the cathode bypass capacitor C3. The stock cap with R5 creates a bass roll-off starting above 100Hz, the first of many bass cuts in the amp circuits. Replace the 0.68uf/250v cap with something in the 5uf-10uf range. You don’t need a high-voltage cap here, (the cathode voltage is only a few volts), but I happened to have a 10uf/200v Mylar cap in a parts bin, and there is room for it, so that’s what I used. You can safely use a 50v electrolytic. This change flattens the 1st stage response and eliminate one of the bass-cut stages.
The amp has two inputs, one “Rock” input for high-gain overdrive, and the more modest gain “Roll” input. Since I often use an external preamp, the Roll input is perfect, but I also use the Rock input without the preamp. The Rock input stage is modified to increase the cathode bypass capacitor C3. The stock cap with R5 creates a bass roll-off starting above 100Hz, the first of many bass cuts in the amp circuits. Replace the 0.68uf/250v cap with something in the 5uf-10uf range. You don’t need a high-voltage cap here, (the cathode voltage is only a few volts), but I happened to have a 10uf/200v Mylar cap in a parts bin, and there is room for it, so that’s what I used. You can safely use a 50v electrolytic. This change flattens the 1st stage response and eliminate one of the bass-cut stages.
In the second stage, which is the Roll input, R4 and C6 create a high-boost network that I suggest leaving intact. It’s only a modest boost that flattens at 2x starting at about 340Hz. This boost is a matter of taste, and effectively adds a bit of a mid-cut that is common in many amps. Next is C7, a 220pf cap that bypasses the volume control. Clearly the intent was to add brightness at low volumes. The problem is it adds too much since the boost kicks in at lower frequencies as the volume setting is reduced. Worst case, when you’re trying to play clean at a low volume setting (knob near 9am), the boost kicks in at about 500Hz and it doubles every octave. I'd suggest you remove this cap. If you want some treble boost at any volume level, just use the treble control – it has a wide range and there is plenty of treble left in the amp (recall the R4 / C6 boost). Some have suggested replacing C7 with something much smaller (~47pf), and that's perfectly reasonable too - size it to taste. It's a common technique in many amps to have a cap bypass the volume control (aka bright cap), so people must like it's effect.
In the second stage, which is the Roll input, R4 and C6 create a high-boost network that I suggest leaving intact. It’s only a modest boost that flattens at 2x starting at about 340Hz. This boost is a matter of taste, and effectively adds a bit of a mid-cut that is common in many amps. Next is C7, a 220pf cap that bypasses the volume control. Clearly the intent was to add brightness at low volumes. The problem is it adds too much since the boost kicks in at lower frequencies as the volume setting is reduced. Worst case, when you’re trying to play clean at a low volume setting (knob near 9am), the boost kicks in at about 500Hz and it doubles every octave. I'd suggest you remove this cap. If you want some treble boost at any volume level, just use the treble control – it has a wide range and there is plenty of treble left in the amp (recall the R4 / C6 boost). Some have suggested replacing C7 with something much smaller (~47pf), and that's perfectly reasonable too - size it to taste. It's a common technique in many amps to have a cap bypass the volume control (aka bright cap), so people must like it's effect.
In the second stage, the cathode bypass cap (C8) also needs to be increased. This is an electrolytic 2.2uf/100v cap and you should either replace it or just parallel it with a 47uf cap with a 50v or 100v rating. The stock network gave another low cut at about 100Hz. The mod moves it to the range of 5Hz. Ok, so we don’t really hear 5Hz, but that’s the -3db point, the cut actually begins higher, and there is no reason to skimp. There are valid concerns about very low frequencies getting into the output stage, but its not of concern at this stage in the amp. Firstly, there is little chance of very low freq signal (< 30 Hz) coming from the guitar, and even if there were, there are still many RC networks that will block anything in that freq range from getting through to to the output stage.
In the second stage, the cathode bypass cap (C8) also needs to be increased. This is an electrolytic 2.2uf/100v cap and you should either replace it or just parallel it with a 47uf cap with a 50v or 100v rating. The stock network gave another low cut at about 100Hz. The mod moves it to the range of 5Hz. Ok, so we don’t really hear 5Hz, but that’s the -3db point, the cut actually begins higher, and there is no reason to skimp. There are valid concerns about very low frequencies getting into the output stage, but its not of concern at this stage in the amp. Firstly, there is little chance of very low freq signal (< 30 Hz) coming from the guitar, and even if there were, there are still many RC networks that will block anything in that freq range from getting through to to the output stage.
The circuit continues from pin 6 of V1 – the output of the second gain stage – into the input of V2 where a cathode follower drives the tone stack. There is no reason (IMO) to modify this stage or the tone section. None of this is show in the posted schematic since there are no mods needed there.
The circuit continues from pin 6 of V1 – the output of the second gain stage – into the input of V2 where a cathode follower drives the tone stack. There is no reason (IMO) to modify this stage or the tone section. None of this is show in the posted schematic since there are no mods needed there.
The output of the tone section feeds the second half of V2 through C14. Similarly, C15 couples the output of V2 to the power tube V3. The C15 cathode cap rolls off at 32 Hz, so it has some impact on the lowest frequencies, but this cut may be important to the character of how the tube breaks up when driven hard. I have not changed this cap and I suggest leaving it alone, at least initially. The Univalve allows for a number of different output tubes, so you can play with the overdrive character that way. I personally like using a EL34 or a 6550. It is great that tube changes are easy with this amp and they do create subtle differences. At the time of this writing, I’m favoring the 6550, but tomorrow, who knows. I keep a shelf of varied preamp and power tubes for this amp.
The output of the tone section feeds the second half of V2 through C14. Similarly, C15 couples the output of V2 to the power tube V3. The C15 cathode cap rolls off at 32 Hz, so it has some impact on the lowest frequencies, but this cut may be important to the character of how the tube breaks up when driven hard. I have not changed this cap and I suggest leaving it alone, at least initially. The Univalve allows for a number of different output tubes, so you can play with the overdrive character that way. I personally like using a EL34 or a 6550. It is great that tube changes are easy with this amp and they do create subtle differences. At the time of this writing, I’m favoring the 6550, but tomorrow, who knows. I keep a shelf of varied preamp and power tubes for this amp.
Lastly, the cathode bypass cap on the power tube is a generous 330uf/100v electrolytic. The image below shows where that cap is found in the amp chassis. I paralleled this cap with a second one of similar value. It’s easy to do since both ends of the cap are readily accessible and although the stock value is adequate, I saw enough signal on it during my sweep tests that I felt the second cap was warranted. If you skip this mod, it will likely have minor impact on the amp sound, but again, caps are cheap, paralleling the existing cap is easy, and it can only help.
Lastly, the cathode bypass cap on the power tube is a generous 330uf/100v electrolytic. The image below shows where that cap is found in the amp chassis. I paralleled this cap with a second one of similar value. It’s easy to do since both ends of the cap are readily accessible and although the stock value is adequate, I saw enough signal on it during my sweep tests that I felt the second cap was warranted. If you skip this mod, it will likely have minor impact on the amp sound, but again, caps are cheap, paralleling the existing cap is easy, and it can only help.
Fortunately, the output transformer on the Univalve is a high quality component and there is no need to change it. The amp also has an L-Pad, or equivalent called a “Hot Plate,” built into it. That means you set the Hot Plate to control the speaker volume, while the volume control is used to determine how much you drive the tubes. In most amps the “drive” and “volume” controls only affect the preamp tube drive. The Univalve arrangement controls the drive to the power tube – a much richer source of non-linear effects and range of harmonics. The Univalve also has a unique "Attitude" control that is part of the V2 driver circuit. It's an interesting circuit that offers some control over the harmonics generated in the driver stage. The effect is most pronounced in high frequencies (It also adds a bit of high-boost) and also leads to the unique nature of the amp sound. If you want the cleanest sound, turn the "attitude" down to nothing. As you increase it you'll hear more of that scratchy growl. Changing the gain of this tube (V2) will likely impact that sound so tube changes are another control for this effect.
Fortunately, the output transformer on the Univalve is a high quality component and there is no need to change it. The amp also has an L-Pad, or equivalent called a “Hot Plate,” built into it. That means you set the Hot Plate to control the speaker volume, while the volume control is used to determine how much you drive the tubes. In most amps the “drive” and “volume” controls only affect the preamp tube drive. The Univalve arrangement controls the drive to the power tube – a much richer source of non-linear effects and range of harmonics. The Univalve also has a unique "Attitude" control that is part of the V2 driver circuit. It's an interesting circuit that offers some control over the harmonics generated in the driver stage. The effect is most pronounced in high frequencies (It also adds a bit of high-boost) and also leads to the unique nature of the amp sound. If you want the cleanest sound, turn the "attitude" down to nothing. As you increase it you'll hear more of that scratchy growl. Changing the gain of this tube (V2) will likely impact that sound so tube changes are another control for this effect.
That’s it. If you have a Univalve or can find a used one somewhere, you can make these simple mods and turn it into a unique balanced sound amp. When you play intervals or simple triads, the notes remain distinctive, yet the harmonics fill in around the notes. You'll also find the attitude control has a bigger impact on the sound once you've made these mods - this was a welcome surprise to me. If you modify a Univalve, based on these suggestions, I’d love to hear your reactions to the result. Enjoy!
That’s it. If you have a Univalve or can find a used one somewhere, you can make these simple mods and turn it into a unique balanced sound amp. When you play intervals or simple triads, the notes remain distinctive, yet the harmonics fill in around the notes. You'll also find the attitude control has a bigger impact on the sound once you've made these mods - this was a welcome surprise to me. If you modify a Univalve, based on these suggestions, I’d love to hear your reactions to the result. Enjoy!
Update 7/13/09 - I'm adding a note about amplifier hum. I've always heard a bit of power supply hum from this amp - even without any input and the volume set to zero. This was never an issue for me until recently when I did some recording using the line out. The hum is most pronounced at low (clean) volume settings. At first I thought the mods had something to do with this, but I no longer believe that's the case. It's at least partially a matter of too little filtering on the high voltage feed to the power tube / output transformer.
Update 7/13/09 - I'm adding a note about amplifier hum. I've always heard a bit of power supply hum from this amp - even without any input and the volume set to zero. This was never an issue for me until recently when I did some recording using the line out. The hum is most pronounced at low (clean) volume settings. At first I thought the mods had something to do with this, but I no longer believe that's the case. It's at least partially a matter of too little filtering on the high voltage feed to the power tube / output transformer.
Adding a 200uf 500v cap has reduced the hum to almost inaudible level so you may want to add this cap if the hum is a problem for you (whether or not you do any other mods). Warning: the power supply produces up to 480 volts, so do not attempt this unless you know about safely working on high voltage circuits. The cap I used has dual 100uf 500v sections and I parallel the two sections to get a 200uf cap. I tried a smaller 47uf cap and it reduced the hum, but I opted for the 200uf cap since it eliminated more hum and I had the physical room for it. The positive cap terminal is wired to the positive end of C23 and the negative end is tied to ground at the ground side of C24. See the picture below to get a complete view of the connections. Use a high voltage insulation rating on the positive wire (red color in my picture) to preclude any arcing to chassis. The picture also shows how a clamp is attached using one of the transformer screws to hold the cap without drilling any new holes in the chassis. I also suggest using a two-sided sticky-pad (or some silicon adhesive) to hold the cap against the chassis and keep it from vibrating.
Adding a 200uf 500v cap has reduced the hum to almost inaudible level so you may want to add this cap if the hum is a problem for you (whether or not you do any other mods). Warning: the power supply produces up to 480 volts, so do not attempt this unless you know about safely working on high voltage circuits. The cap I used has dual 100uf 500v sections and I parallel the two sections to get a 200uf cap. I tried a smaller 47uf cap and it reduced the hum, but I opted for the 200uf cap since it eliminated more hum and I had the physical room for it. The positive cap terminal is wired to the positive end of C23 and the negative end is tied to ground at the ground side of C24. See the picture below to get a complete view of the connections. Use a high voltage insulation rating on the positive wire (red color in my picture) to preclude any arcing to chassis. The picture also shows how a clamp is attached using one of the transformer screws to hold the cap without drilling any new holes in the chassis. I also suggest using a two-sided sticky-pad (or some silicon adhesive) to hold the cap against the chassis and keep it from vibrating.
A word on cap selection - my choice of a 500v rating is a bit tight given the supply produces almost 480v when switches are set for Hi-V and standby. In operation, that drops to 425v, but it's still a small margin. (Lo-V produces a more comfortable 300-315v.) A 600v cap would be more appropriate, if you can find one. I got my cap from Parts Express, and a similar cap shows up on the Mojo web site, among others. These are common in guitar amps, but IMO a 600v cap would be better. I didn't have one at the time, but recently found a 100uf/600v unit at http://www.justradios.com/orderform.html. You may want to consider that if you can figure out a way to mount it.
A word on cap selection - my choice of a 500v rating is a bit tight given the supply produces almost 480v when switches are set for Hi-V and standby. In operation, that drops to 425v, but it's still a small margin. (Lo-V produces a more comfortable 300-315v.) A 600v cap would be more appropriate, if you can find one. I got my cap from Parts Express, and a similar cap shows up on the Mojo web site, among others. These are common in guitar amps, but IMO a 600v cap would be better. I didn't have one at the time, but recently found a 100uf/600v unit at http://www.justradios.com/orderform.html. You may want to consider that if you can figure out a way to mount it.
I hope you enjoy the new sound if you try these mods.
I hope you enjoy the new sound if you try these mods.
There are several clips on the Music page recorded with the ModA version of the Univalve.
There are several clips on the Music page recorded with the ModA version of the Univalve.
These include: Happy.mp3, It Hurts Me Too.mp3, and Slowblues.mp3.
These include: Happy.mp3, It Hurts Me Too.mp3, and Slowblues.mp3.