Hello,
I'm building a Tube Guitar Amp vaguely inspired to Mesa Boogie mk IIC+ with some ideas taken from later models and other amps.
I decided to start by fixing a very coarse amp topology and focus on prototyping the power supply first and once tested I will make a PCB.
Before starting to build, I tried to lay down a grounding scheme. Having read the related books by Merlin Blencowe thoroughly, in particular the chapter on grounding, I decided to go for a "local star approach" as strictly as possible, as the amp is high-gain.
However, I think I'm missing some points (I marked in red my doubts on the schematics).
My concerns are mainly related to safety, avoiding damaging the transformers, and avoiding ground loops.
My transformer has the following windings: (345/55/0/345) (3.15/0/0.15) (5v - no center tap) (12V - no center tap)
The primary mains circuit has a double fuse, double switch, a MOV, a NTC, then goes into the primary winding. The safety ground is bolted to chassis close to the IEC inlet. The only other connection to chassis is at the preamp input (see schematics), with eventually a switchable ground lift circuit.
1) On the first winding I'm using two-phase rectifiers since I have center taps. For the other windings I'm using bridge rectifiers. Is it safe for the transformer to connect their ground together (eg. red mark on the top right in the schematics), or would I short a winding/rectifier somehow? Since I plan to always bring wires coupled with their relative ground, my perception is that the connection marked in red only serves as voltage reference and the big flow of current from 12V / 5V rails would flow directly into their capacitor or rectifier, so they shouldn't produce significant hum. Am I correct?
2) One thing that isn't specified in books or posts I've read on ground loops, is what happen if I have to supply a pre-amp section from a different rail.
As you can see in the pre-amp sketch on the bottom, my tube stages in the pre-amp are fed by the same wires (A+ and A-), however I also have a solid state graphic equalizer in the middle of the pre-amp, which needs 24V that are taken by the bias windings. What is the appropriate grounding in this case? I suspect them to be either D- or the preamp ground bus. However, in the latter case, current from D+ would travel a long way down to A-/main reservoir and then... reach the EQ reservoir again? (something doesn't add up to me). In the former case, current from D+ would traverse the EQ and get back to the Eq reservoir in D- a shorter loop, but then I'm not sure what is the signal reference in that case. What is the solution that allows to generate less ground loops?
3) In this case D+ actually comes from the same windings of A+, so they should naturally be referenced to the same center tap, which is connected to the same physical point as A-, so It might be not a big issue. But what if I were to connect the EQ to the 12V rail?
4) As you can notice, there are two separate connections for power amp HT (B) and pre amp HT (A). This is because I plan to design a power scaling circuit from Merlin's book, which allows to drop the voltage of power amp B+, screens and bias voltage, leaving the rest unaffected. It will stay on a separate board, since I want to try the "stock version" of the amp first to measure actual voltages and actual maximum power. Again, I have to provide two different voltages to two components in the signal chain, and I'm not sure how to think. I maked it as a possible ground loop, but I don't really think it is the case.
I apologize for the wall of text, but I couln't make up my mind. Feel free to point out any other silly mistake you spot on the schematics 🙂.
Thanks!
I'm building a Tube Guitar Amp vaguely inspired to Mesa Boogie mk IIC+ with some ideas taken from later models and other amps.
I decided to start by fixing a very coarse amp topology and focus on prototyping the power supply first and once tested I will make a PCB.
Before starting to build, I tried to lay down a grounding scheme. Having read the related books by Merlin Blencowe thoroughly, in particular the chapter on grounding, I decided to go for a "local star approach" as strictly as possible, as the amp is high-gain.
However, I think I'm missing some points (I marked in red my doubts on the schematics).
My concerns are mainly related to safety, avoiding damaging the transformers, and avoiding ground loops.
My transformer has the following windings: (345/55/0/345) (3.15/0/0.15) (5v - no center tap) (12V - no center tap)
The primary mains circuit has a double fuse, double switch, a MOV, a NTC, then goes into the primary winding. The safety ground is bolted to chassis close to the IEC inlet. The only other connection to chassis is at the preamp input (see schematics), with eventually a switchable ground lift circuit.
1) On the first winding I'm using two-phase rectifiers since I have center taps. For the other windings I'm using bridge rectifiers. Is it safe for the transformer to connect their ground together (eg. red mark on the top right in the schematics), or would I short a winding/rectifier somehow? Since I plan to always bring wires coupled with their relative ground, my perception is that the connection marked in red only serves as voltage reference and the big flow of current from 12V / 5V rails would flow directly into their capacitor or rectifier, so they shouldn't produce significant hum. Am I correct?
2) One thing that isn't specified in books or posts I've read on ground loops, is what happen if I have to supply a pre-amp section from a different rail.
As you can see in the pre-amp sketch on the bottom, my tube stages in the pre-amp are fed by the same wires (A+ and A-), however I also have a solid state graphic equalizer in the middle of the pre-amp, which needs 24V that are taken by the bias windings. What is the appropriate grounding in this case? I suspect them to be either D- or the preamp ground bus. However, in the latter case, current from D+ would travel a long way down to A-/main reservoir and then... reach the EQ reservoir again? (something doesn't add up to me). In the former case, current from D+ would traverse the EQ and get back to the Eq reservoir in D- a shorter loop, but then I'm not sure what is the signal reference in that case. What is the solution that allows to generate less ground loops?
3) In this case D+ actually comes from the same windings of A+, so they should naturally be referenced to the same center tap, which is connected to the same physical point as A-, so It might be not a big issue. But what if I were to connect the EQ to the 12V rail?
4) As you can notice, there are two separate connections for power amp HT (B) and pre amp HT (A). This is because I plan to design a power scaling circuit from Merlin's book, which allows to drop the voltage of power amp B+, screens and bias voltage, leaving the rest unaffected. It will stay on a separate board, since I want to try the "stock version" of the amp first to measure actual voltages and actual maximum power. Again, I have to provide two different voltages to two components in the signal chain, and I'm not sure how to think. I maked it as a possible ground loop, but I don't really think it is the case.
I apologize for the wall of text, but I couln't make up my mind. Feel free to point out any other silly mistake you spot on the schematics 🙂.
Thanks!