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ModB is an extension of ModA. All the ModA changes were in place before I made the ModB changes.
ModB is an extension of ModA. All the ModA changes were in place before I made the ModB changes.
Update 1/16/11 - I've been using the GSP-1101 outputs into the THD and Blues Deluxe of late - a stereo rig. The THD roll input is used and the volume is set to about 1pm. While the Blues Deluxe (driven via power amp input) has some hum and hiss, it's much lower than that of the THD. This seemed odd - the roll-input signal passes through just one triode gain stage and the driver and power tube. What is causing all of the hiss and buzz? I spent a day digging into it and here's what I found. If you are bothered by the THD buzz and hiss, this mod is worth considering.
Update 1/16/11 - I've been using the GSP-1101 outputs into the THD and Blues Deluxe of late - a stereo rig. The THD roll input is used and the volume is set to about 1pm. While the Blues Deluxe (driven via power amp input) has some hum and hiss, it's much lower than that of the THD. This seemed odd - the roll-input signal passes through just one triode gain stage and the driver and power tube. What is causing all of the hiss and buzz? I spent a day digging into it and here's what I found. If you are bothered by the THD buzz and hiss, this mod is worth considering.
First off, I want to be clear that this is not a 60Hz hum issue - see the 7/13/09 update in ModA if that's your problem. I've already done that mod and this problem is strictly hiss and buzz... the kind of static-e background noise the THD exhibits as the volume goes up past the mid-point. Oddly, it actually seemed to decrease a bit as you max the volume - and therein lies a hint of the problem. If you look at the updated schematic below, you see that tube noise can't be the problem.
First off, I want to be clear that this is not a 60Hz hum issue - see the 7/13/09 update in ModA if that's your problem. I've already done that mod and this problem is strictly hiss and buzz... the kind of static-e background noise the THD exhibits as the volume goes up past the mid-point. Oddly, it actually seemed to decrease a bit as you max the volume - and therein lies a hint of the problem. If you look at the updated schematic below, you see that tube noise can't be the problem.
Tubes do create some hiss, but the tube stage is after the volume control so the volume control has no affect on any noise generated by the tube itself. So where is the hiss coming from? The volume control lies between the tube and the input, so there is nothing before the volume control but a few resisters and caps and the input jack. How does that create hiss? I plugged in a shorted input cable to eliminate any input noise coming from outside the amp. The amp was still noisy and the noise level varied with the volume control setting. If you look at the amp internals you see the input jack and volume control are near each other, but the tube is way across the chassis. The tube grid (pin 7) is very high-impedance, so any wires connected to it are very sensitive to electric fields. The wires and PC traces between the volume control and grid are a source of noise pickup. Since the earlier mod (ModA) removed C7, there is nothing on the circuit board between the volume control wiper and the tube grid. I therefore removed the volume control center pin wire and added a shielded cable that runs to the the tube grid. The cable shield is grounded only at the volume control (Vol 3). In addition, I replaced the wire running to the input of the volume control (Vol 1) with a shielded cable. Now the paths to the volume control and between the control and tube grid are shielded. The only unshielded paths that remain are from the input jack to the volume control area - about 2-3 inches of PC traces. These wiring changes significantly reduced (by ~1/2) the hiss and buzz.
Tubes do create some hiss, but the tube stage is after the volume control so the volume control has no affect on any noise generated by the tube itself. So where is the hiss coming from? The volume control lies between the tube and the input, so there is nothing before the volume control but a few resisters and caps and the input jack. How does that create hiss? I plugged in a shorted input cable to eliminate any input noise coming from outside the amp. The amp was still noisy and the noise level varied with the volume control setting. If you look at the amp internals you see the input jack and volume control are near each other, but the tube is way across the chassis. The tube grid (pin 7) is very high-impedance, so any wires connected to it are very sensitive to electric fields. The wires and PC traces between the volume control and grid are a source of noise pickup. Since the earlier mod (ModA) removed C7, there is nothing on the circuit board between the volume control wiper and the tube grid. I therefore removed the volume control center pin wire and added a shielded cable that runs to the the tube grid. The cable shield is grounded only at the volume control (Vol 3). In addition, I replaced the wire running to the input of the volume control (Vol 1) with a shielded cable. Now the paths to the volume control and between the control and tube grid are shielded. The only unshielded paths that remain are from the input jack to the volume control area - about 2-3 inches of PC traces. These wiring changes significantly reduced (by ~1/2) the hiss and buzz.
The next step involves adding a capacitor between the grid and plate of the tube stage. This is already done for the rock-input stage (C1) and is a common approach to reducing RF or other noises on input stages. This new cap increases the intrinsic "Miller Capacitance" of tubes (http://en.wikipedia.org/wiki/Miller_effect). The roll input stage lacks this cap, but the stage gets quieter if you add it. C1 is a 22pf ceramic cap. I added a new cap (Cz) between pins 6 and 7 on V1. Like C1, Cz should be soldered right to the tube pins. The cap should have a high voltage rating (600v or greater). As to value, it's question of taste. I tried a 22pf to match C1 and the amp got very quiet but also very creamy and smooth (yes, this cap has an impact on tone). I like a bright sound, so I settled on a 13pf cap to keep the sound brighter and preserve the "THD" character. I suggest trying values between 10pf and 33pf. The higher the value, the more you reduce the hiss and smoother (less bright) the amp tone.
The next step involves adding a capacitor between the grid and plate of the tube stage. This is already done for the rock-input stage (C1) and is a common approach to reducing RF or other noises on input stages. This new cap increases the intrinsic "Miller Capacitance" of tubes (http://en.wikipedia.org/wiki/Miller_effect). The roll input stage lacks this cap, but the stage gets quieter if you add it. C1 is a 22pf ceramic cap. I added a new cap (Cz) between pins 6 and 7 on V1. Like C1, Cz should be soldered right to the tube pins. The cap should have a high voltage rating (600v or greater). As to value, it's question of taste. I tried a 22pf to match C1 and the amp got very quiet but also very creamy and smooth (yes, this cap has an impact on tone). I like a bright sound, so I settled on a 13pf cap to keep the sound brighter and preserve the "THD" character. I suggest trying values between 10pf and 33pf. The higher the value, the more you reduce the hiss and smoother (less bright) the amp tone.
*** Be careful with this process*** Pin 6 is connected to high voltage! Give the amp 2-3 mins of "off-time" to discharge the power caps before you do this.
*** Be careful with this process*** Pin 6 is connected to high voltage! Give the amp 2-3 mins of "off-time" to discharge the power caps before you do this.
The 13pf cap reduced the hiss to a level comparable to the Blues Deluxe, which I can live with. The 22pf cap was even quieter. The 13pf tone is still noticeably smoother than with no cap. Between the shielding and cap, the amp tone has lost some of the grittiness that it's known for. I'm fine with the changes - less noise and smoother tone... I'll take it. The quiet is great and It now chimes nicely at the top-end, a bit like my vox, but it's still has a THD character. The smoother tone is also evident with the hot plate on and the amp pushed hard into break up. It's less fuzzy, which is an improvement IMO.
The 13pf cap reduced the hiss to a level comparable to the Blues Deluxe, which I can live with. The 22pf cap was even quieter. The 13pf tone is still noticeably smoother than with no cap. Between the shielding and cap, the amp tone has lost some of the grittiness that it's known for. I'm fine with the changes - less noise and smoother tone... I'll take it. The quiet is great and It now chimes nicely at the top-end, a bit like my vox, but it's still has a THD character. The smoother tone is also evident with the hot plate on and the amp pushed hard into break up. It's less fuzzy, which is an improvement IMO.
See the pictures below for a better idea of how I did these mods. The new schematic (above) shows the new cap and gives a hint at the shielding route.
See the pictures below for a better idea of how I did these mods. The new schematic (above) shows the new cap and gives a hint at the shielding route.
This first picture shows how it looked at the end. You can see the two shielded cables at the volume control and the new cap on the V1 socket.
This first picture shows how it looked at the end. You can see the two shielded cables at the volume control and the new cap on the V1 socket.
The next image shows the new Cz cap and the shielded connection to pin 7. Note that the wire from pin7 to the PC board is removed since the new cable now makes that connection to the volume control.
The next image shows the new Cz cap and the shielded connection to pin 7. Note that the wire from pin7 to the PC board is removed since the new cable now makes that connection to the volume control.
The last image shows the connections to the volume control. Note that the cable to the PC board has its shield connected on the board side, and the cable running to V1 has its shield connected to the volume control ground pin (Vol 3).
The last image shows the connections to the volume control. Note that the cable to the PC board has its shield connected on the board side, and the cable running to V1 has its shield connected to the volume control ground pin (Vol 3).
Here are two audio clips of the modded amp. It's just me rambling and trying the new sounds. I played an American Deluxe Telecaster, and you'll hear the pickups switched to get a sample of tone range. In both cases, the left channel is the THD line output. The right channel is the THD input feed coming from the GSP-1101. The GSP is set for direct out - meaning no modeling - just some tremolo and reverb. The THD settings in both clips are volume at max with tone and attitude controls at 1pm. LoV is selected. These clips are unedited so you hear everything - including my wandering aimlessly :-). Notice the hum and hiss levels are pretty low for a cranked volume setting.
Here are two audio clips of the modded amp. It's just me rambling and trying the new sounds. I played an American Deluxe Telecaster, and you'll hear the pickups switched to get a sample of tone range. In both cases, the left channel is the THD line output. The right channel is the THD input feed coming from the GSP-1101. The GSP is set for direct out - meaning no modeling - just some tremolo and reverb. The THD settings in both clips are volume at max with tone and attitude controls at 1pm. LoV is selected. These clips are unedited so you hear everything - including my wandering aimlessly :-). Notice the hum and hiss levels are pretty low for a cranked volume setting.
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