Tube preamp pedal design and safety

[aiyiadam]

Hello all,

I am planning to build myself a two channel preamp pedal. It will have two 12AX7’s. The pedal will be fed with 12VDC and I will try to use a step up booster for the B+.

My problem is the grounding.. The preamp could be grounded with an instrument cable, when it is connected to an amp. But what if I am using the preamp with a cab simulation and a portable headphone amp? I surely could ground the device separately (and maybe add a ground lift switch on the outputs), which the commercial producers don’t do.

How do the commercial producers get away with that safety issue? Am I missing something here?

Thanks,

Evren

[R.G.]

I think you may be missing some of the intent of safety grounding. It's not designed to protect you from high voltage DC, it's specifically to protect you from AC wall-socket voltages.

AC line voltage is inherently ground referenced. In the USA and Canada, and most other places, one of the wall-socket prongs is "line", one is "neutral" and the third is "safety ground". Line is the one with the nominal wall-socket voltage on it. Safety ground is connected to a metallic rod driven into the dirt near the main breaker panel, and neutral is connected to safety ground at the panel. Neutral is only different from safety ground by the voltage offset caused by current flowing in the neutral wires. Inside the device using power, both line and neutral are scrupulously kept from touching anything the user can touch, because you never really know without testing it if the wall socket is wired the right way. It is possible for current to leak from the line wire to the metallic chassis of the equipment. Safety ground tied to the chassis "eats" this leakage without any danger to the user.

In your project, you are powering the whole thing from 12Vdc. So immediately, safety ground plays no part in your grounding scheme. Your high voltage is generated by stepping up the 12Vdc, and that isn't necessarily connected to any kind of safety ground, except by the signal cable. The high voltage inside the pedal is still dangerous, but safety ground can't help you there, no more than it would if you had a stack of 9V batteries in series to make hundreds of volts.

[pdf64]

Much easier to step up using (eg with a 12/240 mini PT) a 12Vac feed
http://www.dogstar.dantimax.dk/tubes...s/mctb1sch.gif

Have a look at how the Fender Reverb decouples circuit common from chassis ground thereby avoiding ground loops (not that your project should need that, as RG noted, provided that the 12Vac feed isn't referenced to mains ground).
CR5, CR6, R23 https://schematicheaven.net/fenderam...erb_manual.pdf

If you're connecting the pedal output to more than one device, to prevent ground loops via the pedal, additional outputs may need the signal common isolated somehow, eg small signal transformer; see RG's geofex site, eg http://www.geofex.com/FX_images/spltr2.gif

[Leo_Gnardo]

Much easier to step up using (eg with a 12/240 mini PT) a 12Vac feed

Agreed - that's what Mesa did with their V-Twin preamp pedal. Whether you like their products or not, it's a clever way of applying power.

[aiyiadam]

Thank you all for the responses.

In your project, you are powering the whole thing from 12Vdc. So immediately, safety ground plays no part in your grounding scheme. Your high voltage is generated by stepping up the 12Vdc, and that isn't necessarily connected to any kind of safety ground, except by the signal cable. The high voltage inside the pedal is still dangerous, but safety ground can't help you there, no more than it would if you had a stack of 9V batteries in series to make hundreds of volts.

My assumption was this: If I fuse the HV and connect the metal case to the safety ground and the negative side of the booster (or the tranny), a wire touching the case would blow the fuse before the HV hurts somebody. Without the safety ground I had considered this approach unsafe.

Anyways, I will just forget about the safety ground and try to design the pedal as safe as I can. Big cap on the guitar input, heat shrink tubing on the plate connectors,... The whole circuit will float in the case. I could maybe connect the "line out" ground to the case for some EMI shielding. Or maybe not..

Much easier to step up using (eg with a 12/240 mini PT) a 12Vac feed

Good Idea.. I will also try a linear power source like this and compare it to the SMPS. Since it will be a 4 stage circuit similar to the Soldano SLO, I might need DC for the heaters. Maybe something like this: https://el34world.com/projects/image...lSchematic.gif

If I can manage the noise level, SMPS could maybe be more advantageous. Those things are smaller, could be cheaper and I could save the DC rectifier for the heaters. For example Kingsley pedals like Jouster (and very probably the new Victory V4’s) use a step up booster.

I had actually ordered this: https://www.aliexpress.com/item/DC-D...3d454c4dDo7c8z

Thanks also for the Fender Reverb and the splitter schematics. I will take a look and grab for ideas. For preventing ground loops I could use the reamping box, which I had built earlier. Good enough for connecting two devices.

Agreed - that's what Mesa did with their V-Twin preamp pedal. Whether you like their products or not, it's a clever way of applying power.

I think Mike Soldano had followed a similar way for his Supercharger Reissue.