Hi all,
I'm currently working on an 'effect' to have adjustable amounts of second order triode distortion. Here you can see the schematic of one channel:
The input OPA amplifies the standard -10dBv to ~11Vpp. This is then fed through the 12-position stepped attenuator P1 giving 11x 4dB steps. The attenuated signal is then fed towards the grids of a paralleled 6SN7 and afterwards again attenuated by P2 to give an overall gain of 1. In the first position, the grids-stoppers are shorted to ground and the input signal is directly fed to the output OPA.
This gives adjustable ~0.05-5% of second order harmonics while keeping the loudness the same.
Here you can also see the layout, in the first image coloured according to the layers, with red facing upwards to the tubes and blue facing down towards the components. In the second they are coloured red for cathode, green for anode, green for grid, blue for heater, magenta for ground and cyan for input and output. The PCB will have a four layer stackup with both inner layers filled with solid ground planes:
I'm now wondering if I have overseen something in the schematic or layout. Specifically, are R3, R4 and R16 necessary? Should I try to make the green traces shorter?
Best regards
Dominic
I'm currently working on an 'effect' to have adjustable amounts of second order triode distortion. Here you can see the schematic of one channel:
The input OPA amplifies the standard -10dBv to ~11Vpp. This is then fed through the 12-position stepped attenuator P1 giving 11x 4dB steps. The attenuated signal is then fed towards the grids of a paralleled 6SN7 and afterwards again attenuated by P2 to give an overall gain of 1. In the first position, the grids-stoppers are shorted to ground and the input signal is directly fed to the output OPA.
This gives adjustable ~0.05-5% of second order harmonics while keeping the loudness the same.
Here you can also see the layout, in the first image coloured according to the layers, with red facing upwards to the tubes and blue facing down towards the components. In the second they are coloured red for cathode, green for anode, green for grid, blue for heater, magenta for ground and cyan for input and output. The PCB will have a four layer stackup with both inner layers filled with solid ground planes:
I'm now wondering if I have overseen something in the schematic or layout. Specifically, are R3, R4 and R16 necessary? Should I try to make the green traces shorter?
Best regards
Dominic
Last edited:
Why did you waste time designing a PCB for an unproven circuit?
PCB looks nice, but might need rework.😳
PCB looks nice, but might need rework.😳
I would add a resistor from the C3 capacitor side of R9 to the ground, to help discharge C3. On your original schematic, C3 will discharge trough the op-amp input.
Both sections of the 6SN7 should have identical parameters to enable a successful parallel. I believe that R3 and R4, due to the very low value, do not help if the sections are dissimilar. A new production tube, or an expensive selected NOS is required. You can remove this requirement by using a single section and maybe add a switch to select the one to use. You may then select the less noisy/best sounding triode of inexpensive unbalanced/vintage used tubes.
Both sections of the 6SN7 should have identical parameters to enable a successful parallel. I believe that R3 and R4, due to the very low value, do not help if the sections are dissimilar. A new production tube, or an expensive selected NOS is required. You can remove this requirement by using a single section and maybe add a switch to select the one to use. You may then select the less noisy/best sounding triode of inexpensive unbalanced/vintage used tubes.
Not sure why you need a multilayer board for this, I'd look more at the layout.
You might find that more 2nd harmonic is available by running at lower voltages, the datasheets will tell you I think.
You might find that more 2nd harmonic is available by running at lower voltages, the datasheets will tell you I think.
4 layer does have the same price as 2 layers on most prototype board services. I've just noticed that the octal socket traces aren't mirrored so tubes will be on the same side of the components. On my builds I place the sockets on the opposite side, to ease the mounting inside a chassis.
@jhstewart9: some PCB prototype services are giving away small boards like this one, you pay only the shipping so it is possible to get boards for free by adding them to a scheduled order for another project. The marketing idea is that they will later get your orders for production runs due to the convenience of having the gerber files already processed and tested by them; this easy trick worked for me several times.
It is also pretty fast and easy to design a PCB, if you are familiar with the software. For simple boards like this one I often spend more time to input the circuit in the schematic capture and made it readable than placing traces; you see that this schematic is pretty raw and does not follow the rules of classic tube circuits. I believe that the primary function is simulation+PCB design rather than human comprehension. This circuit does use SMD operational amplifiers so an experimental point to point built would require adapters; a further incentive to prototype directly on the PCB instead.
I am more puzzled by the choice to not use SMD resistors and SMT C0G capacitors; or at least have the option for both traditional THT and SMD parts. This way at least the the first test could be done with cheap SMD parts that are fast to mount - you may also ask the PCB service to do this for you - and may be discarded with the board when errors have been ironed out and a second iteration of the PCB is ready.
@jhstewart9: some PCB prototype services are giving away small boards like this one, you pay only the shipping so it is possible to get boards for free by adding them to a scheduled order for another project. The marketing idea is that they will later get your orders for production runs due to the convenience of having the gerber files already processed and tested by them; this easy trick worked for me several times.
It is also pretty fast and easy to design a PCB, if you are familiar with the software. For simple boards like this one I often spend more time to input the circuit in the schematic capture and made it readable than placing traces; you see that this schematic is pretty raw and does not follow the rules of classic tube circuits. I believe that the primary function is simulation+PCB design rather than human comprehension. This circuit does use SMD operational amplifiers so an experimental point to point built would require adapters; a further incentive to prototype directly on the PCB instead.
I am more puzzled by the choice to not use SMD resistors and SMT C0G capacitors; or at least have the option for both traditional THT and SMD parts. This way at least the the first test could be done with cheap SMD parts that are fast to mount - you may also ask the PCB service to do this for you - and may be discarded with the board when errors have been ironed out and a second iteration of the PCB is ready.
Thank you all for the input! Maybe this is a more readable schematic?
Rin (P1 in the first schematic) is a 10kOhm stepped attenuator, Rout (P2 in the first schematic) is one with 470kOhm. Please note that the resistors in question (R3, R4 and R16) from the first schematic are not shown in this simulation.
I do hope that this circuit is 'simple' enough to not run into major issues. I choose a PCB because they are cheaper then point-to-point wiring and to be honest, I am not to confident in getting point-to-point right. I choose THT for easy soldering and better rework-ability.
I will use new production Psvane UK-6SN7, so I could remove R3 and R4? I was planning on maybe increasing their values if I run into oscillations. This was also the idea with R16. Would that be worth to try or would cleaner routing be worth removing them?
Best regards
Dominic
Rin (P1 in the first schematic) is a 10kOhm stepped attenuator, Rout (P2 in the first schematic) is one with 470kOhm. Please note that the resistors in question (R3, R4 and R16) from the first schematic are not shown in this simulation.
I do hope that this circuit is 'simple' enough to not run into major issues. I choose a PCB because they are cheaper then point-to-point wiring and to be honest, I am not to confident in getting point-to-point right. I choose THT for easy soldering and better rework-ability.
As you can see clearer in this schematic, C3 is discharged through the stepped attenuator Rout.
I will use new production Psvane UK-6SN7, so I could remove R3 and R4? I was planning on maybe increasing their values if I run into oscillations. This was also the idea with R16. Would that be worth to try or would cleaner routing be worth removing them?
Best regards
Dominic